High-pressure torsion driven phase transformations in Cu-Al-Ni shape memory alloys

Abstract

Severe plastic deformation (SPD) frequently induces phase transformations like decomposition of supersaturated solid solution, dissolution of precipitates, amorphization, nanocrystallization etc. Such diffusive phase transitions are combined with SPD-driven accelerated mass transfer. Displacive (or martensitic) phase transitions can also take place and in combination with diffusive ones have not been investigated in depth in severely deformed materials. The goal of this work is to investigate the combination of displacive (austenite↔martensite) and diffusive (decomposition of supersaturated solid solution) phase transitions in two different Cu–Al–Ni shape memory alloys under the influence of high-pressure torsion (HPT). After homogenization in the one-phase (austenitic) β-area of Cu–Al–Ni phase diagram and quenching, the first alloy was in martensitic state (mainly β′3 martensite with a small amount of γ′3 martensite), and the second one remained austenitic (β3 phase). The HPT of these alloys led to the precipitation of α1-phase in the first case and γ1-phase in the second one (as if they were annealed at an effective temperature Teff = 620 ± 20 °C). As a result of precipitation, the matrix in the first alloy was enriched and in the second one depleted in Al. After HPT, both alloys contained mainly β′3 martensite with a certain amount of γ′3 martensite. Thus, the HPT-driven diffusive transformations (precipitation of α1-and γ1-phase) influence the followed displacive (martensitic) transformation. Simultaneously, a dramatic grain refinement is obtained and the reported results open new possibilities to investigate the superelastic and shape memory effects in nanostructured Cu–Al–Ni alloys.