Hidekazu Sakamoto

Superelasticity effects and stress-induced martensitic transformations in CuAlNi alloys

Superelasticity in CuAlNi single crystals has been extensively studied as a function of temperature and strain rate, using tensile tests, macroscopic and microscopic observations and the back-reflection Laue method. Two distinct superelasticity effects were found depending upon the test temperature and it is concluded that two different stress-induced transformations, β1 ⇄ β′1 and β1 ⇄ γ′1. are responsible for these effects. A new treatment in terms of an effective stress has been presented for analysis of the characteristics of a superelastic loop, such as the presence or absence of a peak at the yield point and the magnitude of the hysteresis. Thus, the difference in the two types of superelastic loops is rationalized from a fundamental point of view. Furthermore, the predictions from the treatment concerning the appearance of the peak and the strain rate dependence of the hysteresis were found to be consistent with experimental observations. The phase relationships between the β1 matrix and the two martensites. β′1 and γ′1, in temperature-stress coordinates have been discussed from a thermodynamic point of view.

Successive stress-induced martensitic transformations and associated transformation pseudoelasticity in Cu-Al-Ni alloys

Abstract Successive stress-induced martensitic transformations in Cu-Al-Ni single crystals have been extensively studied as a function of temperature and strain rate, using tensile tests, optical microscope observation and the back-reflection Laue method. As a result, a phase diagram relating various martensitic phases and the matrix has been determined in temperature and stress coordinates. Complicated stress-strain curves, which drastically change with temperature, have been explained in consistent terms by using the phase diagram. Strong orientation dependence and remarkable elongation (exceeding 15% in total) of the associated multistage pseudoelasticity have been rationalized by the calculations based on the crystallographic theory. The temperature and strain rate dependence of these pseudoelastic curves have been obtained and analyzed by the surface dislocation theory developed by Sumino. It is proposed that the mechanism of successive transformations between martensites with long-period stacking order structures is the successive nucleation of regularly spaced partial dislocations. This is a unique mechanism to account for the above pseudoelastic behavior.

Direct Observation of Martensitic Transformation between Martensites in a Cu-Al-Ni Alloy

The martensitic transformations in metals and alloys usually occur from matrix to martensite by cooling below some critical temperature, that is, from a loosely packed structure to more close packed structure in order to lower the internal energy, except for ferrous alloys where the magnetic energy is more significant. These transformations are usually characterized by the presence of the habit plane, the lattice invariant shear and the lattice rotation, and their crystallographies are well accounted for by the phenomenological theory under the invariant plane strain assumption (1, 2). Now, if stress is couppled as the second variable, even the martensitic transformation between martensites may become possible, in cases where several martensitic structures with similar free energies are available (3). In fact, there have been a few reports on the crystal structure change from one martensite to another by deformation in such alloy systems, Cu-Al (4~7) and Au-49(at%)Cd (8), although the transformation process itself is not fully investigated. Further, the present authors recently found that the martensitic transformation occurs from one martensite to another under certain conditions in a Cu-Al-Ni alloy (9). The purpose of the present paper is to describe in detail on how the transformation between martensites proceed morphologically and crystallographically in such an idealized situation that the starting specimen is a martensite single crystal.