Transitions between stress-induced martensites in Cu Al Ni alloys

Abstract

Stress-induced martensitic transformations have been studied in β₁ Cu Al Ni single crystals at temperatures aboveM _s . Close toM _s γ′ martensite is formed, well aboveM _s β ₁ ′ martensite forms, whilst in an intermediate temperature range β₁′ martensite initially forms and then transforms to γ′ on continued stressing and particularly on unloading, γ′ martensite is also formed when the stress-induced β₁′ is cooled below a critical temperature. The γ′ martensite has a (101) twinned structure. The morphological and crystallographic aspects of the γ₁′→ γ′ transition are discussed in detail. The two twin variants have different habit planes with respect to the β₁′ phase, one being (201)γ′ and the other (001)γ′. A thermodynamic argument is presented to explain the γ₁′→ γ′ transition, taking into account the relative stabilities of the β₁′ and γ′ phases with respect to the β₁, and the relative value for the critical driving force to nucleate the stress-induced β₁′ and γ′ structures from the β₁ phase