Electron-irradiation-induced Changes of martensitic transformation characteristics in TiNi shape memory alloys
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Electron-irradiation-induced Changes of martensitic transformation characteristics in TiNi shape memory alloys X. T. Zu1 ∗, L.P. You2, S. Zhu2, Z. G. Wang1, J. H. Wu1, L. M. Wang2 1 Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu, 610054, P. R. China 2 Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, Michigan 48109, U.S.A. ABSTRACT TiNi shape memory alloy samples were irradiated within R-phase by 1.7 MeV electrons with different doses. The martensitic transformation temperatures were measured by Differential Scanning Calorimeter (DSC). The results indicated that the temperature Ms of the onset of R-phase-to-martensite transformation decreased with increasing the dose. The electron irradiation had a slight effect on the other transformation temperatures. The second lifetime of positrons determined by Positron Annihilation Technology were lowered with an increment of the irradiated dose. Relaxation of the elastic stress fields around the Ti3Ni4 precipitates was the cause of the observed change of the transformation characteristics because of the migration and accumulation of electron irradiation-induced point defects.
INTRODUCTION TiNi-base shape memory alloys (SMAs) have potential applications for fission and fusion engineering and space technology because of their unique shape memory effect, superelasticity and stress relaxation resistance [1~2]. There are some reports on the amorphization of TiNi SMAs induced by neutron and proton and electron irradiation [3~8]. The martensitic transformation characteristics in SMAs are very sensitive to various physical factors, which either directly shift the temperature of thermodynamic phase equilibrium (hydrostatic pressure, mechanical stresses, magnetic field) or exert indirect influence through a change of crystal structure (thermal and mechanical treatments). The neutron irradiation up to about 1 dpa was found to produce a strong decrease of the transition temperatures [3, 9]. This decrease is believed to be due to the pronounced chemical disordering of the crystal lattice [3, 10]. The electron irradiation to about 1.7 × 1021 e/m2 (10-5 dpa) led to a higher equilibrium temperature and martensitic stabilization, had little effect on inverse martensitic transformation temperature in a ternary TiNiCu shape memory alloy [11]. It was found in ultra-high voltage electron microscopy that the electron irradiation decreased inverse martensitic transformation temperature above 7 × 1024 e/m2 [7]. The near-equiatomic TiNi SMAs have the two-step transformation from cubic ∗
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parent phase (B2) to a trigonal phase (R-phase) and then to the monoclinic martensitic phase (B19 ) ׳under appropriate thermal treatment [2]. It is interesting that the study of influence on the martensitic transformation using electron beam irradiation, because it only produces simple point defects (Frenkel pairs) withou
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