Hardness and Phase Transformation Kinetics of Cast Mg and Al Alloys for Aerospace Applications

High integrity cast components such as accessory gear boxes used in jet-engine applications, require higher service temperatures during continuous and peak operating conditions that could exceed 170 and 250^oC respectively. This generates demands for developing new high performance materials with Al and/or Mg-based alloys being the primary candidates in these applications.

This study was aimed to evaluate hardness and transformation kinetics during annealing between 25 and 500^oC in Al-based (A356, F357, C355) and Mg-based (QE22A, EV31A, ZE41A, WE43B) alloys and to determine the maximum temperature the alloys can withstand without drastic loss of hardness. 

The results revealed that A356 and F357 alloys showed softening at 225^oC associated with the hardness drop from 95 to 30 HRB. The C355 alloy exhibited improved hardness retention since the softening was observed above 240^oC resulting in hardness decrease from 95 to 45 HRB. The temperature range of 225-240^oC, corresponding to the over-aging reaction, has been established as the transition temperature above which degradation of hardness was observed in Al-based alloys.

The Mg alloys, QE22A, EV31A, ZE41A and WE43B, subjected to the same testing, showed an improved high temperature performance. The ZE41A and WE43B alloys exhibited gradual softening with a hardness decrease from 80 to 70 HRB and from 80 to 60 HRB for EV31A and QE22A alloys. The dilatometer analysis revealed a visible transformation peak at 275^oC in EV31A alloy, but no similar over-aging reactions were observed in other Mg alloys up to 400^oC. The differences in the performance between Al and Mg alloys have also been observed in tensile characteristics. The microstructural analysis has been carried out to understand the link between the microstructure and mechanical properties of these alloys at elevated temperatures.