NiTi and NiTi-TiC composites: Part IV. Neutron diffraction study of twinning and shape-memory recovery |
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Authors: | D C Dunand D Mari M A M Bourke J A Roberts |
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Affiliation: | (1) Department of Materials Science and Engineering, Massachusetts Institute of Technology, 02139 Cambridge, MA;(2) Composite and Microwave Engineering, CEO, 1015 Lausanne, Switzerland;(3) MST5, 87545 Los Alamos, NM;(4) Los Alamos National Laboratory, LANSCE, 87545 Los Alamos, NM |
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Abstract: | Neutron diffraction measurements of internal elastic strains and crystallographic orientation were performed during compressive
deformation of martensitic NiTi containing 0 vol pct and 20 vol pct TiC particles. For bulk NiTi, some twinning takes place
upon initial loading below the apparent yield stress, resulting in a low apparent Young's modulus; for reinforced NiTi, the
elastic mismatch from the stiff particles enhances this effect. However, elastic load transfer between matrix and reinforcement
takes place above and below the composite apparent yield stress, in good agreement with continuum mechanics predictions. Macroscopic
plastic deformation occurs by matrix twinning, whereby (1 0 0) planes tend to align perpendicular to the stress axis. The
elastic TiC particles do not alter the overall twinning behavior, indicating that the mismatch stresses associated with NiTi
plastic deformation are fully relaxed by localized twinning at the interface between the matrix and the reinforcement. For
both bulk and reinforced NiTi, partial reverse twinning takes place upon unloading, as indicated by a Bauschinger effect followed
by rubberlike behavior, resulting in very low residual stresses in the unloaded condition. Shape-memory heat treatment leads
to further recovery of the preferred orientation and very low residual stresses, as a result of self-accommodation during
the phase transformations. It is concluded that, except for elastic load transfer, the thermal, transformation, and plastic
mismatches resulting from the TiC particles are efficiently canceled by matrix twinning, in contrast to metal matrix composites
deforming by slip. |
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