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Nanoindentation was performed on individual grains of a polycrystalline Mg sample with c-axis declination angles ranging from parallel (0°) to perpendicular (90°) to the c-axis. Hardness was highest at ∼0°, decreased up to ∼55°, and then increased at ∼90° to an intermediate level. At ∼0°, high-density 〈c + a〉 dislocations extended deep into the crystal, contributing to high hardness. At ∼55°, 〈c + a〉 dislocations were confined near the indent, and occasional extension twinning reoriented the crystal to ∼45°, promoting 〈a〉 slip in both matrix and twin, leading to low hardness. At ∼90°, extension twinning reoriented the crystal to ∼0°, inducing texture hardening and intermediate hardness. Despite the complex stress state in nanoindentation, which fundamentally differs from the uniaxial stress in bulk tensile and compression tests, the combined contributions of dislocation and twinning still give rise to measurable hardness anisotropy, suggesting nanoindentation as a high-throughput technique for probing orientation-dependent mechanical behavior in Mg. 

NiTiHf is a class of promising high-temperature shape memory alloys (SMAs) that find many applications. However, their complex martensitic microstructure and attendant thermomechanical properties are not well understood. In this work, we used solution-treated (precipitate-free) and aged (precipitate-bearing) Ni50.3Ti29.7Hf20 (at.%) SMAs as a model system. We observed that the presence of precipitates refines the martensite plates, reduces the number of martensite variants, and changes the orientation relationship between the martensite plates compared with the solution-treated counterpart. Furthermore, the aged samples exhibited higher transformation temperatures, narrower phase transformation temperature windows, improved thermal stability, and retained or even improved actuation strain. The improved thermomechanical properties observed in the aged samples are attributed in part to the reduction of the number of martensite variants and the change in martensite and twin interface characteristics, both of which are induced by the presence of precipitates. The findings of this study offer new information on the processing-property-microstructure relationship in NiTiHf-based SMAs. These insights can guide future materials design efforts, facilitating the development of advanced SMAs tailored for specific high-temperature applications. 

In this work, we performed in situ nanoindentation in TEM to capture the real-time 〈c + a〉 dislocation and twinning activities in pure Mg during loading and unloading. We demonstrated that the screw component of 〈c + a〉 dislocations glides continuously, while the edge components rapidly become sessile during loading. The twin tip propagation is intermittent, whereas the twin boundary migration is more continuous. During unloading, we observed the elastic strain relaxation causes both 〈c + a〉 dislocation retraction and detwinning. Moreover, we note that the plastic zone comprised of 〈c + a〉 dislocations in Mg is well-defined, which contrasts with the diffused plastic zones observed in face-centered cubic metals under the nanoindentation impressions. Additionally, molecular dynamics simulations were performed to study the formation and evolution of deformation-induced crystallographic defects at the early stages of indentation. We observed that, in addition to 〈a〉 dislocations, the I1 stacking fault bounded with a 〈1/2c+p〉 Frank loop can be generated from the plastic zone ahead of the indenter, and potentially serve as a nucleation source for abundant 〈c + a〉 dislocations observed experimentally. These new findings are anticipated to provide new knowledge on the deformation mechanisms of Mg, which are difficult to obtain through conventional ex situ approaches. These observations may serve as a baseline for simulation work that investigate the dynamics of 〈c + a〉 dislocation slip and twinning in Mg and alloys. 

Precession electron diffraction (PED) is a powerful technique for revealing the crystallographic orientation of samples at the nanoscale. However, the quality of orientation indexing is strongly influenced by the quality of diffraction patterns. In this study, we have developed a novel algorithm called Auto-CLAHE (automatic contrast-limited adaptive histogram equalization), which automatically enhances low-intensity diffraction pattern signals using contrast-limited adaptive histogram equalization (CLAHE). The degree of enhancement is dynamically adjusted based on the overall intensity of the diffraction pattern, with greater enhancement applied to patterns with fewer spots (i.e., away from zone axes) and little or no enhancement applied to patterns with many spots (i.e., at a zone axis). By improving the visibility of low-intensity diffraction spots, Auto-CLAHE significantly improves the template matching between experimentally acquired and simulated diffraction patterns, leading to orientation maps with dramatically higher quality and lower noise. We anticipate that Auto-CLAHE provides an efficient and practical solution for preprocessing PED data, enabling higher-quality crystal orientation mapping to be routinely obtained. 

During martensitic phase transformations in thin films, substrates impact hysteresis by introducing an additional interface, which can inhibit martensite/austenite interface motion. In order to reduce hysteresis, we examine 2.9–14.5 μm thick Ni–Mn–Sn films, which in some cases have been delaminated from the substrates before or after annealing. We compare thermal hysteresis and defect densities at the interface. Delaminating films prior to annealing decreases hysteresis, whereas delaminating films after annealing does not significantly impact hysteresis. Substrate effects are attributed to the thermal expansion mismatch between the film and substrate, resulting in the formation of dislocations at the interface and, consequentially, an increase in frictional resistance to martensite/austenite interface motion.