Experimental investigation and thermodynamic calculation of the Ti-Ni-Cu shape memory alloys
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I. INTRODUCTION
TERNARY alloying element additions to near equiatomic TiNi shape memory alloys (SMAs) can provide a wide range of modifications of their shape memory properties by affecting their transformation behavior. Among the alloying elements, Cu is unique since it considerably narrows the transformation temperature hysteresis,[1,2] reduces the sensitivity of MS to the Ni:Ti ratio,[3] increases the strength difference between the parent and martensite phases, and improves the transformational cyclic behavior.[4,5] It has been found that Cu substitution for Ni affects the transformation sequence[1,6–9] When the substitution of Cu for Ni is less than the range between 5 to 8 at. pct, the transformation is the same as in the binary TiNi system, i.e., cubic B2 → monoclinic B198. When the addition of Cu reaches 10 at. pct, a two-step martensitic transformation occurs, i.e., B2 → orthorhombic B19 and then B19 → monoclinic B198. If the addition is in the range from 10 to 20 at. pct Cu, it leads to B2 → B19 only, which is accompanied by a pronounced reduction of the elastic modulus. This indicates that TiNiCu could be also a potential damping material. It is clear that the properties of the ternary TiNiCu SMAs are closely related to the martensitic transformation sequences, which obviously depend on the Cu content. There are limited experimental data that only give a qualitative description of the transformations. For example, it is hard to determine the accurate Cu content at which one stage transformation B2 → B198 is divided into two stages: B2 → B19 and B19 → B198. Similarly, it is not clear what is the Cu content at which the two-stage transformation turns into the one stage transition B2 → B19 only. Besides, some experimental data are difficult to obtain due to, e.g., very small values or low temperatures. Therefore, a thermodynamic evaluation is essential to establish the phase relations among B2, B19, and B198 in the TiNiCu system based on accurate experimental information. Much experimental ¨ M, Professor, and Z.G. W. TANG, Assistant Professor, R. SANDSTRO WEI, Postdoctoral Fellow, are with the Department of Materials Science and Engineering, Royal Institute of Technology, S-100 44 Stockholm, Sweden. S. MIYAZAKI, Professor, is with the Institute of Materials Science, University of Tsukuba, Ibaraki 305-8573, Japan. Manuscript submitted May 18, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
work[1,6–10] has been carried out on the transformations among B2, B198, and B19, and the corresponding crystal structures and microstructures were determined by X-ray diffractometry[8,10] and TEM,[9] respectively. However, systematic experimental data were lacking for performing a reliable thermodynamic evaluation. Data from different sources are scattered due to diverse data background, depending on alloy preparation process, measuring method, instrument accuracy, data interpretation, etc. In addition, few quantitative thermodynamic data are available for the transformation between B19 and B198. Considering the importa
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