The mechanical properties of Ni3AI (γ’) are first reviewed, with particular emphasis on the flow-stress and creep properties. It is shown that the flow stress of Ni3AI has a very unusual temperature dependence: it increases with increasing temperature. This behaviour is shown to be related to the nature of the dislocation dissociation in Ni3AI, the increase in flow stress being the result of thermally activated cross-slip from {111} planes to {010} planes of ½(110) screw dislocations. This model is shown to be in agreement with the observed changes in the flow stress with changes in test temperature, sample orientation, and sense of the applied stress. The effects of compositional changes on the flow stress are also discussed. Stage II work hardening, strain-rate effects, and the creep deformation of γ’ are reviewed and shown to be only poorly understood. The flow strength of γ/y’ alloys is then discussed, and it is shown that many of the anomalies in the behaviour of pure γ’ are also found in γ/γ...

Mechanical properties of Ni3AI and nickel-base alloys with high volume fraction of γ'

An experimental analysis of the γ ↔ γ + γ ′ phase transformations in two single-crystal Nickel-based superalloys is presented. The transformations are characterised macroscopically by differential thermal analysis and dilatometry measurements and microscopically by qualitative and quantitative metallography of samples quenched during various thermal cycles. This fully covers the large changes in precipitate morphology and volume fraction encountered in the upper temperature transformation domain (between 1100 and 1300°C). It is shown that dissolution and precipitation processes are strongly influenced by the local elastic distortions resulting from the lattice mismatch between the two phases. The complex microstructure evolution observed is explained by considering the successive equilibrium shapes of a coherent precipitate growing in more or less strong interaction with its neighbors.

Precipitation and dissolution processes in γ/γ′ single crystal nickel-based superalloys

Apparent and effective creep parameters in single crystals of a nickel base superalloy—II. Secondary creep

Interrupted tensile tests were conducted to fixed plastic strain levels in 100 ordered single crystals of the nickel based superalloy PWA 1480. Testing was done in the range of 20 to 1093 C, at strain rate of 0.5 and 50 percent/min. The yield strength was constant from 20 to 760 C, above which the strength dropped rapidly and became a strong function of strain rate. The high temperature data were represented very well by an Arrhenius type equation, which resulted in three distinct temperature regimes. The deformation substructures were grouped in the same three regimes, indicating that there was a fundamental relationship between the deformation mechanisms and activation energies. Models of the yielding process were considered, and it was found that no currently available model was fully applicable to this alloy. It was also demonstrated that the initial deformation mechanism (during yielding) was frequently different from that which would be inferred by examining specimens which were tested to failure.

C. Carry

Papers

Direct observation of 〈110〉{110} slip in FCC single crystals of a nickel base superalloy