<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 T_eff = 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.

<全文連結>