Use of the Phenomenological Theory for Studying the Mechanism of the Reversible Shape Memory Effect

In the previous studies on the reversible shape memory effect in Cu-Zn-Al alloy, it has been verified that a planar structure consisting of α1′ and β1′ martensites is constructed after heating the specimen under constraint and also that this composite structure is expected to produce special stress field by which the original β1′ martensite is induced on subsequent cooling.In the present paper the above mechanism was crystallographically examined by the phenomenological theory of transformation. The shape deformation due to the two-step transformation, β1→β1′→α1′, and the reverse transformation, α1′→β1′, were first formulated assuming a shear deformation on the close-packed planes in the second transformation, β1′→α1′. The numerical calculations of shape deformation using these formulae with the practical lattice parameters indicate that β1′ martensites with a different orientation from the original one can be formed under the influence of the β1′→β1 reverse transformation of the original one. It is also theoretically confirmed that around the retained α1′ martensites, with which the above β1′ martensites construct the composite structure, the stress field having a sense to produce the original β1′ martensite is reasonably formed.