Aibin Ma

Movement of alloying elements in Mg -8.5 wt-%Li and AZ91 alloys during tensile tests for superplasticity

Abstract Magnesium - 8.5 wt-% lithium and AZ91 alloys, processed using rotary die equal channel angular pressing, exhibit superplasticity. Deformed specimens have been used to investigate the movement of alloying elements during superplastic deformation. Optical microscopy and electron probe microanalysis were used for the investigation of microstructure changes in Mg - 8.5 wt-%Li and AZ91 alloys. In a two phase Mg - 8.5 wt-%Li alloy, the average content of β phase (lithium richer than α) decreased during superplastic deformation. No β phase layer was found at all in the surface area of the deformed specimens. The higher the test temperature, the thicker the layer without β phase is. In addition, the higher the test temperature, the lower the average β phase content is. The results indicate that lithium is lost during superplastic deformation. At too high a temperature of 673 K, lithium oxidises were obtained in the tested alloy. At the optimum conditions of superplasticity, no oxidation was obtained in the specimen. In the AZ91 alloy, zinc was almost lost at the superplastic deformed area. Both Mg - 8.5 wt-%Li and AZ91 alloys show low activation energies derived from the stress - strain rate results, connecting to the microstructure changes during tensile tests. The mechanism of superplasticity of these alloys seems to be grain boundary sliding assisted by the movement of the relatively low activation energy elements during tensile tests.

High Strain Rate Superplasticity in 10vol.% SiC Particulate Reinforced AZ31 Magnesium Alloys

AZ31 magnesium matrix composites reinforced with 10vol.% SiC particulate (2 μ m) were prepared by stirring-cast under high purity argon atmosphere, then extruded with a reduction ratio of 49:1 at 663K. Superplasticity of the composites was investigated at temperature range from 638K to 838K and strain rate from 2,08 x 10 S - to 5.21 × 10 -1 S -1 . High strain rate superplasticity has been obtained in the composites at 798K with a strain rate of 2.08 × 10 - s -1 and at 813K with a strain rate of 5.21 × 10 -1 S -1 , respectively. The maximum total elongation of 228% was obtained at a strain rate of 2.08 × 10 -1 S -1 , The strain rate sensitivity exponent (m) which was higher than 0.3, were observed when the strain rate was higher than 10 - S - at 798K. Increasing the test temperature to 813K, the maximum total elongation exceeding 195% was achieved at a higher strain rate of 5.21 x 10 -1 s -1 than at 798K. It indicates that superplasticity in such a kind of composite can be obtained by adjusting the strain rate and tensile test temperature. One kind of magnesium matrix composites may show high strain rate superplasticity under two tensile strain rates, and correspondingly at two optimum test temperatures. The higher the tensile strain rate is, correspondingly, the higher the optimum temperature is, too.

Severe Plastic Deformation of Commercially Pure Titanium by Rotary-Die Equal Channel Angular Pressing Method

The commercially pure titanium cylindrical samples with a diameter of 11.5mm and a length of 24mm were processed by a new severe plastic deformation process, called the rotary-die equal channel angular pressing (RD-ECAP), under the condition of 773K, 2.4mm/s punch. By the RD-ECAP, ECAP processes of 1-4 passes were possible without sample removal and the temperature of cp-titanium could be simply controlled. After the RD-ECAP process, the cp-titanium samples had no crack. Fine-grained microstructures were observed in the sample on Y plane. Therefore the samples processed by RD-ECAP were expected to have high mechanical strength.