Effects of Boron and Sulfuro on the Ideal Yield Stress of Ni₃Al—A First-Principles Approach

Abstract

The past two decades have witnessed dramatic growth in both the experimental and theoretical activity in intermetallic compounds and ordered alloys, because of the potential use of these systems in a wide variety of technological applications.¹ Of central interest are their high melting temperature, low density, resistance to oxidation,² and the increase of yield stress it displays with increasing temperature,³ in contrast to conventional compounds or disordered alloys. However, as with many other inter-metallics, an inherent drawback to using polycrystalline ordered stoichiometric Ni₃Al alloys as a structural material is the tendency to undergo brittle intergranular fracture,⁴ even though single crystals of Ni₃Al are highly ductile, having an elongation of 20%. Microalloying studies have shown² that doping with certain impurities, which prefer to segregate towards grain boundaries, can significantly improve the ductility of polycrystalline Ni₃Al.³ At room temperature, the addition of only 0.05 wt% (0.25 at%) of boron increases elongation of polycrystalline Ni³Al from a few percent to values of 45–50%. On the other hand, impurities such as sulfur or phosphorus act as strong embrittlers.⁵