Strain Rate Sensitivity Of Flow Stress Research Articles

Plastic anisotropy in a superplastic Al-Li-Mg-Cu alloy

The plastic anisotropy of AA8090 Al-Li-Cu-Mg alloy sheet has been evaluated by tensile testing and by deep drawing at temperatures in the range 200 °C to 525 °C. At temperatures of about 500 °C and strain rates of about 10-3 s-1, this material exhibits a high strain-rate sensitivity of flow stress which reduces any tendency to strain localization in stretching and allows so-called superplastic forming of the sheet. Most models of the material behavior in this regime require highly inhomogeneous deformation on the scale of the material’s grain size. The plastic anisotropy measured in the superplastic regime was similar in form, though of reduced magnitude, to that measured under conditions associated with a much smaller strain-rate sensitivity. Homogeneous slip models predict the correct form of anisotropy, and inclusion of slip-rate senitivity can reduce the magnitude of anisotropy predicted but not sufficiently to give good correlation with the experimental results unless very high values are used. The development of the preferred crystallographic orientation in deep drawing has also been examined. The predictions of homogeneous slip models correlate quite well with experimental results at low temperatures, but the situation is more complex in the superplastic regime where, although there is some evidence of texture changes as predicted, there is a general reduction in the intensity of preferred orientation with deformation. However, the results indicate that a greater contribution of homoeneous slip deformation is involved in superplastic deformation than is assumed in some models of superplasticity.

Anomalous strain-rate sensitivity of flow stress in superconducting Al and Al-Mg alloys

An anomalous strain-rate sensitivity of yield stress in superconducting Al and Al Mg alloys has been found experimentally. It is discussed that this phenomenon is due to an anomalous increase of the yield stress τ s in the superconducting state, induced by the destruction of the Cooper pairs by dislocations. The result agrees well with the calculation by Huffman and Louat. Δτ sn = τ n − τ s , where τ n is the yield stress in the normal state, and the above-mentioned strain-rate sensitivity of Δτ sn are hardly explained by classical theories of the strength of metals and alloys. A new theory of the strength of solid-solution alloys has been worked out. On this basis, Δτ sn and its strain-rate dependence are analytically expressed and compared successfully with experiment.

Some remarks concerning dislocation motion in superconductors

It is pointed out that differences in strain-rate sensitivity of flow stress in superconductors have been reported in some detail several years before, and the vibrating-string model in its present state does not allow the strain-rate sensitivity of the flow stress to be related to the area swept out by a dislocation upon break-away from obstacles or to a quantity called activation volume. (GHT)