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Title: The role of crystal boundaries in creep
Author: Thompson, Noel Brentnall Watson
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 1962
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The importance of the crystal boundary in the high-temperature creep of zinc was-investigated by two distinct methods, namely, (i) by testing polycrystalline specimens under constant strain-rate conditions. (ii) by carrying out constant stress experiments on sections of bicrystals. The polycrystalline tests were performed at strain rates of 1%, 0.1% and 0.01% per hour over a temperature range from 23°C to 200°C. Data relating creep stress and ductility to strain rate and temperature were obtained. In addition, metallographic examination enabled the occurrence and extent of intergranular cavitation to be correlated with the test conditions. From these results, it is shown that secondary creep in zinc obeys a relation of the form:- -(Q-BQ)RT Ae where E is the creep rate 0-is the stress Q is the activation energy for creep. Furthermore, the creep activation energy appears to be considerably less than that for self-diffusion. In these respects, zinc differs from many other metals. Reasons for the anomalous behaviour are discussed and possible creep mechanisms considered. In the investigations on bicrystals, most tests were conducted on specimens containing a high—angle, symmetrical tilt—boundary. The load was, in general, applied in shear parallel to the boundary, but a few specimens were tested in tension. It was demonstrated that a large stress concentration can act at certain parts of the boundary and that the locations of this stress concentration may be related to the slip geometry of the component crystals. It is concluded that many of the characteristics of grain boundary sliding, including the progressive boundary hardening which occurs under constant stress, may be understood in terms of the generation of sub—structure which results from the interaction of crystal slip with the boundary. The relevance of these findings to the process of creep deformation and to the nucleation and growth of intergranular cavities is discussed.
Supervisor: Bell, R. L. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available