Polycrystalline γ-γ′ superalloys with varying grain sizes and unimodal, bimodal, or trimodal distributions of precipitates have been studied. To assess the contributions of specific features of the microstructure to the overall strength of the material, a model that considers solid-solution strengthening, Hall–Petch effects, precipitate shearing in the strong and weak pair-coupled modes, and dislocation bowing between precipitates has been developed and assessed. Cross-slip-induced hardening of the Ni3Al phase and precipitate size distributions in multimodal microstructures are also considered. New experimental observations on the contribution of precipitate shearing to the peak in flow stress at elevated temperatures are presented. Various alloys having comparable yield strengths were investigated and were found to derive their strength from different combinations of microconstituents (mechanisms). In all variants of the microstructure, there is a strong effect of antiphase boundary (APB) energy on strength. Materials subjected to heat treatments below the γ′ solvus temperature benefit from a strong Hall–Petch contribution, while supersolvus heat-treated materials gain the majority of their strength from their resistance to precipitate shearing.

Strengthening Mechanisms in Polycrystalline Multimodal Nickel-Base Superalloys

The influence of the fault energies on the anomalous mechanical behaviour of Ni3Al alloys

Modelling of the influence of alloy composition on flow stress in high-strength nickel-based superalloys

From dislocation cores to strength and work-hardening: a study of binary Ni3Al

The processes controlling the dislocation mobility in micro-alloyed two-phase γ-titanium aluminides have been investigated over a wide temperature range by determining activation parameters of plastic deformation and TEM observations. The deformation behavior of the materials is characterized by a relatively high athermal stress component due to dislocation interactions with grain boundaries and lamellar interfaces. The glide resistance of the dislocations is controlled by several processes. At room temperature, the mobility of ordinary dislocations is determined by a combination of localized pinning and lattice friction. Additional glide resistance arises from nonconservative processes at jogs in screw dislocations and leads to a thermal contribution to work hardening. Dislocation climb processes start above 900 K and seem to initiate the transition from brittle to ductile material behavior.

Thermally activated deformation mechanisms in micro-alloyed two-phase titanium amminide alloys

The strain rate sensitivity (SRS) β = δτ/δ (In σ) of the flow stress τ, has been measured for crystals of Ni3(Al, Hf)B over a range of temperatures T in the yield stress anomaly up to 500 K, and as a function of strain E and of crystal orientation. β obeys a Cottrell-Stokes low when plotted against τ - τy, where τy, is the yield stress at 0·01% strain. The slopes are about 1%, decrease with increasing T and are approximately independent of orientation. The results show that the yield stress and work-hardening rates (WHRs) are controlled by different mechanisms. The flow stress τ=τy + τh, where τh is due to work hardening. τy is a reversible function of T, but independent of pre-strain ∊ and σ τh is a function of ∊, T and σ; it controls the SRS of τ. At yield, both edge and screw dislocations propagate on (111), the latter via lateral glide of edge character superkinks which bypass screws locked by cross-slip on to (010), allowing source operation and slip lines to be formed rapidly. Work hardenin...

S. S. Ezz

Papers

The strain rate sensitivity of the flow stress and the mechanism of deformation of single crystals of Ni3(Al Hf)B