*4.2. Microhardness Evolution*

Microhardness is significantly increased by HPT deformation as discussed in detail in [8] due to the microstructural refinement, the introduction of high dislocation density, and increased content of ω phase. Ageing of both non-deformed and HPT-deformed materials at 400 ◦C resulted in microhardness increase. Moreover, similar microhardness values were observed in both conditions. However, the similar increase of the microhardness can be attributed to different effects in both conditions.

The hardening of the non-deformed material aged at 400 ◦C is caused by the ω particles—the nano-sized ω particles are stabilized by diffusion, their size increases, and they act as much stronger obstacles for motion of dislocations. One may assume that a moving dislocation can pass through (cut) ωath particles (known as Friedel effect [42]) as they are small and coherent. It is well known that the shear stress required for a dislocation to pass through a precipitate increases with its increasing size (within the Friedel's limit) and/or with increasing strength of the obstacle to dislocation motion [42,43]. Due to this and also because of the increasing volume fraction of ω phase, the hardness of the non-deformed material increases with increasing ageing time at 400 ◦C.

In HPT-deformed material, ageing at 400 ◦C already for 1 h results in the precipitation of tiny α phase particles, which are incoherent and cause significant Orowan strengthening. On the other hand, ω phase content is relatively low. The decreasing microhardness of the HPT material aged at 400 ◦C for longer times (4 and 16 h) may be related to the coarsening of the α + β microstructure. Both β matrix grains and α phase precipitates coarsen with increasing ageing time. The same process is even more pronounced during ageing at 500 ◦C. The microhardness was found to monotonically decrease with increasing ageing temperature and time. The maximum microhardness is therefore achieved in the HPT specimen aged at 400 ◦C. In a recent study [44], HPT deformation of Ti15Mo/TiB composite was performed at 400 ◦C and very high microhardness values (650 HV after *N* = 1 HPT revolution) were achieved.

The microhardness of the non-deformed material aged at 500 ◦C is inferior to that of the material aged at 400 ◦C for all ageing times. The relative decrease of microhardness of the non-deformed material with increasing temperature of ageing may be attributed to the decreasing volume fraction and increasing size of ω particles, whose size is well beyond the Friedel's limit.

Severe plastic deformation of the parent β phase and the introduction of high density of defects significantly accelerates the α phase precipitation. An important additional effect of this enhancement is the reduction of the content of ω phase and its disappearance at comparatively low ageing temperatures.
