ADVANCED MATERIALS & PROCESSES | OCTOBER 2024 48 SMJ HIGHLIGHTS March 2024 ULTRA-HIGH TEMPERATURE SHAPE MEMORY BEHAVIOR IN Ni-Ti-Hf ALLOYS O. Benafan, G.S. Bigelow, A. Garg, L.G. Wilson, R.B. Rogers, E.J. Young Dohe, D.F. Johnson, D.A. Scheiman, J.W. Lawson, and Zhigang Wu Shape memory behavior in stoichiometric Ni–Hf–Ti shape memory alloys with high Hf was evaluated. Five alloy compositions with a hafnium content from 30 to 50 at.% were arc melted, homogenized, and tested to reveal microstructure and shape memory properties. Transformation temperatures increased linearly with Hf addition, reaching a maximum austenite finish temperature of 1190°C at 50Hf, measured using differential scanning calorimetry (DSC). Fig. 2 — DSC peak evolution as a function of cycles for the NiTi-30Hf alloy during heating. (a) Entire scan showing both matrix phase transformation around 600°C, and HfO2 oxide phase transformation around 1175°C, and (b) magnified view of the oxide peak evolution with cycling. Ts and Tf are the monoclinic-to-tetragonal transformation start and finish temperatures, respectively, for the HfO2. The low temperature stable microstructures were composed of a majority B33 orthorhombic phase, with traces of B19′ monoclinic structure below the martensite finish temperature, as revealed by x-ray diffraction and transmission electron microscopy. These microstructures convert to a B2 cubic structure at higher temperature. Macroscopically, specimens were tested isothermally at room temperature, and endured stresses as high as 1 GPa in compression. Strain recovery decreased from nearly 100% recovery in the 30Hf alloy, to nearly 0% at 50Hf alloy, as plasticity mechanisms dominated at high temperatures in the higher Hf alloys. Uniaxial constant-force thermal cycling experiments revealed limited work output at high temperatures due to creep-dominant mechanisms simultaneously occurring during the phase transformation process (Fig. 2). June 2024 ELASTOCALORIC EFFECT IN SHAPE MEMORY ALLOYS Lluís Mañosa and Antoni Planes It is widely acknowledged that shape memory alloys have an enormous potential for future developments of an environmentally friendly new solid-state refrigeration technology, thanks to their excellent elastocaloric properties. In this review paper, after a brief summary of the historical milestones that led to the present state-of-the-art of the subject of elastocaloric effect and materials, the authors develop its thermodynamic bases and review recent advances of the elastocaloric effect in non-magnetic and magnetic shape memory alloys. They show that in this last family of alloys, multicaloric effects can occur induced by the combination of mechanical and magnetic fields and that this possibility might open new avenues for applications (Fig. 3). Fig. 3 — Schematic illustration of the inverse elastocaloric effect associated with volume dilatation elastic materials and conventional effect associated with a martensitic transition. 15
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