*3.3. The E*ff*ects of Isothermal Heat Treatments*

Microhardnesses of heat-treated alloys in the initial state and HPT-processed state have been reported in [34]. After homogenization, followed by furnace-cooling or quenching, Mg alloy samples were not only HPT-processed to ε~5 and ε~20, but also heat-treated for up to 29 h, at a constant temperature between 50–220 ◦C in intervals of 20 ◦C. After each isothermal heat treatment, microhardness was measured. Similarly to the results reported by Horky et al. [33] (see also Sections 4.3 and 4.4), further hardness increases were observed in all alloys studied in this work except Mg0.3Ca, as discussed later. The highest hardness was observed after a heat treatment at 100 ◦C (see Figure 8a–e) and at an annealing time of 1 h (Figure 8).

The increases in the IS (furnace-cooled) samples were larger than in the IS (quenched); i.e., heat treatments of the furnace-cooled and HPT-processed samples gave a total increase of microhardness by 40%, while the heat treatments of the quenched and HPT-processed samples gave 15% only.

The binary alloy Mg0.3Ca had its hardness maxima after heat treatments at 75 ◦C and between 150 and 160 ◦C. When exceeding the annealing temperature of 100 ◦C (or 160 ◦C in the case of Mg0.3Ca), microhardness decreased significantly for all alloys; and at annealing temperatures near 220 ◦C, it reached original values before heat treatment.

It is interesting to note that the samples without any HPT-processing also showed a thermally-induced hardness maximum at around the same annealing temperatures, i.e., 100 ◦C and 160 ◦C, although the values remained substantially below those of the HPT-processed samples (Figure 8a–e).

**Figure 8.** Microhardness after heat treatments for 1 h at various temperatures for (**a**) Mg5Zn0.3Ca (**b**) Mg5Zn (**c**) Mg5Zn0.5Ca (**d**) Mg5Zn0.15Ca0.15Zr and (**e**) Mg0.3Ca in the IS (furnace-cooled) (grey), at ε~5 (red) and ε~20 (black).

(**e**)

The peak temperature of 100 ◦C was chosen to investigate how the extension of heat treating time up to 29 h affects hardness of Mg5Zn0.3Ca, Mg5Zn and Mg0.3Ca alloys HPT-processed to ε~5 and ε~20; see Figure 9. For almost all materials, hardness increased after one hour but did not increase further within the next 23 h of heat treatment. Only after longer heat treatment time, substantial decreases in hardness were observed. Figure 10 shows the results for a fixed heat treatment time of 24 h, and varying temperature between 25 and 150 ◦C. For HPT-processed samples, the highest hardness values were observed, most of them again at T = 100 ◦C. Significantly smaller or no hardness maxima at all were found in the case of IS samples heat treated at T = 100 ◦C.

**Figure 9.** Microhardness after heat treatments at the peak temperature of 100 ◦C with varying annealing times up to 29 h for Mg5Zn0.3Ca (black), Mg5Zn (red) and Mg0.3Ca (blue) processed at different strains.

**Figure 10.** Results of microhardness measurements of Mg alloys in the IS (furnace-cooled) and after HPT-processing at 4 GPa and 0.5 rotations, and additional heat treatment for 24 h at varying temperatures. (**a**) Mg5Zn0.3Ca (black), Mg5Zn (red) and Mg0.3Ca (grey); and (**b**) Mg5Zn0.15Ca0.15Zr (red) and Mg5Zn0.15Ca (black).
