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The creep behaviour of nickel under cyclic loading has been studied, the aim being to relate the creep strain to the metallurgical recovery that occurs during each load reduction. Square-wave loading between stresses σ1 (40 or 80 N/mm2) and σ2 (0–σ1) for times t 1 (0.01–0.1 h) and t 2 (0.1–20 h) was applied at 650 or 750° C, and a point of principle was to apply the full stress σ1 initially until the metal was fully work-hardened so that all subsequent changes could be ascribed to recovery. The strain per cycle consists of three components, which could be expressed in terms of experimental conditions: (1) ∆e1, the normal strain in time t 1 at a stress σ1; (2) ∆e2, the strain in time t 2 at stress σ2; e2/e1 = (σ2/σ1)m, where m = 10 at 650° C and 5.2 at 750° C; (3) ∆er, the extra strain on reloading from σ2 to σ1; ∆er = bt 2⅔ + c, where b and c have been determined. The factor c can be negative, probably because strain-ageing occurs. A consequence is that three functions are needed to describe accur...

Zones free of γ′ precipitate formed adjacent to grain boundaries in the gauge-lengths of specimens of Ni–20 % Cr-base alloys subjected to creep tests or slow-strain-rate tensile tests in the temperature range 750–850°C (1023–1123 K) have been studied. Electron-probe microanalysis did not reveal any significant change in composition between the precipitate-free zones and the grain interior. Consideration of the variation in the stress-dependence of minimum creep rate with temperature and grain size indicates that a diffusion creep mechanism may explain the formation of the precipitate-free zones.