Grain boundary character distribution in B2 intermetallics

This paper reports the results of the experimental studies of the effect of chemical composition, texture and processing methods on the grain boundary character distribution evaluated by the electron back-scatter diffraction pattern (EBSD or EBSP) technique, in B2 FeAl and NiAl intermetallic compounds. An alternative method based on the grain boundary surface area for calculating the unbiased fraction of grain boundaries of a given character (Sigma CSL) is proposed. It is shown that the proposed method gives different results than the classical method based on the counting of grain boundary segments. It is found for both FeAl and NiAl that the fraction of low-angle boundaries (Sigma1 LABs) increases with increasing percentage of the <100> and to a lesser extent the <111> texture, up to the upper limit of similar to 20%. In B2 FeAl the fraction of so-called "special" grain boundaries (SGBs) (Sigma3-29) seems to be independent of the percentage of the <100> texture. In B2 NiAl and NiAl+2wt.%HfC alloy the fraction of SGBs decreases continuously with increasing percentage of the <111> texture. The <110> texture does not have strong effect on LABs. Processing of B2 FeAl by shock-loading and subsequent annealing can increase the fraction of LABs to 90-97%. This effect is not observed in B2 NiAl. Instead, the premature abnormal grain growth occurs, accelerated by the accumulated shock strain energy. In B2 compounds the fraction of LABs seems to increase nearly linearly up to the limit of similar to 20% with decreasing grain size from similar to 400 to similar to 100 mum. For grain sizes smaller than similar to 100 mum the fraction of LABs seems to be independent of grain size. The fraction of SGBs does not exhibit any dependence on grain size. (C) 2000 Elsevier Science Ltd. All rights reserved.