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

Abstract

Abstract 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 1273–1473 K 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.