The effects of composition and γ'/γ lattice parameter mismatch on the critical resolved shear stresses for octahedral and cube slip in NiAlCrX alloys

Prototypical single-crystal NiAlCrX superalloys were studied to examine the effects of the common major alloying elements, Co, Mo, Nb, Ta, Ti, and W, on yielding behavior. The alloys contained about 10 at. pct Cr, 60 vol pct of the γ′ phase, and about 3 at. pct of X in the γ′. The critical resolved shear stresses (CRSSs) for octahedral and primary cube slip were measured at 760 °C, which is about the peak strength temperature. The CRSSOct and CRSScube are discussed in relation to those of Ni3 (Al, X) γ′ alloys taken from the literature and the γ′/γ lattice mismatch. The CRSSOct of the γ+γ′ alloys reflected a similar compositional dependence to that of both the CRSSOct of the γ′ phase and the γ′/γ lattice parameter mismatch. The CRSScube of the γ+γ′ alloys also reflected the compositional dependence of the γ′/γ mismatch, but bore no similarity to that of CRSScube for γ′ alloys since it is controlled by the γ matrix. The ratio of CRSScube/CRSSOct was decreased by all alloying elements except Co, which increased the ratio. The decrease in CRSScube/CRSSOct was related to the degree in which elements partition to the γ′ rather than the γ phase.