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Author

Tian Xiao

Date of Award

12-1993

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Materials Engineering

Supervisor

G.P. Johari

Abstract

The temperature and time dependence of the thermoelastic martensitic transformation in Cu-Zn-Al shape memory alloys have been studied by means of internal friction, DSC (Differential Scanning Calorimetry), X-ray diffraction and metallography. A physical model and mathematical treatment have been developed to derive an expression for internal friction associated with the thermoelastic martensitic transformation. This expression has been able to explain the amplitude dependence of the internal friction in various ranges of stress amplitude, where other theories have failed. In the special case where the applied stress amplitude is much higher than the critical stresses for the stress-induced martensitic transformation, the internal friction is found to be linearly proportional to the reciprocal of the stress amplitude, in good agreement with the experimental results. The time evolution of internal friction and shape change has been measured simultaneously in different microstructural states of the shape memory alloys. In the single-martensite phase state, the monotonic decrease of internal friction with time, without any accompanying shape change, has been attributed to the progressive pinning of the martensite/martensite interfaces. When the parent phase is present, the appearance of an internal friction peak, accompanied by an evident shape change, is found to be associated with the stress-induced martensitic transformation and the subsequent pinning of the martensite/parent and martensite/martensite interfaces.

The effects of aging in both martensite and parent phases have been studied. Aging in parent phase caused the precipitation of $\alpha$-phase which hindered the subsequent martensitic transformation and resulted in a suppressed internal friction peak and a deteriorated DSC peak. Aging in the martensite phase resulted in the pinning of the martensite/martensite interfaces by vacancies, which significantly obstructed the subsequent transformations upon heating and cooling. This has been evidenced by both internal friction and DSC measurements.

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