نمونه سوالات آزمون های کارشناسی ارشد مهندسی مواد و متالورژی زبان عمومی و تخصصی (انگلیسی) امتحانات 94-86 - بخش (3)

The martensitic transformations have an important role in defining the properties of shape memory alloys, including the shape memory and superelastic effects. Quenched NiTi alloys with a stoichiometry ratio of 50 at. % Ni show a one-stage-austenite (B2) to martensite (B 19´) transformation. The transformation temperature determines the temperature range where such an effect can be observed. Experimentally it is well know that the martensite transformation temperature is stronglydependent on composition and on ageing treatments. Moreover, when adding alloying elements or when ageing treatments are performed, not only the transformation temperature is changed, but also the transformation paths and the transformation product can be changed. Under these conditions, a two-stage transformation (B2 <-> R <-> B19` or B2 <-> B19 <-> B19`) is usually observed, instead of a one-stage (B2 <-> B19`) transformation. According to the Ti-Ni phase diagram, the NiTi compound, which is an intermetallic compound with B2 order, shows a certain solubility of excess Ni on the Ni-rich side at high temperature but cannot dissolve excess Ti (the Ti-rich side is almost vertical). So, in Ni-rich alloys the transformation temperature is strongly dependent on Ni concentration and the increase in Ni content causes a drastic decrease in the transformation temperature. Conversely, for Ti-rich alloys the transformation temperature is almost unaffected by composition and shows a similar behavior to that of the Ti-50Ni alloy. Therefore, the precise control of overall chemical composition and homogeneity of NiTi alloys plays a very important role in the fabrication of these alloys. It is found that, by changing the ageing temperature, it is possible to adjust the transformation temperature of Ni-rich alloys even after the aloy has been produced due to the (metastable) equilibrium between NiTi and Ni4Ti3 precipitates.