3.2.3. Microhardness Evolution during Ageing

Figure 10 shows the dependence of Vickers microhardness on ageing of the non-deformed and HPT-deformed samples. Ageing of both samples at 400 ◦C resulted in an abrupt increase of microhardness. In the non-deformed specimen, the microhardness increases with increasing ageing time at 400 ◦C. On the other hand, in the HPT-deformed sample aged at 400 ◦C for 1 h the microhardness reaches the maximum (500 HV). With increasing ageing time at 400 ◦C the microhardness continuously decreases. Specimens aged at 500 ◦C exhibit lower microhardness than specimens aged at 400 ◦C. The microhardness of HPT sample aged at 500 ◦C for 16 h even drops below the microhardness of the non-aged HPT specimen.

**Figure 10.** The evolution of the microhardness during ageing in the non-deformed and HPT-deformed samples.

### **4. Discussion**
