Preparation, Deformation and Fracture of Stoichiometric NiAl Single Crystals

Abstract

ABSTRACTA modified Bridgman method involving vacuum treatment prior to the growth process was developed for preparation of high-accuracy stoichiometric NiAl crystals. Single crystals with stoichiometry deviations < 0.2 wt.% Ni characterized by residual resistance ratios R298K/R4.2K > 4 have been prepared. An improved gravimetric analysis ensured the best accuracy among the various wet chemical analysis methods tested and a total error less than 0.1 at.% for Ni determination in NiAl. The stoichiometric NiAl single crystals exhibit high compressive strain in <123> crystallographic axis of the order of 20 % in the as-grown state and 40 % after annealing 1 h at 1073 K. The critical resolved shear stress was reduced from 104 MPa to 45 MPa by annealing. In 3-point bend tests tensile strains to failure up to 10 % have been achieved. Microscopic oxide inclusions near the sample surface were identified as the cause of crack initiation in bent samples.