M. Ahlers

Martensite ageing and its stabilisation in CuZnAl shape memory alloys

Etude experimentale sur les alliages: 67,54% at. Cu-19,33% at. Zn-15,53% at. Al et 66,74% at. Cu-18,52% at. Zn-14,74% at. Al. On constate que la stabilisation de la martensite par vieillissement est due a un abaissement de son energie libre

Stress induced hexagonal martensite in CuZnAl

Abstract It has been shown that it is possible to induce a hexagonal long range ordered martensite phase in a CuZnAl alloy with M S = +50°C and an electron concentration e/a=1.48. From measurements of the critical stress and of the retransformation temperature to β after reheating, it is concluded that the hexagonal and the original 3R martensite have the same formation free energy. This implies that the two structures cannot be described by their difference in stacking fault density (compared to a face centered lattice), and an energy difference which is proportional to it. It is probable that either the interaction between neighbouring stacking faults or an electronic contribution from the Fermi surface interaction with the Jones zone plays an important role.

Transformation hardening and energy dissipation in martensitic β-brass

Abstract The temperature- and stress dependence of the martensitic transformation has been studied for single crystals of β-brass. It is found that the energy which is dissipated during the progression of the transformation is generated mainly by the friction of moving habit plane interfaces when the transformation is induced either by cooling below Ms or by applying a stress at a constant temperature above Ms. The stress which is necessary to move the interface is ≈0.25 kg/mm2. During the progression of the transformation additional energy is stored in the martensite and is released again on retransformation, leading to a lowering of the transformation temperature or an increase in stress (“transformation hardening”) when the amount of martensite grows. The stored energy has been determined and the results are discussed.