Investigation of Martensite Ageing Effect and the Rubber-like Behavior

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The strange phenomenon called "the rubber-like behavior" occurs in the following way. When the alloy is deformed in the martensitic state right after the transformation, it is plastic, while when it is aged in the martensitic state for some time, it becomes pseudoelastic. Although the phenomenon is known for more than 60 years, the origin is still not known yet. To understand the origin and mechanism, the characteristics of the phenomenon was studied in detail by employing various techniques such as tensile tests, DSC,4 circle x-ray diffractometer, internal friction etc. The Au-49.5at%Cd alloy (trigonal martensite) was used mostly, in addition to Au-47.5at%Cd (orthorhombic) and Ti-Ni alloys. Thus, the following basic characteristics were confirmed for this peculiar phenomenon.1) The critical stress for twinning, the Af temperature and Young's modules increase, while the internal friction decreases with ageing.2) The rubber-like behavior is a volume effect rather than a boundary effect. … More 3) The effect of martensite ageing annihilates once the specimen is reverse trasformed to the parent phase.4) The martensite ageing effect is enhanced by the presence of vacancies, and is considered to occur by short range diffusion. i.e.there is no ageing effect in Ti-Ni alloy, where the density of vacancies is very low. The activation energy for the ageing effect was measured to be 0.44eV.5) The average structure and the long range order of a martensite do not change with ageing.Considering these characteristics, most of the 9 models so far proposed for the phenomenon are ruled out, except for the following three models. i) short range order model (Marukawa-Tsuchiya), ii) strain dipole model (T.Suzuki) and iii) vacancy interaction ordering model (Ren and Otsuka, present work). However, all these models have some shortcomings. To get a breakthrough, measurements of defect denities (i.e.the densities of vacancies and anti-structure defects) are very important, and the research for the goal is still intensively continued. We believe the solution for the strange effect will be obtained in the near furture. Less