Shape memory and pseudoelastic behavior of 51.5%Ni-Ti single crystals in solutionized and overaged state

Deformation of nickel rich (51.5%Ni) Ni-Ti single crystals are investigated over a wide range of temperatures (77-440 K) and strain levels in compress ion as high as 9%. These alloys combine high strength with an unusually wid e pseudoleasticity temperature interval (near 200 K) and can be exploited t o suit specific applications. The slip deformation in [001] orientation can not occur due to the prevailing slip systems, as confirmed by transmission electron microscopy. Consequently, the [001] orientation exhibited pseudol eastic deformation at temperatures ranging from 77 to 283 K for the solutio nized case and 273-440 K for the aged condition respectively. The critical transformation stress levels were in the range 800-1800 MPa for the solutio nized case, and 200-1000 MPa for the aged case depending on the temperature and specimen orientation. These stress levels are considerably higher comp ared to the near equiatomic Ni compositions of these class of alloys. On th e other hand, the maximum transformation strains, measured from incremental straining experiments in compression, were lower compared to both the phen omenological theory with Type II twinning and the previous experimental wor k on 50.8%Ni NiTi crystals. A new theory for compound twinning is introduce d with lattice invariant shear as a solution, and relies on the successive austenite phase (132) to intermediate phase (R) to martensite phase (B 19') transformation. The compound twinning model predicts lower transformation strains compared to the Type II twinning case lending an explanation of the experimental transformation strain levels. (C) 2001 Acta Materialia Inc. p ublished by Elsevier Science Ltd. All rights reserved.