MECHANISM OF THE A(S) TEMPERATURE INCREASE BY PRE-DEFORMATION IN THERMOELASTIC ALLOYS

The Ti-Ni-Nb shape memory alloys are attracting recent attention for t he wide transformation temperature hysteresis, which is caused by the A(s) increase due to pre-deformation, since it is suitable for pipe co uplings. To find out the origin for the A(s) increase by pre-deformati on, similar investigation was made by tensile tests, electrical resist ance measurements, optical microscopy and transmission electron micros copy (TEM), for a Cu-13.8%Al-4.0%Ni (mass%) single crystal, and Ti-50% Ni and Ti-50.5%Ni (at%) polycrystals, which exhibit the thermoelastic martensitic transformation. The increase of A(s) due to pre-deformatio n was confirmed in these alloys as well. When the single crystal was e longated to the end of the first stage of the stress-strain curve in t he martensitic state, which corresponds to the rearrangement of marten site variants without permanent strain, A(s) increased. On the other h and, the A(s) increase in polycrystals was found to be closely related with the introduction of the permanent strain. Furthermore, the effec t of pre-deformation was found to be erased by the subsequent reverse transformation, as is reported for Ti-Ni-Nb alloys. For that effect, w e propose a mechanism, by focusing attention on the elastic strain ene rgy stored during the thermoelastic martensitic transformation, which can explain the observed effect well.