Mechanisms of Microsuperplasticity

Abstract

After superplastic deformation of Al-7475 and some other aluminium alloys, straininduced cavities are seen to be associated with long fibres parallel to the tensile direction. These fibres, whiskers or filaments are also observed on the fracture surface. This effect has become known as microsuperplasticity. The whisker characteristics are affected by the deformation conditions, particularly temperature and strain rate. To study the effect of these variables more fully, tensile samples of Al 7475 have been strained to failure at temperatures ranging from 480OC to 530OC and strain rates from 1.0E- 04s-1 to 5.0E-03s-1. Additional samples were deformed at 450OC and 420OC and a single strain rate. Some whiskering was observed under all testing conditions. The longest whiskers were generally seen at high temperatures and low strain rates. A TEM study of macroscopic whiskers produced under conditions of around 540OC and 1.0E-04 /s showed an amorphous structure. Annealing prior to deformation was shown to have little effect on whisker formation. EDX analysis showed the whole surface of the alloy to be enriched in alloying elements compared with the bulk alloy. The high levels of Mg detected were connected with the formation of magnesia as the surface oxide verified using Cr3+ fluorescence microscopy. Use of the differential scanning calorimeter (DSC) showed no conclusive evidence of partial melting below the testing temperatures. Considerations of capillarity and the DSC analysis suggest whiskering did not occur by a mechanism of viscous flow.