The Thermomechanical Behaviour of a Metastable Cu-Al-Ni Single Crystal Alloy

Abstract

This paper presents the results from an experimental and theoretical analysis of the thermomechanical behaviour of a metastable $β$-phase single crystal alloy of composition Cu - 28.3at% Al - 3.8at% Ni. The stress-strain behaviour of this single crystal material was investigated experimentally by tensile testing and this data was used to characterise the stress/ temperature martensite phase equilibria. The same alloy was also the subject of microcalorimetric measurements which enabled some of the enthalpies and entropies of transformation to be directly determined. This thermodynamic data was used to calculate the stress/temperature phase equilibria of the alloy. It is shown that the calculated equilibria agree well with those measured experimentally. It is also shown that such thermodynamic evaluations allow a direct interrogation of free-energy/stress/temperature space which provides a fundamental explanation for the change in stress-induced martensite type as the temperature of the alloy is increased. It is shown that the latter is due primarily to the difference in the shear strains associated with the $β$→ $γ$' and $β$→ $β$' martensitic transformations, with a smaller effect resulting from the entropy difference between the $γ$' and $β$' phases.