Overview No. 1: The nucleation of cavities by plastic deformation

Over the past 15 years important advances have been made in the experimental study of the microstructural changes occurring during the non-linear steady-state creep of single phase crystalline matter at elevated temperatures. Curiously, although the results of these painstaking studies have gone a long way toward elucidating the mechanism of this phenomenon, they have been largely ignored in favour of some simple dislocation mechanisms that are not only inconsistent with these observations, but are also unable to describe correctly the known phenomenology. This review concentrates primarily on the recent experiments on microstructural alterations occurring during creep; however, it also surveys the many mechanistic models that attempt to describe this phenomenon, and finds them all deficient.

Steady-state creep of single-phase crystalline matter at high temperature

Recent experimental data of the stress/creep rate relationship for precipitation-hardened alloys have been surveyed. These materials display typically a higher stress-sensitivity of the creep rate than is observed for pure metals, and solid solutions. Furthermore, their stress/creep rate curves show pronounced breaks, with a stress-sensitivity of the creep rate, ∂ ln έ/∂ ln σ, of ∼4 below the break. The precipitation dispersion affects the creep rate such that it reaches a minimum at an intermediate interparticle spacing, with higher rates for both larger and smaller spacings. Based on the principles of recovery creep, a creep theory for precipitation-hardened alloys has been described, leading to a relationship of the type έ= A(έ-έp)4, where έp represents a back stress due to the particle dispersion. At high creep stresses έp is constant and defined by the stress to operate a particle-cutting mechanism or the Orowan mechanism. At lower stresses the dislocations are able to bypass the particles, s...

The stress/creep rate behaviour of precipitation-hardened alloys

Metals often fail in service under creep conditions because of the formation of cavities on the grain boundaries which are approximately normal to the applied stress. This phenomenon of creep cavitation is becoming of increasing technological importance. As a result a complete understanding of it is desirable so that alloys with improved cavitation resistance can be designed. This paper reviews the development of our present understanding of the phenomenon which is one of nucleation, growth and linkage leading to failure. Several mechanisms of nucleation, such as at grain-boundary ledges or precipitates, have been postulated and experimental evidence in support of each has been cited. Similarly, deformation- or vacancy-controlled growth mechanisms have been discussed. It is apparent from the literature thatno single mechanism is applicable, indeed, the work discussed here suggests that several mechanisms may operate and each may become dominant at different stages of the creep life. Finally, the status of research into nickel-base superalloys is reviewed with reference being made to such effects as regenerative heat-treatment.

Cavitation in creep

Importance of grain boundary sliding to creep intergranular fracture is focussed. Previous metallographic and fractographic studies of creep intergranular fracture on metal bicrystals and polycrystals are briefly reviewed in order to show the close relationship between grain boundary sliding and fracture. Deformation ledge and migration irregularity are shown to be potential sites of stress concentration and crack nucleation on sliding grain boundaries without particles. The effect of grain boundary structure on creep intergranular fracture is discussed on the basis of the effect of grain boundary structure on sliding, the contribution of sliding to the overall creep deformation, and a sliding-fracture diagram. Recent observations of the effect of grain boundary structure on creep intergranular fracture on alpha iron-tin alloy polycrystals are shown.

Grain boundary sliding and stress concentration during creep

The steady creep rate, e s , for polycrystalline copper at 686, zinc at 385, and iron at 923 K varies as the nth power of the applied stress (σ). The value of n was 4.8 for copper and zinc but was 8.3 for iron. Stress-relaxation tests and constant-stress tests in which the stress was progressively reduced gave values of a ‘friction’ stress, σ0, such that for all materials examined ɛ ˙ s ∝ ( − σ 0 ) 4 In all cases, decreasing the stress by a small amount, (∆σ, ≃ 0.1 σ) during steady-state creep resulted in an incubation period of zero creep rate, after which the creep rate increased to a new steady value. When the stre...

Stress-Change Experiments during High-Temperature Creep of Copper, Iron, and Zinc

An equation to represent strain/time relationships during high temperature creep

The true and apparent angular distributions of grain boundary cavities have been measured for OFHC copper deformed over a range of creep rates at 415°C. No effect of strain rate was observed, the f...

P. W. Davies

Papers

Stress-Change Experiments during High-Temperature Creep of Copper, Iron, and Zinc

An equation to represent strain/time relationships during high temperature creep

On the distribution of cavities during creep