Phenomenological Approach to the Coexistence of Two Types of Martensites in Cu–Zn–Al–Mn Alloys

Abstract

Certain compositions of polycrystalline Cu–Zn–Al–Mn alloys have been found to transform, on stress-free cooling, to a mixture of two types of martensites, 18R (β′) and 2H (γ′). Due to the different thermal hysteresis shown by the β\ ightleftarrowsβ′ and β\ ightleftarrowsγ′ transformations, the corresponding reverse transformations take place separately. The coexistence of two types of martensites has been investigated by means of non-conventional differential scanning calorimetry. The relative amount of each phase existing in the martensite product depends on composition and thermal treatment, ageing promoting the β′ martensite at the expense of the γ′. To get a better understanding of this phenomena, a phenomenological model, based on the Gibbs free energy balance currently accepted for thermoelastic transformations, has been applied. The model gives an insight on the influence of different physical effects, either chemical (changes in the relative stability of the related phases) and non-chemical (frictional and elastic energy contributions), on the final relative fraction of each martensite and on the transformation temperatures.