3.2. Deformation Behavior at Elevated Temperatures
EL at elevated temperatures was affected by strain rate, deformation temperature, and type of sample (Figure 5
). ELs measured at a deformation temperature of 750 °C and 2 × 10−4
are not presented, because severe oxidation at the sample surface caused large variability in the measurements. An EL of 400% is regarded as the minimum requirement for superplasticity in metals [19
]. The investigated alloys exhibited a significant EL of over 400% under most conditions, except at the lowest deformation temperature (650 °C) and the highest strain rate (10−2
). The subsequent annealing had an ambivalent effect on the difference between the mechanical properties at RT and those at elevated temperatures: it rarely increased EL in the step-rolled alloys at elevated temperatures, and it even degraded the ductility at 650 °C.
The stress–strain curves of STEP-0, STEP-5, and STEP-6 were affected by the deformation temperature and strain rate (Figure 6
). At 750 °C and 10−3
, three specimens showed weak flow hardening behavior. This suggests that continuous dynamic recrystallization (CDRX) could not suppress grain growth, thereby diminishing the superplasticity [20
]. The plateau region was confirmed during the deformation at 700 °C, indicating that CDRX strongly suppressed grain growth [21
]. To obtain the highest and most efficient superplasticity, this plateau behavior should be achieved. Usually, EL is higher at 750 °C than at 700 °C, but in our results the ELs were similar. In this case, severe grain growth and oxidation degraded the superplasticity at 750 °C. At a deformation temperature of 650 °C, the specimens presented a flow softening followed by the plateau region. Despite the decreasing deformation temperature, they still exhibited significant EL values, suggestive of a low-temperature superplasticity.
The highest superplastic EL of STEP-0 among the investigated step-rolled alloys can be understood in terms of slope m
(i.e., strain-rate sensitivity) in a log-scale plot of stress and strain rate (Figure 7
). In general, UFG Ti-6Al-4V alloy demonstrates three regions with different slopes in this plot [6
]. Among them, region II is characterized by (i) an intermediate 10−4
≤ 2 × 10−3
, (ii) high m
, and (iii) significant superplastic EL. The high m
of the STEP-0 sample indicates inhibition of the transition from region II to region III at a high strain rate of 10−2
], supporting a superplasticity obtained by the step rolling. Accordingly, the STEP-0 sample had a uniformly high m
in the entire range of investigation employed in this work: m
= 0.34 at 650 °C, 0.40 at 700 °C, and 0.55 at 750 °C. These m
are comparable to or higher than those of UFG Ti-6Al-4V alloys fabricated by SPD processes: 0.34 ≤
≤ 0.43 [4
Step-rolled UFG Ti-6Al-4V yielded excellent superplasticity (i.e., EL up to 960%) that is comparable to the EL obtained using other fabrication methods. ECAP yielded UFG Ti-6Al-4V that had d
= 0.3 μm, EL = 356% at 700 °C, and 10−3
]. HPT produced Ti-6Al-4V alloy that had d
~0.03 μm and EL = 820% under the same conditions [7
]. Another HPT study produced d
= 0.3 μm and the sample had EL = 676% at 725 °C and 10−3
]. Another UFG Ti-6Al-4V alloy (d
= 0.4 μm) produced by hot rolling had EL = 400% at 700 °C and 10−3
]. A combination of forging and warm rolling refined the grains to d
= 0.3 μm; the sample had EL = 550–900% at 700 °C [25
Superplasticity is highly dependent on microstructural features. Grain refinement is regarded as the most important factor to activate superplastic behavior. Grain refinement expands the superplastic regime to lower temperatures and higher
, because the increasing fraction of grain boundaries provides increasing sources for GBS [5
]. Grain morphology also affects superplasticity. Equiaxed grains yield a higher superplastic EL than elongated grains [26
]. Dislocations distributed at grain boundaries also contribute to superplasticity by providing a fast diffusion path at high temperatures and by increasing the grain-boundary diffusion coefficient [28
]. All of these microstructural features that are beneficial to superplasticity appeared in STEP-0, so its significant EL at 650–750 °C is understandable.
Comparison of the microstructural features at strains of ε
= 0%, 100%, and 300% provided insight into the microstructural evolution during superplastic deformation at elevated temperatures (Figure 8
). STEP-0 and STEP-6 were selected for this comparison, because STEP-6 was expected to show more distinct changes in microstructure than STEP-5 due to the higher annealing temperature of STEP-6. In the EBSD map, the STEP-0 and STEP-6 samples showed similar grain sizes before the deformation, whereas the former exhibited a smaller grain size after applying strains of 100% and 300%.
Application of ε
affected LAB fraction and grain size differently in the two samples (Figure 9
). During the early stage of deformation, up to a 100% strain, the LAB fraction was considerably reduced, by 12.2%, in the STEP-0 sample but only by 6.1% in the STEP-6 sample. During this stage, grain size in the STEP-0 sample also increased by only 0.16 μm, whereas in the STEP-6 sample grain size increased by 0.68 μm. Moving from a 100% to a 300% strain, STEP-0 and STEP-6 exhibited a similar reduction in LAB fraction (5.6%) as well as a similar increase in grain size (0.46 μm).
The distinct difference between the microstructural features of STEP-0 and those of STEP-6 arose from the active occurrence of CDRX in the STEP-0 sample. A continuous accumulation of dislocations during high-temperature deformation drove the formation of subgrains surrounded by LAB. Further straining generated recrystallized grains by increasing the misorientation of the subgrain boundaries [17
]. The higher amount of pre-existing LAB in STEP-0 than in STEP-6 provided a higher driving force for CDRX in comparison with the annealed samples. This result suggests that CDRX of the STEP-0 sample occurred actively during the early stage of deformation, up to a 100% strain.
The hypothesis of the active occurrence of CDRX in STEP-0 during superplastic deformation is supported by the distribution of grain sizes (Figure 10
). The STEP-0 sample showed an increasing fraction of grains with a size of ~1 μm in exchange for a decreasing fraction of coarse grains. In contrast, in the STEP-6 sample, the peak of the distribution curve shifted towards a higher grain size after a 100% strain at 650 °C and 10−3
. This is the typical trend that results from dominant grain-growth behavior at an elevated temperature. Such results suggest that the conversion of coarse grains into several fine grains (i.e., CDRX), occurred more actively in STEP-0 than in STEP-6. Consequently, active CDRX during the early stage of deformation in STEP-0 suppressed grain growth during elevated-temperature deformation and thereby contributed to its superior superplasticity.
particles also assisted in the increase in superplasticity in the STEP-0 sample. The sample possessed twice as much β
phase as the STEP-5 and STEP-6 samples. Due to the difference in their crystallographic structures, grain-boundary diffusivity is over 100 times higher in the β
phase than in the α
]. Such a high diffusivity reduces the resistance to GBS activation. Moreover, resistance to GBS is lower at α
interfaces than at α
]. Therefore, the high β
fraction was the secondary factor that increased the superplasticity of the STEP-0 sample.