The antiphase boundary energy of γ precipitates in nickel-based superalloys

Abstract

For a series of γ' strengthened nickel-based superalloys the antiphase boundary energy γ of the γ' particles has been determined by two independent methods: firstly, by measuring the critical resolved shear stress of peak-aged single crystals and, secondly, by determining the minimum size of stable Orowan loops. γ is found to increase with increasing volume fraction f of the γ' particles, that is δγ/δf is positive. This is tentatively interpreted as being due to the formerly observed excess of Cr in the outer layers of the γ' particles. The argument involves two steps: firstly, the Cr excess is assumed to decrease if f is raised, that is δc^γ _Cr/δf<0, where c^γ' _Cr is the Cr concentration in the γ' particles; secondly, on the basis of Miedema's procedures for the calculation of alloy formation enthalpies, it is rationalized that Cr lowers γ, that is δγ/δc^γ' _Cr <0. Since δγ/δf equals.the product of the two latter derivatives, δγ/δf is positive.