G. W. Greenwood

The threshold stress for creep controlled by dislocation-particle interaction

Abstract A model based on the dissociation of lattice dislocations into interfacial dislocations, when they enter the matrix-particle interface for climb bypass, is proposed to explain the attractive dislocation-particle interaction during the creep of dispersion-strengthened alloys. This model predicts a threshold stress for creep which depends on the particle radius, the interparticle spacing and the reduction in dislocation self-energy because of dissociation. A minimum energy reduction of about 36% is required through the dissociation to offset the increase in line length energy because of the local climb configuration. There is reasonable agreement between theoretical predictions and experimental data on threshold stresses. The model is also supported by transmission electron microscopy evidence of departure-side pinning and contrast at the matrix-particle interface.

An analysis of the effect of multiaxial stresses and grain shape on Nabarro-Herring creep

Abstract The rate of Nabarro-Herring creep is known to depend strongly on grain size and shape since it involves the diffusion distances of vacancies between their sources and sinks. The anisotropy of creep rate by this mechanism is thus apparent in materials in which grains are not equiaxed. An analysis is presented which shows that the response to multiaxial stress is also strongly influenced by such anisotropy. From the analysis, formulae are derived which permit the creep rate along three axes parallel to the three principal stresses to be calculated for orthorhombic grains of different shapes.

On the transition between dislocation and diffusion creep

Abstract It has become common to represent different creep mechanisms as areas on ‘maps’ with sharp boundaries delineating such areas and indicating the conditions of predominance of each mechanism. These boundaries represent transition stages and the present contribution attempts to clarify the nature of the transition between dislocation and diffusion creep. It is concluded that dislocation- and diffusion-creep mechanisms may occur simultaneously over a wide range of strain rate, but in distinct regions of each grain, and that the form and extent of their individual contributions when combined in a physically plausible manner lead to a gradual change in stress exponent. Two consequences arising from this approach are that new methods of extrapolation of creep data become apparent and that the relative contributions of dislocation and diffusion creep may conceivably be identified by the regions which become free of particles in certain materials.

The effect of stress orientation on the formation of precipitate free zones during low stress creep

Abstract The variation in orientation of precipitate free zones at grain boundaries in the neighbourhood of drilled holes is shown to be related to the change in direction of the tensile stress over these regions in support of the formation of these zones by a diffusional creep mechanism.

On the threshold stress for diffusional creep in pure metals

Abstract A temperature-dependent threshold stress [sgrave] is often observed for diffusional creep in pure metals and existing experimental data are examined to identify the factors which affect it. The analysis indicates that the threshold stress is inversely proportional to the stacking fault energy. A temperature dependence, characterised by an energy of 22-5 kJ mol−1, is obtained by incorporating the effect of stacking fault energy as well as elastic modulus and Burgers vector in normalising [sgrave]. A model based on grain boundary dislocation climb by jog nucleation and movement is proposed to account for the origin of threshold stress and its temperature dependence.

Observation and interpretation of some microstructural features of low stress creep

Abstract Alternative mechanisms have been invoked to explain behaviour cited as evidence for diffusional creep. Consideration of microstructural changes detected during testing, in terms of both scratch displacements and precipitate redistribution, demonstrates that the alternative mechanisms suggested cannot give rise to the measured strain.

Creep and cavity growth under multiaxial stresses

Abstract A consideration of cavity growth by the acquisition of vacancies diffusing along grain boundaries in a polycrystalline material under stress at elevated temperatures leads to the evaluation of the influence of a multiaxial stress system on cavities on grain boundaries at any angle to the principal stresses. From this, the special features are examined of the form of creep process that operates to accommodate the progressive increase in volume of the growing cavities and conditions are determined where it may produce a creep strain exceeding that caused by the more common creep processes that are independent of cavity growth. In making these comparisons, it is important to take into account the major differences of the form of creep to accommodate cavity growth from other creep processes because it does not operate under conditions of constant volume.

The coarsening of intergranular bubbles containing sparingly soluble monatomic gas

Abstract When gas bubbles of varying sizes exist in a material, the higher concentration of gas in solution around the smaller bubbles results in a net migration of gas atoms towards the larger bubbles which grow at the expense of the smaller ones. The driving force is derived from the consequent increase in ontropy of the system. When the bubbles are situated on grain boundaries and the gas is preferentially segregated at the boundaries, the gas migration may be confined mainly to the boundary planes. The bubble-coarsening process in such a situation is analysed in torms of gas atom diffusion along the grain boundaries and an equation is derived for the increase in mean bubble size. The possible significance of the coarsening mechanism in the development of intergranular porosity and life limitation of doped tungsten lamp filaments operating at high temperature is suggested.

Tensile creep behaviour of coarse grained copper foils at high homologous temperatures and low stresses

Abstract Tensile creep behaviour of OFHC copper in the temperature range 850 to 1074°C (0·83 to 0·99Tm) under low stress (0·1 to 0·6 MPa) has been investigated in tension for 0·4 and 0·6 mm thick foils with grain size ∼1 mm, in the plane of the foils. Increases in creep rate per unit stress at 0·99Tm were two orders of magnitude higher than predicted for Nabarro–Herring diffusional creep and were nearer to values expected from the operation of grain size independent Harper–Dorn creep, but the stress exponent n was closer to 2 than to the n=1 expected in this mechanism. Observations on specimen surfaces revealed some widely spaced slip bands, some small grain boundary movements and occasional cavitation on grain boundaries nearly perpendicular to the stress. Creep rates were comparable with predictions of the movement of dislocations, controlled by the rate of their generation at Bardeen–Herring sources at a spacing similar to that of the observed slip lines.

Changes in precipitate distribution during the low-stress creep of a hydrided magnesium-zirconium alloy

Abstract Microstructural changes occurring in hydrided Mg-Zr alloy (ZR55) during creep in the stress range 1.7–9.5 MPa at temperatures between 673 and 733 K have been critically examined by optical microscopy and by scanning electron microscopy. Particular attention has been given to the distribution of precipitates with respect to the grain boundaries to elucidate the operative mechanism of deformation. Geometrical features of the formation of precipitate-denuded zones and of the concurrent accumulation of precipitates have been explored in relation to the conditions of creep and the direction of tensile stress. Additional information has been obtained from microanalysis within the grains and near grain boundaries. Clear evidence is provided that, at the lower stress levels, all such microstructural changes are consistent with those expected from a deformation mechanism controlled by diffusional flow. The measured strain rates, however, exceeded those estimated from diffusional creep theory in this alloy and the microstructural observations provide some indication of reasons for this discrepancy.