Optimization of Composition for the Appearance of Pseudoelasticity due to Consecutive β<SUB>1</SUB>\ ightleftarrowsβ<SUB>1</SUB>′\ ightleftarrowsα<SUB>1</SUB>′ Transformations in Cu–Al–Ni Alloy Single Crystals

Abstract

The effect of Al and Ni contents on the pseudoelastic behavior of Cu–Al–Ni alloys above Af has been investigated using rapidly quenched single crystals with nearly 〈001〉 orientation. The results obtained are as follows: One-stage pseudoelasticity due to consecutive β1\ ightleftarrowsβ1′\ ightleftarrowsα1′ transformations on loading and unloading appears in Cu-13.9Al-5.0Ni (mass%) alloy above a critical temperature of the transformation type, from β1→β1′ to β1′→α1′, Tc,1=2L. However, such complete one-stage pseudoelasticity does not appear in Cu–14.4Al–3.6Ni and Cu–14.5Al–3.0Ni alloys because the α1′ martensite is stabilized by the occurrence of slip deformation inside it and some strain remains on unloading, although one-stage yielding accompanied by a prior peak is observed due to the consecutive transformations on loading. The one-stage yielding on loading is not observed in Cu–14.1Al–4.2Ni alloy, since Tc,1=2L of the alloy reaches a temperature where some aging effect occurs. The effect of compositions on those consecutive or successive transformations has been examined in detail with respect to various parameters such as Ms for γ1′ martensite, critical stresses for the β1→β1′, (σ1L), and β1′→α1′, (σ2L), transformations on loading, Tc,1=2L, critical stress of transformation type, σc,1=2L and extrapolated temperture of a line expressing the σ1L vs temperature relation to zero stress, T1L (σ=0)(which corresponds to a hypothetical Ms for the β1′ martensite). Taking these effects into account, the composition for the appearance of consecutive transformations has been optimized and the origin of the high Tc,1=2L in Cu–14.1Al–4.2Ni alloy is discussed.