Ji-Cheng Zhao - Academia.edu (original) (raw)

Papers by Ji-Cheng Zhao

Research paper thumbnail of Nonlinear Arrhenius behavior of self-diffusion in β−Ti and Mo

Physical Review Materials

While anomalous diffusion coefficients with non-Arrhenius like temperature dependence are observe... more While anomalous diffusion coefficients with non-Arrhenius like temperature dependence are observed in a number of metals, a conclusive comprehensive framework of explanation has not been brought forward to date. Here, we use first-principles calculations based on density functional theory to calculate self-diffusion coefficients in the bcc metals Mo and-Ti by coupling quasiharmonic transition state theory and large displacement phonon calculations and show that anharmonicity from thermal expansion is the major reason for the anomalous temperature dependence. We use a modified Debye approach to quantify the thermal expansion over the entire temperature range and introduce a method to relax the vacancy structure in a mechanically unstable crystal such as-Ti. Thermal expansion is found to weakly affect the activation enthalpy but has a strong effect on the prefactor of the diffusion coefficient, reproducing the non-linear, non-Arrhenius "anomalous" self-diffusion in both bcc systems with good agreement between calculation and experiment. The proposed methodology is general and simple enough to be applicable to other mechanically unstable crystals.

Research paper thumbnail of Improved model for the thermal conductivity of binary metallic systems

Cornell University - arXiv, Jun 29, 2016

We extended and updated Mott's two-band model for the composition-dependence of thermal and elect... more We extended and updated Mott's two-band model for the composition-dependence of thermal and electrical conductivity in binary metal alloys based on high-throughput time-domain thermoreflectance (TDTR) measurements on diffusion multiples and scatterer-density calculations from first principles. Examining Au-Cu, Au-Ag, Pd-Ag, Pd-Cu, Pd-Pt, Pt-Rh, and Ni-Rh binary alloys, we found that the nature of the two dominant scatterer-bands considered in the Mott model changes with the alloys, and should be interpreted as a combination of the dominant element-specific sand/or d-orbitals. Using calculated orbital and element-resolved density-of-states values calculated with density functional theory as input, we determined the correct orbital mix that dominates electron scattering for all examined alloys and found excellent agreement between fitted models and experiments. This general model of the composition dependence of the thermal and electrical resistivity can be readily implemented into the CALPHAD framework.

Research paper thumbnail of Microstructure and Fracture Toughness of an Aluminum-Steel Impact Weld and Effect of Thermal Exposure

Metallurgical and Materials Transactions A, 2021

Welding of aluminum alloys to steel is increasingly important in manufacturing, however the use o... more Welding of aluminum alloys to steel is increasingly important in manufacturing, however the use of fusion welding is difficult because of disparate melting points and the possibility of intermetallic compound (IMC) formation. Here, an impact welding technique, vaporizing foil actuator welding, was utilized to produce solid-state joints between AA1100-O and 1018 mild steel. The relationship between weld processing conditions, microstructure, and mechanical properties were investigated. For this purpose, the welds were annealed between 300°C and 600°C and a combination of optical and scanning electron microscopy, along with image analysis was performed to characterize the weld microstructure and monitor IMC growth. Wedge testing was applied to understand the effect of annealing on the weld fracture toughness. A numerical model incorporating the Fick's laws of diffusion, grain boundary diffusion, and grain growth kinetics was also developed to simulate the IMC growth. The heterogeneity in the original microstructure caused persistent differences in IMC growth, as initial IMC seemed to increase nucleation and growth. Simulation results indicated short circuit diffusion to be the major contributor to IMC growth since it is consistently faster then experimental IMC growths compared with the computational results that used lattice diffusion only. Wedge testing reveals increased weld toughness for modest anneals of 300°C, possibly due to homogeneity at the weld interface while avoiding IMC growth.

Research paper thumbnail of Rapid Synthesis and Sintering of Metals from Powders

Advanced Science, 2021

Powder to bulk processes, such as additive manufacturing and metal injection molding (MIM), have ... more Powder to bulk processes, such as additive manufacturing and metal injection molding (MIM), have enabled great potential for complex metal designing and manufacturing. However, additive manufacturing process normally introduces a high residue stress and textures due to the locally intense temperature. MIM is an excellent batch manufacturing process; nevertheless, it is not suitable for rapid screening and development of new metal compositions and structures due to the slow sintering process. Herein, an ultrafast high-temperature sintering (UHS) process is reported that enables the rapid synthesis and sintering of bulk metals/alloys and intermetallic compounds. In this process, elemental powders are mixed and pressed into pellets, followed by UHS sintering in just seconds at a temperature between 1000 and 3000°C. Three representative compositions, including pure metals, intermetallics, and multielement alloys, are demonstrated with a broad range of melting points. The UHS process for metal sintering is nonmaterials specific, in addition to being extremely rapid, which make it suitable for materials discovery. Furthermore, the sintering method does not apply pressure to the samples, making it compatible with 3D printing and other additive manufacturing processes of complex structures. This rapid sintering technique will greatly facilitate the development and manufacturing of metals and alloys. Powder metallurgy is a widely used technology to manufacture metal components from their powders. Unlike traditional metal processing techniques (e.g., casting), powder metallurgy does

Research paper thumbnail of A simple yet general model of binary diffusion coefficients emerged from a comprehensive assessment of 18 binary systems

Research paper thumbnail of Diffusion Coefficients and Phase Equilibria of the Cu-Zn Binary System Studied Using Diffusion Couples

Journal of Phase Equilibria and Diffusion, 2020

The diffusion behavior and phase equilibria in the Cu-Zn binary system were investigated using so... more The diffusion behavior and phase equilibria in the Cu-Zn binary system were investigated using solidsolid and solid-liquid diffusion couples. Heat treatments at temperatures ranging from 100 to 750°C were performed and the samples were examined using optical microscopy, energy dispersive x-ray spectroscopy, and electron probe microanalysis to identify the phases and to obtain composition profiles. Solubility limits of both solid solution and intermetallic phases were then evaluated, and a forwardsimulation analysis (FSA) was applied to extract interdiffusion coefficients. The composition profiles from Hoxha et al. were also re-analyzed using FSA to obtain more reliable diffusion coefficient data without the assumption of constant diffusion coefficients for the intermetallic phases. A comprehensive assessment of the interdiffusion coefficients in three intermetallic phases of the Cu-Zn system was performed based on the results from the current study as well as those in the literature. Activation energies and Arrhenius pre-factors were evaluated for each phase as a function of composition. The fitted equations based on the comprehensive assessment have the capabilities of computing the interdiffusion coefficients of each of the phases at a given composition and temperature. Suggested modifications to the Cu-Zn binary phase diagram were presented based on the new experimental information gathered from the present study. A clear explanation is provided for the puzzling low Zn concentrations often observed in the Cu-rich fcc phase of Cu-Zn diffusion couples in comparison with the expected high solubility values based on the equilibrium Cu-Zn phase diagram.

Research paper thumbnail of On the quantitative characterization of weld microstructures

Scripta Materialia, 2020

A new technique to calculate spatial variations of grain shape, grain size, and grain boundary cu... more A new technique to calculate spatial variations of grain shape, grain size, and grain boundary curvature in the fusion zone (FZ) of refractory metal alloys has been developed. This technique was applied to quantitatively evaluate changes in microstructures of simulated molybdenum welds available in the literature. Use of this technique showed that a 50% increase in weld speed more than doubled the fraction of grain segments oriented between 0 °and 30 °to the normal to the weld direction along with a decrease in grain size from approximately 120 μm to 84 μm in the weld direction at the FZ centerline.

Research paper thumbnail of Phase Equilibria and Diffusion in the Ni-Cr-Pt System at 1200 °C

Journal of Phase Equilibria and Diffusion, 2019

Pt modified NiAl, which is often simply called Pt aluminide, is widely used in jet engines as a p... more Pt modified NiAl, which is often simply called Pt aluminide, is widely used in jet engines as a protective coating against oxidation and hot corrosion as well as a bond coat for thermal barrier coatings. Phase equilibria and diffusion coefficients in the Ni-Cr-Pt ternary system are thus valuable data for modeling the behavior of Pt aluminide coatings on Ni-based superalloys in which Cr is an essential alloying element. A Ni-Cr-Pt diffusion multiple was made and annealed at 1200°C for 100 h to obtain the first reliable isothermal section phase diagram for this ternary system. Interdiffusion coefficients were extracted from the measured diffusion concentration profiles using a forward simulation analysis for all the single-phase regions of the Ni-Cr, Ni-Pt and Cr-Pt binary systems. The impurity diffusion coefficient data obtained from this study are combined with literature data to assess reliable Arrhenius equations for the impurity diffusion coefficients of Pt in Ni, Ni in Pt, and Cr in Ni. An Arrhenius equation for the impurity diffusion coefficient of Cr in Pt is recommended for the first time by combining a fitted prefactor from experimental data and an activation energy value from first principles calculations. Keywords binary diffusion Á diffusion couples Á diffusivity coefficient Á experimental phase equilibria Á impurity diffusivity Á phase diagram This invited article is part of a special issue of the Journal of Phase Equilibria and Diffusion in honor of the 2018 J. Willard Gibbs Phase Equilibria Award winner Dr. John Morral. The award was presented to Dr. Morral during MS&T'18, October 14-18, 2018, in Columbus, Ohio, ''for fundamental and applied research on topology of phase diagrams and theory of phase equilibria resulting in major advances in the calculation and interpretation of phase equilibria and diffusion.''

Research paper thumbnail of Anisotropic thermal conductivity of magnetocaloric AlFe2B2

Materialia, 2018

The thermal conductivity () and heat capacity (C p) of the magnetocaloric AlFe 2 B 2 compound wer... more The thermal conductivity () and heat capacity (C p) of the magnetocaloric AlFe 2 B 2 compound were investigated to assess its room temperature heat transfer and thermal management potential relative to other magnetocaloric materials such as FeRh, Gd, Gd 5 (Si,Ge) 4 La(Fe,Si) 13 and (Mn,Fe) (P,As). The room temperature thermal conductivity was determined along the three principal crystallographic directions of AlFe 2 B 2 using the timedomain thermoreflectance (TDTR) method on six randomly oriented AlFe 2 B 2 grains, resulting in an observed anisotropic thermal conductivity with a 40% larger along the c-axis: x = 4.7 ± 0.1 W/mK, y = 4.4 ± 0.1 W/mK, and z = 6.8 ± 0.3 W/mK.

Research paper thumbnail of pydiffusion: A Python Library for Diffusion Simulation and Data Analysis

Journal of Open Research Software, 2019

pydiffusion is a free and open-source Python library designed to solve diffusion problems for bot... more pydiffusion is a free and open-source Python library designed to solve diffusion problems for both singlephase and multi-phase binary systems. The key features of pydiffusion include fast simulation of multiphase diffusion and extraction of diffusion coefficients from experimental concentration profiles using forward simulation analysis. pydiffusion also provides various mathematical models for diffusion profile smoothing, diffusion coefficient evaluation, and data optimization. In pydiffusion, diffusion profiles and various phases are easy to define or read from the experimental datasets. Visualization tools based on Matplotlib are also provided to help users present or refine their simulations and analysis.

Research paper thumbnail of First measurement of the full elastic constants of Ni-based superalloy René 88DT

Scripta Materialia, 2018

The full elastic constants of Ni-based superalloy René 88DT were measured for the first time usin... more The full elastic constants of Ni-based superalloy René 88DT were measured for the first time using a recently developed method that matches the computed surface acoustic wave (SAW) velocities from an elastodynamic model with experimental velocities of SAWs that are generated and detected with a femtosecond laser. The experimental measurements were performed on several grains of a polycrystalline sample without the need of growing a single crystal. The computed Young's modulus from the resultant elastic constants agrees with the literature values, lending confidence on our measured elastic constants: C 11 = 267.1, C 12 = 170.5, and C 44 = 107.6 (GPa).

Research paper thumbnail of Recommendation for reliable evaluation of diffusion coefficients from diffusion profiles with steep concentration gradients

Materialia, 2018

Essentially all diffusion coefficients, except for a tiny fraction from tracer experiments, are e... more Essentially all diffusion coefficients, except for a tiny fraction from tracer experiments, are evaluated from diffusion concentration profiles that are predominantly measured via wavelength or energy dispersive spectroscopy of characteristic X-rays of the pertinent elements. X-ray generation (interaction) volume simulation and an analysis of experimental profiles show that the extracted diffusion coefficients can be inflated by up to three orders of magnitude in very steep gradient regions due to artificial broadening of the concentration profiles. A recommendation is made to trust only diffusion coefficients from concentration gradients less than 1 at.% per micron unless very special corrections are made.

Research paper thumbnail of Effective evaluation of interfacial energy by matching precipitate sizes measured along a composition gradient with Kampmann-Wagner numerical (KWN) modeling

Scripta Materialia, 2019

A dual-anneal diffusion multiple was utilized to generate a composition gradient via a first anne... more A dual-anneal diffusion multiple was utilized to generate a composition gradient via a first anneal at a high temperature followed by a second anneal at a lower temperature to induce phase precipitation as a function of composition/supersaturation. By adjusting the interfacial energy value in simulations using the classical nucleation and growth theories as implemented in the Kampmann-Wagner numerical (KWN) model and matching the simulated average precipitate sizes at different compositions with the experimental measurements along the composition gradient, the Ni 3 Al/fcc interfacial energy in the Ni-Al system at 700°C was effectively determined to be~12 mJ/m 2 .

Research paper thumbnail of First Reliable Diffusion Coefficients for Mg-Y and Additional Reliable Diffusion Coefficients for Mg-Sn and Mg-Zn

Metallurgical and Materials Transactions A, 2017

Forward-simulation analysis was performed on composition profiles collected from liquid-solid and... more Forward-simulation analysis was performed on composition profiles collected from liquid-solid and solid-solid diffusion couples to obtain diffusion coefficients of Mg-Sn, Mg-Y, and Mg-Zn. Reliable impurity diffusion coefficient of Y in Mg is obtained for the first time, showing that Y diffusion in Mg is about the same as Sn below 390°C. The first sets of interdiffusion coefficients of Mg-Sn and Mg-Y are also obtained together with wider temperature-range data for Mg-Zn.

Research paper thumbnail of Measurement of Diffusion Coefficients in the bcc Phase of the Ti-Sn and Zr-Sn Binary Systems

Metallurgical and Materials Transactions A, 2019

Sn and Zr are two important alloying elements in low-modulus biomedical titanium alloys, yet expe... more Sn and Zr are two important alloying elements in low-modulus biomedical titanium alloys, yet experimental data on Sn diffusion in bcc Ti and bcc Zr are still very limited in the literature, and thus were systematically measured in the present study using solid-solid diffusion couples (SSDCs) and novel liquid-solid diffusion couples (LSDCs). Both interdiffusion and impurity diffusion coefficients in the bcc phase of the Ti-Sn and Zr-Sn systems were extracted with forward-simulation analysis (FSA), and show good agreement with existing literature. The interdiffusion coefficients in the Ti-Sn system at six temperatures, from 950°C to 1200°C, extend experimental measurements to a much wider composition range, and the interdiffusion coefficients in the Zr-Sn system at 1200°C, 1150°C, and 1100°C are the first such data experimentally measured. The data obtained from this study, together with our previous data for the Ti-X (X = Cr, Hf, Mo, Nb, V, Zr) systems, provide reliable experimental inputs to improve the mobility databases for advanced Ti alloys development. In addition, the excellent agreement between the LSDCs and SSDCs results validates the reliability of this novel approach by combining LSDC experiments and FSA in evaluating diffusion coefficients at elevated temperatures.

Research paper thumbnail of Experimental Determination of Impurity and Interdiffusion Coefficients in Seven Ti and Zr Binary Systems Using Diffusion Multiples

Metallurgical and Materials Transactions A, 2018

Diffusion coefficients of seven binary systems (Ti-Mo, Ti-Nb, Ti-Ta, Ti-Zr, Zr-Mo, Zr-Nb, and Zr-... more Diffusion coefficients of seven binary systems (Ti-Mo, Ti-Nb, Ti-Ta, Ti-Zr, Zr-Mo, Zr-Nb, and Zr-Ta) at 1200°C, 1000°C, and 800°C were experimentally determined using three Ti-Mo-Nb-Ta-Zr diffusion multiples. Electron probe microanalysis (EPMA) was performed to collect concentration profiles at the binary diffusion regions. Forward simulation analysis (FSA) was then applied to extract both impurity and interdiffusion coefficients in Ti-rich and Zr-rich part of the bcc phase. Excellent agreements between our results and most of the literature data validate the high-throughput approach combining FSA with diffusion multiples to obtain a large amount of systematic diffusion data, which will help establish the diffusion (mobility) databases for the design and development of biomedical and structural Ti alloys.

Research paper thumbnail of Facile measurement of single-crystal elastic constants from polycrystalline samples

npj Computational Materials, 2017

Elastic constants are among the most fundamental properties of materials. Simulations of microstr... more Elastic constants are among the most fundamental properties of materials. Simulations of microstructural evolution and constitutive/micro-mechanistic modeling of materials properties require elastic constants that are predominately measured from single crystals that are labor intensive to grow. A facile technique is developed to measure elastic constants from polycrystalline samples. The technique is based upon measurements of the surface acoustic wave velocities with the help of a polydimethylsiloxane film grating that is placed on a polished surface of a polycrystalline sample to confine surface acoustic waves that are induced by a femtosecond laser and measured using pump-probe time-domain thermoreflectance. Electron backscatter diffraction is employed to measure the crystallographic orientation along which the surface acoustic wave propagates in each grain (perpendicular to the polydimethylsiloxane grating). Such measurements are performed on several grains. A robust mathematica...

Research paper thumbnail of Experimental determination of the Ni–Cr–Ru phase diagram and thermodynamic reassessments of the Cr–Ru and Ni–Cr–Ru systems

Intermetallics, 2015

Ruthenium addition was found to be beneficial in inhibiting the formation of the detrimental topo... more Ruthenium addition was found to be beneficial in inhibiting the formation of the detrimental topologically closed-packed (TCP) phases in Ni-based single crystal superalloys; thus, the NieCreRu phase stability is very valuable for better understanding of the constitutional effects of Ru in multicomponent Ni-base superalloys. Four isothermal sections of the NieCreRu system at 800 C, 900 C, 1000 C and 1200 C were constructed from results obtained from diffusion multiples using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The measured phase equilibria together with prior experimental results were used to optimize a CALPHAD thermodynamic description of the NieCr eRu ternary system using thermodynamic parameters of the NieCr and NieRu systems published in the literature and modified CreRu parameters from the current study. The intermetallic compound sigma phase in the CreRu system was described with a simplified two-sublattice model, (Cr,Ru) 20 (Cr,Ru) 10 , by taking into account future extension of the model into higher order systems. Comparison between the calculated results and the experimental measurements was made to demonstrate the reliability of the current thermodynamic parameters for the NieCreRu system. For easy implementation of the thermodynamic parameters into some existing thermodynamic databases, a version of the thermodynamic description of the NieCreRu system with an older sublattice model, (Cr,Ni,Ru) 10 (Cr) 4 (Cr,Ni,Ru) 16 , for the sigma phase was also developed.

Research paper thumbnail of Measurement of interdiffusion and impurity diffusion coefficients in the bcc phase of the Ti–X (X = Cr, Hf, Mo, Nb, V, Zr) binary systems using diffusion multiples

Journal of Materials Science, 2016

The design and development of novel titanium alloys for structural and biomedical applications re... more The design and development of novel titanium alloys for structural and biomedical applications require reliable thermodynamic and kinetic databases. In this study, diffusion behaviors of six Ti-X (X = Cr, Hf, Mo, Nb, V, Zr) binary systems were systematically investigated at temperatures from 800 to 1200°C using a set of five Ti-TiAl-Cr-Hf-Mo-Nb-V-Zr diffusion multiples. Concentration profiles of the six Ti-X binary systems were collected from binary regions of the diffusion multiples using electron probe microanalysis (EPMA). Both interdiffusion and impurity (dilute) diffusion coefficients in the Ti-rich bcc phase of these systems were extracted from the concentration profiles using the forward-simulation method. Twenty impurity diffusion coefficients of all the six elements in bcc Ti as well as Ti in bcc Zr at different temperatures obtained from this study are in excellent agreement with the literature data. The interdiffusion coefficients obtained from this study are also in good agreement with previous literature results. The large amount of new experimental data obtained from this study will be essential for establishing the mobility databases for the design and development of advanced titanium alloys.

Research paper thumbnail of Thermal Conductivity Degradation and Microstructural Damage Characterization in Low-Dose Ion Beam-Irradiated 3C-SiC

Metallurgical and Materials Transactions E, 2017

This study assesses the impact of low-dose and low-temperature irradiation on the properties of c... more This study assesses the impact of low-dose and low-temperature irradiation on the properties of cubic silicon carbide (3C-SiC). 3C-SiC was irradiated with Kr ions to different fluences at 420 K (147°C). Raman spectroscopy was used to investigate the impact of irradiation-induced defects on vibrational modes and time-domain thermoreflectance (TDTR) was used to measure thermal conductivity. We observe a noticeable reduction in thermal conductivity with increasing fluence. Analysis of Raman spectra reveals the longitudinal optical (LO) and transverse optical (TO) modes with noticeable peak broadening of LO mode with increasing dosage. We also notice a decrease of ratio of peak intensities of LO and TO modes in irradiated samples. We observe a correlation between the thermal conductivity reduction and the decrease in the peak intensity ratio and attribute this to the accumulation of charged vacancy defects.

Research paper thumbnail of Nonlinear Arrhenius behavior of self-diffusion in β−Ti and Mo

Physical Review Materials

While anomalous diffusion coefficients with non-Arrhenius like temperature dependence are observe... more While anomalous diffusion coefficients with non-Arrhenius like temperature dependence are observed in a number of metals, a conclusive comprehensive framework of explanation has not been brought forward to date. Here, we use first-principles calculations based on density functional theory to calculate self-diffusion coefficients in the bcc metals Mo and-Ti by coupling quasiharmonic transition state theory and large displacement phonon calculations and show that anharmonicity from thermal expansion is the major reason for the anomalous temperature dependence. We use a modified Debye approach to quantify the thermal expansion over the entire temperature range and introduce a method to relax the vacancy structure in a mechanically unstable crystal such as-Ti. Thermal expansion is found to weakly affect the activation enthalpy but has a strong effect on the prefactor of the diffusion coefficient, reproducing the non-linear, non-Arrhenius "anomalous" self-diffusion in both bcc systems with good agreement between calculation and experiment. The proposed methodology is general and simple enough to be applicable to other mechanically unstable crystals.

Research paper thumbnail of Improved model for the thermal conductivity of binary metallic systems

Cornell University - arXiv, Jun 29, 2016

We extended and updated Mott's two-band model for the composition-dependence of thermal and elect... more We extended and updated Mott's two-band model for the composition-dependence of thermal and electrical conductivity in binary metal alloys based on high-throughput time-domain thermoreflectance (TDTR) measurements on diffusion multiples and scatterer-density calculations from first principles. Examining Au-Cu, Au-Ag, Pd-Ag, Pd-Cu, Pd-Pt, Pt-Rh, and Ni-Rh binary alloys, we found that the nature of the two dominant scatterer-bands considered in the Mott model changes with the alloys, and should be interpreted as a combination of the dominant element-specific sand/or d-orbitals. Using calculated orbital and element-resolved density-of-states values calculated with density functional theory as input, we determined the correct orbital mix that dominates electron scattering for all examined alloys and found excellent agreement between fitted models and experiments. This general model of the composition dependence of the thermal and electrical resistivity can be readily implemented into the CALPHAD framework.

Research paper thumbnail of Microstructure and Fracture Toughness of an Aluminum-Steel Impact Weld and Effect of Thermal Exposure

Metallurgical and Materials Transactions A, 2021

Welding of aluminum alloys to steel is increasingly important in manufacturing, however the use o... more Welding of aluminum alloys to steel is increasingly important in manufacturing, however the use of fusion welding is difficult because of disparate melting points and the possibility of intermetallic compound (IMC) formation. Here, an impact welding technique, vaporizing foil actuator welding, was utilized to produce solid-state joints between AA1100-O and 1018 mild steel. The relationship between weld processing conditions, microstructure, and mechanical properties were investigated. For this purpose, the welds were annealed between 300°C and 600°C and a combination of optical and scanning electron microscopy, along with image analysis was performed to characterize the weld microstructure and monitor IMC growth. Wedge testing was applied to understand the effect of annealing on the weld fracture toughness. A numerical model incorporating the Fick's laws of diffusion, grain boundary diffusion, and grain growth kinetics was also developed to simulate the IMC growth. The heterogeneity in the original microstructure caused persistent differences in IMC growth, as initial IMC seemed to increase nucleation and growth. Simulation results indicated short circuit diffusion to be the major contributor to IMC growth since it is consistently faster then experimental IMC growths compared with the computational results that used lattice diffusion only. Wedge testing reveals increased weld toughness for modest anneals of 300°C, possibly due to homogeneity at the weld interface while avoiding IMC growth.

Research paper thumbnail of Rapid Synthesis and Sintering of Metals from Powders

Advanced Science, 2021

Powder to bulk processes, such as additive manufacturing and metal injection molding (MIM), have ... more Powder to bulk processes, such as additive manufacturing and metal injection molding (MIM), have enabled great potential for complex metal designing and manufacturing. However, additive manufacturing process normally introduces a high residue stress and textures due to the locally intense temperature. MIM is an excellent batch manufacturing process; nevertheless, it is not suitable for rapid screening and development of new metal compositions and structures due to the slow sintering process. Herein, an ultrafast high-temperature sintering (UHS) process is reported that enables the rapid synthesis and sintering of bulk metals/alloys and intermetallic compounds. In this process, elemental powders are mixed and pressed into pellets, followed by UHS sintering in just seconds at a temperature between 1000 and 3000°C. Three representative compositions, including pure metals, intermetallics, and multielement alloys, are demonstrated with a broad range of melting points. The UHS process for metal sintering is nonmaterials specific, in addition to being extremely rapid, which make it suitable for materials discovery. Furthermore, the sintering method does not apply pressure to the samples, making it compatible with 3D printing and other additive manufacturing processes of complex structures. This rapid sintering technique will greatly facilitate the development and manufacturing of metals and alloys. Powder metallurgy is a widely used technology to manufacture metal components from their powders. Unlike traditional metal processing techniques (e.g., casting), powder metallurgy does

Research paper thumbnail of A simple yet general model of binary diffusion coefficients emerged from a comprehensive assessment of 18 binary systems

Research paper thumbnail of Diffusion Coefficients and Phase Equilibria of the Cu-Zn Binary System Studied Using Diffusion Couples

Journal of Phase Equilibria and Diffusion, 2020

The diffusion behavior and phase equilibria in the Cu-Zn binary system were investigated using so... more The diffusion behavior and phase equilibria in the Cu-Zn binary system were investigated using solidsolid and solid-liquid diffusion couples. Heat treatments at temperatures ranging from 100 to 750°C were performed and the samples were examined using optical microscopy, energy dispersive x-ray spectroscopy, and electron probe microanalysis to identify the phases and to obtain composition profiles. Solubility limits of both solid solution and intermetallic phases were then evaluated, and a forwardsimulation analysis (FSA) was applied to extract interdiffusion coefficients. The composition profiles from Hoxha et al. were also re-analyzed using FSA to obtain more reliable diffusion coefficient data without the assumption of constant diffusion coefficients for the intermetallic phases. A comprehensive assessment of the interdiffusion coefficients in three intermetallic phases of the Cu-Zn system was performed based on the results from the current study as well as those in the literature. Activation energies and Arrhenius pre-factors were evaluated for each phase as a function of composition. The fitted equations based on the comprehensive assessment have the capabilities of computing the interdiffusion coefficients of each of the phases at a given composition and temperature. Suggested modifications to the Cu-Zn binary phase diagram were presented based on the new experimental information gathered from the present study. A clear explanation is provided for the puzzling low Zn concentrations often observed in the Cu-rich fcc phase of Cu-Zn diffusion couples in comparison with the expected high solubility values based on the equilibrium Cu-Zn phase diagram.

Research paper thumbnail of On the quantitative characterization of weld microstructures

Scripta Materialia, 2020

A new technique to calculate spatial variations of grain shape, grain size, and grain boundary cu... more A new technique to calculate spatial variations of grain shape, grain size, and grain boundary curvature in the fusion zone (FZ) of refractory metal alloys has been developed. This technique was applied to quantitatively evaluate changes in microstructures of simulated molybdenum welds available in the literature. Use of this technique showed that a 50% increase in weld speed more than doubled the fraction of grain segments oriented between 0 °and 30 °to the normal to the weld direction along with a decrease in grain size from approximately 120 μm to 84 μm in the weld direction at the FZ centerline.

Research paper thumbnail of Phase Equilibria and Diffusion in the Ni-Cr-Pt System at 1200 °C

Journal of Phase Equilibria and Diffusion, 2019

Pt modified NiAl, which is often simply called Pt aluminide, is widely used in jet engines as a p... more Pt modified NiAl, which is often simply called Pt aluminide, is widely used in jet engines as a protective coating against oxidation and hot corrosion as well as a bond coat for thermal barrier coatings. Phase equilibria and diffusion coefficients in the Ni-Cr-Pt ternary system are thus valuable data for modeling the behavior of Pt aluminide coatings on Ni-based superalloys in which Cr is an essential alloying element. A Ni-Cr-Pt diffusion multiple was made and annealed at 1200°C for 100 h to obtain the first reliable isothermal section phase diagram for this ternary system. Interdiffusion coefficients were extracted from the measured diffusion concentration profiles using a forward simulation analysis for all the single-phase regions of the Ni-Cr, Ni-Pt and Cr-Pt binary systems. The impurity diffusion coefficient data obtained from this study are combined with literature data to assess reliable Arrhenius equations for the impurity diffusion coefficients of Pt in Ni, Ni in Pt, and Cr in Ni. An Arrhenius equation for the impurity diffusion coefficient of Cr in Pt is recommended for the first time by combining a fitted prefactor from experimental data and an activation energy value from first principles calculations. Keywords binary diffusion Á diffusion couples Á diffusivity coefficient Á experimental phase equilibria Á impurity diffusivity Á phase diagram This invited article is part of a special issue of the Journal of Phase Equilibria and Diffusion in honor of the 2018 J. Willard Gibbs Phase Equilibria Award winner Dr. John Morral. The award was presented to Dr. Morral during MS&T'18, October 14-18, 2018, in Columbus, Ohio, ''for fundamental and applied research on topology of phase diagrams and theory of phase equilibria resulting in major advances in the calculation and interpretation of phase equilibria and diffusion.''

Research paper thumbnail of Anisotropic thermal conductivity of magnetocaloric AlFe2B2

Materialia, 2018

The thermal conductivity () and heat capacity (C p) of the magnetocaloric AlFe 2 B 2 compound wer... more The thermal conductivity () and heat capacity (C p) of the magnetocaloric AlFe 2 B 2 compound were investigated to assess its room temperature heat transfer and thermal management potential relative to other magnetocaloric materials such as FeRh, Gd, Gd 5 (Si,Ge) 4 La(Fe,Si) 13 and (Mn,Fe) (P,As). The room temperature thermal conductivity was determined along the three principal crystallographic directions of AlFe 2 B 2 using the timedomain thermoreflectance (TDTR) method on six randomly oriented AlFe 2 B 2 grains, resulting in an observed anisotropic thermal conductivity with a 40% larger along the c-axis: x = 4.7 ± 0.1 W/mK, y = 4.4 ± 0.1 W/mK, and z = 6.8 ± 0.3 W/mK.

Research paper thumbnail of pydiffusion: A Python Library for Diffusion Simulation and Data Analysis

Journal of Open Research Software, 2019

pydiffusion is a free and open-source Python library designed to solve diffusion problems for bot... more pydiffusion is a free and open-source Python library designed to solve diffusion problems for both singlephase and multi-phase binary systems. The key features of pydiffusion include fast simulation of multiphase diffusion and extraction of diffusion coefficients from experimental concentration profiles using forward simulation analysis. pydiffusion also provides various mathematical models for diffusion profile smoothing, diffusion coefficient evaluation, and data optimization. In pydiffusion, diffusion profiles and various phases are easy to define or read from the experimental datasets. Visualization tools based on Matplotlib are also provided to help users present or refine their simulations and analysis.

Research paper thumbnail of First measurement of the full elastic constants of Ni-based superalloy René 88DT

Scripta Materialia, 2018

The full elastic constants of Ni-based superalloy René 88DT were measured for the first time usin... more The full elastic constants of Ni-based superalloy René 88DT were measured for the first time using a recently developed method that matches the computed surface acoustic wave (SAW) velocities from an elastodynamic model with experimental velocities of SAWs that are generated and detected with a femtosecond laser. The experimental measurements were performed on several grains of a polycrystalline sample without the need of growing a single crystal. The computed Young's modulus from the resultant elastic constants agrees with the literature values, lending confidence on our measured elastic constants: C 11 = 267.1, C 12 = 170.5, and C 44 = 107.6 (GPa).

Research paper thumbnail of Recommendation for reliable evaluation of diffusion coefficients from diffusion profiles with steep concentration gradients

Materialia, 2018

Essentially all diffusion coefficients, except for a tiny fraction from tracer experiments, are e... more Essentially all diffusion coefficients, except for a tiny fraction from tracer experiments, are evaluated from diffusion concentration profiles that are predominantly measured via wavelength or energy dispersive spectroscopy of characteristic X-rays of the pertinent elements. X-ray generation (interaction) volume simulation and an analysis of experimental profiles show that the extracted diffusion coefficients can be inflated by up to three orders of magnitude in very steep gradient regions due to artificial broadening of the concentration profiles. A recommendation is made to trust only diffusion coefficients from concentration gradients less than 1 at.% per micron unless very special corrections are made.

Research paper thumbnail of Effective evaluation of interfacial energy by matching precipitate sizes measured along a composition gradient with Kampmann-Wagner numerical (KWN) modeling

Scripta Materialia, 2019

A dual-anneal diffusion multiple was utilized to generate a composition gradient via a first anne... more A dual-anneal diffusion multiple was utilized to generate a composition gradient via a first anneal at a high temperature followed by a second anneal at a lower temperature to induce phase precipitation as a function of composition/supersaturation. By adjusting the interfacial energy value in simulations using the classical nucleation and growth theories as implemented in the Kampmann-Wagner numerical (KWN) model and matching the simulated average precipitate sizes at different compositions with the experimental measurements along the composition gradient, the Ni 3 Al/fcc interfacial energy in the Ni-Al system at 700°C was effectively determined to be~12 mJ/m 2 .

Research paper thumbnail of First Reliable Diffusion Coefficients for Mg-Y and Additional Reliable Diffusion Coefficients for Mg-Sn and Mg-Zn

Metallurgical and Materials Transactions A, 2017

Forward-simulation analysis was performed on composition profiles collected from liquid-solid and... more Forward-simulation analysis was performed on composition profiles collected from liquid-solid and solid-solid diffusion couples to obtain diffusion coefficients of Mg-Sn, Mg-Y, and Mg-Zn. Reliable impurity diffusion coefficient of Y in Mg is obtained for the first time, showing that Y diffusion in Mg is about the same as Sn below 390°C. The first sets of interdiffusion coefficients of Mg-Sn and Mg-Y are also obtained together with wider temperature-range data for Mg-Zn.

Research paper thumbnail of Measurement of Diffusion Coefficients in the bcc Phase of the Ti-Sn and Zr-Sn Binary Systems

Metallurgical and Materials Transactions A, 2019

Sn and Zr are two important alloying elements in low-modulus biomedical titanium alloys, yet expe... more Sn and Zr are two important alloying elements in low-modulus biomedical titanium alloys, yet experimental data on Sn diffusion in bcc Ti and bcc Zr are still very limited in the literature, and thus were systematically measured in the present study using solid-solid diffusion couples (SSDCs) and novel liquid-solid diffusion couples (LSDCs). Both interdiffusion and impurity diffusion coefficients in the bcc phase of the Ti-Sn and Zr-Sn systems were extracted with forward-simulation analysis (FSA), and show good agreement with existing literature. The interdiffusion coefficients in the Ti-Sn system at six temperatures, from 950°C to 1200°C, extend experimental measurements to a much wider composition range, and the interdiffusion coefficients in the Zr-Sn system at 1200°C, 1150°C, and 1100°C are the first such data experimentally measured. The data obtained from this study, together with our previous data for the Ti-X (X = Cr, Hf, Mo, Nb, V, Zr) systems, provide reliable experimental inputs to improve the mobility databases for advanced Ti alloys development. In addition, the excellent agreement between the LSDCs and SSDCs results validates the reliability of this novel approach by combining LSDC experiments and FSA in evaluating diffusion coefficients at elevated temperatures.

Research paper thumbnail of Experimental Determination of Impurity and Interdiffusion Coefficients in Seven Ti and Zr Binary Systems Using Diffusion Multiples

Metallurgical and Materials Transactions A, 2018

Diffusion coefficients of seven binary systems (Ti-Mo, Ti-Nb, Ti-Ta, Ti-Zr, Zr-Mo, Zr-Nb, and Zr-... more Diffusion coefficients of seven binary systems (Ti-Mo, Ti-Nb, Ti-Ta, Ti-Zr, Zr-Mo, Zr-Nb, and Zr-Ta) at 1200°C, 1000°C, and 800°C were experimentally determined using three Ti-Mo-Nb-Ta-Zr diffusion multiples. Electron probe microanalysis (EPMA) was performed to collect concentration profiles at the binary diffusion regions. Forward simulation analysis (FSA) was then applied to extract both impurity and interdiffusion coefficients in Ti-rich and Zr-rich part of the bcc phase. Excellent agreements between our results and most of the literature data validate the high-throughput approach combining FSA with diffusion multiples to obtain a large amount of systematic diffusion data, which will help establish the diffusion (mobility) databases for the design and development of biomedical and structural Ti alloys.

Research paper thumbnail of Facile measurement of single-crystal elastic constants from polycrystalline samples

npj Computational Materials, 2017

Elastic constants are among the most fundamental properties of materials. Simulations of microstr... more Elastic constants are among the most fundamental properties of materials. Simulations of microstructural evolution and constitutive/micro-mechanistic modeling of materials properties require elastic constants that are predominately measured from single crystals that are labor intensive to grow. A facile technique is developed to measure elastic constants from polycrystalline samples. The technique is based upon measurements of the surface acoustic wave velocities with the help of a polydimethylsiloxane film grating that is placed on a polished surface of a polycrystalline sample to confine surface acoustic waves that are induced by a femtosecond laser and measured using pump-probe time-domain thermoreflectance. Electron backscatter diffraction is employed to measure the crystallographic orientation along which the surface acoustic wave propagates in each grain (perpendicular to the polydimethylsiloxane grating). Such measurements are performed on several grains. A robust mathematica...

Research paper thumbnail of Experimental determination of the Ni–Cr–Ru phase diagram and thermodynamic reassessments of the Cr–Ru and Ni–Cr–Ru systems

Intermetallics, 2015

Ruthenium addition was found to be beneficial in inhibiting the formation of the detrimental topo... more Ruthenium addition was found to be beneficial in inhibiting the formation of the detrimental topologically closed-packed (TCP) phases in Ni-based single crystal superalloys; thus, the NieCreRu phase stability is very valuable for better understanding of the constitutional effects of Ru in multicomponent Ni-base superalloys. Four isothermal sections of the NieCreRu system at 800 C, 900 C, 1000 C and 1200 C were constructed from results obtained from diffusion multiples using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The measured phase equilibria together with prior experimental results were used to optimize a CALPHAD thermodynamic description of the NieCr eRu ternary system using thermodynamic parameters of the NieCr and NieRu systems published in the literature and modified CreRu parameters from the current study. The intermetallic compound sigma phase in the CreRu system was described with a simplified two-sublattice model, (Cr,Ru) 20 (Cr,Ru) 10 , by taking into account future extension of the model into higher order systems. Comparison between the calculated results and the experimental measurements was made to demonstrate the reliability of the current thermodynamic parameters for the NieCreRu system. For easy implementation of the thermodynamic parameters into some existing thermodynamic databases, a version of the thermodynamic description of the NieCreRu system with an older sublattice model, (Cr,Ni,Ru) 10 (Cr) 4 (Cr,Ni,Ru) 16 , for the sigma phase was also developed.

Research paper thumbnail of Measurement of interdiffusion and impurity diffusion coefficients in the bcc phase of the Ti–X (X = Cr, Hf, Mo, Nb, V, Zr) binary systems using diffusion multiples

Journal of Materials Science, 2016

The design and development of novel titanium alloys for structural and biomedical applications re... more The design and development of novel titanium alloys for structural and biomedical applications require reliable thermodynamic and kinetic databases. In this study, diffusion behaviors of six Ti-X (X = Cr, Hf, Mo, Nb, V, Zr) binary systems were systematically investigated at temperatures from 800 to 1200°C using a set of five Ti-TiAl-Cr-Hf-Mo-Nb-V-Zr diffusion multiples. Concentration profiles of the six Ti-X binary systems were collected from binary regions of the diffusion multiples using electron probe microanalysis (EPMA). Both interdiffusion and impurity (dilute) diffusion coefficients in the Ti-rich bcc phase of these systems were extracted from the concentration profiles using the forward-simulation method. Twenty impurity diffusion coefficients of all the six elements in bcc Ti as well as Ti in bcc Zr at different temperatures obtained from this study are in excellent agreement with the literature data. The interdiffusion coefficients obtained from this study are also in good agreement with previous literature results. The large amount of new experimental data obtained from this study will be essential for establishing the mobility databases for the design and development of advanced titanium alloys.

Research paper thumbnail of Thermal Conductivity Degradation and Microstructural Damage Characterization in Low-Dose Ion Beam-Irradiated 3C-SiC

Metallurgical and Materials Transactions E, 2017

This study assesses the impact of low-dose and low-temperature irradiation on the properties of c... more This study assesses the impact of low-dose and low-temperature irradiation on the properties of cubic silicon carbide (3C-SiC). 3C-SiC was irradiated with Kr ions to different fluences at 420 K (147°C). Raman spectroscopy was used to investigate the impact of irradiation-induced defects on vibrational modes and time-domain thermoreflectance (TDTR) was used to measure thermal conductivity. We observe a noticeable reduction in thermal conductivity with increasing fluence. Analysis of Raman spectra reveals the longitudinal optical (LO) and transverse optical (TO) modes with noticeable peak broadening of LO mode with increasing dosage. We also notice a decrease of ratio of peak intensities of LO and TO modes in irradiated samples. We observe a correlation between the thermal conductivity reduction and the decrease in the peak intensity ratio and attribute this to the accumulation of charged vacancy defects.