EBSD Study of Microstructural and Textural Changes of Hot-Rolled Ti-6Al-4V Sheet After Annealing at 800 °C

doi:10.1007/s40195-018-0768-7

Abstract

Abstract:

In this paper, electron backscatter diffraction and various other characterization and analysis techniques including X-ray diffraction, electron channeling contrast imaging and energy-dispersive spectrometry were jointly employed to investigate microstructural and textural changes of a hot-rolled Ti-6Al-4V (TC4) sheet after annealing at 800 °C for 5 h. In addition, the hardness variation induced by the annealing treatment is rationalized based on revealed microstructural and textural characteristics. Results show that the TC4 sheet presents a typical dual-phase (α?+?β) microstructure, with α-Ti as the major phase and short-rod-shaped β-Ti (minority) uniformly distributed throughout the α matrix. Most of α grains correspond to the un-recrystallized structures with a typical rolling texture (c//TD and <11-20>//ND) and dense low angle boundaries (LABs). After the annealing, the stored energy in the as-received specimen is significantly reduced, along with greatly decreased LABs density. Also, the annealing allows recrystallization and grain growth to occur, leading to weakening of the initial texture. Furthermore, the water quenching immediately after the annealing triggers martensitic transformation, which makes the high-temperature β phases be transformed into submicron α plates. The hardness of the annealed specimen is 320.5 HV, lower than that (367.0 HV) of the as-received specimen, which could be attributed to reduced LABs, grain growth and weakened texture. Nevertheless, the hardening effect from the fine martensitic plates could help to suppress a drastic hardness drop.

Key words: Ti alloy, Electron backscatter, diffraction, Recrystallization, Texture, Hardness

Ji-Ying Xia, Lin-Jiang Chai, Hao Wu, Yan Zhi, Yin-Ning Gou, Wei-Jiu Huang, Ning Guo. EBSD Study of Microstructural and Textural Changes of Hot-Rolled Ti-6Al-4V Sheet After Annealing at 800 °C[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(11): 1215-1223.

Figures/Tables 9

Table 1 Chemical composition of the experimental material (wt%)

Al	V	Fe	C	N	H	O	Ti
6.10	3.98	0.21	0.011	0.027	0.05	0.12	Bal.

Fig. 1 XRD patterns of the as-received and the 800 °C-annealed specimens

Fig. 2 ECC observations of the as-received a, the 800 °C-annealed c specimens; white boxes in a, c are magnified in b, d, respectively, with different phases labeled

Fig. 3 EDS analyses of the as-received specimen ECC images of microstructures of the annealed specimen are presented in Fig. 2c, d, from which remarkable changes can be noticed. After annealing at 800 °C for 5 h, the matrix grains are found to be evidently coarsened and the prior β phases distributed along α grain boundaries (Fig. 2a) seem to be completely replaced by a type of equiaxed structures. Figure 2d further reveals that a large number of fine plates exist inside the equiaxed structures, with an average value of plate widths measured to be ~?240 nm. Malinov et al. [24] demonstrated rapid cooling like quenching in cold water was able to trigger martensitic β?→?α transformation in TC4 alloy. Considering the suppressed diffraction peaks of the β phase (Fig. 1) in the annealed specimen, the fine plate structures in Fig. 2d are believed to correspond to martensites (α′) transformed from grown β phases.

Fig. 4 EBSD characterization: a band contrast (BC), b inverse pole figure (IPF) maps of the as-received specimen; c BC, d IPF maps of the 800 °C-annealed specimen. Black and gray lines in b, d indicate grain boundaries with θ?>?15° and 2°?<?θ?<?15°, respectively

Fig. 5 Misorientation angle and rotation axis distributions of a the as-received, b the 800 °C-annealed specimens

Table 2 Fraction and specific lengths of LABs and HABs corresponding to the EBSD maps in Fig. 4

Microstructural parameters	As-received		800 °C-annealed
Microstructural parameters	LABs	HABs	LABs	HABs
Fraction (%)	65.8	34.2	49.4	50.6
Length per area (1/μm)	1.01	0.52	0.47	0.47

Fig. 6 Pole figures of a the as-received, d the 800 °C-annealed specimens; b, e recalculated pole figures for the circled orientations in a, d, respectively; c, f IPF maps corresponding to b, e, respectively

Fig. 7 Hardness variations of the specimens before and after annealing

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