Local texture changes associated with grain growth

Abstract

Recently it has been shown that grain growth is accompanied by macroscopic texture changes. Such changes will necessarily produce modifications in the proportions of ‘special’ grain boundaries and consequently may affect significantly the kinetics of grain growth and particularly anomalous growth. After new contributions from computer modelling processes are taken into account, it becomes clear that the onset of grain growth for a fixed driving force is a complex function of particle pinning, effects from solutes, surfaces and residual strains, grain-size distribution and texture-boundary mobility. The aim of this paper is to provide insight into the relation between grain texture-grain misorientation texture and anomalous grain growth in the alloy Nimonic PE16 where, for the particular heat-treatment conditions employed, particle pinning may be ignored. Macroscopic orientation measurements reveal only the overall texture; the present work considers microstructural regions of anomalous growth and texture measurements arecollated on a grain-specific basis (microtexture). From this grain-specific data the grain-misorientation textures (GMT) (i. e. the grain-boundary counterparts of an inverse pole figure) are also computed. The experimental results demonstrate that two types of microtexture may exist for different regions of anomalous growth within the same specimen. These differences are rationalized by noting that further grain growth-inducing heat treatments cause a change from the first texture to the second. Clear evidence is thus provided that there are micro textural changes associated with anomalous grain growth. The grain-boundary textural data show some deviation from a random distribution, particularly for boundaries between small grains (SS boundaries) as compared to boundaries between large and small grains (LS boundaries). These differences between the SS and ls groups extend to the proportions of geometrically special boundaries in each group, with the SS group containing a higher proportion of special (therefore higher than average mobility and lower energy) boundaries than the ls group. This trend was particularly apparent for boundaries that fell into the second texture group. The implication from these results is that in this case anomalous growth will tend to stagnate because the boundaries of small grains surrounding the large grain (SS boundaries) are on average of lower energy than the boundaries that border the large grain (LS boundaries). Finally, it is suggested, by analogy with primary recrystallization, that the definition of grain growth in terms of an energy-reduction criterion should be extended to encompass the sum of all available sources of energy minimization. Hence both grain-boundary textural rearrangements and grain-boundary migration are included.