Biomedical Engineering Reference
In-Depth Information
of the microhardness are seen to be non-monotonic and its increase from
4200 up to 4780 МPа takes place when the annealing temperature is
increased to 500
С. Figure 1.5 displays the increase in microhardness with
increasing annealing temperature up to 500
°
С and decrease in microhard-
ness at higher temperatures. h e high microhardness observed at 500
°
С in
[199] at er annealing of the UFG Ti-6Al-4V alloy is unusual. h e authors
associated these values with relative structure stability and changes of the
proportions and structures of the α and β phases, with the β phase volume
fraction being slightly increased. In this case strengthening of the UFG
Ti-6Al-4V ELI alloy at er annealing at 500
°
С can be associated with the
aging process as well. h is process is known to be accompanied by decay of
the metastable β-phase and precipitation of secondary particles of α-phase
of various morphologies in conventional alloys [200]. Herewith, any defor-
mation more or less increases the possibility of metastable phase break-
ing up due to high crystal lattice distortions. However, investigation of the
nature of this aging in the UFG alloy requires more thorough study.
Figures 1.6a-c represent the alloy's structure at er ECAP and extrusion.
SPD processing leads to considerable rei nement and formation of a com-
plex UFG structure with grains and subgrains having a mean size of about
300 nm. h ese grains have irregular form, a great number of various defects
in the crystal lattice, and a high level of internal elastic stresses. h e elon-
gated form of structural elements created by extrusion straining is seen in
°
Figure 1.6
Microstructure of the UFG Ti-6Al-4V ELI alloy before (а, b, c) and at er
annealing at 500
о
С, 1 hour (d, e, f ). Longitudinal section. а, d and c, f - bright-i eld and
dark-i eld images respectively, b, e - dif raction patterns. TEM.
