Calorimetric measurements on the β⇌γ' and β⇌β' martensitic transformations in a cu-al-ni single crystal subjected to uniaxial tensile stress

Abstract

This paper describes a high-sensitivity calorimeter which is capable of carrying out measurements on samples whilst they are subjected to uniaxial tensile stress. It is shown how this device can be used to investigate the β⇌γ' and β⇌β' martensitic transformations in a Cu-27.8 at.% Al-3.8 at.% Ni single crystal and an experimental stress-temperature phase diagram for this alloy is presented which contains both the forward and the reverse transformation phase boundaries. Direct calorimetric determinations of the enthalpy and entropy changes of the β⇌γ' and β⇌β' transformations are presented, and it is shown that the thermal hysteresis associated with the β⇌γ' transformation exhibits a slight dependence on the level of applied stress. The latter is shown to be associated with differences in the entropy change between the forward and reverse transformations.