On the strengthening of Ni₃Al by hafnium additions

Abstract

To interpret the notable strengthening of Ni₃Al due to Hf additions in the strength anomaly domain, the dislocation features of a 3at.% Hf compound are characterized. Since the general microstructure does not exhibit obvious differences from that observed in similar compounds, the superdislocation core is investigated to find reasons for this effect. Various weak beam conditions are tested which never yield more than 3 peaks for the intensity profiles. The latter are interpreted for the chosen g, ng conditions (with 3 < n < 6) after extensive computer image simulations. The different fault energies related to the core are determined and found to be γ₁₁₁ = 300mJm⁻², γ₀₁₀ = 250mJm⁻² at 300K while γ_CSF exhibits very high values (larger than or equal to 460mJm⁻²). This explains the peculiar dislocation images. A comparison of the flow stress—temperature plots with those corresponding to a binary and a 1at.% Ta compounds confirms that the shifts observed for the flow stress in the anomaly domain and those for the peak temperature can be correlated well with the γ_CSF values, but not with the antiphase boundary anisotropy ratio. γ_CSF appears to be the key parameter for dislocation locking in the strength anomaly domain. Other solid solution strengthening effects operate in addition, without hindering the influence of γ_CSF. This interpretation of the differences in mechanical properties agrees with previous studies on similar compounds, but it is shown to hold even when these differences are large. In addition it is strongly supported by data about dislocation exhaustion rates which are measured in the Hf, the Ta and the binary compounds through repeated load relaxation experiments at 575K. The high ability of superpartials to cross-slip in this large γ_CSF Hf compound also explains the rather large minimum dislocation character observed for dislocations lying on the octahedral plane.