Study of cooperative grain boundary sliding by using macroscopic marker lines
- PDF / 1,771,244 Bytes
- 4 Pages / 613 x 788.28 pts Page_size
- 96 Downloads / 144 Views
M.G. ZELIN, Postdoctoral Fellow, and A.K. MUKHERJEE, Professor, are with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95626-5294. Manuscript submitted August 1, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
at shear surfaces (Figures 1(d) through (f)) and the fact that the marker lines are still undisturbed in between these shear surfaces indicate that grain groups were sliding as blocks at the shear surfaces. One can see that an average length of marker line segments, into which they were broken by GBS and which reflects an average size of sliding grain groups, decreases when strain increases. (2) Transverse marker lines. Figure 2(a) shows a montage of low magnification SEM micrographs taken from the prepolished surface of the SP-deformed specimen, on which marker lines were drawn perpendicular to the tensile axis. Initially, straight maker lines (see the insert in the upper-left corner in Figure 2(a)) became curved. Such "buckling" of the transverse marker lines owes its origin to sliding of grain groups. The nature of the offsets of macroscopic marker line offsets (arrowed in Figure 2(c)) assumes sliding of grain groups at some grain boundary surfaces. The same directionality in marker line offsets, seen in Figure 2(c), which shows the regions with crosses in Figure 2(b) under higher magnification, confirms CGBS. (3) Oblique marker lines. Figure 3 shows a prepolished surface of the SP-deformed specimen on which marker lines were inscribed at an angle close to 45 deg with respect to the tensile axis. Initially, straight marker lines were disturbed by shear surfaces, traces of which are clearly seen (arrowed in Figure 3). Observations performed at the other prepolished surface, on which marker lines were inscribed at - 4 5 deg with respect to the tensile axis (as schematically shown in the insert given in the upper-left comer in Figure 3), revealed the second system of shear surfaces. The orientation of these shear surfaces is about 45 to 60 deg with respect to the tensile axis. (4) Coarse macroscopic marker lines and tiny marker lines. Figures 4(a) and (b) show a deformed network of marker lines inscribed by using a diamond needle and transverse marker lines inscribed by using diamond paste with particle size 0.5 /~m, respectively. Disturbance of marker lines drawn by a diamond needle reflects macroscopic character of deformation; however, these scratch marks are not sensitive enough to reveal GBS. There is evidence of numerous as well as significant (in amount) of offsets of tiny marker lines drawn by a diamond paste (Figure 4(b)). The net consequence is that it becomes difficult to get a total picture of the general trend in GBS. This indicates that tiny marker lines are too sensitive to GBS: their offsets can be explained not only by CGBS, but also by accommodational GBS that takes place inside sliding grain groups, in order to maintain compatibility between groups of grains. Figures 1 through 3 exhibit clear evidence of sliding of grain groups. Quantita
Data Loading...
