ADVANCED MATERIALS & PROCESSES | MARCH 2025 42 12 SMJ HIGHLIGHTS December 2024 EFFECT OF Cu ALLOYING AND HEAT TREATMENT PARAMETERS ON NiTi ALLOY PHASE STABILITY AND CONSTITUTIVE BEHAVIOR S. Cai, J.E. Schaffer, T. Shi, J. Gao, and L. Kadeřávek NiTiCu alloys with Cu-content up to 20 at.% were successfully made into fine wire with cold-drawn area reduction of 70%. These materials were heat treated and tested at different conditions to study phase transformation and constitutive behavior. It is found that: (1) At Cu-content less than 7.5 at.%, increasing Cu stabilizes the B2 austenite, suppresses R-phase, and slightly decreases B19′ transformation temperatures. At higher level, Cu suppresses B19′-phase but promotes B19 martensitic phase. (2) Increasing Cu decreases thermal temperature hysteresis and lattice strains during phase transformation. (3) The effect of Cu on stress hysteresis depends on test conditions. A continuous increase in Cu does not guarantee a continuously decreased stress hysteresis. (4) Heat treatment affects Ti2(Ni,Cu)3 precipitation. The highest phase transformation temperatures were obtained after annealing at ~600°C. (5) The {102}B19 and {120}B19 martensite texture comes from the {111}B2 austenite texture after phase transformation through {111} and/or {011} martensite twins. These texture patterns explain the relatively short transformation strains and suggest potential for large improvement through processing-induced texture optimization. (6) The combined effects of Cu addition and cold work strengthening largely increase thermal cycling stabilities of NiTiCu alloys and make them promising candidates for actuator applications (Fig. 2). Fig. 2 — SEM images of (a) as-cast alloy 5Cu, (b) as-cast alloy 20Cu, and (c) homogenized alloy 20Cu. Fig. 3 — Total energy profiles along different shear deformation paths (a) for stoichiometric Ni2MnGa alloy, (b) for off-stoichiometric Ni50Mn31.25Ga18.75 alloy. (a) (b) SHEAR DEFORMATION OF NON-MODULATED Ni2MnGa MARTENSITE: AN AB INITIO STUDY Martin Heczko, Petr Šesták, Hanuš Seiner, and Martin Zelený The impact of shear deformation in (101)[101] system of non-modulated (NM) martensite in Ni2MnGa ferromagnetic shape memory alloy is investigated by means of ab initio atomistic simulations. The shear system is associated with twinning of NM lattice and intermartensitic transformation to modulated structures. The stability of the NM lattice increases with increasing content of Mn. The most realistic shear mechanism for twin reorientation can be approximated by the simple shear mechanism, although the lowest barriers were calculated for pure shear mechanism. The energy barrier between twin variants further reduces due to spontaneous appearance of lattice modulation or, in other words, the nanotwins with thickness of two atomic planes. Such nanotwins appear also on the generalized planar fault energy (GPFE) curve calculated using a newly developed advanced procedure and exhibits even lower energy than the defect free NM structure. These nanotwin double layers are also basic building blocks of modulated structures and play an important role in intermartensitic transformation (Fig. 3).
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