Y. Murakami

Self-accomodation and morphology of 14M (7R) martensites in an Ni370at.%Al alloy

The self-accomodation mechanism and morphology of the unique 14M (7R) martensite was studied using single crystals of an Ni370at.%Al alloy. The self-accomodating group was identified by the X-ray diffraction technique, assisted by computer simulations, and it was found that the elastic strains around martensites are effectively cancelled out by making a variant group around the {110&}B2 pole, being similar to other β phase alloys. As a result of extensive observations using a scanning electron microscope and an optical microscope, the basic morphology of the self-accomodating group was found to consist of parallelogram bounded by type I and type II twin bounderies, which is different from the conventional diamond morphology. This basic morphology was supported by the theoretical considerations based on the phenomenological crystallographic theory, and it was shown to be favored over the diamond morphology. Furthermore, the developing mechanism of self-accomodation is proposed, on the basis of the above results.

Characteristics and mechanism of martensite ageing effect in Au−Cd alloys

Abstract Characteristics of the martensite ageing effect was extensively studied for the ζ′2 (trigonal) martensite in Au-49.5 and 50.0at.%Cd alloys. Considerable changes in various physical properties were observed with ageing the ζ′2 martensite, such as the increase of the critical stress for the rearrangement of martensite variants and of the reverse transformation temperature etc., in addition to the appearance of the rubber-like behavior, which were shown to originate from the same origin. The ageing effect was further investigated with respect to the temperature dependence, composition dependence and the influence by heat-treatment, from which a close relation with a diffusion mechanism in the martensite was derived. The activation energy for the martensite ageing effect was determined to be 0.44 eV for the ζ′2 martensite in a furnace-cooled Au-49.5at.%Cd alloy, which was found to correspond to activation energy for migration of a vacancy in the martensite. The structural change associated with the ageing effect was examined both for the ζ′2 and γ′2 (orthorhombic) martensites in Au−Cd alloys by careful X-ray diffraction measurements. Only a small and local structural change, which was achieved by some short range diffusion, was found to be responsible for the ageing effect.

New martensite structures and composition dependence of martensitic transformations in Ni50AlxMn50−x alloys

Transformation temperatures and the structures of martensites were extensively studied for Ni50AlxMn50−x (16.0 ⩽ x ⩽ 20.5) alloys. By electron microscopy observations, all martensites were found to be long-period stacking-order structures of closed-packed planes, including new martensite structures such as 10M(15R) and 12M(36R). DSC measurements revealed two peaks for composition x < 17.5 and these were interpreted to correspond to two successive martensitic transformations. Based on these results, the map of transformation temperatures and martensite structures as a function of composition is proposed.

Effect of quenched-in vacancies on the martensitic transformation

In most of the thermoelastic alloys to exhibit the martensitic transformation, such as Cu-Al-Ni, Cu-Zn-Al, and Ni-Al alloys etc., the {beta} phase (parent phase) exists in equilibrium only in the high temperature region, i.e. it decomposes by a slow cooling (furnace-cooling). Hence, a quenching from the {beta} phase region is a necessary operation to observe the martensitic transformation in those alloys. The quenching also freezes a high concentration of vacancies (quenched-in vacancies) or a disordered structure into the lower temperature region (e.g. room-temperature). The Au-Cd alloy with equi-atomic composition is the ideal system in which the {beta}{sub 2} parent phase (B2) exists in equilibrium even around room-temperature without a decomposition. Hence, the martensitic transformation can be observed both in the quenched and furnace-cooled specimens. The purpose of the present paper is to report the effect of the quenched-in vacancies on the martensitic transformation, which was derived from both the quenched and furnace-cooled Au-49.5at%Cd alloys.

Composition dependence of the rubber-like behavior in ζ2′-martensite of AuCd alloys

Abstract The effect of aging on the rubber-like behavior of martensite in a Au–50.5 at.% Cd alloy was studied, and compared with stoichiometric Au–50% at.% Cd and a Au–49.5 at.% Cd alloy. It is found that for all these alloys the critical stress for domain reversion increases with aging, and gradually saturates at long aging time, following a Chipman function. The magnitude and the rate of the aging effect are strongly dependent on composition. The magnitude decreases monotonically with increasing Cd content, but the aging rate reaches a minimum at the stoichiometric composition. These results suggest that point defects (anti-structure defects and vacancies) play a central role in the martensite aging effect, and anti-structure defects contribute much more to the magnitude of the aging effect than vacancies do.