Effect of multi-pass deformation on microstructure evolution of spark plasma sintered TC4 titanium alloy

3.2.1 Effect of deformation on microstructure

At low strain rate, the stress–strain curve remained relatively stable at each pass, and the peak stress value did not change much, so the strain rate was selected to be 0.001 s⁻¹. In Figure 4(a), the grain boundary was obvious, and the lamellar α was parallel to each other in the β grains, showing the Widmanstatten structure distinctly. This was because the degree of the deformation was low at one-pass (20% deformation) with the temperature at 1,050°C. The gradual phase transformation from the prior β grains through the β-transformed phase to the primary α phase results in a large number of lamellar α and acicular α, and the degree of coarsening was low, which were not sufficient to further transform into lath α. With the increase in deformation and the decrease in temperature, the lamellar α gradually increased and the acicular α gradually decreased (Figure 4(b)). The grain boundaries became inconspicuous at this time. In the case of three-pass deformation, the microstructure of phase included coarse colonies with the typical weave-basket structure. It showed that the lamellar α gradually formed at the discontinuous α phase after the deformation (shown in the rectangle in Figure 4(c)). Therefore, it could be clearly concluded that as the number of passes increased and the temperature decreased, the Widmanstatten structure gradually changed to the weave-basket structure. However, at this low strain rate (0.001 s⁻¹), the equiaxed α phase did not appear. Correspondingly, the plasticity of the TC4 titanium alloy had not been improved to a large extent. The lamellar structure contributes little to the flow softening [19]. Therefore, the phenomenon of lamellar α phase coarsening has little effect on the stress–strain curve, which was consistent with the data of strain rate of 0.001 s⁻¹ in Figure 2.

Figure 4

SEM of the samples at the strain rate of 0.001 s⁻¹: (a) one-pass, (b) two-pass and (c) three-pass.

Figure 5 shows the XRD patterns of the TC4 titanium alloy before and after the deformation. In terms of diffraction peak intensity, it remained stable except for (0002) and (10−11). The difference was that the peak intensity increased with the increase in deformation at the α lattice plane of (10−11). For the α lattice plane of (0002), the peak intensity increased first and then decreased. This feature was clearly demonstrated by comparing the XRD patterns of the deformations at 20% and 70%. An increase in the intensity of the diffraction peak indicated an increase in the corresponding phase shown by the diffraction peak. Therefore, it could be proved that as the amount of deformation increased and the temperature decreased, the α phase gradually increased, which was consistent with the previous characteristics of microstructure. The α + α′ phase could be shown as the diffraction peak of the plane of (0002). This was a transition phase from the β-transformed phase to the α phase, in which hexagonal α′ martensite appeared as lath martensite in the microstructure. This feature could explain the change in lath α in Figure 4. Compared with the deformation of one-pass, the content of lath α greatly increased in Figure 4(b). For lath α in Figure 4(c), the growth rate was greatly reduced, and the evolution trend of lath α at this time changed from the β-transformed phase to the α phase, with the coarsening of lath α. The discontinuous lath α began to appear.

Figure 5

XRD patterns of the TC4 titanium alloy at different amounts of deformation: (a) one-pass, (b) two-pass and (c) three-pass.

In terms of the XRD pattern before and after the deformation, the main changes were expressed at the diffraction peak in the (10−10) of the α phase and (110) of the β phase. As shown in the XRD pattern in Figure 5, the (10−10) diffraction peak exhibited two phase structures of α″ and α + α′. The content of the phase structure is related to the element [20]. α″ is an orthorhombic martensite, and its stability is related to the Al element [21], shown as an acicular martensite structure. The Al element makes an effect and can stabilize the orthorhombic martensite. Also, the diffraction peak of the β phase gradually appeared at the diffraction peak of (110) as the amount of deformation increased. It remained stable after the 50% deformation. The increase in this β phase was attributed to the grain refinement at the time of deformation. This grain refinement provided greater resistance to dislocation movement and enhanced the strength of the material [22].

3.2.2 Effect of strain rate on microstructure

The OM images of the TC4 titanium alloy deformed at 1,050°C are shown in Figure 6. The amount of one-pass deformation was 20% (2.4 mm). It can be seen that all images exhibited coarse initial β grains with elongated and parallel lamellar α phases on them. In addition, at the strain rate of 1 s⁻¹, the phenomenon of dynamic recrystallization (DRX; shown in ellipse) occurred as shown in Figure 6(c). The grain size was about 30 µm, compared with the grain size larger than 120 µm (Figure 6(a and d)). The grain size of DRX grew relatively as shown in Figure 6(b) and was about 40 µm. Therefore, the degree of grain refinement increased first and then decreased as the strain rate increased. The appearance of this feature was related to the softening mechanism [23]. The initial α martensite was composed of acicular martensite platelets and had a large number of defects due to displacement during hot deformation. Recrystallized grains began to nucleate in the platelets and gradually developed with strain at the process of deformation. During the subsequent cooling process, the acicular martensite transforms into the lamellar α phase. It was remarkable that the discontinuous α phase appeared in Figure 6(d). This kind of lamellar α phase had the tendency to transform into the weave-basket structure.

Figure 6

OM images of the samples at the deformation of one-pass: (a) 0.01 s⁻¹, (b) 0.1 s⁻¹, (c) 1 s⁻¹ and (d) 5 s⁻¹.

The OM images of the TC4 titanium alloy deformed at 1,000°C are shown in Figure 7. The amount of two-pass deformation was 30% (3.6 mm). As the temperature decreased, the characteristic of the weave-basket structure was exhibited during the two-pass deformation (Figure 7(a)). At the same time, the tendency of change from the α phase to the discontinuous α phase was found in the grain (indicated in the ellipse), compared with the grain which exhibited the weave-basket structure. This change in tendency was related to temperature and deformation [21,24]. Upon comparing Figure 7(b) with Figure 7(c), it can be seen that as the strain rate increased, the grain refinement effect was remarkable, and the feature of DRX (circle area) was presented apparently. The finer grains of DRX could reach 25 µm in Figure 7(c). As the strain rate increased to 5 s⁻¹, the lamellar α structure began the process of globularization, shown in the elliptical region (Figure 7(d)), during which, the equiaxed α phase was just beginning to appear. However, the α phase could not form the equiaxed structure only by the change of temperature, and it could not be spheroidized by cyclic heating like steel. Therefore, the appearance of the globularization process could explain to a certain extent that the sample had undergone sufficient plastic deformation and the process of DRX [25]. In addition, the 50% deformation also provided the condition for the occurrence of the equiaxed α phase.

Figure 7

OM images of the samples at the deformation of two-pass: (a) 0.01 s⁻¹, (b) 0.1 s⁻¹, (c) 1 s⁻¹ and (d) 5 s⁻¹.

Figure 8 shows the IPF and the misorientation angles of the TC4 titanium alloy at different strain rates under two-pass deformation. The different colors in the figures represented different crystal orientations, and the coloring principle is shown in the IPF scale in Figure 8(b). In order to avoid the decrease in confidence factor caused by data noise and the deviation of grain boundary identification, the orientation less than 2° was not considered [26]. At the strain rate of 5 s⁻¹, more characteristics of DRX between the grain boundaries appeared in Figure 8(b). It confirmed the characteristics of grain structure in the OM images. In addition, these grain boundaries showed the serrated feature. The main reason of this feature was the process of elongation and deformation accompanied by the migration of grain boundaries. It is worth noting that in the IPF diagram, there were two regions in a grain that were similar in color but were not divided into new grains. The region of A1 and A2 and the region of B1 and B2 are shown in Figure 8(b), respectively. Sun et al. [12] considered that it was covalent grain boundary, which appeared in the different gray levels in the same grain. This phenomenon provided a larger space for dislocation movement. Besides, it was indicated that the microstructure inside of the grains was inhomogeneous during deformation, which caused the occurrence of DRX. Figure 8(c and d) provides the evidence for this conclusion. Therefore, the covalent grain boundary affected the dislocations and promoted the DRX of the grains, which increased the strength of the material.

Figure 8

Inverse pole figure of the samples at the deformation of two-pass: (a) 1 s⁻¹; (b) 5 s⁻¹, misorientation angles of the samples at the deformation of two-pass; (c) 1 s⁻¹; and (d) 5 s⁻¹.

In order to better explore the impact of covalent grain boundary, the grain orientation of (10−12) was used to explore this phenomenon. Fan et al. [27] mentioned that the appearance of different gray levels would have an effect on the appearance of subgrains. Showing the lattice phase, a polyline was drawn to better explain the misorientation in Figure 9 and to understand the inhomogeneity of the microstructure. This low angle of dislocation relationship existed even within the same grain orientation. Thus, such low-angle dislocations were consumed in large quantities by DRX, so that the grain structure could be refined. As the amount of deformation increased and the temperature decreased, the gradual globularization of the lamellar α phase was also regarded as DRX.

Figure 9

Inverse pole figure of the sample at the grain orientation of (10−12) after deformation, and the misorientation along the black line.

The OM images of the TC4 titanium alloy deformed at 950°C are shown in Figure 10. The amount of three-pass deformation was 20% (2.4 mm). Overall, the grain size decreased with the increase in strain rate in Figure 10. It refined the grains from about 150 µm (Figure 10(a)) to about 15 µm (Figure 10(b and c)). The degree of grain refinement was significantly improved compared to the grain of one-pass and two-pass deformations. A large number of characteristics of DRX were clearly presented at the grain boundaries in Figure 10(c). It was remarkable that the elongated dynamic recovery (DRV) grains appeared in Figure 10(d), and the process of DRX was produced during the continuous deformation of the DRV grains (shown in the rectangle). Moreover, the DRX grains also grew at the grain boundaries (shown in the ellipse of Figure 10(c)). Combined with the stress–strain curve of Figure 2, it could be seen that at higher strain rates (5 s⁻¹), the grains undergone more DRV processes, especially when the deformation reached 70%. In addition, with the increase in strain rates, the tendency was shown about the transformation from the lamellar α phase to the equiaxed α phase at this higher deformation (shown in ellipse of Figure 10(d)). It could not be ignored that the temperature of 950°C at this time also had a large influence with the appearance of the equiaxed α phase.

Figure 10

OM images of the samples at the deformation of three-pass: (a) 0.01 s⁻¹, (b) 0.1 s⁻¹, (c) 1 s⁻¹ and (d) 5 s⁻¹.

Figure 11 is represented from the perspective of EBSD and is consistent with the view of the OM. It could be clearly seen that after 70% hot deformation, the increase in strain rate had a great influence on the microstructure of the TC4 titanium alloy. At the high strain rate (5 s⁻¹), DRX existed not only in the grains but also at the grain boundaries (Figure 11(c)). With the decrease in strain rate, the DRX grains gradually became larger. At the strain rate of 0.1 s⁻¹, the DRX grains grew into fine grains with the size of 10–20 µm. However, the grains at this time had the HAGB, which meant more dislocation accumulation. The increase in dislocation accumulation would directly affect the strength and plasticity of the TC4 titanium alloy. For Figure 11(e and f), there were low-angle grain boundaries, and two of the figures were close. Therefore, the sensitivity of the response from the strain rate to the changes of microstructure and grain refinement decreased, when it was higher than 0.1 s⁻¹.

Figure 11

Inverse pole figure of the samples at the deformation of two-pass: (a) 0.1 s⁻¹, (b) 1 s⁻¹, (c) 5 s⁻¹, misorientation angles of the samples at the deformation of two-pass: (d) 0.1 s⁻¹, (e) 1 s⁻¹ and (f) 5 s⁻¹.

3.2.3 Evolution of microstructure

In this part, the changes of microstructure and grain refinement were mainly discussed under the influence of hot deformation. In the previous comparison, it was believed that the change of temperature had more important effect on the microstructure of the TC4 titanium alloy. As it was known that the lath α and the lamellar α tended to be the equiaxed α phase when the TC4 titanium alloy was cooled from the high temperature, and the discontinuous α phase was the transition phase of this process. The variation in this feature is illustrated in Figure 12. In Figure 12(a), the area of line scan was divided into three nodes, which represented the β-transformed phase. It is shown as bright silver in the figure according to the color contrast. The part between these three nodes was the α phase. It could be found that the α phase at this time was lamellar α, and the lamellar α near node 1 thickened in the figure which showed the beginning of the roughened α-phase, which was shown in the ellipse in Figure 12(a). As the deformation continued, squeezing and stacking began to occur between two adjacent thicken lamellar α. In Figure 12(b), three equiaxed α phases were used as the main regions, and it could be seen that there were β-transformed phases between the three equiaxed α phases. Many white area were shown in the SEM figure that the area between the α phases, which were obtained by squeezing and stacking between the two thicken α phases mentioned above (shown in the ellipse). Therefore, the dislocations formed, and the method of energy conversion was to transfer from high potential energy to low potential energy. During the thickening of the α phase, due to the accumulation of more and more α phase and β-transformed phase affected by dislocations, the potential energy continued to increase under the influence of high temperature. But when the temperature gradually decreased, the lamellar α gradually tended to be equiaxed. However, if the potential energy was not enough to deform it, the morphology of the lamellar α was maintained. The evolution process is shown in Figure 13. The effect of strain rate on the grain refinement of TC4 titanium alloy was more obvious. This change was very limited for the microstructure mainly composed of lamellar α phase and martensite. When the lamellar α phase was gradually transformed into the equiaxed α phase, the high dislocation density during hot deformation contributed to grain refinement and significantly promoted uniform nucleation.

Figure 12

The line scan of energy-dispersive X-Ray spectroscopy on the SEM of the TC4 titanium alloy.

Figure 13

The evolution process of the α phase and β-transformed phases.