Microscopic shear localization in nickel
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I.
INTRODUCTION
A number of recent publications have established that large strain plastic deformation produces localized deformation in the form of microbands and shear bands.It-T] In recent work by Korbel et al.,14J crystallographically oriented microbands were seen to be precursor to formation of noncrystallographic shear bands which propagate across several grains. Shear band formation accelerates premature failure during deformation processing.[~'6] The post-working deformation properties should be enhanced by reducing the extent of localization during processing. One possible deformation scheme to accomplish this is slightly elevated temperature working. It has been proposed that the formation of microbands is a result of dynamic recovery which occurs concomitant with strain and results in both local softening and local increase in strain rate. [7] The present study was performed on nominally pure nickel to follow microstructural changes with strain and working temperature (room temperature and slightly elevated temperature rolling). Features of interest were microstructure, thermal stability, and texture evolution. It is proposed that the ease with which cross slip occurs and high vacancy concentration are critical factors for the formation of well-defined microbands. II.
EXPERIMENTAL
The material used in this study was 12.7 mm diameter nickel bar. The chemical analysis was 0.12MR, 0. l i F e , 0.11Co, 0.08Mn, 0.07Ca, 0.02A1, 0.02Si, 0.006Cr, bal Ni (wt pct). The initially equiaxed grain size was 80/zm, measured by the linear intercept method. Rolling histories are listed in Table I. The initial sample thickness was varied to produce a 1.6 m m final thickness independent of rolling strain. Samples worked at elevated temperature (200 ~ and 300 ~ were preheated for 30 minutes before rolling. They were reduced in thickness approximately 10 pct per pass with 5 minutes reheating at YOUNG WOO KIM, formerly Graduate Research Assistant with Materials Science and Engineering Program, The University of Texas, Austin, TX 78712, is with Advanced Engineering and Research Laboratory (Mabook-Ri. Lab), 140-2, Ke-dong, Jongro-Gu, Seoul, 110 Korea. D.L. BOURELL is Associate Professor in the Center for Materials Science and the Department of Mechanical Engineering, ETC 5.160, The University of Texas, Austin, TX 78712. Manuscript submittedJuly 31, 1987. METALLURGICALTRANSACTIONSA
the rolling temperature between each pass. The roll speed was increased between passes to fix the mean strain rate at 5 s -l, as described elsewhere, tsl These samples were postheated for 10 minutes after the final pass, then air cooled. The total true thickness strain accumulated during rolling was calculated as the natural logarithm of the ratio of final to initial thicknesses. T r a n s m i s s i o n electron m i c r o s c o p y ( 2 0 0 K V J E O L 200CX) was used to analyze thin foils of the rolled nickel. The foils were prepared parallel to the rolling plane by twin jet polishing in a mixture of 1 : 3 nitric acid and methanol at - 10 ~
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