In this research, we systematically investigated equiatomic CoCrFeNi and CoCrFeMnNi high-entropy ... more In this research, we systematically investigated equiatomic CoCrFeNi and CoCrFeMnNi high-entropy alloys (HEAs). Both of these HEA systems are single-phase, face-centered-cubic (FCC) structures. Specifically, we examined the tensile response in as-cast quaternary CoCrFeNi and quinary CoCrFeMnNi HEAs at room temperature. Compared to CoCrFeNi HEA, the elongation of CoCrFeMnNi HEA was 14% lower, but the yield strength and ultimate tensile strength were increased by 17% and 6%, respectively. The direct real-time evolution of structural defects during uniaxial straining was acquired via in situ neutron-diffraction measurements. The dominant microstructures underlying plastic deformation mechanisms at each deformation stage in as-cast CoCrFeNi and CoCrFeMnNi HEAs were revealed using the Convolutional Multiple Whole Profile (CMWP) software for peak-profile fitting. The possible mechanisms are reported.
Abstract Plastic anisotropy and deformation-induced phase transformation of additively manufactur... more Abstract Plastic anisotropy and deformation-induced phase transformation of additively manufactured (AM) stainless steels were investigated via in-situ neutron diffraction, electron backscatter diffraction, metallography, and fractography. Two types of tensile specimens were manufactured: (1) One sample was vertically fabricated with its tensile axis parallel to the z-direction (AM-V), (2) The other sample was horizontally fabricated with its tensile axis perpendicular to the z-direction (AM-H). A commercial 15-5PH stainless steel (CA) was used for comparison. AM steel revealed enhanced yield strength, tensile strength, and uniform elongation over CA, which was mainly due to grain refinement and transformation induced plasticity (TRIP). Different onsets of strain nonlinearity between AM-V and AM-H were closely related to martensitic phase transformation. Stresses estimated from lattice strains measured by neutron diffraction matched well with the applied stress-strain curves. After plastic deformation, voids were formed and congregated near the solidified line where fine grains were populated. Higher dislocation density was observed in the fine grain zone, and lower density was shown in the relatively coarse grain zone. AM steels exhibited significant anisotropic fracture behavior in terms of loading direction. In contrast to isotropic failure for CA and AM-V, AM-H revealed anisotropic failure with elliptical formation of the fracture feature. The fracture surface of AM-H possessed many secondary cracks propagating perpendicular to the building direction. The occurrence of secondary cracks in AM-H resulted in rapid load drop during tensile loading after necking.
Abstract Insights into the phase transformation kinetics and lattice dynamics associated with the... more Abstract Insights into the phase transformation kinetics and lattice dynamics associated with the newly discovered confined martensitic transformation are of great significance to the in-depth understanding of the phase transformation behavior responsible for the rich new physical phenomena in shape memory alloys and could shed light on the design of novel multifunctional properties through tuning the confined martensitic transformation. Here, we employ in-situ elastic and inelastic neutron scattering techniques to reveal the characteristics of transformation kinetics and lattice dynamics in two Ni 55-x Co x Fe 18 Ga 27 magnetic shape memory alloys with x = 5.5 and 6.5. With a remarkable difference in transformation temperature, these two alloys show similar vibrational properties. Phonon softening of the TA 2 [ξξ0] phonon branch, manifested as a dip at ξ ∼ 0.33 on the phonon dispersion curves, was disclosed by inelastic neutron scattering experiments; but this phonon softening is much less pronounced than that observed in stoichiometric Ni 2 MnGa or near-stoichiometric Ni–Fe–Ga Heusler alloys. The dispersion curves for the TA 2 [ξξ0] phonon branch are only weakly dependent on temperature. The less pronounced phonon softening and very weak temperature dependence of the phonon dispersion curves could be attributed to the martensitic transformation which is confined by local inhomogeneities. This confined martensitic transformation shows a sluggish nature and occurs in a broad temperature range. The sluggish transformation was evidenced by the existence of residual austenite at low temperatures far below the martensitic transformation start temperature.
Langmuir : the ACS journal of surfaces and colloids, Jan 30, 2014
The structures of C- and N-terminally monoPEGylated human parathyroid hormone fragment hPTH(1-34)... more The structures of C- and N-terminally monoPEGylated human parathyroid hormone fragment hPTH(1-34) as well as their unmodified counterparts, poly(ethylene glycol) (PEG) and hPTH(1-34), have been studied by small-angle neutron scattering (SANS). The scattering results show that free hPTH(1-34) in 100 mM phosphate buffer (pH 7.4) aggregates into clusters. After conjugation with PEG, the PEG-peptide conjugates self-assemble into a supramolecular core-shell structure with a cylindrical shape. The PEG chains form a shell around the hPTH(1-34) core to shield hPTH(1-34) from the solvent. The detailed structural information on the self-assembled structures is extracted from SANS using a model of the cylindrical core with a shell of Gaussian chains attached to the core surface. On the basis of the data, because of the charge-dipole interactions between the conjugated PEG chain and the peptide, the conjugated PEG chain forms a more collapsed conformation compared to free PEG. Moreover, the siz...
Shielded metal arc (SMA) and gas tungsten arc (GTA) weldments were investigated to study the weld... more Shielded metal arc (SMA) and gas tungsten arc (GTA) weldments were investigated to study the welding effects on the mechanical behavior of 308L austenitic stainless steel weldments, respectively. Both SMA and GTA weldments showed dendritic microstructure. The observed austenitic stainless steel welds solidified to give primary ferrite and secondary austenite as the ferritic-austenitic solidification mode (FA-mode) solidification. However, the lower heat input with larger Cr-versus-Ni ratio in SMA weld process led to lathy ferrite morphology and more residual ferrite in the SMA welds, while vermicular ferrite morphology was shown in GTA weldments. The yield strength of the welds significantly increased with decreasing elongation, which was mainly due to the dual phase strengthening effect after rapid solidification during welding
An experimental investigation, combining synchrotron X-ray powder diffraction, small-angle neutro... more An experimental investigation, combining synchrotron X-ray powder diffraction, small-angle neutron scattering and transmission electron microscopy, has been undertaken to study the microstructure of nanoprecipitates in a nickel-based superalloy. Upon increasing the ageing time during a heat-treatment process, the average size of the precipitates first decreases before changing to a monotonically growth stage. Possible reasons for this observed structural evolution, which is predicted thermodynamically, are suggested.
ABSTRACT This study simulates the Wigner Effect of nuclear-grade graphite in a High Temperature G... more ABSTRACT This study simulates the Wigner Effect of nuclear-grade graphite in a High Temperature Gas-cooled Reactor (HTGR). The graphite was artificially irradiated with 3 MeV C2+ ions to mimic the fast neutron-radiation damage of the HTGR core environment. The irradiation temperatures were controlled between the range of 500–800 °C in a high vacuum environment of 10−7 torr. This high-dosage radiation creates enormous amounts of Frenkel pairs, which induce lattice swelling. These Frenkel vacancies and interstitials generate new strain fields and, hence, store energy in the distorted crystalline structure. The structural integrity of nuclear grade graphite was quantified using high-resolution transmission electron microscopy (HRTEM). The microstructure was estimated by the fast Fourier transform of HRTEM images. Within the samples irradiated with 10 dpa at 600 °C, the d-spacing of {0 0 0 2} expanded from 0.336 nm to 0.396 nm accompanying with the greatest distorted graphite microstructure. The c-axis of graphite swelled approximately 18% and the disorder coefficient was 1.10 ± 0.17 (1/nm). The synchrotron X-ray experimental results, gauged from 500 μm3 volume, suggesting that the ion-implanted graphite only deformed locally and epitaxially. This study also presents possible mechanisms.
ABSTRACT Morphological evolution and phase transformation of metastable intermediate precipitates... more ABSTRACT Morphological evolution and phase transformation of metastable intermediate precipitates are critical to their mechanical properties for the non-isothermal processing. During the non-isothermal precipitation, the formation of the new phases usually couples with structural evolution. Traditional structural characterization has limitation to resolve comprehensive changes simultaneously. In this study, we report direct observation, precipitation sequence, and the details of concurrent morphological and structural changes of various intermediate precipitates during non-isothermal heating in the Al-Cu systems with different pretreatments. The structural heterogeneity during the non-isothermal precipitation processes is resolved into coexistence of two different precipitate phases and quantitatively studied in terms of the phase transition and the morphological evolution. This paper presents the in situ small- and wide-angle synchrotron x-ray scattering (SAXS and WAXS) to refine and to identify the mixed structural information during multiple precipitation stages. The WAXS results show that the precipitation sequence is theta '' -> (theta '' + theta ') -> theta ' -> (theta ' + theta) -> theta upon heating. Due to the fact of the specifically oriented SAXS intensity, the evolution of the aforementioned phase transformation is resolved by the refinement of the SAXS intensity integrated over the selected area. These methods reveal multiscale information that is not trivial comparing to the traditional characterization methods.
ABSTRACT The microstructural characterization of the Zr60.0Ti14.7Nb5.3Cu5.6Ni4.4Be10.0 bulk-metal... more ABSTRACT The microstructural characterization of the Zr60.0Ti14.7Nb5.3Cu5.6Ni4.4Be10.0 bulk-metallic-glass-matrix composites is investigated using high-energy synchrotron x-ray diffraction. The convoluted diffraction-intensity distribution in the azimuthal direction is naturally yielded from the spatial arrangements of the crystalline dendrites and their amorphous matrix. We facilitate the area selection and the intensity integration of the diffraction collected from a two-dimensional detector to characterize the diffraction intensity of the amorphous matrix. The results enable us to apply the modified Williamson–Hall plots for using the peak width to study the microstrain and micromechanism of the deformation of the crystalline phase.
The plastic behavior of an annealed HASTELLOY C-22HS alloy, a face-centered cubic (FCC), nickel-b... more The plastic behavior of an annealed HASTELLOY C-22HS alloy, a face-centered cubic (FCC), nickel-based superalloy, was examined by in-situ neutron-diffraction measurements at room temperature. Both monotonic-tension and low-cycle-fatigue experiments were conducted. Monotonic-tension straining and cyclic-loading deformation were studied as a function of stress. The plastic behavior during deformation is discussed in light of the relationship between the stress and dislocation-density evolution. The calculated dislocation-density evolution within the alloy reflects the strain hardening and cyclic hardening/softening. Experimentally determined lattice strains are compared to verify the hardening mechanism at selected stress levels for tension and cyclic loadings. Combined with calculations of the dislocation densities, the neutron-diffraction experiments provide direct information about the strain and cyclic hardening of the alloy.
In this research, we systematically investigated equiatomic CoCrFeNi and CoCrFeMnNi high-entropy ... more In this research, we systematically investigated equiatomic CoCrFeNi and CoCrFeMnNi high-entropy alloys (HEAs). Both of these HEA systems are single-phase, face-centered-cubic (FCC) structures. Specifically, we examined the tensile response in as-cast quaternary CoCrFeNi and quinary CoCrFeMnNi HEAs at room temperature. Compared to CoCrFeNi HEA, the elongation of CoCrFeMnNi HEA was 14% lower, but the yield strength and ultimate tensile strength were increased by 17% and 6%, respectively. The direct real-time evolution of structural defects during uniaxial straining was acquired via in situ neutron-diffraction measurements. The dominant microstructures underlying plastic deformation mechanisms at each deformation stage in as-cast CoCrFeNi and CoCrFeMnNi HEAs were revealed using the Convolutional Multiple Whole Profile (CMWP) software for peak-profile fitting. The possible mechanisms are reported.
Abstract Plastic anisotropy and deformation-induced phase transformation of additively manufactur... more Abstract Plastic anisotropy and deformation-induced phase transformation of additively manufactured (AM) stainless steels were investigated via in-situ neutron diffraction, electron backscatter diffraction, metallography, and fractography. Two types of tensile specimens were manufactured: (1) One sample was vertically fabricated with its tensile axis parallel to the z-direction (AM-V), (2) The other sample was horizontally fabricated with its tensile axis perpendicular to the z-direction (AM-H). A commercial 15-5PH stainless steel (CA) was used for comparison. AM steel revealed enhanced yield strength, tensile strength, and uniform elongation over CA, which was mainly due to grain refinement and transformation induced plasticity (TRIP). Different onsets of strain nonlinearity between AM-V and AM-H were closely related to martensitic phase transformation. Stresses estimated from lattice strains measured by neutron diffraction matched well with the applied stress-strain curves. After plastic deformation, voids were formed and congregated near the solidified line where fine grains were populated. Higher dislocation density was observed in the fine grain zone, and lower density was shown in the relatively coarse grain zone. AM steels exhibited significant anisotropic fracture behavior in terms of loading direction. In contrast to isotropic failure for CA and AM-V, AM-H revealed anisotropic failure with elliptical formation of the fracture feature. The fracture surface of AM-H possessed many secondary cracks propagating perpendicular to the building direction. The occurrence of secondary cracks in AM-H resulted in rapid load drop during tensile loading after necking.
Abstract Insights into the phase transformation kinetics and lattice dynamics associated with the... more Abstract Insights into the phase transformation kinetics and lattice dynamics associated with the newly discovered confined martensitic transformation are of great significance to the in-depth understanding of the phase transformation behavior responsible for the rich new physical phenomena in shape memory alloys and could shed light on the design of novel multifunctional properties through tuning the confined martensitic transformation. Here, we employ in-situ elastic and inelastic neutron scattering techniques to reveal the characteristics of transformation kinetics and lattice dynamics in two Ni 55-x Co x Fe 18 Ga 27 magnetic shape memory alloys with x = 5.5 and 6.5. With a remarkable difference in transformation temperature, these two alloys show similar vibrational properties. Phonon softening of the TA 2 [ξξ0] phonon branch, manifested as a dip at ξ ∼ 0.33 on the phonon dispersion curves, was disclosed by inelastic neutron scattering experiments; but this phonon softening is much less pronounced than that observed in stoichiometric Ni 2 MnGa or near-stoichiometric Ni–Fe–Ga Heusler alloys. The dispersion curves for the TA 2 [ξξ0] phonon branch are only weakly dependent on temperature. The less pronounced phonon softening and very weak temperature dependence of the phonon dispersion curves could be attributed to the martensitic transformation which is confined by local inhomogeneities. This confined martensitic transformation shows a sluggish nature and occurs in a broad temperature range. The sluggish transformation was evidenced by the existence of residual austenite at low temperatures far below the martensitic transformation start temperature.
Langmuir : the ACS journal of surfaces and colloids, Jan 30, 2014
The structures of C- and N-terminally monoPEGylated human parathyroid hormone fragment hPTH(1-34)... more The structures of C- and N-terminally monoPEGylated human parathyroid hormone fragment hPTH(1-34) as well as their unmodified counterparts, poly(ethylene glycol) (PEG) and hPTH(1-34), have been studied by small-angle neutron scattering (SANS). The scattering results show that free hPTH(1-34) in 100 mM phosphate buffer (pH 7.4) aggregates into clusters. After conjugation with PEG, the PEG-peptide conjugates self-assemble into a supramolecular core-shell structure with a cylindrical shape. The PEG chains form a shell around the hPTH(1-34) core to shield hPTH(1-34) from the solvent. The detailed structural information on the self-assembled structures is extracted from SANS using a model of the cylindrical core with a shell of Gaussian chains attached to the core surface. On the basis of the data, because of the charge-dipole interactions between the conjugated PEG chain and the peptide, the conjugated PEG chain forms a more collapsed conformation compared to free PEG. Moreover, the siz...
Shielded metal arc (SMA) and gas tungsten arc (GTA) weldments were investigated to study the weld... more Shielded metal arc (SMA) and gas tungsten arc (GTA) weldments were investigated to study the welding effects on the mechanical behavior of 308L austenitic stainless steel weldments, respectively. Both SMA and GTA weldments showed dendritic microstructure. The observed austenitic stainless steel welds solidified to give primary ferrite and secondary austenite as the ferritic-austenitic solidification mode (FA-mode) solidification. However, the lower heat input with larger Cr-versus-Ni ratio in SMA weld process led to lathy ferrite morphology and more residual ferrite in the SMA welds, while vermicular ferrite morphology was shown in GTA weldments. The yield strength of the welds significantly increased with decreasing elongation, which was mainly due to the dual phase strengthening effect after rapid solidification during welding
An experimental investigation, combining synchrotron X-ray powder diffraction, small-angle neutro... more An experimental investigation, combining synchrotron X-ray powder diffraction, small-angle neutron scattering and transmission electron microscopy, has been undertaken to study the microstructure of nanoprecipitates in a nickel-based superalloy. Upon increasing the ageing time during a heat-treatment process, the average size of the precipitates first decreases before changing to a monotonically growth stage. Possible reasons for this observed structural evolution, which is predicted thermodynamically, are suggested.
ABSTRACT This study simulates the Wigner Effect of nuclear-grade graphite in a High Temperature G... more ABSTRACT This study simulates the Wigner Effect of nuclear-grade graphite in a High Temperature Gas-cooled Reactor (HTGR). The graphite was artificially irradiated with 3 MeV C2+ ions to mimic the fast neutron-radiation damage of the HTGR core environment. The irradiation temperatures were controlled between the range of 500–800 °C in a high vacuum environment of 10−7 torr. This high-dosage radiation creates enormous amounts of Frenkel pairs, which induce lattice swelling. These Frenkel vacancies and interstitials generate new strain fields and, hence, store energy in the distorted crystalline structure. The structural integrity of nuclear grade graphite was quantified using high-resolution transmission electron microscopy (HRTEM). The microstructure was estimated by the fast Fourier transform of HRTEM images. Within the samples irradiated with 10 dpa at 600 °C, the d-spacing of {0 0 0 2} expanded from 0.336 nm to 0.396 nm accompanying with the greatest distorted graphite microstructure. The c-axis of graphite swelled approximately 18% and the disorder coefficient was 1.10 ± 0.17 (1/nm). The synchrotron X-ray experimental results, gauged from 500 μm3 volume, suggesting that the ion-implanted graphite only deformed locally and epitaxially. This study also presents possible mechanisms.
ABSTRACT Morphological evolution and phase transformation of metastable intermediate precipitates... more ABSTRACT Morphological evolution and phase transformation of metastable intermediate precipitates are critical to their mechanical properties for the non-isothermal processing. During the non-isothermal precipitation, the formation of the new phases usually couples with structural evolution. Traditional structural characterization has limitation to resolve comprehensive changes simultaneously. In this study, we report direct observation, precipitation sequence, and the details of concurrent morphological and structural changes of various intermediate precipitates during non-isothermal heating in the Al-Cu systems with different pretreatments. The structural heterogeneity during the non-isothermal precipitation processes is resolved into coexistence of two different precipitate phases and quantitatively studied in terms of the phase transition and the morphological evolution. This paper presents the in situ small- and wide-angle synchrotron x-ray scattering (SAXS and WAXS) to refine and to identify the mixed structural information during multiple precipitation stages. The WAXS results show that the precipitation sequence is theta '' -> (theta '' + theta ') -> theta ' -> (theta ' + theta) -> theta upon heating. Due to the fact of the specifically oriented SAXS intensity, the evolution of the aforementioned phase transformation is resolved by the refinement of the SAXS intensity integrated over the selected area. These methods reveal multiscale information that is not trivial comparing to the traditional characterization methods.
ABSTRACT The microstructural characterization of the Zr60.0Ti14.7Nb5.3Cu5.6Ni4.4Be10.0 bulk-metal... more ABSTRACT The microstructural characterization of the Zr60.0Ti14.7Nb5.3Cu5.6Ni4.4Be10.0 bulk-metallic-glass-matrix composites is investigated using high-energy synchrotron x-ray diffraction. The convoluted diffraction-intensity distribution in the azimuthal direction is naturally yielded from the spatial arrangements of the crystalline dendrites and their amorphous matrix. We facilitate the area selection and the intensity integration of the diffraction collected from a two-dimensional detector to characterize the diffraction intensity of the amorphous matrix. The results enable us to apply the modified Williamson–Hall plots for using the peak width to study the microstrain and micromechanism of the deformation of the crystalline phase.
The plastic behavior of an annealed HASTELLOY C-22HS alloy, a face-centered cubic (FCC), nickel-b... more The plastic behavior of an annealed HASTELLOY C-22HS alloy, a face-centered cubic (FCC), nickel-based superalloy, was examined by in-situ neutron-diffraction measurements at room temperature. Both monotonic-tension and low-cycle-fatigue experiments were conducted. Monotonic-tension straining and cyclic-loading deformation were studied as a function of stress. The plastic behavior during deformation is discussed in light of the relationship between the stress and dislocation-density evolution. The calculated dislocation-density evolution within the alloy reflects the strain hardening and cyclic hardening/softening. Experimentally determined lattice strains are compared to verify the hardening mechanism at selected stress levels for tension and cyclic loadings. Combined with calculations of the dislocation densities, the neutron-diffraction experiments provide direct information about the strain and cyclic hardening of the alloy.
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