Microstructure and microtexture in powder-extruded monolithic NiAl and NiAl-HfC alloy

The microstructure and microtexture of powder-extruded and subsequently annealed monolithic NiAl and an NiAl-HfC alloy were studied using an Electron Backscattering Pattern system and an image analyzer fully integrated with a scanning electron microscope. A microstructural examination of the as-extruded microstructure shows that during powder-extrusion the NiAl-HfC alloy has much stronger tendency for exhibiting an abnormal grain growth which is manifested by the existence of coarse-grained 'islands' in its microstructure. The as-extruded monolithic NiAl exhibits the dominant [110] and [111], components of a fiber microtexture while its NiAl-HfC counterpart does not display any dominant component. Annealing at the 1273-1473K range results in an increase of the [111] at the expense of the [110] component in both monolithic NiAl and NiAl-HfC. An abnormal grain growth is observed during annealing, particularly profound in the NiAl-HfC alloy. The latter seems to be related to the coarse HfC particles acting as nucleation sites for abnormally growing grains as well as a free-surface of the specimen. Most of abnormally grown grains in the monolithic NiAl mainly exhibit the [112] and some of them [111] orientation while abnormal grains in the NiAl-HfC alloy exhibit mostly the [111] orientation. However, the general changes of texture and orientations of abnormally growing grains after annealing are not accompanied by any measurable changes in the grain boundary misorientation angle distribution. (C) 1998 Elsevier Science Limited. All rights reserved.