Internal Friction Measurements During Stress Induced Martensitic Transformation of Cu Zn Al Shape Memory Single Crystal

The shape memory alloys present, in the high temperature phase, a super-elastic behaviour : during a super-elastic test, the martensitic transformation is induced by the stress, and a large reversible deformation is observed. With a single crystal, the maximum superelastic deformation reaches 8 to 10%. During the tensile test on a single crystal, only one variant of martensite is induced in the shape of many parallel needles. Using a specific tensile machine, we have measured the friction due to the movement of the interfaces between the austenite and the martensitic phases. These measurements have been realised during a tensile test on single crystals of CuZnAl. The movement of the interfaces is obtained by a low sinusoidal stress that is superimposed to the tensile stress. Results show that the interfaces are progressively pinned when the super-elastic strain is maintained constant. This pinning disappears when the strain is slightly increased.