John Perepezko - Academia.edu (original) (raw)

Papers by John Perepezko

Journal of Non-Crystalline Solids, 2003

Journal of Applied Physics, Mar 15, 2012

The formation of Al nanocrystals from an amorphous Al 92 Sm 8 alloy involves kinetic phenomena wi... more The formation of Al nanocrystals from an amorphous Al 92 Sm 8 alloy involves kinetic phenomena with very different characteristic length and timescales, including initial nucleation and later growth and coarsening. Insight into these processes can be derived from the evolution of the sizes of nanocrystals as a function of time. Synchrotron small angle x-ray scattering (SAXS) experiments provide information about the evolution of the nanocrystal size distribution, particularly at times after nucleation has reached saturation. Accurately interpreting the distribution of intensity measured using SAXS requires a nanoparticle model consisting of nanocrystalline core of pure Al surrounded by a shell enriched in Sm. With this approach, statistical parameters derived from SAXS are independent of detailed assumptions regarding the distribution of Sm around the nanocrystals and allow the maximum radius of nanocrystals within the distribution to be determined unambiguously. Sizes determined independently using transmission electron microcopy are in excellent agreement with the SAXS results. The maximum radius obtained from SAXS is proportional to the cube root of time at large sizes and long times, consistent with a coarsening model. The diffusivity of Al within the Al-Sm alloy is obtained from a quantitative analysis of the coarsening process. Further analysis with this diffusivity and a particle growth model provides a satisfactory account for the particle size evolution at early times before the kinetic transition to coarsening. V

Frontiers in Materials, 2021

Strategies to change the properties of metallic glass by controlling the crystallization and the ... more Strategies to change the properties of metallic glass by controlling the crystallization and the glass transition behavior are essential in promoting the application of these materials. Aside from changing the composition approaches to stabilize the glass and frustrate the nucleation and growth of crystals, new strategies at a fixed glass composition are of special interest. In this review, some recent work is summarized on new strategies to tune the properties of metallic glasses without changing composition. First, the nanocrystallization strategy is introduced that is based on the nanocrystallized microstructures such as those that develop in marginal Al-based metallic glasses. The heterogeneous and transient nucleation effects in the nanocrystallization reactions in Al-based metallic glasses are systematically investigated and can be assessed by the determination of delay time based on Flash DSC measurements. These results provide a basis to understand the strong effect of minor...

Computational Materials Science, 2021

We have developed models of metallic alloy glass forming ability based on newly computationally a... more We have developed models of metallic alloy glass forming ability based on newly computationally accessible features obtained from molecular dynamics simulations. Since the discovery of metallic glasses, there have been efforts to predict glass forming ability (GFA) for new alloys. Effective evaluations of GFA have been obtained but generally relied on knowledge of alloy characteristic temperatures like the glass transition, crystallization, and liquidus temperatures but are of limited utility because these features require synthesizing and characterizing the alloy of interest. More recently, machine learning approaches to predict GFA have employed more accessible model features such as the elemental properties of constituent elements. However, these more accessible features generally provide less predictive accuracy than their less accessible counterparts. In this work we showed that it is possible to increase the predictive value of GFA models by using input features obtained from molecular dynamics simulations. Such features require only relatively straightforward and scalable simulations, making them significantly easier and less expensive to obtain than experimental measurements. We generated a database of molecular dynamics critical cooling rates along with associated candidate features that are inspired from previous research on GFA. Out of the list of 9 proposed GFA features, we identify two as being the most important to performance through a LASSO model. Enthalpy of crystallization and icosahedral-like fraction at 100 K showed promise because they enable a significant improvement to model performance and because they are accessible to flexible ab initio quantum mechanical methods readily applicable to almost all systems. This advancement in computationally accessible features for machine learning predictions GFA will enable future models to more accurately predict new glass forming alloys.

Intermetallics, 2020

In order to evaluate the impact of an oxidation resistant coating on the structural performance o... more In order to evaluate the impact of an oxidation resistant coating on the structural performance of a Mo-9Si-8B alloy tensile creep experiments were conducted at 1200 °C. After a plastic strain of 6 % the creep rates of the coated samples compared favorably with the reported values for uncoated samples. Moreover, the coating structure was maintained during creep deformation and the coating exhibited a self-healing capability.

ECS Meeting Abstracts, 2016

Ni-Cr and Ni-Cr-Mo alloys owe their outstanding corrosion resistance to the surface enrichment of... more Ni-Cr and Ni-Cr-Mo alloys owe their outstanding corrosion resistance to the surface enrichment of passivating Cr(III) oxides and synergistic effect of Cr and Mo in case of Mo-containing alloys [1-3]. However, the specific roles of minor elements are not well understood especially with respect to precise location of Mo relative to the oxide/metal interface, nor the atomistic processes responsible for protective-oxide layer growth and breakdown. The composition, structure and thickness of the passivating oxide films are challenging to characterize considering their nanoscale dimensions and the high electric field imposed during growth in solution. Key processes that take place within the oxide and regulate passivation are controlled by defect interactions that are atomic, ionic, and electronic in nature and currently poorly understood, often needed to be studied at the resolution and detection limits of experimental methods [4]. Common electrochemical methods such as AC and DC electro...

Acta Materialia, 2020

Oxidation and corrosion have a significant economic footprint. Mo-based alloys are a strong candi... more Oxidation and corrosion have a significant economic footprint. Mo-based alloys are a strong candidate for structural materials with oxidation resistance at high temperatures. However, understanding of the mechanisms remains limited as experimental techniques do not reach atomic-scale resolution. We examined the mechanism of oxidation of Mo 3 Si (A15 phase) in MoÀSiÀB alloys, the emergence of a superficial silica scale, and explain available experimental data up to the large nanometer scale using chemically detailed reactive simulations. We introduce new simulation protocols for layer-by-layer oxidation and simple force fields for the reactants, intermediates, and products. Growth of thin superficial silica layers as a function of temperature and oxidation rate on the (001) surface involves the formation of silica clusters, rings, and chains with pore sizes of 0 to 2 nm. An increase in temperature from 800 to 1000°C slightly decreased the pore size and lead to less accumulation of Mo oxides at the interface, consistent with observations by electron tomography and energy dispersive X-ray spectroscopy (EDS). The elimination of gaseous MoO x is essential to form open channels and much larger pores up to 100 nm size as observed by 3D tomography, in-situ transmission electron microscopy (TEM) and scanning electron microscopy (SEM) as the oxide phase grows. According to the simulation, these large pores would otherwise be closed. The rate of oxidation, represented by successive oxidation of layers of variable thickness per unit time, influences the structure and cohesion of silica layers. High rates of oxidation can destabilize and break apart the silica layer, supported by a very wide pore size distribution in electron tomography. Limitations of the simulations in time scale currently restrict the analysis to few-layer oxidation. Within these bounds, the proposed simulation protocols can provide insight into the oxidation of (hkl) surfaces, grain boundaries, and various alloys compositions up to the 100 nm scale in atomic-level detail.

Corrosion, 2018

Results of in situ transmission electron microscopy experiments on the early stage oxidation of N... more Results of in situ transmission electron microscopy experiments on the early stage oxidation of Ni-Cr and Ni-Cr-Mo alloys are reported. An epitaxial rock-salt oxide with compositions outside the conventional solubility limits initiated at the surface of both alloys, progressing by a layer-by-layer mode. Kirkendall voids were found in Ni-Cr alloys near the metal/oxide interface, but were not seen in the Ni-Cr-Mo. The voids initiated in the oxide then diffused to the metal/oxide interface, driven by the misfit stresses in the oxide. A sequential oxide initiation was observed in NiCr alloys: rock-salt → spinel → corundum; however, for NiCrMo alloys, the metastable Ni2-xCrxO3 (corundum structure) phase formed shortly after the growth of the rock-salt phase. Chemical analysis shows that solute atoms were captured in the initial oxide before diffusing and transforming to more thermodynamically stable phases. The results indicate that Mo doping inhibits the formation of Kirkendall voids vi...

The Journal of Physical Chemistry C, 2018

The effects of crystal orientation and prior etching on the polarization and repassivation behavi... more The effects of crystal orientation and prior etching on the polarization and repassivation behavior of Ni− Cr and Ni−Cr−Mo alloys have been investigated in acidic chloride environment using dc potentiostatic and ac singlefrequency electrochemical impedance spectroscopy. Tests were conducted within the passive region at potentials where local oxide breakdown was possible. Surface morphologies of grains across a wide range of orientations were measured before and after passivation using atomic force microscopy. Oxide growth was monitored on isolated lowand high-index crystallographic planes as a function of repassivation time, enabling the independent measurement of the oxidation and total anodic current densities. Grains exhibited the tendency to either passivate with significant or minimal oxidation or instead resist active passivation and repassivation. The oxidation performance was a function of the crystallographic orientation of the exposed grain surface. The oxides grown on various-orientated surfaces differed in their film growth kinetics, morphology, and steady state thicknesses, even given similar total anodic current densities due to dissimilar oxidation efficiencies. For Ni-11 wt % Cr, only orientations close to (1 0 1) were able to form stable passive films, aided by nanofaceted surfaces with a matchstick-type morphology. On the other hand, all orientations of the Ni-11 wt % Cr-6 wt % Mo alloys formed electrochemically stable oxides films owing to the beneficial influence of Mo on repassivation. Passivation, breakdown behavior, and oxide properties vary with crystal orientation, and this work provides insight into the effects of crystallographic orientation and oxide film morphology on the passivation mechanism of Ni−Cr and Ni−Cr−Mo alloys.

Imaging and Applied Optics, 2011

Space borne IR remote sensing instruments in the future will require higher accuracies than now a... more Space borne IR remote sensing instruments in the future will require higher accuracies than now available, and the ability to perform absolute on-orbit calibrations traceable to SI standards. An On-Orbit Absolute Radiance Standard (OARS) is being developed to provide these on-orbit calibrations. The OARS uses a high emissivity (>0.999) blackbody cavity with imbedded thermistor temperature sensors that can be periodically calibrated using the transient melt signatures from three different reference materials also imbedded in the cavity. Emissivity measurements of the blackbody can be made periodically by measuring the reflection of a carefully baffled heated cylinder placed in front of the cavity.

Electrochemical Studies of Passive Film Stability on Fe49.7Cr17.7Mn1.9Mo7.4W1.6B15.2C3.8Si2.4 Amo... more Electrochemical Studies of Passive Film Stability on Fe49.7Cr17.7Mn1.9Mo7.4W1.6B15.2C3.8Si2.4 Amorphous Metal in Seawater at 90oCElectrochemical Studies of Passive Film Stability on Fe49.7Cr17.7Mn1.9Mo7.4W1.6B15.2C3.8Si2.4 Amorphous Metal in Seawater at 90C61616;C

Dekker Encyclopedia of Nanoscience and Nanotechnology, Second Edition - Six Volume Set (Print Version), 2008

Dekker Encyclopedia of Nanoscience and Nanotechnology, Second Edition - Six Volume Set (Print Version), 2004

MRS Proceedings, 2006

ABSTRACTThe passive film stability of several Fe-based amorphous metal formulations have been fou... more ABSTRACTThe passive film stability of several Fe-based amorphous metal formulations have been found to be comparable to that of high-performance Ni-based alloys, and superior to that of stainless steels, based on electrochemical measurements of the passive film breakdown potential and general corrosion rates. Chromium (Cr), molybdenum (Mo) and tungsten (W) provide corrosion resistance; boron (B) enables glass formation; and rare earths such as yttrium (Y) lower critical cooling rate (CCR). The high boron content of this particular amorphous metal also makes it an effective neutron absorber, and suitable for criticality control applications, as discussed in companion publications. Corrosion data for SAM2X5 (Fe49.7Cr17.7Mn1.9Mo7.4W1.6B15.2C3.8Si2.4) is discussed here.

SPIE Proceedings, 2010

NASA's anticipated plan for a mission dedicated to Climate (CLARREO) will hinge upon the ability ... more NASA's anticipated plan for a mission dedicated to Climate (CLARREO) will hinge upon the ability to fly SI traceable standards that provide irrefutable absolute measurement accuracy. As an example, instrumentation designed to measure spectrally resolved infrared radiances will require high-emissivity calibration blackbodies that have absolute temperature uncertainties of better than 0.045K (3 sigma). A novel scheme to provide absolute calibration of temperature sensors onorbit, that uses the transient melt signatures from multiple phase change materials, has been demonstrated in the laboratory at the University of Wisconsin and is now undergoing technology advancement under NASA Instrument Incubator Program funding. Using small quantities of phase change material (less than half of a percent of the mass of the cavity), melt temperature accuracies of better than 10 mK have been demonstrated for mercury, water, and gallium (providing calibration from 233K to 303K). Refinements currently underway focus on ensuring that the melt materials in their sealed confinement housings perform as expected in the thermal and microgravity environment of a multi-year spaceflight mission. Thermal soak and cycling tests are underway to demonstrate that there is no dissolution from the housings into the melt materials that could alter melt temperature, and that there is no liquid metal embrittlement of the housings from the metal melt materials. In addition, NASA funding has been recently secured to conduct a demonstration of this scheme in the microgravity environment of the International Space Station.

Science, 2009

Alloys based on molybdenum or niobium may allow the high-temperature components of jet engines to... more Alloys based on molybdenum or niobium may allow the high-temperature components of jet engines to run hotter and more efficiently.

Nanotechnology, 2013

A new class of materials: Au-based nanograined metallic glasses (NGMGs) were synthesized using ma... more A new class of materials: Au-based nanograined metallic glasses (NGMGs) were synthesized using magnetron sputtering with powder targets. A detailed study by x-ray diffraction and high-resolution transmission electron microscopy (TEM) documents the unique nanoscale granular structure of the Au-based NGMG. This material inherited the good mechanical properties of metallic glasses, showing a high hardness of ∼5.3 GPa and a low elastic modulus of ∼79 GPa. In addition, in contrast to most MGs the nanoglassy particles can deform along the loading direction, exhibiting unique tensile elongation up to 100%. During thermal crystallization of NGMG material, even smaller sized Au solid solution nanocrystals are formed within the glassy nanograins, offering a new way for production of the nanocomposites with tailored structural length scales.

Metallurgical and Materials Transactions A, 2009

Incorporated SiC nanoparticles are demonstrated to influence the solidification of magnesiumzinc ... more Incorporated SiC nanoparticles are demonstrated to influence the solidification of magnesiumzinc alloys resulting in strong, ductile, and castable materials. By ultrasonically dispersing a small amount (less than 2 vol pct) of SiC nanoparticles, both the strength and ductility exhibit marked enhancement in the final casting. This unusual ductility enhancement is the result of the nanoparticles altering the selection of intermetallic phases. Using transmission electron microscopy (TEM), the MgZn 2 phase was discovered among SiC nanoparticle clusters in hypoeutectic compositions. Differential thermal analysis showed that the MgZn 2 formation resulted in elimination of other intermetallics in the Mg-4Zn nanocomposite and reduced their formation in Mg-6Zn and Mg-8Zn nanocomposites.

Materials Science and Engineering: A, 2004

The nanocrystalline state is often viewed in terms of isolated nanocrystalline particles, but an ... more The nanocrystalline state is often viewed in terms of isolated nanocrystalline particles, but an equally important form of nanostructured alloys is based upon a dispersion of a high number density of nanocrystals or upon a bulk nanocrystalline microstructure. In a number of marginal glass-forming alloys with compositions that limit the solute content to <15 at.%, nanocrystal densities of 10 21 to 10 23 m −3 can develop during primary crystallization and offer exceptional magnetic and structural performance. There is also a remarkable thermal stability of the dispersed nanocrystal and amorphous matrix microstructure to significant change in size scale. Several proposals involving solute effects, phase separation or quenched-in nuclei and heterogeneous nucleation have been advanced to account for the high nanocrystal density. Alternatively, other approaches involving alloying by intense cold-rolling reveal that a deformation-induced amorphization can be achieved for marginal glass-forming alloy compositions. In other systems, a deformation-induced nanocrystal synthesis can be observed during the cold-rolling of amorphous ribbons. These developments represent a major level of microstructure control.