The effect of interfaces on mechanical properties is considered. Elastic and plastic compatibilities at boundaries are treated. Specific influences at both low and high temperatures are discussed, with emphasis on dislocation mechanisms and the atomic scale structure of boundaries.

The influence of grain boundaries on mechanical properties

The relationship between stress and strain rate is often sigmoidal in superplastic materials, with a low strain rate sensitivity at low and high strain rates (regions I and III, respectively) and a high strain rate sensitivity at intermediate strain rates (region II) where the material exhibits optimal superplasticity This relationship is examined in detail, with reference both to the conflicting results reported for the Zn-22 pct Al eutectoid alloy and to the significance of the three regions of flow

The mechanical properties of superplastic materials

It was first shown in 1960 that rapid quenching of certain alloys from the melt could form completely extended solid solutions, new metastable crystalline phases, and amorphous solid phases. These effects were attributable to the high cooling rates (>or approximately=105 K s-1) from the melt achieved by new techniques of rapidly spreading a few milligrams of liquid alloy into a thin layer in close contact with a good heat conductor (splat cooling). A comprehensive review of developments to data which have resulted from the application of this technique in laboratories throughout the world is presented. It includes an analysis of the range of methods now available for quenching the melt and of present understanding of how they form and quench specimens. The structural features observed as-quenched are then discussed under the divisions of microstructure and extended solid solubility, and of formation of both metastable crystalline and amorphous solid phases. A survey of the response of the as-quenched microstructure to annealing then precedes treatment of the properties and applications of splat-cooled alloys. It is concluded that splat cooling is being established in ever widening fields of study as a tool for controlling structure and properties, and that further applications can be anticipated.

http://iopscience.iop.org/article/10.1088/0034-4885/36/11/002/pdf

Splat cooling and metastable phases

The deformation behavior of a superplastic Ti-6A1-4V alloy at 927°C has been characterized by means of constant strain-rate tensile tests up to large plastic strain. Significant hardening has been recorded in the course of deformation. Microstructural studies on deformed samples indicate the occurrence of simultaneous strain-rate induced grain growth, which explains nearly all of the hardening. A small amount of hardening may also be expected from grain elongation or grain clustering effects. As a result of concurrent grain growth, the strain-rate sensitivity is found to decrease with strain, thus indicating that stress-strain rate behavior determined initially may not be applicable after large amounts of plastic strain. The stressJstrain-rate data obtained from step strain-rate test for a variety of grain sizes, together with the grain growth kinetics plots, provide a means for developing a constitutive description for this material at large strains.

Mechanical behavior and hardening characteristics of a superplastic Ti-6AI-4V alloy

On the mechanism of strain-enhanced grain growth during superplastic deformation

Stress-strain curves have been obtained for the superplastically deformed Al-Cu eutectic tested in tension under constant true strain-rate conditions. It is shown that constant flow stress conditions do not obtain and that, after an initial transient, the flow stress is linearly related to natural tensile strain. Optical metallography has been employed to follow the variation of both inter-phase particle separation and α-Al grain size with strain and it is concluded that the observed strain hardening is due mainly to grain coarsening.

B. M. Watts

Papers

The variation in flow stress and microstructure during superplastic deformation of the Al-Cu eutectic