Thermoelastic martensitic transformation in Cu-Zn-Al alloy

Abstract

Thermoelastic martensitic transformation in a Cu-29% Zn-3% Al alloy has been observed with optical and electron microscopy, including 1.0–1.2 MV high-voltage transmission electron microscopy (TEM), and treated with the phenomenological theory suggested before. The observation showed that the transformation was conducted in three stages: parallel plates growing, self-accommodating variants advancing, and plates merging and/or tiny plates forming in carved-up parent phase areas. TEM showed that the martensite consisted of a huge number of packets with constant size and distinct interfaces. By using the phenomenological theory, the free-energy function as well as the friction quasi-enthalpy and friction quasi-entropy are obtained in SI units. Comparing with classical theory, the free energy can be broken into three parts: the chemical free energy, the interfacial energy and the elastic strain energy. In the range of 20–70% martensite in the alloy, the interfacial energy per unit of martensite formation is constant and the corresponding elastic strain energy is a linear function of martensite percentage. Some possible explanations for this energetics relating to the observation are given.