Martensite transformation and shape memory effect in Ni--Ti alloy. [50. 3 at. percent Ni]
A polycrystalline Ti--50.3 at. percent Ni alloy was investigated. It was found that the high temperature phase possesses a CsCl-structure (B2 type). During quenching from high temperature, excess Ni precipitates in the form of very fine highly coherent ordered particles or clusters prior to the martensitic transformation. The CsCl-structure transforms by inhomogeneous (110) (110) shear in a thermoelastic manner into a two-layer close-packed structure which has a distorted orthorhombic unit cell (martensite). The martensitic phase forms mostly in self-accommodating groups. Each member of a group is internally twinned and members of a group are themselves twin-related along the same plane. Deformation within the recoverable strain of a partially transformed material occurs by modes related to the transformation process itself. These modes are: (i) Stress-induced growth of the most favorably oriented existing martensite at the expense of unfavorably oriented martensite and retained high temperature phase (CsCl-structure). (ii) Stress-induced re-orientation of an existing martensite variant by utilizing the most favorable twinning mode which involves de-twinning and re-twinning. (iii) Stress-induced twin boundary migration within a martensite variant. The memory effect can be explained in terms of the high reversibility of the transformation and the limitation of the deformation modes within the recoverable strain range to those involved in the transformation process itself. 37 figures, 132 references.
- Research Organization:
- California Univ., Berkeley (USA). Lawrence Berkeley Lab.
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 7267616
- Report Number(s):
- LBL-5112
- Resource Relation:
- Other Information: Thesis
- Country of Publication:
- United States
- Language:
- English
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