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Strain induced martensitic transformation in Cu-Al-Ni Oishi, Kazumasa


A study has been made of super-elasticity and the strain-memory effect in Cu-Al-Ni alloys in the composition range 14 wt. % Al and 2 to 6 wt. % Ni. These alloys have a bcc structure on quenching and show a low temperature transformation to a body-centered orthorhombic martensitic structure. It is this transformation that is responsible for the super-elastic and strain-memory effects. Tests on both single and polycrystalline specimens showed that the maximum super-elasticity occurred close to As. At higher temperatures the effect gradually decreased, whilst at lower temperatures it decreased very quickly. The magnitude of the effect was large in single crystal specimens (> 6%), but small in polycrystal specimens (< 1.5%). The super-elastic effect was caused by stress-induced martensite (SIM). Two types of SIM were observed: thin plates of thermoelastic martensite which was always reversible, and wide plates of burst-type martensite. This burst-type martensite was responsible for the major portion of SIM, and whether it was reversible or not on removal of the stress controlled the amount of super-elasticity observed. The strain-memory effect occurred on deformation either in the martensitic state (temperature <Mf) or in the temperature range where the martensite once formed was stable (temperatures close to Ms). Deformation caused reorientation of the martensite plates and when the specimen was heated, the martensite disappeared and the specimen reverted back to its original shape. This effect was explained on the basis of development of martensite plates of favorable orientation on stressing.

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