Factors influencing the stress-induced fcc ↔ hcp martensitic transformation in Co–32Ni single crystal

Abstract

This study investigated the mechanical reversibility of the stress-induced face-centred cubic (fcc) ↔ hexagonal close-packed (hcp) martensitic transformation in Co–32Ni single crystal. The as-fabricated single crystal, which was predominantly in the fcc γ-phase, contained a large number of stacking faults at room temperature, effectively containing a small portion of the hcp phase. The transformation to hcp martensite upon cooling was incomplete, resulting in a mixed structure of fcc and hcp regions. Consequently, the alloy showed stress-induced fcc → hcp martensitic transformation on cooling and stress-induced hcp → fcc reverse transformation on heating from room temperature. The stress-induced transformations were found to stabilise quickly, i.e. the transformation volume diminished rapidly after a small number of cycles. This is attributed to the trapping of mobile partial dislocations at internal pinning sites and the interlocking of boundaries of hcp martensite variants. It is also unique that the alloy exhibited transformation-induced strains in the same direction of the applied stress for both the forward and the reverse transformations, in contrast to conventional shape memory alloys. This is attributed to the multiplicity of the return path of the hcp martensite to the fcc austenite. Such condition jeopardises the transformation system to serve as a mechanism for shape memory effect and related properties. This conclusion should be universal for all alloy systems exhibiting fcc ↔ hcp martensitic transformations.