Investigation of the Dynamic Recovery and Recrystallization of Near-β Titanium Alloy Ti-55511 during Two-Pass Hot Compression

Round 1

Reviewer 1 Report

This study aims to investigate the microstructure features of near-β alloy during two-stage thermal compression on a Gleeble 3800. Authors studied grain refining mechanisms: dynamic recovery and dynamic recrystallization at different stages and at different temperatures of deformation. However, the more global purpose of the study is not disclosed. Also there are some comments:

1. The practical significance and applicability of this study has not been explained.
2. What method was used to determine the temperature of the β -transus?
3. In the description for Figure 1 there is information about the shape of grains of α -phase and their sizes are given, but which shape and size does grains of β-phase have?
4. For what purpose was stage H used and how was the holding time at 950 C (3 minutes) chosen?
5. How was the strain rate of 0.1 s-1 chosen? Have preliminary tests been carried out for this?
6. Describe in more detail what is the reason for grain growth with an increase in the deformation temperature from 700 to 750 °C?

Author Response

1. (line 76-83) Multi-pass forging is the main processing method of Ti-55511 ingots, and there is limited research on the interactions between deformation mechanisms during multi-pass deformation. In this study, 4 types of two-pass hot-compression methods were designed to investigate the influence of the first-pass deformation restoration behavior on subsequent deformation. This work for Ti-55511 provides essential references for process parameters optimizing and microstructure controlling under multi-pass forging conditions.
2. (line 86) The β-transition temperature was about 875 ºC by using the metallographic method.
3. Figure 1 is replaced with a metallographic image of β grains and a clearer SEM image of α phase. (line 86-89) Figure. 1 shows the original microstructure composed of large β grains with an average size of 300 µm. Inside β grains, there are α duplex microstructure including 1) an equiaxed α grain with a diameter of 5–8 µm and 2) a lamellar α grain with a length of 3–5 µm.
4. In order to ensure that the core and edge temperatures of the sample before the second-pass deformation are 950 ºC, 3 minutes is the most suitable time for stage-H.
5. The deformation rate during the forging process in the factory is close to 0.1s^-1. This work is a simple simulation of the forging process.
6. (line 178-183)

   Figure. 7a shows the sample microstructure after being deformed at 700 ºC and then holding at 950 ºC for 3 min. The β grain boundaries show the zigzag lines, and the β grain size is significantly finer than other samples  This is because the β grain size is related to the recrystallization nucleation rate and the grain boundary migration rate [33]. A high nucleation rate and a small grain boundary migration rate are conducive to grain refinement. Meanwhile, the rate of grain boundary migration (v) and the effective pressure (P_Eff) on the grain boundary satisfy equation:

   v=mPEff

   (1)

   Effective pressure (P_Eff) is the sum of driving pressure (P_D) and retarding pressure (P_R). Driving pressure (P_D) is related to dislocation density:

   PD=(ρ2-ρ1)τ

   (2)

   Where τ is the energy of per unit length dislocation, ρ the dislocation density. A large amount of recrystallized nucleation has been observed in the sample deformed at 700°C, and the generation of β recrystallized grains reduces the dislocation density in the β matrix. This results in the β grain size in Figure. 7a is much smaller than other samples. 

Reviewer 2 Report

This paper investigates recovery and recrystallization of beta Ti alloys processed by two-pass hot compression. There are some interesting recovery behavior and this paper is suitable for publication after the following rivision.

1. The SEM image is not clear. Please replace to a clear image. Please show the grain size of the initial beta phase.
2. Line 132, “the sample deformed at 700C contains less alpha phase.” Does this mean 750 C? The volume fraction of the alpha phase in the sample deformed at 700C is minimum among the tested alloys.
3. In Fig, 5, please show the crystal lattice direction map.
4. 5 and 6 indicates high strain in the sample deformed at 850C, but generally, deformation at high temperature in alpha + beta phase progress recrystallization. So the trend of your observation seems the opposite trend. How does the authors think about this?
5. Are Fig. 7, Fig. 8, and Fig. 9 IPF maps? Or is the color of grains randomly used? Please clarify which EBSD map is shown. If they are IPF map, please show the crystal lattice direction map.
6. In Fig. 8, How does the author identify CDRX and DDRX?
7. Line 242, “line 1 and line in Figure 6a and 6c” I can not see any lines in Fig. 6. Please clarify the lines if there are in Fig. 6.
8. L1 and L2 in Figure 9 are analyzed in Fig. 10. So, please represent L1 and L2 instead of line 1 and line 2 in Fig. 10.

Author Response

1. Figure 1 is replaced with a metallographic image of β grains and a clearer SEM image of α phase. (line 86-89) Figure. 1 shows the original microstructure composed of large β grains with an average size of 300 µm. Inside β grains, there are α duplex microstructure including 1) an equiaxed α grain with a diameter of 5–8 µm and 2) a lamellar α grain with a length of 3–5 µm.
2. (line 136) The sample deformed at 750 ºC contains less α phase.
3. The crystal lattice direction maps are added to Figure. 5.
4. Recrystallization will preferentially appear in the places with higher deformation energy, such as grain boundaries and phase interfaces. Figures 5 and 6 only show the recrystallization conditions inside the β grains. Since there are few phase interfaces, it is not conducive to recrystallization.
5. The crystal lattice direction maps are added to Figure. 7, Figure. 8 and Figure. 9.
6. DDRX has obvious nucleation and growth stages. The most common microstructure is new grains bulging out from boundaries. CDRX evolves relatively homogeneously throughout the entire grains, without recognisable nucleation and growth of the recrystallized grains.
7. (line 256) There are in Figure. 9.
8. L1 and L2 have been changed