Impurity and Stoichiometry Effects in the Preparation and Mechanical Properties of Intermetallic NiAl Single Crystals

Abstract: NiAl single crystals with small deviation from stoichiometry and low impurity content were grown by a modified Bridgman technique. The starting material was melted in a vacuum induction furnace with a composition slightly hyper-stoichiometric in Al. The adjustment of stoichiometry was effected by preferred A1 evaporation during a preceding vacuum treatment of the melt in the Bridgman apparatus. Theoretical and experimental investigations of the Ni and Al distribution in the single crystals revealed macro-segregations with increasing concentration of the surplus element in the length direction as well as near the crystal surface. Crystals with low deviation from stoichiometry had a nearly constant composition over most of their length and significant macro-segregations only in the last-solidifying part, whereas crystals with larger deviations from stoichiometry showed a gradual increase in concentration of the surplus element with increasing slope as a function of the length co-ordinate. The macro-segregations caused corresponding variations of the microhardness HV0.05 in both the axial and radial directions. Room temperature compression tests of soft-oriented single crystals (sample axes in the <I l l>-, <123>- and <112>-direction) exhibited values of yield strength and critical resolved shear stress comparable with those of high purity single crystals reported in the literature and ductilities of the order of 225 %. 1