The grain boundary migration in Ag induced by thermal strain
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The Grain Boundary Migration in Ag Induced by Thermal Strain CHA-YOUNG YOO, SANG-CHUL HAN, and DUKYONG YOON The recrystallization of a deformed solid occurs by nucleation and growth of strain-free grains. When the deformation is relatively small (on the order of about 5 pct), the existing grain boundaries can migrate, leaving behind them strain-free regions. Such a strain-induced grain boundary migration has been observed in aluminumtll and other metalsY ,3] The thermal stress during rapid cooling can produce
CHA-YOUNG YOO, formerly Graduate Student, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, is Associate Staff Researcher, Samsung Electronics Co., Ltd., Suwon 449-900, Korea. SANG-CHUL HAN, Postdoctoral Student, and DUK-YONG YOON, Professor, are with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejon 305-701, Korea. Manuscript submitted November 28, 1994. 304~-VOLUME 26A, NOVEMBER 1995
Fig. 1--The microstructure of an Ag specimen quenched after annealing at 800 ~ for 48 h and heat treated at 800 ~ for 1 h in N z atmosphere after rapid heating. (The arrows indicate the initial grain boundary locations marked by chemical etching.)
a plastic strain at the surface of a soft polycrystal, and such a thermally produced strain can also induce grain boundary migration. The purpose of this work is to test experimentally such a possibility in Ag. The Ag specimens were made by melting Ag pieces of 99.99 pct purity at 1000 ~ in N2 atmosphere. The cast ingot was cold-rolled to a thickness of 4 mm and cut into approximately 1 cm 1 cm plates. After annealing at 800 ~ in N 2 for 48 hours, the specimens were either quenched in water or slowly cooled to room temperature for 30 hours at about 24 ~ After electropolishing, the specimens were etched in a solution of 5 mL H202 and 40 mL NH4OH in order to mark the grain boundaries. After this annealing treatment, the average grain diameter was about 200 /zm. When a specimen that was quenched after the annealing treatment was rapidly heated to 800 ~ by pushing into the center of a preheated tube furnace and held for 1 hour in N2 atmosphere, the grain boundaries were observed to migrate, as shown in Figure 1. The initial grain boundary locations marked by chemical etching are indicated by arrows. The final grain boundary locations marked by thermal grooving were fainter. As can be seen in Figure 1, the twin boundaries were either extended or obliterated with the grain boundary migration. A specimen heat treated in Oz atmosphere with the same temperature cycle also showed as grain boundary migration of approximately the same magnitude. When a specimen that was quenched after the annealing treatment was rapidly heated to 520 ~ and held for 1 hour, the grain boundaries again migrated, but the migration distance was much shorter than that obtained after holding at 800 ~ for 1 hour (Figure 1). When the holding time at 520 ~ was increased to 7 hours, the migration distance was a
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