Zhigang Wu

Fully compensated ferrimagnets do not create any magnetic stray field and allow for a completely polarized current of charges. As a result, these alloys show promising prospects for applications as spintronic devices. In this paper, we investigated the phase stability, the site preference, the tetragonal distortion and the influence of symmetry from the crystal structure and chemical environments of magnetic ions on the magnetic properties of Cr₂YZ and Mn₂YZ (Y = void, Ni, Cu, and Zn; Z = Ga, Ge, and As) full Heusler alloys by first-principles calculations. We found that the selected Cr₂-based alloys, except for Cr₂NiGa and Cr₂NiGe, prefer to crystallize in the centrosymmetric L2₁-type structure, while the selected Mn₂-based alloys, except for Mn₂CuAs, Mn₂ZnGe and Mn₂ZnAs, tend to crystallize in the non-centrosymmetric XA-type structure. Due to the symmetry, the antiferromagnetism of the selected L2₁-type alloys is very stable, and no spin-polarized density of states could be generated. In contrast, the magnetic moment of the selected XA-type alloys depends heavily on the number of valence electrons and tetragonal distortion, and spin-polarized density of states is generated. Therefore, the selected alloys with L2₁-type structures and their tetragonal-distorted structure are potential candidates for conventional antiferromagnets, while those with XA-type structure and their tetragonal-distorted structure are promising candidates for (fully) compensated ferrimagnets.

NiTi shape memory alloys with quasilinear and slim pseudoelasticity are desirable for high fatigue life and easy controllability. This paper reports that the selective laser melting (SLM) technique can assist with formation of quasilinear pseudoplasticity with small hysteresis of NiTi. The NiTi parts are found to present a high density of dislocations in the as-fabricated state. A few deformation cycles of the defect-containing SLM NiTi parts promote the formation of a unique microstructure consisting of nanocrystalline and amorphous-like phases. Such microstructure leads to a continuous and quasilinear type pseudoelasticity with small hysteresis. It is found that the scanning speed of laser affects the dislocation density and transformation behaviour, thus influencing the pseudoelastic behaviour of the SLM-fabricated NiTi. The transformation temperatures are shown to decrease while the transformation intervals to increase with increasing the scanning speed due to the increased dislocation density. The mechanically cycled SLM fabricated NiTi with a scanning speed of 850 mm/s exhibits a stable quasilinear pseudoelasticity of 5.8% and a small energy dissipation of 1.4 MJ/m 3 .

This paper investigates the deformation mechanism and energy absorption behaviour of 316 L triply periodic minimal surface (TPMS) structures with uniform and graded wall thicknesses fabricated by the selective laser melting technique. The uniform P-surface TPMS structure presents a single-level stress plateau for energy absorption and a localized diagonal shear cell failure. A graded strategy was employed to break such localized geometrical deformation to improve the overall energy absorption and to provide a double-level function. Two segments with different wall thicknesses separated by a barrier layer were designed along the compression direction while keeping the same relative density as the uniform structure. The results show that the crushing of the cells of the graded P-surface TPMS structure occurs first within the thin segment and then propagates to the thick segment. The stress–strain response shows apparent double stress plateaus. The stress level and length of each plateau can be adjusted by changing the wall thickness and position of the barrier layer between the two segments. The total energy absorption of the gradient TPMS structure was also found slightly higher than that of the uniform TPMS counterparts. The gradient design of TPMS structures may find applications where the energy absorption requires a double-level feature or a warning function.

Poor mechanical properties severely impede the practical application of ferromagnetic shape memory alloys. In this Letter, we report cold deformation-induced B2-γ phase transformation in polycrystalline FeMnGa alloys. Due to this property, the alloy achieved a high strength of ∼1000 MPa and a ductility of ∼75% in a compressive test, and a 90% deformation in thickness in a cold rolling experiment, indicating excellent cold-workability. The recrystallization annealing transformed the cold rolled γ phase to a textured B2 phase, which exhibited magnetic-field-induced phase transformation and anisotropic transformation strains. These properties indicate that FeMnGa is a promising candidate for future applications in actuator devices.

Isothermal annealing of a eutectic dual phase Ni–Mn–Sn–Fe alloy was carried out to encourage grain growth and investigate the effects of grain size of the γ phase on the martensitic transformation behaviour and mechanical properties of the alloy. It is found that with the increase of the annealing time, the grain size and volume fraction of the γ phase both increased with the annealing time predominantly by the inter-diffusion of Fe and Sn elements between the γ phase and the Heusler matrix. The isothermal anneals resulted in the decrease of the e/a ratio and suppression of the martensitic transformation of the matrix phase. The fine γ phase microstructure with an average grain size of 0.31 μm showed higher fracture strength and ductility values by 28% and 77% compared to the coarse-grained counterpart with an average grain size of 3.31 μm. The fine dual phase microstructure shows a quasi-linear superelasticity of 4.2% and very small stress hysteresis during cyclic loading, while the coarse dual phase counterpart presents degraded superelasticity of 2.6% and large stress hysteresis. These findings indicate that grain size refinement of the γ phase is an effective approach in improving the mechanical and transformation properties of dual phase Heusler alloys.

This article investigates the energy absorption performance and deformation mechanism of 316L stainless steel (SS316L) triply periodic minimal surface (TPMS) cellular structures fabricated by a selective laser melting (SLM) technique. The as‐built specimens are subjected to abrasive blasting treatment to improve the surface quality of the printed parts, in order to reveal the true surface and mechanical characteristics of the TPMS structures. It is found that the P‐type structure outperforms the G‐type structure with a higher energy absorption capability at low relative densities (<0.35). The macroscopic examination of these micro‐architectures reveals that the P‐type structure develops a rapid local cell deformation following the diagonal shear geometry on the face sheet, whereas the G‐type structure experiences continuous strain hardening along the stress plateau and deforms in a gradual manner during compression. The apparent strain hardening effect of the G‐type structure is caused by the development of many macro‐localities with extreme geometry distortion and cell wall self‐contacting during compression. The findings in this study may provide valuable insight into design, fabrication, and post‐fabrication treatment of metallic TPMS structures for the applications of high compression performance.