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

在马氏体起始温度(Ms)以上观察到许多反常现象，如剪切常数软化、内耗增大等。然而，关于马氏体相变前母相电子系统的变化的信息很少，这应该是所有预马氏体相现象的根源。本文利用对物质电子结构变化非常敏感的正电子寿命谱研究了NiTi合金中的预马氏体现象。采用电弧熔炼的近等原子NiTi合金在Ar气氛中进行180K的均匀化处理，然后在冰水中淬火。正电子寿命测量是在125~320K温度范围内用时间分辨率(FWHM)为190ps的快-快时间符合系统进行的。为了确定合金的Ms温度，在150~350K的温度范围内进行了差示扫描量热(DSC)测量。我们发现，在Ni51Ti49的马氏体相变温度以上，正电子寿命发生了反常的变化：即使在高于合金ms温度100度的温度下，正电子寿命也随着温度的降低而反常地增加。在加热过程中也观察到了几乎相同的正电子寿命变化，几乎没有滞后，这清楚地表明，所观察到的正电子寿命的预马氏体变化是由于NiTi合金的本征电子结构变化造成的，这种变化导致了所谓的预马氏体现象。

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.