Grain boundary faceting and abnormal grain growth in nickel

Abstract

A correlation between grain boundary faceting and abnormal grain growth has been observed in recrystallized polycrystalline Ni at varying annealing temperatures, with or without C added. Carburized Ni specimens deformed to 50 pct show faceted grain boundaries and abnormal grain growth when annealed at temperatures below 0.7 T m , where T m is the melting point of Ni in absolute scale. When annealed at or above 0.7 T m , the grain boundaries are smoothly curved and, therefore, have a rough structure, and normal grain growth is observed. In the specimens annealed in vacuum without carburization, all grain boundaries are faceted at 0.55 T m , and some of them become defaceted at higher temperatures. The specimens annealed in vacuum at temperatures between 0.55 and 0.95 T m show abnormal grain growth. When the grain boundaries have a rough structure and are, therefore, nearly isotropic, normal grain growth is indeed expected, as shown by the simulation and analytical treatment. When all or a fraction of the grain boundaries are faceted, with the facet planes corresponding to the singular cusp directions in the variation of the boundary energy against the inclination angle, abnormal grain growth can occur either because some grain boundary junctions become immobile due to a torque effect, or the growth occurs by a step mechanism.

Temperature Dependence of Grain Boundary Structure and Grain Growth in Bulk Silicon-Iron.

“…Normal grain growth occurred at these temperatures. This correlation between the grain boundary structure and grain growth is similar to those found in other metals [13][14][15][16][17][18] and oxides. 19,20) The results of this study thus confirm that AGG can arise from the singular grain boundary structure without any precipitates, surface energy anisotropy, or texture.…”

“…It is likely that these grain boundaries were actually rough with curved shapes at the annealing temperatures and the fine scale h&v structures formed during cooling. If the grain boundaries undergo roughening-singular transitions, their structures at low temperatures will depend on the cooling rate.The correlation between the grain boundary roughening transition and the change of grain growth mode from AGG to NGG is also consistent with the previous observations in metals 13,14,[16][17][18] and oxides. 19,20) As proposed earlier, 31,32) if the singular grain boundaries migrate by the movement of existing steps or those produced by two dimensional nucleation, [21][22][23][24][25][26] v(F) will be non-linear as observed by Tsurekawa et al 30) In polycrystals with singular grain boundaries, non-linear v(F) will cause AGG as confirmed by simulation.…”

“…As pointed out earlier, [13][14][15][16][17][18][19][20]31,32) some flat segments of the h&v grain boundaries are likely to be singular corresponding to the cusps in the grain boundary Wulff plot. 33) The kinked shapes are the limiting cases of the h&v shapes, because the h&v shapes can evolve to the kinked shapes by a coarsening process.…”

“…It was recently observed that AGG and normal grain growth (NGG) in bulk polycrystalline metals [13][14][15][16][17][18] and oxides [19][20] were related to grain boundary structures. At low temperatures or in the presence of additives, grain boundaries had hill-and-valley (h&v) shapes with singular segments and AGG occurred.…”

“…At high temperatures or with different additives, all grain boundaries were defaceted and NGG was observed. 13,14,[16][17][18][19][20] The occurrence of AGG was attributed to the step growth of singular grain boundaries. [21][22][23][24][25][26] The purpose of this work is to determine experimentally if the same correlation between the grain growth and grain boundary structure exists in pure Si (about 3 wt%) iron at different heat-treatment temperatures.…”