Hot isostatic pressing (hipping) can be used for upgrading castings, densifying presintered components, consolidating powders, and interfacial bonding. It involves the simultaneous application of a high pressure and elevated temperature in a specially constructed vessel. The pressure is applied with a gas (usually inert) and, so, is isostatic. Under these conditions of heat and pressure, internal pores or defects within a solid body collapse and diffusion bond. Encapsulated powder and sintered components alike are densified to give improved mechanical properties and a reduction in the scatter band of properties. In this article, the basic science of sintering and hipping is summarized and contrasted. The current state of understanding and modeling of hipping is then reviewed. Models can be classified either as microscopic or macroscopic in their approach. In the microscopic approach, the various mechanisms of densification are analyzed in terms of a single particle and its surroundings. In the macroscopic approach, the compact is treated as a continuous medium. In hipping, although the pressure is isostatic, shrinkage is not generally isotropic, particularly if containment is used. However, the shrinkage can now be well predicted, provided that the material and container properties are accurately known.

Fundamental aspects of hot isostatic pressing: An overview

In this paper, recent progress in magnesium matrix composite technologies is reviewed. The conventional and new processes for the fabrication of magnesium matrix composites are summarized. The composite microstructure is subsequently discussed with respect to grain refinement, reinforcement distribution, and interfacial characteristics. The mechanical properties of the magnesium matrix composites are also reported.

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Review of recent studies in magnesium matrix composites

It is now recognized that grain boundary sliding (GBS) is often an important mode of deformation in polycrystalline materials. This paper reviews the developments in GBS over the last four decades including the procedures available for estimating the strain contributed by sliding to the total strain, ξ, and the division into Rachinger GBS in conventional creep and Lifshitz GBS in diffusion creep. It is shown that Rachinger GBS occurs under two distinct conditions in conventional creep depending upon whether the grain size, d, is larger or smaller than the equilibrium subgrain size, λ. A unified model is presented leading to separate rate equations for Rachinger GBS in power-law creep and superplasticity. It is demonstrated that these two equations are in excellent agreement with experimental observations. There are additional recent predictions, not fully resolved at the present time, concerning the role of GBS in nanostructured materials.

Grain boundary sliding revisited: Developments in sliding over four decades

Boundary defects, and atomistic aspects of boundary sliding and diffusional creep

https://publikationen.sulb.uni-saarland.de/bitstream/20.500.11880/24025/1/ea198303.pdf

Interface controlled diffusional creep

The possible effects on diffusion creep of some limitation of grain boundaries as vacancy sources or sinks

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.

The threshold stress for creep controlled by dislocation-particle interaction

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.

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

Creep behaviour of 7075 aluminium alloy has been investigated in the temperature range 350–410 °C and at applied tensile stresses between 1.8 and 6.3 MPa. The as-received material was thermomechanically processed to obtain 48 μm grain size, with grain growth to 78 μm during creep and a thixoforged sample was also investigated. A transition from a region of low stress exponent (≈1) to a higher stress exponent (≈5) was observed on increasing the stress above 5 MPa. The measured creep rates at low stresses were normalised for grain growth and found to be within a factor of 2 of the rates expected from the Nabarro–Herring equation. The value of activation energy for creep was found to be close to 147 kJ/mol. The measured rates were found to conform to rates measured at constant strain rates and comparably low stresses for superplastic 7075 aluminium alloy with comparable microstructure.

G. W. Greenwood

Papers

The possible effects on diffusion creep of some limitation of grain boundaries as vacancy sources or sinks

The threshold stress for creep controlled by dislocation-particle interaction

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

Low stress creep behaviour of 7075 high strength aluminium alloy

Negative creep in a nickel-base superalloy