The role of planar fault energy in the yield anomaly in L12 intermetallics

Abstract

We have used ab initio quantum mechanics to calculate planar fault energies in a number of L12 intermetallic compounds. Used in conjunction with a diagram of Saada and Veyssiè showing elastic and antiphase-boundary anisotropies we show that such calculations may be used to predict the ocurrence of the yield strength anomaly. The less common ‘normal’ yield behaviour of Pt3Al is shown to have a different origin from what was previously thought. We are able to make predicted estimates of the size of the dissociated core of screw superdislocations. The nature of the activated process is also discussed. Microscopic factors determining fault energy are described, and we separate faults into those whose energy is chemical and those whose energy isstructural in origin. Use is made of the axial Ising model to analyse structural fault energies. We find a marked difference between the properties of Ni₃Si and Ni₃Ge and show that this arises because the bonding is more ionic in Ni₃Si and more covalent in Ni₃Ge. In spite of these wide differences, both show a strong yield anomaly.