NiTi and NiTi-TiC composites: Part IV. Neutron diffraction study of twinning and shape-memory recovery
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I,
INTRODUCTION
THE shape-memory effect exhibited by near-stoichiometric NiTi-based alloys originates from the thermoelastic phase transformation between an ordered, high-temperature phase B2 (austenite) and an ordered, low-temperature phase B 19' (martensite) deforming by twinning. The conclusions of several recent reviewsv-8] are summarized in the following. Upon transformation from austenite to martensite in the absence of external or internal stresses, the martensite microstructure consists of equal proportions of the 24 crystaUographically possible martensitic variants with different orientations, as a result of self-accommodation reducing the internal transformation strains. Upon mechanical loading, variant conversion takes place, such that the variant resulting in the largest strain in the direction of the applied stress grows at the expense of the other, less favorably oriented variants. The conversion between variants, which is equivalent to a twinning operation, results in a macroscopic strain and a preferred orientation of the martensite. While, in some cases, partial reverse twinning is observed upon mechanical unloading (rubberlike behavior), the twinning strain is usually conserved upon removal of the external stress. Upon subsequent transformation of the twinned martensite, austenite is formed with the same orientation as that prior to deformation, because a unique lattice correspondence exists between the two ordered phases and because D.C. DUNAND, AMAX Associate Professor, is with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. D. MAR1, CEO, is with Advanced Composite and Microwave Engineering, 1015 Lausanne, Switzerland. M.A.M. BOURKE, Technical Staff Member, MST5, and J.A. ROBERTS, Acting Center Leader, LANSCE, are with the Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted January 23, 1995. 2820~VOLUME 27A, SEPTEMBER 1996
the transformation is crystallographically reversible. Therefore, the macroscopic twinning strain developed in the deformed martensite phase is recovered and the specimen reverts to its previous, undeformed shape (shape-memory effect). If, upon subsequent cooling, the martensitic variants formed during the transformation are again randomly oriented, the strain recovery is conserved (one-way shapememory effect). However, if the previously oriented martensite variants are formed as a result of biasing by internal stresses, the macroscopic strain recovery is partially or fully canceled (two-way shape-memory effect). Because the NiTi phase transformation is thermoelastic, it is very sensitive to extemal or internal stresses. Internal mismatch stresses produced by the reinforcing second phase in NiTi-based metal matrix composites may thus affect both the matrix transformation behavior (e.g., hysteresis, thermal cycling, shape-memory recovery, and stressinduced martensite formation) and its deformation behavior (twinning, rubberlike effect and pseudoelastic deformation). In part I of this arti
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