Effect of Ni Concentration on Transformation Pseudoelasticity in Cu–Al–Ni Alloy Single Crystals

Abstract

Single crystals with ⟨001⟩ orientation of Cu-14.1Al-xNi (x=0, 1.0, 2.1, 3.2 and 4.3 mass%) alloys have been tensile-tested at various temperatures above and below M_s, except for the binary Cu–Al alloy tested only at 243 K below M_s. Critical stresses on loading, σ^L, and on unloading, σ^U, for the γ₁′\ ightleftarrowsβ₁″(β₁′), β₁″(β₁′)\ ightleftarrowsα₁′ and β₁\ ightleftarrowsβ₁′ transformations have been measured as a function of temperature. Then, the equilibrium stress, σ^E (=(σ^L+σ^U)/2), vs. temperature, T, phase diagram, which involves four phases of β₁ parent and γ₁′, β₁′ (β₁″) and α₁′ martensites, has been constructed for the four kinds of alloys with different Ni contents, and the critical and equilibrium stresses have been plotted as a function of Ni content. As a result, σ^L extrapolated to 0% Ni for the γ₁′→β₁″ transformation at 243 K was found to be higher than that for the β₁″→α₁′ transformation, and the β₁′ (β₁″) phase region in the σ^E vs. T diagram was found to become narrower with decreasing Ni content. The stress region of the β₁′(β₁″) phase at 243 K, which was extrapolated to 0% Ni, was between ∼320 MPa and ∼410 MPa. Hence, the γ₁′→α₁′ direct transformation so far reported for binary Cu–Al alloys was revealed to be due to the consecutive γ₁′→β₁″→α₁′ transformations in which the β₁″ martensite stress-induced from the γ₁′ martensite immediately transforms into the α₁′ martensite. Meanwhile, the temperature where slip deformation took place in the α₁′ martensite was found to decrease from far above A_f to below M_f with decreasing Ni content. Hence, the reason for no appearance of pseudoelasticity in the binary Cu–Al alloys was ascribed to the effect that the slip deformation in α₁′ martensite lowered the critical stress for reverse transformation below zero stress.