Structural anelasticity of NiTi during two-stage martensitic transformation

The two-staged thermoelastic martensitic transformation (TMT) B2-->R-->B19' in polycrystalline equiatomic NiTi has been studied by means of measuremen ts of strain amplitude-independent and amplitude-dependent internal frictio n (ADIF), Young's modulus and amplitude-dependent modulus defects. The inte rnal friction measurements were performed at a frequency of about 100 kHz, rendering negligible the transient internal friction component and allowing one to investigate the structural internal friction, much less dependent o n the external parameters such as the heating/cooling rate or the frequency of vibrations. Attention is focussed on the amplitude-dependent anelastici ty. Based on the data obtained, the anelasticity is associated with the dis locations inside the martensitic variants, not with the interfaces or inter face dislocations, as is traditionally done. The ADIF and anelastic strain in the R phase have been found to be an order of magnitude higher than in t he B19' martensitic phase. This observation is explained by a much higher d ensity of the dislocations inside the variants of the R phase as compared w ith that of the B19' phase. (C) 2000 Elsevier Science S.A. All rights reser ved.