The compositional dependence of antiphase-boundary energies and the mechanism of anomalous flow in Ni₃ Al alloys

Abstract

The theory of the anomalous flow behaviour of Ll₂ compounds has developed over the last 30 years. This theory has a foundation in the early estimates of the crystallographic anisotropy of antiphase-boundary (APB) energy in these compounds. In spite of this critical aspect of the theory, it is only in the last 5 years that electron microscopy has been employed to quantify the APB energies and to determine the detailed nature of dislocation structures at each stage of deformation. The present study examined binary Ni₃Al single crystals having compositions which span the single-phase region, and selected ternary compositions expected to have a dominant influence on either {001} or {111} plane APB energies. Crystals were deformed in compression at an orientation near the [001] direction over the temperature range from — 196 to 700°C. Weak-beam dark-field microscopy was employed to quantify the APB energies and the APB energy anisotropy over the composition range. Experimental uncertainties associated with the weak-beam dark-field technique were evaluated with regard to their influence on determining the chemical dependences of the fault energies. Results from the deformation experiments suggest that the mechanism governing the increase in flow stress with temperature is operative at temperatures as low as — 196°C. The findings indicate a lack of correlation between the APB energy anisotropy ratio and the strength of the alloy. Further, little correlation exists between the magnitude of the individual fault energies and the flow stress.