The effect of small plastic deformation and annealing on the properties of polycrystals: Part I. Experimental observatio
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I.
INTRODUCTION
D U R I N G plastic deformation of polycrystals, lattice dislocations are trapped by grain boundaries. If the temperature of deformation is lower than 0.2 Tm for pure metals and 0.5 Tm for alloys, then these dislocations retain their identity and their contrast is visible in the TEM. Such dislocations have been termed as Extrinsic Grain Boundary Dislocations (EGBD's). ul In situ TEM observations, for example References 2, 3, and 4, indicate that on annealing at higher temperatures, the image of the EGBD's on random boundaries widens and eventually disappears. It has been shown that the kinetics of the widening process is controlled by grain boundary diffusion.[a] The driving force for this process is the decrease in the elastic energy of EGBD's which is believed to result either due to the delocalization or spreading of the c o r e [3] or due to the dissociation of EGBD's into partials with small Burgers vectors, tS~ From the above, it can be concluded that the instability of EGBD's at higher temperatures leads to changes in the structure of random grain boundaries. Assuming that the boundaries in a material prior to deformation are in an equilibrium state of low energy, one can expect that after the absorption of EGBD's the grain boundaries attain a higher energy metastable state. Therefore, grain boundaries containing EGBD's have been termed as nonequilibrium grain boundaries, t31It is known that these nonequilibrium boundaries exhibit properties different from those observed for grain boundaries without EGBD's (termed as equilibrium grain boundaries), tri Some experimental observations tTI on the recovery of yield stress suggest that the energy state of boundaries may play an important role in the deformation of polycrystals. It has been shown [7] that the annealing of prestrained specimens of an austenitic stainless steel results in significant decrease of yield stress. Such a drop in the yield stress in a high stacking fault energy material would be normally ascribed to recovery by cross slip. However, as discussed in more detail later, decrease in the internal stress by recovery in micrograined 316L steel is not significant due to the difficulty of cross slip after small prestrains and at annealK.J. KURZYDLOWSKI, Research Associate (on leave of absence from Institute of Materials Science and Engineering, Warsaw University of Technology, 02-524 Warszawa, Narbutta 85, Poland), S. SANGAL, Graduate Student, and K. TANGRI, Professor of Metallurgy, are with the Department of Mechanical Engineering, University of Manitoba, Winnipeg, MB, Canada, R3T 2N2. Manuscript submitted January 4, 1988.
METAI.I.URGICAL TRANSACTIONS A
ing temperatures employed in this study. As such another mechanism must be sought for explaining the experimental observation. Therefore, it has been postulated that the recovery of the yield stress observed in austenitic stainless steel is the result of transformation of nonequilibrium grain boundaries to their equilibrium state which in turn is due to the annihilation of EGBD's
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