Light alloys like magnesium and titanium, which have a hexagonal crystal structure, only have a limited number of easy slip systems, which is believed to limit formability. Although room temperature formability is poor for magnesium alloys, it improves with increasing temperature. Titanium, on the other hand, shows good ductility and drawability at room temperature, which decreases during warm forming. In titanium, spring-back is difficult to predict. In this paper we present results of two studies aimed at understanding the different forming behaviour of these two different light alloys. High resolution image correlation (HRDIC) was used in combination with EBSD to study the deformation of these materials at the microstructural scale.
Magnesium alloy AZ31 was tested in uniaxial tension. The local deformaiton was measured using HRDIC. The results showed that although deformation is dominated by easy basal slip, deformation incompatibility is accommodated by the activation of non-easy slip, which together with small amounts of twinning ensures compatibility is maintained during deformation. A commercial purity titanium alloy (grade 2) was also tested in tension and studied using HRDIC. Unlike the Mg alloy, CP-Ti showed extensive prismatic slip in addition to basal slip. This difference helps reduce the level of strain localisation, improving formability. However, poorly aligned grains remain undeformed even above 6% macroscopic strain, which can help explain the difficult to predict spring-back behaviour. These observations suggest that microstructure control could be a powerful method for improving the ductility of these difficult-to-form alloys.