Study of pre-martensitic phenomena in shape memory alloys by positron annihilation lifetime spectroscopy

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Many anomalous phenomena have been observed above martensite start(Ms) temperature, such as softening of the shear constants and increasing of internal friction. However, little information has been ohtained about the change in the electron system of the parent phase prior to the martensitic transformation, which should be the origin of all pre-martensitic phenomena. In this work, the pre-martensitic phenomena in NiTi alloys have been studied by positron lifetime spectroscopy which is quite sensitive to electronic-structural changes of matter.Near-equiatomic NiTi alloys produced by arcmelting were annealed for 180 ks at 1273 K in argon atmosphere for homogenization and subsequently quenched in iced water. The positron lifetime measurements were made at temperatures between 125 and 320 K using a fast-fast timing coincidence system with a time resolution (FWHM) of 190 ps. In order to determine the Ms temperatures the differential scanning calorimetry (DSC) measurements were also carried out in the temperature range between 150 and 350 K.We have found an anomalous positron lifetime change above martensitic transformation temperature of Ni51Ti49: positron lifetime increases anomalously with decreasing temperature even at temperatures 100 degrees higher than the Ms temperature of the alloy. Almost the same change in positron lifetime also has been observed on the heating run with little hysteresis, which clearly indicates that the observed pre-martensitic change of positron lifetime is due to the intrinsic electronic-structural change of NiTi alloys which causes so-called pre-martensitic phenomena.