Microstructure Evolution and Phase Transformation of Ti-6.5Al-2Zr-Mo-V Alloy During Thermohydrogen Treatment

doi:10.1007/s40195-015-0226-8

Abstract

Abstract:

Thermohydrogen treatment (THT) is an effective way to refine microstructure and improve the mechanical properties of the titanium alloys. In the current work, as-cast Ti-6.5Al-2Zr-Mo-V alloy was hydrogenated with different hydrogen contents and processed solution aging. Accordingly, the microstructure evolution and phase transformation were analyzed. Results show that during solution aging, eutectoid decomposition occurs and the product is a mixture of coarse primary α, fine eutectoid product and undecomposed β_H. The size of primary α is closely dependent on the hydrogen content, and large primary α can be obtained at medium hydrogen content. Further, the influence of hydrogen content on the growth of primary α phase was revealed. The primary α is much fine, and the eutectoid product is relatively homogeneous with 0.984 wt% H. After THT, the ultimate strength is beyond 1,100 MPa that has increased by 23.15% compared with that in as-cast state.

Key words: Titanium alloy, Hydrogen, Heat treatment, Phase transformation, Eutectoid decomposition, Mechanical property

Hong-Chao Kou, Han-Lei Zhang, Yu-Dong Chu, Dong Huang, Hai Nan, Jin-Shan Li. Microstructure Evolution and Phase Transformation of Ti-6.5Al-2Zr-Mo-V Alloy During Thermohydrogen Treatment[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(4): 505-513.

Figures/Tables 10

Table 1 Chemical compositions of the as-cast Ti-6.5Al-2Zr-Mo-V alloy (wt%)

Ti	Al	Zr	Mo	V	Fe	Si	C	N	H	O	Other
Balance	6.0	2.0	1.7	2.2	0.04	0.05	0.02	0.01	0.001	0.14	0.30

Fig. 1 a Photographs of the as-cast rod and the cylindrical sample, b hydrogenation and solution aging, c various hydrogen contents obtained during hydrogenation

Fig. 2 XRD patterns of the as-cast microstructure and the hydrogenated microstructures

Fig. 3 Optical micrographs of the as-cast microstructure a and the hydrogenated microstructure (0.984 wt% H) b

Fig. 4 XRD patterns of the aged microstructures

Fig. 5 SEM micrographs of the aged microstructures. a 0.259 wt% H, b 0.430 wt% H, c 0.569 wt% H, d 0.820 wt% H, e 0.984 wt% H

Fig. 6 a TEM micrograph of the aged microstructure (with 0.984 wt% H), b SADP of a coarse primary phase, c SADP of a fine primary phase

Fig. 7 a Content of βH at the end of hydrogenation and aging, b size of primary α phase at the end of aging

Fig. 8 Effect of hydrogen content on the size of primary α phase. Curve 1 shows effect of undercooling on α size, curve 2 shows effect of eutectoid decomposition, and curve 3 shows sum of the two effects. A pseudo-Ti-H diagram is presented to show undercooling and formation of δ. Formation of primary α at three different hydrogen contents (low, medium and high) is presented. Rp0.2 is yield strength, Rm is ultimate strength, A is unit extension, and Z is reduction of area

Fig. 9 Tensile properties of the as-cast sample and the sample processed with THT

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