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 Al 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 <111>, <123> and <112>) 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 ≥25%.