Strengthening of Ni₃Al by Ternary Additions

Abstract

It is important for a number of reasons to have a basic understanding of the mechanisms by which ternary additions strengthen Ni₃Al. First of all, since the basic strength-controlling mechanisms in pure Ni₃Al are different from those in pure metals and substitutional solid solutions, it is expected that the mechanisms of solid solution strengthening will also be different in Ni₃Al. Secondly, such an understanding will provide valuable insights into the properties of nickel-base superalloys in which Ni₃Al is a key constituent. In addition, since new alloys based on an ordered Ni₃Al matrix are being developed, it is important to understand the strengthening mechanisms in such alloys. In the present study, flow stress measurements have been performed on single crystals of Ni₃Al containing additions of Hf and Ta, and on binary Ni rich, Ni₃A1 used as a reference alloy. The data have been collected over a wide range of temperatures, for different orientations within the unit triangle, and as a function of the sense of the applied uniaxial stress. The effect of such additions on the critical resolved shear stress (CRSS) for octahedral slip has been determined and combined with previous data. An attempt is then made to clarify whether a lattice parameter/modulus mismatch effect or a dislocation core effect is the dominant mechanism for the strength increase with compositional changes. It appears that a lattice parameter/modulus mismatch is the dominant mechanism for orientations in which the tension/compression flow stress asymmetry disappears.