Biomedical Engineering Reference
In-Depth Information
1400
(2)
1200
(3)
1000
800
Ti-6AI-4V ELI
(1)
600
1
CG
UFG
UFG + annealing 500°C, 1 h
400
2
3
200
0
0
2 4
6
8
0
2
4
6
8
0
2
4
Elongation (%)
Figure 1.7
Elongation stress−strain curves of the Ti-6Al-4V ELI alloy: as-received state
(1); UFG state before (2) and at er annealing at 500 °С (3).
the longitudinal section in Figure 1.6а and Figure 1.6c. Grain boundaries
are not clear in these images due to large crystal lattice microdistortions as
a result of severe plastic deformation.
Figures 1.6d-f demonstrate that annealing at 500
С of the alloy sub-
jected to ECAP and extrusion leads to signii cant structural changes,
particularly in the rod's longitudinal section, which is characterized by for-
mation of more equiaxed grains with a mean grain size of about 250 nm
and thin boundaries (Figure 1.6d). h e decrease in azimuthal spot spread-
ing seen in SAED patterns (Figure 1.6e) shows the considerable decrease in
internal elastic stresses as a result of processes of dislocation redistribution
and recovery during annealing.
Figure 1.7 displays typical stress-strain curves for the CG and UFG alloy,
which show the signii cant strengthening of the alloy at er SPD process-
ing due to microstructural rei nement. Tensile elongation of the UFG alloy
(curve 2) is reduced from 17% down to 9% compared to the as-received
state (curve 1). Figure 1.7 demonstrates that subsequent annealing at the
500
°
С increased strength and ductility (up to 12%), with uniform elonga-
tion of about 4%. h e results of tensile tests are consistent with the data
on microhardness measurement (Figure 1.5). Ductility enhancement in
the UFG alloy at er annealing is obviously conditioned by such factors as
decrease in internal elastic stresses and dislocation density. As mentioned
above, additional strengthening of the alloy can be associated with decay
of metastable β-phase during cooling from the annealing temperature. Its
°
