In situ TEM study of irradiation-induced transformation in TiNi shape memory alloys
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In situ TEM study of irradiation-induced transformation in TiNi shape memory alloys X. T.
Zu1*,
F.R. Wan2, S. Zhu3, L. M. Wang3
1
Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu, 610054, P. R. China 2 Department of Materials Physics, University of Science and Technology Beijing, Beijing, 100083, China 3 Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA
ABSTRACT TiNi shape memory alloy (SMA) has potential applications for nuclear reactors and its phase stability under irradiation is becoming an important topic. Some irradiation-induced diffusion-dependent phase transformations, such as amorphization, have been reported before. In the present work, the behavior of diffusion-independent phase transformation in TiNi SMA was studied by electron irradiation at room temperature. The effect of irradiation on the martensitic transformation of TiNi shape memory alloys was studied by Transmission Electron Microscopy (TEM) with in-situ observation and differential scanning calorimeter (DSC). The results of TEM and DSC measurements show that the microstructure of samples is R phase at room temperature. Electron irradiations were carried out using several different TEM with accelerating voltage of 200 kV, 300 kV, 400 kV and 1000 kV. Also the accelerating voltage in the same TEM was changed to investigate the critical voltage for the effect of irradiation on phase transformation. It was found that a phase transformation occurred under electron irradiation above 320 kV, but never appeared at 300 kV or lower accelerating voltage. Such phase transformation took place in a few seconds of irradiation and was independent of atom diffusion. The mechanism of Electron-irradiation-induced the martensitic transformation due to displacements of atoms from their lattice sites produced by the accelerated electrons.
INTRODUCTION The first observation of a crystalline-amorphous transition of metals was discovered in TiNi shape memory alloys (SMAs) when they are irradiated with high energy electrons (more than 1 MeV) in 1982 [1]. The amorphization of intermetallic compounds has been researched widely since then [1~10]. 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, 11]. Mori et al. reported that electron *
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irradiation first converts the monoclinic martensite phase to the cubic B2 austenite phase below the Ms temperature (Ms is the martensitic transformation start temperature) and then induces the amorphization of the B2 phase at a critical dose of electrons. Above the Ms temperature, the amorphization of the B2 phase is produced directly [6]. A preferential amorphous transition by electron irradiation is observed at dislocations and twin boundaries in TiNi SMAs [7]. Proton-irradiation-induced cry
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