Application of mechanical state relations at low and high homologous temperatures
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and recovery are included in the model, they a r e not incorporated in description of the relaxation data. In o r d e r t o develop the experimentally observed temperature dependent curvature, t/art's model requires the operation of at l e a s t two parallel rate-dependent mechanisms. When difficulty was encountered in a p plying that model t o load relaxation data for some metals, the discrepancy was attributed t o the p r e s ence of transient components which could obscure the true plastic state.~,1° At h i g h e r temperatures (T//Tm < 0.5) t r a d i tional interpretations of mechanical behavior have a s s u m e d that microstructural changes will o c c u r as a consequence of time-dependent t h e r m a l recovery. If the structure changes d u r i n g load relaxation, the experiment cannot be utilized t o characterize r a t e dependent plastic behavior in a given mechanical state. Explanation of some s t r e s s relaxation results in t e r m s of recovery provides an alternative and possibly more physical interpretation than does H a r t ' s m o d e l of constant-structure rate-dependent plastic i t y . n Recently, U. F. Kooks12 proposed a model of rate-dependent plasticity in which structural changes a r e included by incorporation of an evolutionary structure variable into the power law description of dislocation dynamics. The model was shown t o be reasonably successful~3 in reproducing H a r t ' s data9 for relaxation in pure aluminum at intermediate temperatures. Consider Kooks' explanation of relaxation in t e r m s of recovery 13 together with our study of the statefunction nature of the power law when structure is constant:4'v t h e r e e m e r g e s a b a s i s for hope that equation-of-state concepts can be applied to metals undergoing significant structural changes. A thorough study of the strain-rate sensitivity of a single m e t a l over a wide r a n g e of temperature with the objective of obtaining equation of s t a t e information has only very recently been reported. ~4'1s In that s t u d y , g r e a t care was taken t o ensure that the structure remained constunt during s t r e s s relaxation. The present study was formulated to determine the extent to which r e l a x a tion data can be described by the power law; at low temperatures the structure is expected t o be constant, and at high temperatures the structure is expected t o change due t o recovery d u r i n g relaxation. We b y pass the experimental difficulties associated with very-high absolute-temperature testing by selecting VOLUME 8A, APRIL 1977-577
an alloy (50 pct Sn-50 pct In) which has a low absolute melting temperature. The alloy nevertheless is of practical interest for glass-to-metal sealing, and as such, the information generated in this study should also be of v a l u e in determination of mechanical stresses in joints fabricated from this alloy. The low melting temperature (390 K) permitted temperature control by immersion of the specimens in various liquid baths, and in this manner w e were able to obtain load relaxatio
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