The mechanisms of fatigue life improvement by pre-twinning process in a commercial rolled magnesi... more The mechanisms of fatigue life improvement by pre-twinning process in a commercial rolled magnesium (Mg) alloy have been investigated using real-time in situ neutron diffraction under a continuous-loading condition. It is found that by introducing the excess twinned grains through pre-compression along the rolling direction the fatigue life was enhanced approximately 50%, mainly resulting from the prolonged detwinning process and inhibited dislocation slip during reverse tension. Moreover, after pre-twinning, the reduction of the rapid strain hardening during reverse tension leads to a compressive mean stress value and more symmetric shape of stressstrain hysteresis loop. The pre-twinning has significant impacts on the twinning-detwinning characteristics and deformation modes during cyclic loading and greatly facilitates the twinningdetwinning activities in plastic deformation. The cyclic straining leads to the increase of contribution of tensile twinning deformation in overall plastic deformation in both the asreceived and pre-twinned sample. The mechanisms of load partitioning in different groups of grains are closely related to the deformation modes in each deformation stage, while the fatigue cycling has little influence on the load sharing. The pre-twinning process provides an easy and cost-effective route to improve the low-cycle fatigue life through manufacturing and processing, which would advance the wide application of lightweight wrought Mg alloys as structural materials.
Deformation twinning plays a critical role on improving metals or alloys ductility, especially fo... more Deformation twinning plays a critical role on improving metals or alloys ductility, especially for hexagonal close-packed materials with low symmetry crystal structure. A rolled Mg alloy was selected as a model system to investigate the extension twinning behaviors and characteristics of parent-twin interactions by nondestructive in situ 3D synchrotron X-ray microbeam diffraction. Besides twinning-detwinning process, the "twinning-like" lattice reorientation process was captured within an individual grain inside a bulk material during the strain reversal. The distributions of parent, twin, and reorientated grains and sub-micron level strain variation across the twin boundary are revealed. A theoretical calculation of the lattice strain confirms that the internal strain distribution in parent and twinned grains correlates with the experimental setup, grain orientation of parent, twin, and surrounding grains, as well as the strain path changes. The study suggests a novel deformation mechanism within the hexagonal close-packed structure that cannot be determined from surface-based characterization methods.
In the present research, the real-time in-situ neutron diffraction measurements under a continuou... more In the present research, the real-time in-situ neutron diffraction measurements under a continuous-loading condition and elastic-viscoplastic self-consistent (EVPSC) polycrystal modeling were employed to study the deformation dynamics and the effect of the deformation history on plastic deformation in a wrought magnesium alloy. The experimental results reveal that pre-deformation delays the activation of the tensile twinning during the subsequent compression, mainly resulting from the residual strains. Detwinning does not occur until the applied stress exceeds the tensile yield strength during reverse loading. It is believed that the grain rotation plays an important role in the elastic region during reverse loading. The EVPSC model, which has been recently updated by implementing the twinning and detwinning model, was employed to characterize the deformation mechanism during strain-path changes. The simulation result predicts well the experimental observation from the real-time in-situ neutron diffraction measurements. The present study provides a new insight of the nature of deformation mechanisms in a hexagonal close-packed (HCP) structured polycrystalline wrought magnesium alloy, which has significant implications for future work on studying the deformation mechanisms of HCP structured materials.
In the current study, the deformation mechanisms of a rolled magnesium alloy were investigated un... more In the current study, the deformation mechanisms of a rolled magnesium alloy were investigated under cyclic loading using the real-time in-situ neutron diffraction under a continuous-loading condition. The relationship between the macroscopic cyclic deformation behavior and the microscopic response at the grain level has been established. The neutron diffraction results indicate that more and more grains are involved in the twinning and detwinning deformation process with the increase of fatigue cycles. The residual twins appear in the early fatigue life, which is responsible for the cyclic hardening behavior. The asymmetric shape of the hysteresis loop is attributed to the early exhaustion of the detwinning process during compression, which leads to the activation of dislocation slips and rapid strain hardening. The critical resolved shear stress for the activation of tensile twinning closely depends on the residual strain developed during cyclic loading. In the cycle before the sample fractured, the dislocation slips became active in tension, although the sample was not fully twinned. The increased dislocation density leads to the rise of the stress concentration at weak spots, which is believed to be the main reason for the fatigue failure. The deformation history greatly influences the deformation mechanisms of hexagonal close-packed structured magnesium alloy during cyclic loading.
The deformation dynamics and the effect of deformation history on plastic deformation in a wrough... more The deformation dynamics and the effect of deformation history on plastic deformation in a wrought magnesium alloy at room temperature have been studied by real-time in situ neutron diffraction measurements under a continuous loading condition. The experimental results reveal that no detwinning occurred during unloading after compression and even in an elastic region during reverse tension. It is found that the serration behavior is closely related to the twinning-and detwinning-dominated deformation.
We experimentally and numerically study the large-strain free-end torsion of a rolled magnesium a... more We experimentally and numerically study the large-strain free-end torsion of a rolled magnesium alloy. It is found that a torsion sample with its axial direction parallel to the normal direction elongates axially, while a sample with its axial direction along the rolling direction contracts axially. It is shown that this Swift effect, i.e., the second-order axial effect under free-end torsion, is mainly due to extension twinning.
The low-cycle fatigue behavior of an as-extruded AM50 magnesium alloy has been investigated. The ... more The low-cycle fatigue behavior of an as-extruded AM50 magnesium alloy has been investigated. The cyclic stress response of the alloy strongly depends on the imposed strain amplitude. It is also noted that at the higher total strain amplitudes, the alloy exhibits a pronounced anisotropic deformation behavior in the direction of tension and compression, where the width of the σ-ε hysteresis
In-situ neutron diffraction was used to investigate the twinning and detwinning behavior during f... more In-situ neutron diffraction was used to investigate the twinning and detwinning behavior during fatigue-crack propagation on a wrought magnesium alloy, AZ31B, compact-tension specimen, where fatigue loading was applied parallel to the plate normal. Reversible twinning and detwinning characteristics were observed as a function of the distance from the crack tip. While twinning was activated above a certain critical stress value, detwinning occurred immediately upon unloading. It is thought that the development of compressive residual stresses generated around the crack tip during unloading is responsible for the detwinning behavior. Neutron bulk texture measurements were performed at several locations away from the crack tip (i.e. locations behind, right in front of, and far away from the crack tip) to quantitatively examine the volume fraction of {10.2}/10.1S extension twins in the vicinity of the crack tip. The texture analyses demonstrated that the texture in the fatiguewake region did not change significantly, compared to that in the undeformed region far away from the crack tip, and approximately 11% of the residual twins were left behind the crack tip. The current results reveal that the reversible twinning and detwinning are the dominant deformation mechanisms for the studied material subjected to cyclic loading, and only a small amount of residual twins remain after the crack propagation. The spatial distribution of twinning/detwinning transitions correlates well with our previous predictions of the stress fields in the vicinity of a fatigue crack tip.
ABSTRACT Novel Ni-free Zr-based bulk metallic glasses (BMGs) with low Cu contents (~ 10 at.%) hav... more ABSTRACT Novel Ni-free Zr-based bulk metallic glasses (BMGs) with low Cu contents (~ 10 at.%) have been developed, which are attractive for biomedical applications. Potentially toxic elements (i.e., Ni and Cu) are reduced or eliminated in order to improve the biocompatibility of the alloys. Glass formation and material properties including compressive strength and plasticity, elastic constants, density, corrosion resistance, and in vitro cytotoxicity are systematically evaluated. The BMGs developed in the present study integrate suitable properties of high glass-forming ability, excellent mechanical properties, high corrosion resistance, and good cytocompatibility, which suggest their promising potential to serve as biomedical materials. The good biocompatibility can be attributed to their amorphous structure-related high corrosion resistance as well as their unique compositions, i.e. low Cu contents and free of Ni.
Journal of Biomedical Materials Research Part B: Applied Biomaterials, 2012
Zr-based bulk metallic glasses (BMGs) possess attractive properties for prospective biomedical ap... more Zr-based bulk metallic glasses (BMGs) possess attractive properties for prospective biomedical applications. The present study designs Ni-free Zr-Cu-Al-Nb-Pd BMGs and investigates their in vitro biocompatibility by studying mechanical properties, bio-corrosion resistance, and cellular responses. The Ti-6Al-4V alloy is used as a reference material. It is found that the Zr-based BMGs exhibit good mechanical properties, including high strengths above 1600 MPa, high hardness over 4700 MPa, and low elastic moduli of 85-90 GPa. The Zr-based BMGs are corrosion resistant in a simulated body environment, as revealed by wide passive regions, low passive current densities, and high pitting overpotentials. The formation of ZrO 2-rich surface passive films of the Zr-based BMGs contributes to their high corrosion resistance , whereas their pitting corrosion in the phosphate buffered saline solution can be attributed to the sensitivity of the ZrO 2 films to the chloride ion. The general biosafety of the Zr-based BMGs is revealed by normal cell adhesions and cell morphologies. Moreover, the Zr/Cu content ratio in the alloy composition affects the biocompatibility of the Zr-based BMGs, by increasing their corrosion resistance and surface wettability with the increase of the Zr/Cu ratio. Effects of Zr/ Cu ratios can be used to guide the future design of biocompatible Zr-based BMGs. V
A specially designed semi-circular notch specimen was employed in the current study to generate t... more A specially designed semi-circular notch specimen was employed in the current study to generate the various strain conditions, including uniaxial, biaxial, shear, and plane strains, which was utilized to explore the evolution of different deformation twinning systems under complex loading conditions. Using in situ synchrotron X-ray diffraction mapping method, it was found that the extensive double twins were activated during loading, while nearly no extension twinning activity was detected. After the formation of {10.1} and {10.3} compression twins, they transformed into {10.1}-{10.2} and {10.3}-{10.2} double twins instantaneously at the early stage of deformation. The lattice strain evolutions in different hkls were mapped at selected load levels during the loading-unloading sequence. The relationship between the macroscopic straining and microscopic response was established.
Deformation twinning plays a critical role on improving metals or alloys ductility, especially fo... more Deformation twinning plays a critical role on improving metals or alloys ductility, especially for hexagonal close-packed materials with low symmetry crystal structure. A rolled Mg alloy was selected as a model system to investigate the extension twinning behaviors and characteristics of parent-twin interactions by nondestructive in situ 3D synchrotron X-ray microbeam diffraction. Besides twinning-detwinning process, the " twinning-like " lattice reorientation process was captured within an individual grain inside a bulk material during the strain reversal. The distributions of parent, twin, and reor-ientated grains and sub-micron level strain variation across the twin boundary are revealed. A theoretical calculation of the lattice strain confirms that the internal strain distribution in parent and twinned grains correlates with the experimental setup, grain orientation of parent, twin, and surrounding grains, as well as the strain path changes. The study suggests a novel deformation mechanism within the hexagonal close-packed structure that cannot be determined from surface-based characterization methods.
A specially designed semi-circular notch specimen was employed in the current study to generate t... more A specially designed semi-circular notch specimen was employed in the current study to generate the various strain conditions, including uniaxial, biaxial, shear, and plane strains, which was utilized to explore the evolution of different deformation twinning systems under complex loading conditions. Using in situ synchrotron X-ray diffraction mapping method, it was found that the extensive double twins were activated during loading, while nearly no extension twinning activity was detected. After the formation of {10.1} and {10.3} compression twins, they transformed into {10.1}e{10.2} and {10.3}e{10.2} double twins instantaneously at the early stage of deformation. The lattice strain evolutions in different hkls were mapped at selected load levels during the loading-unloading sequence. The relationship between the macroscopic straining and microscopic response was established.
—In the current study, the deformation mechanisms of a rolled magnesium alloy were investigated u... more —In the current study, the deformation mechanisms of a rolled magnesium alloy were investigated under cyclic loading using real-time in situ neutron diffraction under a continuous-loading condition. The relationship between the macroscopic cyclic deformation behavior and the microscopic response at the grain level was established. The neutron diffraction results indicate that more and more grains are involved in the twinning and detwinning deformation process with the increase of fatigue cycles. The residual twins appear in the early fatigue life, which is responsible for the cyclic hardening behavior. The asymmetric shape of the hysteresis loop is attributed to the early exhaustion of the detwinning process during compression, which leads to the activation of dislocation slips and rapid strain-hardening. The critical resolved shear stress for the activation of tensile twinning closely depends on the residual strain developed during cyclic loading. In the cycle before the sample fractured, the dislocation slips became active in tension, although the sample was not fully twinned. The increased dislocation density leads to the rise of the stress concentration at weak spots, which is believed to be the main reason for the fatigue failure. The deformation history greatly influences the deformation mechanisms of hexagonal-close-packed-structured magnesium alloy during cyclic loading. Published by Elsevier Ltd. on behalf of Acta Materialia Inc.
The mechanisms of fatigue life improvement by pre-twinning process in a commercial rolled magnesi... more The mechanisms of fatigue life improvement by pre-twinning process in a commercial rolled magnesium (Mg) alloy have been investigated using real-time in situ neutron diffraction under a continuous-loading condition. It is found that by introducing the excess twinned grains through pre-compression along the rolling direction the fatigue life was enhanced approximately 50%, mainly resulting from the prolonged detwinning process and inhibited dislocation slip during reverse tension. Moreover, after pre-twinning, the reduction of the rapid strain hardening during reverse tension leads to a compressive mean stress value and more symmetric shape of stressestrain hysteresis loop. The pre-twinning has significant impacts on the twinning-detwinning characteristics and deformation modes during cyclic loading and greatly facilitates the twinning-detwinning activities in plastic deformation. The cyclic straining leads to the increase of contribution of tensile twinning deformation in overall plastic deformation in both the as-received and pre-twinned sample. The mechanisms of load partitioning in different groups of grains are closely related to the deformation modes in each deformation stage, while the fatigue cycling has little influence on the load sharing. The pre-twinning process provides an easy and cost-effective route to improve the low-cycle fatigue life through manufacturing and processing, which would advance the wide application of lightweight wrought Mg alloys as structural materials.
The deformation dynamics and the effect of deformation history on plastic deformation in a wrough... more The deformation dynamics and the effect of deformation history on plastic deformation in a wrought magnesium alloy at room temperature have been studied by real-time in situ neutron diffraction measurements under a continuous loading condition. The experimental results reveal that no detwinning occurred during unloading after compression and even in an elastic region during reverse tension. It is found that the serration behavior is closely related to the twinning-and detwinning-dominated deformation.
In-situ neutron diffraction was used to investigate the twinning and detwinning behavior during f... more In-situ neutron diffraction was used to investigate the twinning and detwinning behavior during fatigue-crack propagation on a wrought magnesium alloy, AZ31B, compact-tension specimen, where fatigue loading was applied parallel to the plate normal. Reversible twinning and detwinning characteristics were observed as a function of the distance from the crack tip. While twinning was activated above a certain critical stress value, detwinning occurred immediately upon unloading. It is thought that the development of compressive residual stresses generated around the crack tip during unloading is responsible for the detwinning behavior. Neutron bulk texture measurements were performed at several locations away from the crack tip (i.e. locations behind, right in front of, and far away from the crack tip) to quantitatively examine the volume fraction of {10.2}/10.1S extension twins in the vicinity of the crack tip. The texture analyses demonstrated that the texture in the fatiguewake region did not change significantly, compared to that in the undeformed region far away from the crack tip, and approximately 11% of the residual twins were left behind the crack tip. The current results reveal that the reversible twinning and detwinning are the dominant deformation mechanisms for the studied material subjected to cyclic loading, and only a small amount of residual twins remain after the crack propagation. The spatial distribution of twinning/detwinning transitions correlates well with our previous predictions of the stress fields in the vicinity of a fatigue crack tip.
Journal of Biomedical Materials Research Part B: Applied Biomaterials, 2012
Zr-based bulk metallic glasses (BMGs) possess attractive properties for prospective biomedical ap... more Zr-based bulk metallic glasses (BMGs) possess attractive properties for prospective biomedical applications. The present study designs Ni-free Zr-Cu-Al-Nb-Pd BMGs and investigates their in vitro biocompatibility by studying mechanical properties, bio-corrosion resistance, and cellular responses. The Ti-6Al-4V alloy is used as a reference material. It is found that the Zr-based BMGs exhibit good mechanical properties, including high strengths above 1600 MPa, high hardness over 4700 MPa, and low elastic moduli of 85-90 GPa. The Zr-based BMGs are corrosion resistant in a simulated body environment, as revealed by wide passive regions, low passive current densities, and high pitting overpotentials. The formation of ZrO(2)-rich surface passive films of the Zr-based BMGs contributes to their high corrosion resistance, whereas their pitting corrosion in the phosphate buffered saline solution can be attributed to the sensitivity of the ZrO(2) films to the chloride ion. The general biosafety of the Zr-based BMGs is revealed by normal cell adhesions and cell morphologies. Moreover, the Zr/Cu content ratio in the alloy composition affects the biocompatibility of the Zr-based BMGs, by increasing their corrosion resistance and surface wettability with the increase of the Zr/Cu ratio. Effects of Zr/Cu ratios can be used to guide the future design of biocompatible Zr-based BMGs.
In the present research, the real-time in-situ neutron diffraction measurements under a continuou... more In the present research, the real-time in-situ neutron diffraction measurements under a continuous-loading condition and elastic-viscoplastic self-consistent (EVPSC) polycrystal modeling were employed to study the deformation dynamics and the effect of the deformation history on plastic deformation in a wrought magnesium alloy. The experimental results reveal that pre-deformation delays the activation of the tensile twinning during the subsequent compression, mainly resulting from the residual strains. Detwinning does not occur until the applied stress exceeds the tensile yield strength during the reverse loading. It is believed that the grain rotation plays an important role in the elastic region during the reverse loading. The EVPSC model, which has been recently updated by implementing the twinning and detwinning model, was employed to characterize the deformation mechanism during the strain-path changes. The simulation result predicts well the experimental observation from the real-time in-situ neutron diffraction measurements. The present study provides a new insight of the nature of deformation mechanisms in a hexagonal close-packed (HCP) structured polycrystalline wrought magnesium alloy, which has significant implications for future work on studying the deformation mechanisms of HCP-structured materials.
The low-cycle fatigue behavior of an as-extruded AM50 magnesium alloy has been investigated. The ... more The low-cycle fatigue behavior of an as-extruded AM50 magnesium alloy has been investigated. The cyclic stress response of the alloy strongly depends on the imposed strain amplitude. It is also noted that at the higher total strain amplitudes, the alloy exhibits a pronounced anisotropic deformation behavior in the direction of tension and compression, where the width of the σ-ε hysteresis
The mechanisms of fatigue life improvement by pre-twinning process in a commercial rolled magnesi... more The mechanisms of fatigue life improvement by pre-twinning process in a commercial rolled magnesium (Mg) alloy have been investigated using real-time in situ neutron diffraction under a continuous-loading condition. It is found that by introducing the excess twinned grains through pre-compression along the rolling direction the fatigue life was enhanced approximately 50%, mainly resulting from the prolonged detwinning process and inhibited dislocation slip during reverse tension. Moreover, after pre-twinning, the reduction of the rapid strain hardening during reverse tension leads to a compressive mean stress value and more symmetric shape of stressstrain hysteresis loop. The pre-twinning has significant impacts on the twinning-detwinning characteristics and deformation modes during cyclic loading and greatly facilitates the twinningdetwinning activities in plastic deformation. The cyclic straining leads to the increase of contribution of tensile twinning deformation in overall plastic deformation in both the asreceived and pre-twinned sample. The mechanisms of load partitioning in different groups of grains are closely related to the deformation modes in each deformation stage, while the fatigue cycling has little influence on the load sharing. The pre-twinning process provides an easy and cost-effective route to improve the low-cycle fatigue life through manufacturing and processing, which would advance the wide application of lightweight wrought Mg alloys as structural materials.
Deformation twinning plays a critical role on improving metals or alloys ductility, especially fo... more Deformation twinning plays a critical role on improving metals or alloys ductility, especially for hexagonal close-packed materials with low symmetry crystal structure. A rolled Mg alloy was selected as a model system to investigate the extension twinning behaviors and characteristics of parent-twin interactions by nondestructive in situ 3D synchrotron X-ray microbeam diffraction. Besides twinning-detwinning process, the "twinning-like" lattice reorientation process was captured within an individual grain inside a bulk material during the strain reversal. The distributions of parent, twin, and reorientated grains and sub-micron level strain variation across the twin boundary are revealed. A theoretical calculation of the lattice strain confirms that the internal strain distribution in parent and twinned grains correlates with the experimental setup, grain orientation of parent, twin, and surrounding grains, as well as the strain path changes. The study suggests a novel deformation mechanism within the hexagonal close-packed structure that cannot be determined from surface-based characterization methods.
In the present research, the real-time in-situ neutron diffraction measurements under a continuou... more In the present research, the real-time in-situ neutron diffraction measurements under a continuous-loading condition and elastic-viscoplastic self-consistent (EVPSC) polycrystal modeling were employed to study the deformation dynamics and the effect of the deformation history on plastic deformation in a wrought magnesium alloy. The experimental results reveal that pre-deformation delays the activation of the tensile twinning during the subsequent compression, mainly resulting from the residual strains. Detwinning does not occur until the applied stress exceeds the tensile yield strength during reverse loading. It is believed that the grain rotation plays an important role in the elastic region during reverse loading. The EVPSC model, which has been recently updated by implementing the twinning and detwinning model, was employed to characterize the deformation mechanism during strain-path changes. The simulation result predicts well the experimental observation from the real-time in-situ neutron diffraction measurements. The present study provides a new insight of the nature of deformation mechanisms in a hexagonal close-packed (HCP) structured polycrystalline wrought magnesium alloy, which has significant implications for future work on studying the deformation mechanisms of HCP structured materials.
In the current study, the deformation mechanisms of a rolled magnesium alloy were investigated un... more In the current study, the deformation mechanisms of a rolled magnesium alloy were investigated under cyclic loading using the real-time in-situ neutron diffraction under a continuous-loading condition. The relationship between the macroscopic cyclic deformation behavior and the microscopic response at the grain level has been established. The neutron diffraction results indicate that more and more grains are involved in the twinning and detwinning deformation process with the increase of fatigue cycles. The residual twins appear in the early fatigue life, which is responsible for the cyclic hardening behavior. The asymmetric shape of the hysteresis loop is attributed to the early exhaustion of the detwinning process during compression, which leads to the activation of dislocation slips and rapid strain hardening. The critical resolved shear stress for the activation of tensile twinning closely depends on the residual strain developed during cyclic loading. In the cycle before the sample fractured, the dislocation slips became active in tension, although the sample was not fully twinned. The increased dislocation density leads to the rise of the stress concentration at weak spots, which is believed to be the main reason for the fatigue failure. The deformation history greatly influences the deformation mechanisms of hexagonal close-packed structured magnesium alloy during cyclic loading.
The deformation dynamics and the effect of deformation history on plastic deformation in a wrough... more The deformation dynamics and the effect of deformation history on plastic deformation in a wrought magnesium alloy at room temperature have been studied by real-time in situ neutron diffraction measurements under a continuous loading condition. The experimental results reveal that no detwinning occurred during unloading after compression and even in an elastic region during reverse tension. It is found that the serration behavior is closely related to the twinning-and detwinning-dominated deformation.
We experimentally and numerically study the large-strain free-end torsion of a rolled magnesium a... more We experimentally and numerically study the large-strain free-end torsion of a rolled magnesium alloy. It is found that a torsion sample with its axial direction parallel to the normal direction elongates axially, while a sample with its axial direction along the rolling direction contracts axially. It is shown that this Swift effect, i.e., the second-order axial effect under free-end torsion, is mainly due to extension twinning.
The low-cycle fatigue behavior of an as-extruded AM50 magnesium alloy has been investigated. The ... more The low-cycle fatigue behavior of an as-extruded AM50 magnesium alloy has been investigated. The cyclic stress response of the alloy strongly depends on the imposed strain amplitude. It is also noted that at the higher total strain amplitudes, the alloy exhibits a pronounced anisotropic deformation behavior in the direction of tension and compression, where the width of the σ-ε hysteresis
In-situ neutron diffraction was used to investigate the twinning and detwinning behavior during f... more In-situ neutron diffraction was used to investigate the twinning and detwinning behavior during fatigue-crack propagation on a wrought magnesium alloy, AZ31B, compact-tension specimen, where fatigue loading was applied parallel to the plate normal. Reversible twinning and detwinning characteristics were observed as a function of the distance from the crack tip. While twinning was activated above a certain critical stress value, detwinning occurred immediately upon unloading. It is thought that the development of compressive residual stresses generated around the crack tip during unloading is responsible for the detwinning behavior. Neutron bulk texture measurements were performed at several locations away from the crack tip (i.e. locations behind, right in front of, and far away from the crack tip) to quantitatively examine the volume fraction of {10.2}/10.1S extension twins in the vicinity of the crack tip. The texture analyses demonstrated that the texture in the fatiguewake region did not change significantly, compared to that in the undeformed region far away from the crack tip, and approximately 11% of the residual twins were left behind the crack tip. The current results reveal that the reversible twinning and detwinning are the dominant deformation mechanisms for the studied material subjected to cyclic loading, and only a small amount of residual twins remain after the crack propagation. The spatial distribution of twinning/detwinning transitions correlates well with our previous predictions of the stress fields in the vicinity of a fatigue crack tip.
ABSTRACT Novel Ni-free Zr-based bulk metallic glasses (BMGs) with low Cu contents (~ 10 at.%) hav... more ABSTRACT Novel Ni-free Zr-based bulk metallic glasses (BMGs) with low Cu contents (~ 10 at.%) have been developed, which are attractive for biomedical applications. Potentially toxic elements (i.e., Ni and Cu) are reduced or eliminated in order to improve the biocompatibility of the alloys. Glass formation and material properties including compressive strength and plasticity, elastic constants, density, corrosion resistance, and in vitro cytotoxicity are systematically evaluated. The BMGs developed in the present study integrate suitable properties of high glass-forming ability, excellent mechanical properties, high corrosion resistance, and good cytocompatibility, which suggest their promising potential to serve as biomedical materials. The good biocompatibility can be attributed to their amorphous structure-related high corrosion resistance as well as their unique compositions, i.e. low Cu contents and free of Ni.
Journal of Biomedical Materials Research Part B: Applied Biomaterials, 2012
Zr-based bulk metallic glasses (BMGs) possess attractive properties for prospective biomedical ap... more Zr-based bulk metallic glasses (BMGs) possess attractive properties for prospective biomedical applications. The present study designs Ni-free Zr-Cu-Al-Nb-Pd BMGs and investigates their in vitro biocompatibility by studying mechanical properties, bio-corrosion resistance, and cellular responses. The Ti-6Al-4V alloy is used as a reference material. It is found that the Zr-based BMGs exhibit good mechanical properties, including high strengths above 1600 MPa, high hardness over 4700 MPa, and low elastic moduli of 85-90 GPa. The Zr-based BMGs are corrosion resistant in a simulated body environment, as revealed by wide passive regions, low passive current densities, and high pitting overpotentials. The formation of ZrO 2-rich surface passive films of the Zr-based BMGs contributes to their high corrosion resistance , whereas their pitting corrosion in the phosphate buffered saline solution can be attributed to the sensitivity of the ZrO 2 films to the chloride ion. The general biosafety of the Zr-based BMGs is revealed by normal cell adhesions and cell morphologies. Moreover, the Zr/Cu content ratio in the alloy composition affects the biocompatibility of the Zr-based BMGs, by increasing their corrosion resistance and surface wettability with the increase of the Zr/Cu ratio. Effects of Zr/ Cu ratios can be used to guide the future design of biocompatible Zr-based BMGs. V
A specially designed semi-circular notch specimen was employed in the current study to generate t... more A specially designed semi-circular notch specimen was employed in the current study to generate the various strain conditions, including uniaxial, biaxial, shear, and plane strains, which was utilized to explore the evolution of different deformation twinning systems under complex loading conditions. Using in situ synchrotron X-ray diffraction mapping method, it was found that the extensive double twins were activated during loading, while nearly no extension twinning activity was detected. After the formation of {10.1} and {10.3} compression twins, they transformed into {10.1}-{10.2} and {10.3}-{10.2} double twins instantaneously at the early stage of deformation. The lattice strain evolutions in different hkls were mapped at selected load levels during the loading-unloading sequence. The relationship between the macroscopic straining and microscopic response was established.
Deformation twinning plays a critical role on improving metals or alloys ductility, especially fo... more Deformation twinning plays a critical role on improving metals or alloys ductility, especially for hexagonal close-packed materials with low symmetry crystal structure. A rolled Mg alloy was selected as a model system to investigate the extension twinning behaviors and characteristics of parent-twin interactions by nondestructive in situ 3D synchrotron X-ray microbeam diffraction. Besides twinning-detwinning process, the " twinning-like " lattice reorientation process was captured within an individual grain inside a bulk material during the strain reversal. The distributions of parent, twin, and reor-ientated grains and sub-micron level strain variation across the twin boundary are revealed. A theoretical calculation of the lattice strain confirms that the internal strain distribution in parent and twinned grains correlates with the experimental setup, grain orientation of parent, twin, and surrounding grains, as well as the strain path changes. The study suggests a novel deformation mechanism within the hexagonal close-packed structure that cannot be determined from surface-based characterization methods.
A specially designed semi-circular notch specimen was employed in the current study to generate t... more A specially designed semi-circular notch specimen was employed in the current study to generate the various strain conditions, including uniaxial, biaxial, shear, and plane strains, which was utilized to explore the evolution of different deformation twinning systems under complex loading conditions. Using in situ synchrotron X-ray diffraction mapping method, it was found that the extensive double twins were activated during loading, while nearly no extension twinning activity was detected. After the formation of {10.1} and {10.3} compression twins, they transformed into {10.1}e{10.2} and {10.3}e{10.2} double twins instantaneously at the early stage of deformation. The lattice strain evolutions in different hkls were mapped at selected load levels during the loading-unloading sequence. The relationship between the macroscopic straining and microscopic response was established.
—In the current study, the deformation mechanisms of a rolled magnesium alloy were investigated u... more —In the current study, the deformation mechanisms of a rolled magnesium alloy were investigated under cyclic loading using real-time in situ neutron diffraction under a continuous-loading condition. The relationship between the macroscopic cyclic deformation behavior and the microscopic response at the grain level was established. The neutron diffraction results indicate that more and more grains are involved in the twinning and detwinning deformation process with the increase of fatigue cycles. The residual twins appear in the early fatigue life, which is responsible for the cyclic hardening behavior. The asymmetric shape of the hysteresis loop is attributed to the early exhaustion of the detwinning process during compression, which leads to the activation of dislocation slips and rapid strain-hardening. The critical resolved shear stress for the activation of tensile twinning closely depends on the residual strain developed during cyclic loading. In the cycle before the sample fractured, the dislocation slips became active in tension, although the sample was not fully twinned. The increased dislocation density leads to the rise of the stress concentration at weak spots, which is believed to be the main reason for the fatigue failure. The deformation history greatly influences the deformation mechanisms of hexagonal-close-packed-structured magnesium alloy during cyclic loading. Published by Elsevier Ltd. on behalf of Acta Materialia Inc.
The mechanisms of fatigue life improvement by pre-twinning process in a commercial rolled magnesi... more The mechanisms of fatigue life improvement by pre-twinning process in a commercial rolled magnesium (Mg) alloy have been investigated using real-time in situ neutron diffraction under a continuous-loading condition. It is found that by introducing the excess twinned grains through pre-compression along the rolling direction the fatigue life was enhanced approximately 50%, mainly resulting from the prolonged detwinning process and inhibited dislocation slip during reverse tension. Moreover, after pre-twinning, the reduction of the rapid strain hardening during reverse tension leads to a compressive mean stress value and more symmetric shape of stressestrain hysteresis loop. The pre-twinning has significant impacts on the twinning-detwinning characteristics and deformation modes during cyclic loading and greatly facilitates the twinning-detwinning activities in plastic deformation. The cyclic straining leads to the increase of contribution of tensile twinning deformation in overall plastic deformation in both the as-received and pre-twinned sample. The mechanisms of load partitioning in different groups of grains are closely related to the deformation modes in each deformation stage, while the fatigue cycling has little influence on the load sharing. The pre-twinning process provides an easy and cost-effective route to improve the low-cycle fatigue life through manufacturing and processing, which would advance the wide application of lightweight wrought Mg alloys as structural materials.
The deformation dynamics and the effect of deformation history on plastic deformation in a wrough... more The deformation dynamics and the effect of deformation history on plastic deformation in a wrought magnesium alloy at room temperature have been studied by real-time in situ neutron diffraction measurements under a continuous loading condition. The experimental results reveal that no detwinning occurred during unloading after compression and even in an elastic region during reverse tension. It is found that the serration behavior is closely related to the twinning-and detwinning-dominated deformation.
In-situ neutron diffraction was used to investigate the twinning and detwinning behavior during f... more In-situ neutron diffraction was used to investigate the twinning and detwinning behavior during fatigue-crack propagation on a wrought magnesium alloy, AZ31B, compact-tension specimen, where fatigue loading was applied parallel to the plate normal. Reversible twinning and detwinning characteristics were observed as a function of the distance from the crack tip. While twinning was activated above a certain critical stress value, detwinning occurred immediately upon unloading. It is thought that the development of compressive residual stresses generated around the crack tip during unloading is responsible for the detwinning behavior. Neutron bulk texture measurements were performed at several locations away from the crack tip (i.e. locations behind, right in front of, and far away from the crack tip) to quantitatively examine the volume fraction of {10.2}/10.1S extension twins in the vicinity of the crack tip. The texture analyses demonstrated that the texture in the fatiguewake region did not change significantly, compared to that in the undeformed region far away from the crack tip, and approximately 11% of the residual twins were left behind the crack tip. The current results reveal that the reversible twinning and detwinning are the dominant deformation mechanisms for the studied material subjected to cyclic loading, and only a small amount of residual twins remain after the crack propagation. The spatial distribution of twinning/detwinning transitions correlates well with our previous predictions of the stress fields in the vicinity of a fatigue crack tip.
Journal of Biomedical Materials Research Part B: Applied Biomaterials, 2012
Zr-based bulk metallic glasses (BMGs) possess attractive properties for prospective biomedical ap... more Zr-based bulk metallic glasses (BMGs) possess attractive properties for prospective biomedical applications. The present study designs Ni-free Zr-Cu-Al-Nb-Pd BMGs and investigates their in vitro biocompatibility by studying mechanical properties, bio-corrosion resistance, and cellular responses. The Ti-6Al-4V alloy is used as a reference material. It is found that the Zr-based BMGs exhibit good mechanical properties, including high strengths above 1600 MPa, high hardness over 4700 MPa, and low elastic moduli of 85-90 GPa. The Zr-based BMGs are corrosion resistant in a simulated body environment, as revealed by wide passive regions, low passive current densities, and high pitting overpotentials. The formation of ZrO(2)-rich surface passive films of the Zr-based BMGs contributes to their high corrosion resistance, whereas their pitting corrosion in the phosphate buffered saline solution can be attributed to the sensitivity of the ZrO(2) films to the chloride ion. The general biosafety of the Zr-based BMGs is revealed by normal cell adhesions and cell morphologies. Moreover, the Zr/Cu content ratio in the alloy composition affects the biocompatibility of the Zr-based BMGs, by increasing their corrosion resistance and surface wettability with the increase of the Zr/Cu ratio. Effects of Zr/Cu ratios can be used to guide the future design of biocompatible Zr-based BMGs.
In the present research, the real-time in-situ neutron diffraction measurements under a continuou... more In the present research, the real-time in-situ neutron diffraction measurements under a continuous-loading condition and elastic-viscoplastic self-consistent (EVPSC) polycrystal modeling were employed to study the deformation dynamics and the effect of the deformation history on plastic deformation in a wrought magnesium alloy. The experimental results reveal that pre-deformation delays the activation of the tensile twinning during the subsequent compression, mainly resulting from the residual strains. Detwinning does not occur until the applied stress exceeds the tensile yield strength during the reverse loading. It is believed that the grain rotation plays an important role in the elastic region during the reverse loading. The EVPSC model, which has been recently updated by implementing the twinning and detwinning model, was employed to characterize the deformation mechanism during the strain-path changes. The simulation result predicts well the experimental observation from the real-time in-situ neutron diffraction measurements. The present study provides a new insight of the nature of deformation mechanisms in a hexagonal close-packed (HCP) structured polycrystalline wrought magnesium alloy, which has significant implications for future work on studying the deformation mechanisms of HCP-structured materials.
The low-cycle fatigue behavior of an as-extruded AM50 magnesium alloy has been investigated. The ... more The low-cycle fatigue behavior of an as-extruded AM50 magnesium alloy has been investigated. The cyclic stress response of the alloy strongly depends on the imposed strain amplitude. It is also noted that at the higher total strain amplitudes, the alloy exhibits a pronounced anisotropic deformation behavior in the direction of tension and compression, where the width of the σ-ε hysteresis
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