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Phase transition and deformation in cold-rolled and heat-treated Ni 50. 6 Ti 49. 4 alloys are explored by in situ multi-scale measurements with simultaneous X-ray diffraction and optical digital image correlation. Macroscale stress − strain curves, mesoscale strain fields and microscale X-ray diffraction peaks are obtained. Electron backscatter diffraction and transmission electron microscopy are also conducted on samples before and after tension as complements. For cold-rolled samples, since martensite nucleation is strongly associated with the widely distributed preexisting dislocations, homogeneous martensite phase transition takes place, in contrast with the localized mode in heat-treated samples. For localized phase transition, the intense growth of martensite results in clear Lüders bands. The beginning and the end of phase transition plateau on the stress − strain curve are associated with the initiation and complete propagation of Lüders bands. For the homogeneous phase transition, broad growth of martensite leads obscure Lüders bands and uniform strain fields, without any phase transition plateau. Given the initial {110} 〈 110 〉 and {111} 〈 110 〉 sheet textures in the heat-treated samples, different martensite variants with different preferred orientations are activated with respect to the loading directions. [ABSTRACT FROM AUTHOR]

Phase transition and deformation in cold-rolled and heat-treated Ni 50. 6 Ti 49. 4 alloys are explored by in situ multi-scale measurements with simultaneous X-ray diffraction and optical digital image correlation. Macroscale stress − strain curves, mesoscale strain fields and microscale X-ray diffraction peaks are obtained. Electron backscatter diffraction and transmission electron microscopy are also conducted on samples before and after tension as complements. For cold-rolled samples, since martensite nucleation is strongly associated with the widely distributed preexisting dislocations, homogeneous martensite phase transition takes place, in contrast with the localized mode in heat-treated samples. For localized phase transition, the intense growth of martensite results in clear Lüders bands. The beginning and the end of phase transition plateau on the stress − strain curve are associated with the initiation and complete propagation of Lüders bands. For the homogeneous phase transition, broad growth of martensite leads obscure Lüders bands and uniform strain fields, without any phase transition plateau. Given the initial {110} 〈 110 〉 and {111} 〈 110 〉 sheet textures in the heat-treated samples, different martensite variants with different preferred orientations are activated with respect to the loading directions. [ABSTRACT FROM AUTHOR]

In situ scanning electron microscope (SEM) and electron back scattering diffraction (EBSD) are used to investigate detwinning deformation of pre-twinned titanium under uniaxial tension. An electro-polishing procedure is optimized to prepare SEM speckles on the Ti samples with minimum interference with EBSD characterization. Strain fields are obtained from the SEM images via digital image correlation. The experimental results suggest that the rate of detwinning decreases with increasing plastic strain, and detwinning deformation is governed by the Schmid's law. Strain localization mainly occurs in the detwinning regions during the early deformation. As deformation proceeds, localized plastic strain tends to accumulate rapidly around the { 10 1 ‾ 2 } extension twins with a low Schmid factor. However, the strain values inside the { 10 1 ‾ 2 } extension twins are relatively low, regardless of their Schmid factors and the applied tensile strain level. [ABSTRACT FROM AUTHOR]

Dynamic (up to 5500 s−1) and quasi-static compression tests are conducted on a 15 wt% B 4 C particle-reinforced Al (B 4 C/Al) composite. In situ , high-speed synchrotron X-ray phase contrast imaging and digital image correlation are employed to map mesoscale deformation fields at μm and μs scales. The bulk stress–strain curves show significant strain and strain-rate hardening under dynamic compression. The strain-rate sensitivity exponent is an order of magnitude higher at high strain rates (> 10 3 s−1) than that at low strain rates (< 10 − 2 s−1). Strain field mapping demonstrates distinct compressive strain localizations for both quasi-static and dynamic loading. Nevertheless, compressive strain localizations appear denser in spacing under dynamic loading, owing to spontaneous dislocation nucleation in both weak and strong zones. This results in a higher density of geometrically necessary dislocations, which contributes to the higher strain and strain-rate hardening of B 4 C/Al under dynamic loading. The ratio of the maximum local strain to the bulk average is ~ 1.5, and the local strain-rate enhancement cannot explain the increased rate sensitivity of B 4 C/Al under dynamic loading. Therefore, the rate-dependent deformation heterogeneity dominates the strain-rate hardening of B 4 C/Al. Postmortem analyses help correlate deformation features to particle- and grain-scale microstructures, yielding consistent results with mesoscale strain fields. [ABSTRACT FROM AUTHOR]

Dynamic and quasi-static tension experiments are conducted on Ti-6Al-4V alloys, with in situ , synchrotron-based, high-speed, x-ray phase contrast imaging implemented to characterize the dynamic deformation and fracture process of Ti alloys at the Advanced Photon Source. X-ray digital imaging correlation (XDIC) is applied for strain field mapping. The size distribution of x-ray speckles are quantified via a morphological analysis, with a mean of ∼20 μ m. Systematic error analyses of displacement and strain field measurements are firstly conducted for XDIC, and demonstrate that the displacement and strain errors can be controlled below 0.01 pixel and 0.1%, respectively. Mesoscale strain characteristics measured via XDIC are consistent with and reveal mechanisms for the bulk-scale stress–strain responses. Under dynamic tension, a sharp transition to strain softening occurs when the bulk strain exceeds about 0.04, which leads to a lower dynamic fracture strain (0.08) than the quasi-static one (0.1). The corresponding strain filed mapping demonstrates that shear deformation localizations grow and coalesce rapidly into a narrow shear deformation band under dynamic loading, while tensile deformation and necking progresses gradually under quasi-static loading. Scanning electron microscopy shows that void nucleation occurs mainly at the interface of α and β phases for both quasi-static and dynamic tension. However, microvoids coalesce preferentially along colony boundaries or the boundary α phase under quasi-static tension, but along the maximum shear stress direction (across colonies) under dynamic tension. Fractography of recovered samples also shows consistent features. [ABSTRACT FROM AUTHOR]

Dynamic split Hopkinson pressure bar experiments with in situ synchrotron x-ray imaging and diffraction are conducted on a rolled magnesium alloy at high strain rates of ~5500 s −1 . High speed multiscale measurements including stress–strain curves (macroscale), strain fields (mesoscale), and diffraction patterns (microscale) are obtained simultaneously, revealing strong anisotropy in deformation across different length scales. {10 1 ¯ 2} extension twinning induces homogenized strain fields and gives rise to rapid increase in strain hardening rate, while dislocation motion leads to inhomogeneous deformation and a decrease in strain hardening rate. During the early stage of plastic deformation, twinning is dominant in dynamic compression, while dislocation motion prevails in quasi-static loading, manifesting a strain-rate dependence of deformation. [ABSTRACT FROM AUTHOR]

In situ synchrotron x-ray imaging and diffraction are used to investigate deformation of a rolled magnesium alloy under uniaxial compression at room and elevated temperatures along two different directions. The loading axis (LA) is either perpendicular or parallel to the normal direction, and these two cases are referred to as LA ⊥ 〈 c 〉 and LA ∥ 〈 c 〉 loading, respectively. Multiscale measurements including stress–strain curves (macroscale), strain fields (mesoscale), and diffraction patterns (microscale) are obtained simultaneously. Due to initial texture, { 10 1 ¯ 2 } extension twinning is predominant in the LA ⊥ 〈 c 〉 loading, while dislocation motion prevails in the LA ∥ 〈 c 〉 loading. With increasing temperature, fewer { 10 1 ¯ 2 } extension twins are activated in the LA ⊥ 〈 c 〉 samples, giving rise to reduced strain homogenization, while pyramidal 〈 c + a 〉 slip becomes readily activated, leading to more homogeneous deformation for the LA ∥ 〈 c 〉 loading. The difference in the strain hardening rates is attributed to that in strain field homogenization for these two loading directions. [ABSTRACT FROM AUTHOR]

Deformation and phase transformation of a cold-rolled, textured, shape memory alloy sheet Ni 50.6 Ti 49.4 are investigated via real-time, in situ synchrotron X-ray diffraction and digital imaging correlation. The specimens are subjected to tensile loading along two orthogonal directions. Bulk stress–strain curves (macroscale), strain fields (mesoscale) and X-ray diffraction patterns (microscale) are obtained simultaneously. Martensite nucleation is highly correlated with the preexisting defects. Different from the widely observed Lüders bands within defect-free NiTi, uniform martensite phase transformation occurs in the cold-rolled alloy as a result of the relatively widely distributed preexisting defects, giving rise to progressive growth of martensite, relatively uniform strain fields, the sigmoidal stress–strain curves and increased strain hardening rate. Different favorably oriented martensite variants are induced due to the initial texture, and the martensite phase transformation is more active for loading along the less favorable orientation, giving rise to the anisotropy in the phase transformation. [Display omitted] • The state-of-the-art, real time, in situ multiscale measurements are adopted. • Martensite nucleation is highly correlated with the preexisting defects. • The nature of the uniform and anisotropic martensite phase transition is studied. • The initial texture affects the inducement of martensite variants. [ABSTRACT FROM AUTHOR]

Dynamic tensile experiments are conducted on 15% and 30% in weight percentage B 4 C/Al composites with a split Hopkinson tension bar, along with high-speed synchrotron x-ray digital image correlation (XDIC) to map strain fields at μm and μs scales. As manifested by bulk-scale stress–strain curves, a higher particle content leads to a higher yield strength but lower ductility. Strain field mapping by XDIC demonstrates that tension deformation and tensile fracture, as opposed to shear and shear failure, dominate deformation and failure of the composites. The fractographs of recovered samples show consistent features. The particle-matrix interfaces are nucleation sites for strain localizations, and their propagation and coalescence are diffused by the Al matrix. The reduced spacing between strain localization sites with increasing particle content, facilitates their coalescence and leads to decreased ductility. Designing a particle-reinforced, metallic-matrix composite with balanced strength and ductility should consider optimizing the inter-particle distance as a key parameter. [ABSTRACT FROM AUTHOR]

Dynamic compression experiments are conducted on micron-sized SiC powders of different initial densities with a split Hopkinson pressure bar. Digital image correlation is applied to images from high-speed X-ray phase contrast imaging to map dynamic strain fields. The X-ray imaging and strain field mapping demonstrate the degree of heterogeneity in deformation depends on the initial powder density; mesoscale strain field evolution is consistent with softening or hardening manifested by bulk-scale loading curves. Statistical analysis of the strain probability distributions exhibits exponential decay tail similar to those of contact forces, which are supposed to lead to the grain-scale heterogeneity of granular materials. [ABSTRACT FROM AUTHOR]

Strain localization in commercially pure titanium under uniaxial tension is investigated with in situ digital image correlation (DIC) at different length scales. Speckles in three different sizes are prepared on the surface of titanium specimens, referred to as microscale, mesoscale and macroscale speckles. The microscale and mesoscale speckles for scanning electron microscopy DIC analysis are fabricated by different electro-polishing procedures, and spray painting is used to prepare the macroscale speckles for the optical DIC measurement. Electron back scattering diffraction (EBSD) characterization is conducted for the microscale speckle case. The strain fields obtained at different length scales combined with the microscale EBSD examination establish the relationship between microscale strain localization and macroscale strain concentration bands. Strain localization tends to form at grain boundaries and inside deformation twins, especially at the large-angle grain boundaries, and the amplitude and rate of strain accumulation at grain boundaries are higher than inside deformation twins. The microscale strain-localized regions dispersed at grain boundaries and within deformation twins present themselves as the strain concentration bands observed with the macroscale speckles, and both the strain localization regions and strain concentration bands are at ±45∘ with the tensile direction. • Speckles in three different sizes are fabricated by electro-polishing and spray painting. • Digital image correlation measurements over a wide length scales are achieved. • Microscale scattered strain-localized regions present themselves as strain concentration bands on macroscale. • Strain localization tends to form at the large-angle grain boundaries. • Amplitude and rate of strain accumulation at grain boundaries are higher than inside twins. [ABSTRACT FROM AUTHOR]

We investigate deformation dynamics in a textured magnesium alloy under uniaxial tension and compression along two different directions. The loading axis (LA) is either parallel or perpendicular to the normal direction (ND), referred to as LA ∥ ND tension/compression and LA ⊥ ND tension/compression, respectively. Stress − strain curves are measured simultaneously with X-ray diffraction and strain field mapping via synchrotron X-ray digital image correlation. Massive { 10 1 ̄ 2 } extension twins are observed in the LA ∥ ND tension and LA ⊥ ND compression specimens. The growth rate of tension-induced twins is lower than that of compression-induced twins, likely because the pyramidal 〈 c + a 〉 slips accommodate partial deformation along the c -axis in the LA ∥ ND tension specimen. Both the tension- and compression-induced deformation twins can slow down the process of strain localization, and thus an increase in strain hardening rate as well as the anisotropy in plastic deformation. The plastic strain ratios depend on strain localization and the relative orientation of the loading direction with respect to the basal texture, and are much larger in tension than in compression. [ABSTRACT FROM AUTHOR]

Interactions between slip bands (SBs) and interfaces in a compressed duplex stainless steel (DSS) are investigated regarding the roles of grain boundaries (GBs) and phase boundaries (PBs) in plastic deformation. Digital image correlation based on scanning electron microscopy, and electron backscatter diffraction are utilized to characterize SBs in ferrite and austenite phases, and the interactions between interfaces and SBs: slip transmission, slip blockage and non-intersection. Most ferrite SBs have lower local strains than austenite SBs, and exhibit weak interactions with interfaces. For austenite GBs and PBs, the strain gradient across interfaces with slip transmission is much lower than that across interfaces with slip blockage. Moreover, GBs/PBs following the twin/Kurdjumov–Sachs relationship exhibit a higher probablity of slip transmission, but a considerably lower probability of slip blockage, compared to general GBs/PBs. This is attributed to the different geometries of slip systems for different types of interfaces. Therefore, twin GBs and PBs following the Kurdjumov–Sachs relationship play a lesser role in restraining plastic flow and promoting the strength of DSS than general GBs and PBs. In addition, the effect of loading orientation is minor for the SB-GB interactions, but pronounced for SB-PB interactions. [ABSTRACT FROM AUTHOR]

• First direct observation of high-speed penetration dynamics in carbonate sands. • XDIC and XPIV are employed to obtain particle-scale deformation. • Wet sands and small container reduce penetration depths. • Mechanisms for moisture and container size effect are analyzed. [Display omitted] Penetration into granular materials such as carbonate sand is of fundamental importance to granular physics, and impact and civil engineering, but research along this line suffers considerably from the lack of direct experimental observations. Here we investigate such penetration dynamics of carbonate sand using a vertical gas gun implemented with in situ , high-speed, synchrotron-based X-ray phase contrast imaging. The experiments yield the first direct observation on the penetration dynamics (projectile trajectory/velocity) and particle-scale deformation in real or natural granular materials during sphere penetration, at high temporal and spatial resolutions. The particle displacement fields around the projectile are obtained via X-ray digital image correlation, and the ejecta velocity fields, with particle image velocimetry. Effects of the moisture content and container size on the penetration process are discussed. The enhanced cohesion between wet particles results in particle clustering during splash and a smaller amount of ejecta. Therefore, the particle rearrangement is limited and the projectile penetration depth is smaller. A small container size confines the lateral movement of particles and yields a much more pronounced compaction of particles beneath the projectile, which results in a smaller penetration depth as well. [ABSTRACT FROM AUTHOR]