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 supe r-elastic behaviour : during a super-elastic test, the martensitic tra nsformation is induced by the stress, and a large reversible deformati on is observed. With a single crystal, the maximum superelastic deform ation reaches 8 to 10%. During the tensile test on a single crystal, o nly one variant of martensite is induced in the shape of many parallel needles. Using a specific tensile machine, we have measured the frict ion due to the movement of the interfaces between the austenite and th e martensitic phases. These measurements have been realised during a t ensile test on single crystals of CuZnAl. The movement of the interfac es is obtained by a low sinusoidal stress that is superimposed to the tensile stress. Results show that the interfaces are progressively pin ned when the super-elastic strain is maintained constant. This pinning disappears when the strain is slightly increased.