Análisis de Objetivos

Following compression testing which was discussed in Chapter 4, specimens deformed at 300°C, 10-1 s-1, representing a high Z condition (low T and high ) and 500°C, 10-3 s-1, representing a low Z conditions (high T and low ), were solution treated at 540°C for 2 h, an industrial standard solution treatment parameter. In this study, EBSD was utilised to examine the static softening behaviour. Similar to the previous examinations, in this analysis, low angle grain boundaries (LAGBs), 0 - 8o [102], are indicated with grey lines,

medium-angle grain boundaries (MAGBs), 8 – 15o [102], are indicated with white lines and high-angle grain boundaries (HAGBs), >15o [102], are indicated with black lines.

Fig. 6.1. Effect of temperature and strain rate on the microstructural development after solution treatment at 540°C for 2 h on the materials deformed at temperature of 300°C with

a strain rate of 10-1 s-1 (left) and 500oC with a strain rate of 10-3 s-1 (right); (a-b) matrix alloy, (c-d) 0.2Ti alloy and (e-f) 0.2Ti-4.5TiB2 composite

(a) (b)

(c) (d)

0 10 20 30 40 50 60 0.00 0.01 0.02 0.03 0.04 0.05 0.06 Number Fr action

Misorientation Angle (degrees)

0 10 20 30 40 50 60

Misorientation Angle (degrees)

Fig. 6.2. Comparison of misorientation profile of as-compressed (left) and as-annealed (right) 0.2Ti-4.5TiB2 specimens deformed at 500°C with a strain rate of 10-3 s-1

Compared with the as-compressed microstructure which has been described in Chapter 4, it can be clearly seen that in general, all of the studied materials were generally fully recrystallised following annealing at 540°C for 2 h. This was indicated by as-annealed boundaries which mostly consisted of HAGBs as shown in Fig. 6.1. This was confirmed by the misorientation profile (which is listed in Appendix C for brevity) which showed an increment of HAGBs following annealing at 540°C for 2 h. However, an interesting finding was observed in the 0.2Ti-4.5TiB2 composite. If we compared its microstructure with the as-compressed as presented in Chapter 4, a significant change in the grain orientation was evident following annealing of a specimen deformed at 300°C, 10-1 s-1. A fine grain structure was observed along the grain boundaries of the 0.2Ti-4.5TiB2 composite suggesting recrystallisation occurred following the annealing treatment. In contrast to that, the deformed microstructure seemed to be retained following annealing of the specimen deformed at 500°C, 10-3 s-1 as no significant change was observed in this microstructure. However, by comparing the misorientation degree between as-deformed and as-annealed microstructures, a lower density of LAGBs and a higher density of MAGBs and HAGBs (Fig. 6.2) were observed in the as-annealed microstructure. This might suggest a coarsening of subgrains and/or recrystallisation.

Furthermore, coarsening of the grain size was evident following subsequent annealing of a deformed specimen at high temperature and low strain rate as shown in Fig. 6.3a. A coarser grain size was achieved in all studied materials following annealing of the specimen deformed at 500°C with a strain rate of 10-3 s-1 (low Z) compared to the specimen

deformed at 300°C with a strain rate of 10-1 s-1(high Z). According to the literature [30,175], a higher dislocation density was created during deformation at a low temperature and high strain rate (high Z) compared to the deformation at high temperature and low strain rate (low Z). The higher flow stress achieved during deformation at high Z as described in Chapter 4 gave evidence to this theory. This higher dislocation density obtained during deformation leads to the higher stored energy, providing a higher driving force for static recrystallisation (SRX) on subsequent annealing. And hence, finer grains were achieved on the subsequent annealing of a specimen deformed at 300°C with a strain rate of 10-1 s-1. In contrast to that, dislocations are more mobile during deformation at low Z conditions, leading to dislocation annihilation [30,102]. This was reflected in the low flow stress achieved on deformation at low Z as has been explained in Chapter 4. Moreover, extensive dynamic recovery (DRV) observed on the deformed specimen also supported this postulation. As a result, less energy will be stored as a driving force for SRX, and hence coarse grains were observed upon annealing of a specimen deformed at 500°C with a strain rate of 10-3 s-1. In other words, the amount of recovery occurring before recrystallisation during annealing in this particular deformed specimen was dominant, resulting in coarser grains. The same results was observed in previous observations by Sheppard [191] and M. Ferry [192].

Fig. 6.3. (a)Recrystallised grain size following annealing at 540°C for 2 h as a function of compression parameter showing larger grains on the subsequent annealed specimen which was deformed at high temperature and a low strain rate (low Z); (b) recrystallised grain

size as a function of flow stress

0 50 100 150 200 250 300 500oC, 10-3 s-1 Grai n Siz e (  m) Compression Parameter 0.2Ti-4.5TiB2 0.2Ti matrix alloy 300o_{C, 10}-1 _s-1 0 20 40 60 80 100 120 140 160 20 40 60 80 100 120 140 160 180 200 0.2Ti-4.5TiB₂ 0.2Ti matrix alloy Grai n Siz e (  m)

Flow stress (MPa)

The above observation leads the author of this thesis to the hypothesis in which the static recrystallisation behaviour of the prior hot deformed specimen is related to its flow stress. Consequently, Fig. 6.3b is presented to investigate this hypothesis. It can be clearly seen that in all the studied materials, a higher flow stress achieved during deformation led to finer recrystallised grains on subsequent annealing. Similar observations were also obtained in earlier research by Sheppard and Vierod [191]. They stated that the flow stress achieved on deformation is related to the energy stored for static recrystallisation upon annealing. It has been observed by them that any decrement in the flow stress resulted in a decrement in the driving force for static recrystallisation. Consequently, it could be understood why coarser grains were achieved on the as-annealed specimen following deformation at a low flow stress levels. This observation could be very useful for industry in which if the stress is kept low during processing, nucleation will be retarded and a coarse microstructure and/or retainment of the deformed microstructure will be obtained during subsequent heat treatment.

6.2.2. Nucleation of Recrystallisation

It has been known that during annealing after hot deformation, nucleii for recrystallisation can be formed at various sites, depending on its hot deformation parameters and initial microstructure [175]. In the matrix alloy and 0.2Ti alloy, fine recrystallised grains at grain boundaries (indicated with black arrows in Fig. 6.1a-c) were evident upon annealing following deformation at a low temperature and a high strain rate (high Z). This suggests that recrystallised grains might nucleate at grain boundaries during annealing of a specimen deformed at high Z. Shear bands observed on the as-compressed specimen might also act as nucleii for recrystallised grains. Furthermore, recrystallised grains were also estimated nucleating in the vicinity of Al3Ti particles in the 0.2Ti alloy. Upon annealing after hot deformation at high temperature and low strain rates (low Z) in which recrystallisation was sluggish, indicated by coarse grains, the nucleation sites were estimated to be the initial grain boundaries. As has been explained above, a high amount of recovery was expected to occur before recrystallisation during annealing in this specimen which led to the retardation of recrystallisation. Consequently, recrystallised grains observed upon annealing in this specimen were estimated to be a result of subgrain boundary migration which transform into grain boundaries during annealing.

0 20 40 60 80 100 0 5 10 15 20 25

Line Intercept Grain Size (m)

Freq

ue

nc

y

(%)

Fig. 6.4. (a)Annealed microstructure and (b) grain size distribution of the 0.2Ti-4.5TiB2 composite deformed at 300°C, 10-1 s-1, showing heterogeneity of recrystallised grains

On the other hand, in the 0.2Ti-4.5TiB2 composite, fine recrystallised grains were observed nucleating in the vicinity of clusters of TiB2 particles located at the grain boundaries upon subsequent annealing of specimen prior hot deformed at high Z as is shown in Fig. 6.4a. Compared with the other two alloys, it was noted that heterogeneous recrystallised grains were obtained in the 0.2Ti-4.5TiB2 composite as illustrated in Fig. 6.4b. Fine recrystallised grains were mainly observed at the grain boundaries, whereas in other areas, large recrystallised grains were observed. Heterogeneous recrystallised grains achieved upon annealing of this particular specimen were hypothesised due to heterogeneous deformation zones which occured during deformation. The area adjacent to the TiB2 clusters at grain

(a)

boundaries was postulated to have a higher accumulated strain. In addition to that, the discrepancy of thermal expansion between TiB2 clusters and the matrix will create local deformation at the interfaces. On the subsequent annealing, subgrain boundaries will be formed in this area and convert into recrystallised grains [176,193].

0 10 20 30 40 50 60 70 Recovery Freq ue nc y (%) Recrystallisation

Fig. 6.5. Microstructure of the 0.2Ti-4.5TiB2 composite previously deformed at 500°C at a strain rate of 10-3 s-1 and subsequently annealed at 540°C for 2 h. Black areas indicate TiB2

particles

In contrast to that, similar to the other two alloys, with increasing deformation temperature and decreasing strain rate, nucleation tended to be less effective in the 0.2Ti-4.5TiB2 composite. It was proved by Fig. 6.5 that recovery dominated following annealing of a specimen deformed at high temperature and a low strain rate. Discontinuous

(a)

recrystallisation was suppressed during annealing. Fig. 6.5 shows the microstructure of the annealed microstructure of the material deformed at a high temperature and a low strain rate which contained a mixture of HAGBs and LAGBs. It was noted that the HAGBs were mostly observed associated with TiB2 clusters (indicated with a black arrow). As was explained in Chapter 4, deformation at this temperature and strain rate involved the formation of subgrains. Highly misoriented subgrains were observed adjacent to TiB2 clusters upon deformation and hence during annealing, these subgrains will convert into new grains.