Peter Collins - Academia.edu (original) (raw)
Papers by Peter Collins
John Wiley & Sons, Inc. eBooks, May 6, 2016
Titanium aluminide (TiAl) alloys have been studied for many years with the intent to replace heav... more Titanium aluminide (TiAl) alloys have been studied for many years with the intent to replace heavier Ni-based superalloys currently used in aerospace structural parts. Various additive manufacturing techniques are being explored to produce near net shape components of TiAl and other materials. Due to the multiple cycles of heating and cooling at different rates, a complex microstructure evolution and resulting mechanical response is expected for materials made by these techniques. In this work, the microstructure of as-deposited Ti-48Al-2Cr-2Nb was documented at different length scales in addition to evaluating the fracture toughness and fatigue crack growth behavior.
Microscopy and Microanalysis, Aug 1, 2014
Journal of Materials Engineering and Performance, Jul 28, 2021
Additive manufacturing is a complex multi-parameter process. Electron beam additive manufacturing... more Additive manufacturing is a complex multi-parameter process. Electron beam additive manufacturing of titanium (Ti-6Al-4V), which consists of a multitude of layers of deposited metal, exhibits significant variability in many key aspects including composition, microstructure, and mechanical properties. When establishing methods to predict material properties of these builds, it is necessary to consider both geometry and microstructure. Specifically, the material property of interest is the yield stress. The constitutive equation that is used to predict the yield stress of specimens subjected to stress relief annealing in the α+β phase field has been developed previously. The yield stress equation contains random variables which are modeled with appropriate cumulative distribution functions that characterize their statistical observations. Subsequently, these distributions functions are incorporated into the physically based model using P.C. Collins and D.G. Harlow standard simulation techniques. The main purpose of this integrated modeling and statistical analysis is to begin to characterize the yield stress, especially in the extreme lower tail which is critical for high reliability estimation and prediction. To manage uncertainty and improve the estimation of the yield stress, an established methodology for calibration of the distribution function for the yield stress using experimental data is applied.
Materials
Research on the additive manufacturing of metals often neglects any characterization of the compo... more Research on the additive manufacturing of metals often neglects any characterization of the composition of final parts, erroneously assuming a compositional homogeneity that matches the feedstock material. Here, the composition of electron-beam-melted Ti-6Al-4V produced through three distinct scanning strategies (linear raster and two point melting strategies, random fill and Dehoff fill) is characterized both locally and globally through energy-dispersive spectroscopy and quantitative chemical analysis. As a result of the different scanning strategies used, differing levels of preferential vaporization occur across the various parts, leading to distinct final compositions, with extremes of ~5.8 wt.% Al and ~4.8 wt.% Al. In addition, energy-dispersive spectroscopy composition maps reveal specific features in both the XY and XZ planes (with Z being the build direction) as a result of local inhomogeneous preferential vaporization. The subsequent change in composition significantly mod...
Journal of Materials Science
Defects in parts produced by additive manufacturing, instead of simply being perceived as deleter... more Defects in parts produced by additive manufacturing, instead of simply being perceived as deleterious, can act as important sources of information associated with the complex physical processes that occur during materials deposition and subsequent thermal cycles. Indeed, they act as materials-state ‘fossil’ records of the dynamic AM process. The approach of using defects as epoch-like records of prior history has been developed while studying additively manufactured Ti–6Al–4V and has given new insights into processes that may otherwise remain either obscured or unquantified. Analogous to ‘epochs,’ the evolution of these defects often is characterized by physics that span across a temporal length scale. To demonstrate this approach, a broad range of analyses including optical and electron microscopy, X-ray computed tomography, energy-dispersive spectroscopy, and electron backscatter diffraction have been used to characterize a raster-scanned electron beam Ti–6Al–4V sample. These anal...
Journal of Materials Science, 2016
Journal of Materials Engineering and Performance, 2021
MATEC Web of Conferences, 2020
Additively manufactured materials are gaining wide attention owing to the manufacturing benefits ... more Additively manufactured materials are gaining wide attention owing to the manufacturing benefits as it results in near net shape components. It is well known that the manufacturing processes affects the performance of the components via microstructural features and the mechanical properties. There is an urgent need to understand the processing-structure-property-performance relationship for the materials manufactures via such innovative techniques. Strategies are needed to quantify and modify the mechanical properties. This study assists to design and tailor the process parameters based on the final properties required. Current work predicts the yield strength of additively manufactured Ti-6Al-4V with different post heat treatments. A thermal model predicted by ABAQUS is fed into an implementation of Langmuir equation that predicts the chemistry which is then used in a phenomenological equation predicting the yield strength. The model is confirmed via experiments showing less than 2...
BMC Chemistry, 2019
Laser Engineered Net Shaping (LENS ™ ) was used to produce a compositionally graded Ti-xMo (0 ≤ x... more Laser Engineered Net Shaping (LENS ™ ) was used to produce a compositionally graded Ti-xMo (0 ≤ x ≤ 12 wt %) specimen and nine Ti-15Mo (fixed composition) specimens at different energy densities to understand the compositionprocessing-microstructure relationships operating using additive manufacturing. The gradient was used to evaluate the effect of composition on the prior-beta grain size. The specimens deposited using different energy densities were used to assess the processing parameters influence the microstructure evolutions. The gradient specimen did not show beta grain size reduction with the Mo content. The analysis from the perspective of the two grain refinement mechanisms based on a model known as the Easton & St. John, which was originally developed for aluminum and magnesium alloys shows the lower bound in prior-beta grain refinement with the Ti-Mo system. The low growth restriction factor for the Ti-Mo system of Q = 6,5C 0 explains the unsuccessful refinement from the solute-based mechanism. The energy density and the grain size are proportional according to the results of the nine fixed composition specimens at different energy densities. More energy absorption from the material represents bigger molten pools, which in turn relates to lower cooling rates.
International Journal of Solids and Structures, 2018
A microscale phase field model developed in and is slightly advanced for different and anisotropi... more A microscale phase field model developed in and is slightly advanced for different and anisotropic elastic moduli of phases and is employed for the study of the stressinduced cubic-monoclinic phase transition in NiTi single crystal involving all 12 martensitic variants. The model is scale-independent, without the gradient term, and it is applicable for any scale greater than 100 nm. This model includes strain softening and the corresponding transformation strain localization, and it reproduces a discrete martensitic microstructure. The model only tracks finite-width interfaces between austenite and the mixture of martensitic variants, and does not consider the interfaces between martensitic variants. The model is implemented as a UMAT subroutine in a commercial finite element (FE) package, ABAQUS. Multiple problems for a uniaxial cyclic loading are solved to study the effect of mesh, strain rate, crystal orientation, different numbers of pre-existing nuclei, and the magnitude of the athermal threshold on the stress-strain responses as well as the microstructure evolution. The obtained results exhibit that the microstructure and global stress-strain responses are practically independent of mesh discretization and the applied strain rate for relatively small strain rates. While the presence of the initial nuclei in the sample decreases the nucleation stress, it slightly increases the total energy dissipation. The observed microstructure, the sudden drop in the stress-strain curve after initiation of the martensitic transformation, and the absence of a similar jump for the reverse phase transformation are in qualitative agreement with known experiments. Changing the crystallographic orientation of the sample varies the entire behavior, namely, the variants which are involved in the phase transformation, the morphology of the associated microstructure, the stress-strain curve, and the total dissipation. Athermal threshold, in addition to the expected increase in the magnitude of hysteresis, leads to some strain hardening for the direct phase transformation.
Journal of Alloys and Compounds, 2017
To date, Zn has not been used as an alloying addition in structural Ti alloys. The main obstacle ... more To date, Zn has not been used as an alloying addition in structural Ti alloys. The main obstacle has been the disparity between their melting and vaporization temperatures. A novel processing technique was developed to create a Ti-Zn compound. The equilibrium phases and microstructures were studied by electron microscopy and x-ray diffraction techniques. Results show the presence of pearlitic domains of α-Ti (hexagonal closed packed crystal structure) and Ti2Zn (body center tetragonal structure) in regions that have a near eutectoid composition. Solutionizing and water quenching results in the formation of martensite along with intermetallic laths, suggesting that the eutectoid transformation is active.
Journal of Materials Science, 2016
Adopting a high-throughput combinatorial approach, a compositionally graded Ti-xAl (0 B x B 8 wt%... more Adopting a high-throughput combinatorial approach, a compositionally graded Ti-xAl (0 B x B 8 wt%) specimen was prepared to conduct a rapid systematic investigation of the influence of composition and exposure time on the oxidation performance of the titanium-rich section of the binary Ti-Al system. The compositionally graded specimen was solution heat treated and subjected to oxidation tests at 650 °C for different exposure times. The morphology, structure, and composition of the oxide scale as well as the microstructural changes in the base material were studied across the entire composition range, using a suite of characterization techniques. The observations revealed the presence of Al 2 O 3 in the topmost layer of the oxide scale in addition to TiO 2 , indicating its early formation during oxidation. An increase in Al concentration improves the scaling rate of Ti; however, this is observed only for extended exposure times (i.e., 50 and 100 h), and a parabolic oxidation law is obeyed in the composition-time domain. The formation of the a 2 phase (Ti 3 Al) also takes place for relatively higher Al contents (i.e., 8 wt%).
Journal of Electronic Materials, 2017
Philosophical Magazine, 2016
Philosophical Magazine Letters, 2015
Fatigue crack growth experiments were conducted in humid air (RH~45%) at 25 °C on 29-mm-thick pla... more Fatigue crack growth experiments were conducted in humid air (RH~45%) at 25 °C on 29-mm-thick plate samples of an aluminium-magnesium (Al-Mg) 5083-H131 alloy in the long transverse (LT) direction. Samples were tested in both the as-received condition and after sensitization at 175 °C for 100 h. Delamination along some grain boundaries was observed in the short transverse plane after fatigue testing of the sensitized material, depending upon the level of ΔK and K max . Orientation microscopy using electron backscattering diffraction and chemical analyses using transmission electron microscopy and energy dispersive spectroscopy of grain boundaries revealed that Mg segregation and the orientation of grains had key roles in the observed grain boundary delamination of the sensitized material.
John Wiley & Sons, Inc. eBooks, May 6, 2016
Titanium aluminide (TiAl) alloys have been studied for many years with the intent to replace heav... more Titanium aluminide (TiAl) alloys have been studied for many years with the intent to replace heavier Ni-based superalloys currently used in aerospace structural parts. Various additive manufacturing techniques are being explored to produce near net shape components of TiAl and other materials. Due to the multiple cycles of heating and cooling at different rates, a complex microstructure evolution and resulting mechanical response is expected for materials made by these techniques. In this work, the microstructure of as-deposited Ti-48Al-2Cr-2Nb was documented at different length scales in addition to evaluating the fracture toughness and fatigue crack growth behavior.
Microscopy and Microanalysis, Aug 1, 2014
Journal of Materials Engineering and Performance, Jul 28, 2021
Additive manufacturing is a complex multi-parameter process. Electron beam additive manufacturing... more Additive manufacturing is a complex multi-parameter process. Electron beam additive manufacturing of titanium (Ti-6Al-4V), which consists of a multitude of layers of deposited metal, exhibits significant variability in many key aspects including composition, microstructure, and mechanical properties. When establishing methods to predict material properties of these builds, it is necessary to consider both geometry and microstructure. Specifically, the material property of interest is the yield stress. The constitutive equation that is used to predict the yield stress of specimens subjected to stress relief annealing in the α+β phase field has been developed previously. The yield stress equation contains random variables which are modeled with appropriate cumulative distribution functions that characterize their statistical observations. Subsequently, these distributions functions are incorporated into the physically based model using P.C. Collins and D.G. Harlow standard simulation techniques. The main purpose of this integrated modeling and statistical analysis is to begin to characterize the yield stress, especially in the extreme lower tail which is critical for high reliability estimation and prediction. To manage uncertainty and improve the estimation of the yield stress, an established methodology for calibration of the distribution function for the yield stress using experimental data is applied.
Materials
Research on the additive manufacturing of metals often neglects any characterization of the compo... more Research on the additive manufacturing of metals often neglects any characterization of the composition of final parts, erroneously assuming a compositional homogeneity that matches the feedstock material. Here, the composition of electron-beam-melted Ti-6Al-4V produced through three distinct scanning strategies (linear raster and two point melting strategies, random fill and Dehoff fill) is characterized both locally and globally through energy-dispersive spectroscopy and quantitative chemical analysis. As a result of the different scanning strategies used, differing levels of preferential vaporization occur across the various parts, leading to distinct final compositions, with extremes of ~5.8 wt.% Al and ~4.8 wt.% Al. In addition, energy-dispersive spectroscopy composition maps reveal specific features in both the XY and XZ planes (with Z being the build direction) as a result of local inhomogeneous preferential vaporization. The subsequent change in composition significantly mod...
Journal of Materials Science
Defects in parts produced by additive manufacturing, instead of simply being perceived as deleter... more Defects in parts produced by additive manufacturing, instead of simply being perceived as deleterious, can act as important sources of information associated with the complex physical processes that occur during materials deposition and subsequent thermal cycles. Indeed, they act as materials-state ‘fossil’ records of the dynamic AM process. The approach of using defects as epoch-like records of prior history has been developed while studying additively manufactured Ti–6Al–4V and has given new insights into processes that may otherwise remain either obscured or unquantified. Analogous to ‘epochs,’ the evolution of these defects often is characterized by physics that span across a temporal length scale. To demonstrate this approach, a broad range of analyses including optical and electron microscopy, X-ray computed tomography, energy-dispersive spectroscopy, and electron backscatter diffraction have been used to characterize a raster-scanned electron beam Ti–6Al–4V sample. These anal...
Journal of Materials Science, 2016
Journal of Materials Engineering and Performance, 2021
MATEC Web of Conferences, 2020
Additively manufactured materials are gaining wide attention owing to the manufacturing benefits ... more Additively manufactured materials are gaining wide attention owing to the manufacturing benefits as it results in near net shape components. It is well known that the manufacturing processes affects the performance of the components via microstructural features and the mechanical properties. There is an urgent need to understand the processing-structure-property-performance relationship for the materials manufactures via such innovative techniques. Strategies are needed to quantify and modify the mechanical properties. This study assists to design and tailor the process parameters based on the final properties required. Current work predicts the yield strength of additively manufactured Ti-6Al-4V with different post heat treatments. A thermal model predicted by ABAQUS is fed into an implementation of Langmuir equation that predicts the chemistry which is then used in a phenomenological equation predicting the yield strength. The model is confirmed via experiments showing less than 2...
BMC Chemistry, 2019
Laser Engineered Net Shaping (LENS ™ ) was used to produce a compositionally graded Ti-xMo (0 ≤ x... more Laser Engineered Net Shaping (LENS ™ ) was used to produce a compositionally graded Ti-xMo (0 ≤ x ≤ 12 wt %) specimen and nine Ti-15Mo (fixed composition) specimens at different energy densities to understand the compositionprocessing-microstructure relationships operating using additive manufacturing. The gradient was used to evaluate the effect of composition on the prior-beta grain size. The specimens deposited using different energy densities were used to assess the processing parameters influence the microstructure evolutions. The gradient specimen did not show beta grain size reduction with the Mo content. The analysis from the perspective of the two grain refinement mechanisms based on a model known as the Easton & St. John, which was originally developed for aluminum and magnesium alloys shows the lower bound in prior-beta grain refinement with the Ti-Mo system. The low growth restriction factor for the Ti-Mo system of Q = 6,5C 0 explains the unsuccessful refinement from the solute-based mechanism. The energy density and the grain size are proportional according to the results of the nine fixed composition specimens at different energy densities. More energy absorption from the material represents bigger molten pools, which in turn relates to lower cooling rates.
International Journal of Solids and Structures, 2018
A microscale phase field model developed in and is slightly advanced for different and anisotropi... more A microscale phase field model developed in and is slightly advanced for different and anisotropic elastic moduli of phases and is employed for the study of the stressinduced cubic-monoclinic phase transition in NiTi single crystal involving all 12 martensitic variants. The model is scale-independent, without the gradient term, and it is applicable for any scale greater than 100 nm. This model includes strain softening and the corresponding transformation strain localization, and it reproduces a discrete martensitic microstructure. The model only tracks finite-width interfaces between austenite and the mixture of martensitic variants, and does not consider the interfaces between martensitic variants. The model is implemented as a UMAT subroutine in a commercial finite element (FE) package, ABAQUS. Multiple problems for a uniaxial cyclic loading are solved to study the effect of mesh, strain rate, crystal orientation, different numbers of pre-existing nuclei, and the magnitude of the athermal threshold on the stress-strain responses as well as the microstructure evolution. The obtained results exhibit that the microstructure and global stress-strain responses are practically independent of mesh discretization and the applied strain rate for relatively small strain rates. While the presence of the initial nuclei in the sample decreases the nucleation stress, it slightly increases the total energy dissipation. The observed microstructure, the sudden drop in the stress-strain curve after initiation of the martensitic transformation, and the absence of a similar jump for the reverse phase transformation are in qualitative agreement with known experiments. Changing the crystallographic orientation of the sample varies the entire behavior, namely, the variants which are involved in the phase transformation, the morphology of the associated microstructure, the stress-strain curve, and the total dissipation. Athermal threshold, in addition to the expected increase in the magnitude of hysteresis, leads to some strain hardening for the direct phase transformation.
Journal of Alloys and Compounds, 2017
To date, Zn has not been used as an alloying addition in structural Ti alloys. The main obstacle ... more To date, Zn has not been used as an alloying addition in structural Ti alloys. The main obstacle has been the disparity between their melting and vaporization temperatures. A novel processing technique was developed to create a Ti-Zn compound. The equilibrium phases and microstructures were studied by electron microscopy and x-ray diffraction techniques. Results show the presence of pearlitic domains of α-Ti (hexagonal closed packed crystal structure) and Ti2Zn (body center tetragonal structure) in regions that have a near eutectoid composition. Solutionizing and water quenching results in the formation of martensite along with intermetallic laths, suggesting that the eutectoid transformation is active.
Journal of Materials Science, 2016
Adopting a high-throughput combinatorial approach, a compositionally graded Ti-xAl (0 B x B 8 wt%... more Adopting a high-throughput combinatorial approach, a compositionally graded Ti-xAl (0 B x B 8 wt%) specimen was prepared to conduct a rapid systematic investigation of the influence of composition and exposure time on the oxidation performance of the titanium-rich section of the binary Ti-Al system. The compositionally graded specimen was solution heat treated and subjected to oxidation tests at 650 °C for different exposure times. The morphology, structure, and composition of the oxide scale as well as the microstructural changes in the base material were studied across the entire composition range, using a suite of characterization techniques. The observations revealed the presence of Al 2 O 3 in the topmost layer of the oxide scale in addition to TiO 2 , indicating its early formation during oxidation. An increase in Al concentration improves the scaling rate of Ti; however, this is observed only for extended exposure times (i.e., 50 and 100 h), and a parabolic oxidation law is obeyed in the composition-time domain. The formation of the a 2 phase (Ti 3 Al) also takes place for relatively higher Al contents (i.e., 8 wt%).
Journal of Electronic Materials, 2017
Philosophical Magazine, 2016
Philosophical Magazine Letters, 2015
Fatigue crack growth experiments were conducted in humid air (RH~45%) at 25 °C on 29-mm-thick pla... more Fatigue crack growth experiments were conducted in humid air (RH~45%) at 25 °C on 29-mm-thick plate samples of an aluminium-magnesium (Al-Mg) 5083-H131 alloy in the long transverse (LT) direction. Samples were tested in both the as-received condition and after sensitization at 175 °C for 100 h. Delamination along some grain boundaries was observed in the short transverse plane after fatigue testing of the sensitized material, depending upon the level of ΔK and K max . Orientation microscopy using electron backscattering diffraction and chemical analyses using transmission electron microscopy and energy dispersive spectroscopy of grain boundaries revealed that Mg segregation and the orientation of grains had key roles in the observed grain boundary delamination of the sensitized material.