Design concept of hardenable magnesium-lean alloys processed by high-speed extrusionFriday (10.11.2017) 11:15 - 11:35 Part of:
Magnesium (Mg) alloys have received considerable attention as lightweight structural materials due to their high strength−to−weight ratio. For a wider applicability however, and in order to be competitive to their aluminum counterparts, improved mechanical strength is required. The addition of alloying elements beneficial to improved mechanical properties limits the forming processability therewith increasing production costs. In this context, alloys with an aging potential are beneficial as they allow high hot forming rates while exhibiting hardening in a subsequent aging treatment. The addition of Al and Ca was shown to lead to the desired aging potential, which significantly increases the strength by the formation of monolayered Guinier-Preston (G.P.) zones made of Al2Ca phase . Moreover, with the addition of Mn an effective grain refinement can be obtained through grain boundary pinning by Al8Mn5 dispersoids during hot forming , which in turn leads to a further increase in strength. In order to benefit from both strengthening effects careful balancing of the Al−Ca−Mn (AXM) ratio and the thermomechanical processing parameters are required.
In this study, an AXM composition is designed by balancing the alloying element ratios while keeping their total content below 1at.% to foster low-cost and high-speed production. Furthermore, a thermomechanical processing route is defined to achieve a fine dispersion of Al8Mn5 precipitates in view of an effective grain refinement.
Billets of a new AXM-lean alloy were cast, heat treated and extruded and their age hardening response evaluated in terms of Vicker’s hardness and uniaxial tensile testing. Despite the very low alloying elements’ content, a significant aging potential was realized with a 70% increase of the tensile yield strength for T6-temper by ~100MPa. In order to understand the microstructural features underlying the mechanical properties, detailed electron microscopy studies are performed using transmission electron microscopy (TEM) equipped with an energy dispersive X-ray spectrometer (EDS) for chemical mapping. Analysis revealed the presence of finely distributed Al−Mn-rich precipitates in high number density as well as the formation of Al−Ca-rich G.P. zones in the aged condition.