*4.3. The E*ff*ect of Post-HPT Heat Treatments on Strength*

The most interesting observation in this work has been the additional substantial strength increase due to a heat treatment, after HPT-processing at RT. Such an effect has been already reported by Horky et al. [33] and Ojdanic et al. [34]. When thermally treating the samples for 1 h (Figure 8), a significant hardness peak was observed for all the furnace-cooled and quenched Mg5Zn0.3Ca and Mg5Zn samples at a temperature of around 100 ◦C (corresponding to T = 0.4 Tm); only Mg0.3Ca showed a peak at 75 ◦C. This effect may be attributed to the fact that only in the latter alloy Zn was missing.

In further experiments the peak temperature of 100 ◦C was set constant, and the annealing time was extended beyond 1 h (Figure 9), which revealed that hardness slightly increases up to 24 h and then decreases. Further heat treatments with the same annealing times of 24 h were done for all alloys for other annealing temperatures too (Figure 10). Again, like in case of only 1 h annealing, the significant hardness peak was found for the annealing treatment at 100 ◦C.

To sum up, heat treatments at T = 100 ◦C can increase the hardness of the HPT-processed samples by ~30% (for Mg5Zn0.3Ca and Mg5Zn) and even up to 75% for Mg0.3Ca. The alloys Mg5Zn0.15Ca and Mg5Zn0.15Ca0.15Zr show no response to the heat treatments. For the non-processed samples, hardness increases of up to 50% can be reached, starting from a much lower initial hardness level than in case of all the HPT-processed samples, however.
