Transformation Characteristics of Aged Ni-rich Ti48Ni52 and Ti48.5Ni41.5Cu10 Shape Memory Alloys
Date Issued
2015
Date
2015
Author(s)
Kuang, Chai-Hsin
Abstract
In this study, transformation characteristics of aged Ni-rich Ti48Ni52 and Ti48.5Ni51.5Cu10 SMAs are investigated. For Ti48Ni52 aged at 250~600°C, schematic diagrams for phases I~III are proposed to interpret the observed transformation behavior. In phase I, Ti3Ni4 ppts distribute homogeneously and show B2↔R, B2↔R↔B19’1 or B2↔B19’1 transformations with/without R phase depending on the existence of coherent interface around Ti3Ni4. In phase II, Ti3Ni4 and Ti2Ni3 coexist, and Ti3Ni4 depletion zones occur around Ti2Ni3 plates to induce B2↔B19’2. In phase III, only Ti2Ni3 ppts and B2↔B19’2 are observed. According to this study, the occurrence of coherent stress field around Ti3Ni4 depends both on aging temperature and time. The hardness vs. aging time curves of Ti48Ni52 aged at 250~350°C show a step before reaching maximum; while those aged at 400~500°C have one maximum; and those aged at 550~600°C never show a hardness increase. For Ti48.5Ni41.5Cu10, the as-quenched specimen exhibits B2↔B191 and B19↔B19’, and both transformation temperatures are lower than those of Ti50Ni40Cu10. The DSC curves of specimens aged at 250~400°C for different times are almost the same as the as-quenched ones; while those aged at 500~600°C precipitate plate-like Ti(Ni,Cu)2 to induce a B2↔B192 transformation simultaneously. As the aging time increases, plate-like Ti(Ni,Cu)2 ppts fill up the matrix, and no B2↔B191 occurs. The aging hardening effect is not significant. Tanδ value of the maximum hardness specimen reaches 0.15 with an obvious softening storage modulus during B2→B19 transformation.
Subjects
Ni-rich TiNi shape memory alloy, aging, transformation sequence, precipitates, hardness, damping capacity
Type
thesis
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