Ultrasonic investigation of the B2 ↔ B19 martensitic transition in a Ni40Ti50Cu10 alloy

The elastic constants C′, C44 and CL of a single crystal of the Ni40Ti50Cu10 (numbers indicate at.%) alloy have been measured in the premartensitic region (320–350 K) and have been found to exhibit softening on approaching the B2→B19 transition from high temperature. This means that these elastic constants decrease with decreasing T. The anisotropy factor A=C44/C′ is relatively small (≅2.2) over all the temperature range and slightly increases with decreasing temperature down to the transition start-temperature Ms. Thus, its temperature coefficient ∂A/∂T has opposite sign of that exhibited by the binary Ni50.8Ti49.2 alloy prior to the B2→B19′ transition. The present results support the view recently expressed by some authors that the B2→B19′ transition occurring in binary Ni–Ti alloys is a consequence of concomitant lattice instabilities against {1 1 0}〈1 1̄ 0〉B2 and {0 0 1}〈 1 1̄ 0〉B2 shear strains respectively corresponding to the C′ and C44 ultrasonic propagation modes.