Evolution of microstructure and phase composition of Ti-3Al-5Mo-4.5V alloy with varied β phase stability

doi:10.1016/j.jmst.2018.04.002

Abstract

Abstract:

The microstructure evolution and phase composition of an α + β titanium alloy, Ti-3Al-5Mo-4.5V (wt.%), have been investigated. Electron probe micro analysis (EPMA) quantitative results manifest that the stability of β phase decreases with increasing quenching temperature, which is influenced by the significant variation of β-stabilizing elements concentration. Detailed microstructure analysis shows that the β → ω phase transformation does occur when quenching at 750 °C and 800 °C. The ω-reflections change from incommensurate ω-spots (750 °C) to ideal ω-spots (800 °C) as the β stability of the alloy decreases. Further the decrease of β phase stability encourages the formation of athermal α′′ martensite, which has the following orientation relationships: [111]_β//[110]_α′′, [100]_β//[100]_α′′ and [-110]_β//[00-1]_α′′ with respect to the β matrix.

Key words: Dual-phase titanium alloy, β phase stability;, ω phase;, α′′ martensite;, Microstructure

Q. Xue, Y.J. Ma, J.F. Lei, R. Yang, C. Wang. Evolution of microstructure and phase composition of Ti-3Al-5Mo-4.5V alloy with varied β phase stability[J]. J. Mater. Sci. Technol., 2018, 34(12): 2325-2330.

Figures/Tables 9

Table 1 Chemical analysis of Ti-3Al-5Mo-4.5V (wt.%).

elements	Al	Mo	V	Fe	O	N	H
wt.%	2.92	5.00	4.47	0.034	0.090	0.0070	0.0050

Fig. 1. SEM micrographs of Ti-3Al-5Mo-4.5V alloy solution treated at different temperatures: (a) ST700, (b) ST750, (c) ST800 and (d) ST880, respectively.

Fig. 2. XRD profiles obtained from the samples (a) ST700, (b) ST750, (c) ST800 and (d) ST880 respectively.

Fig. 3. TEM analysis of (a) bright field (BF) image and SAED pattern (inset) of ST700, (b) SAED pattern (inset) and corresponding dark field image of ω phase in ST750, (c) SAED pattern and corresponding dark field image of ω phase in ST800. All diffraction patterns were taken with the beam direction parallel to [110]β. (d) schematic diagram illustrating β and ω spots in ST750 and ST800, respectively.

Fig. 4. TEM images of ST800: (a) bright field image; (b) SAED pattern along [110]β phase zone axis; (c) corresponding dark-field image of athermal martensite taken from the circled spot in (b); (d) indexed diagram corresponding to β phase, ω phase and α″ phase.

Fig. 5. Detailed TEM analysis of ST800: (a) SAED pattern along [111]β zone axis; (b) dark field image taken from the circled spot in (a) showing the athermal martensite; (c) SAED pattern along [100]β zone axis; (d) dark-field image of athermal martensite taken from [100]β zone axis in ST800.

Table 2 Phase composition and volume fraction of prior α phase of Ti-3Al-5Mo-4.5V alloy with different heat treatment schemes.

Heat treatment Schemes	Phase composition	Volume fraction of prior α phase
ST700	α + β	51.6%
ST750	α + β + ω	38.8%
ST800	α + β + ω + α″	22.7%
ST880	α″	0

Fig. 6. Element distributions of Ti-3Al-5Mo-4.5V alloy by EPMA with quenching temperature 700 °C (ST700), 750 °C (ST750), 800 °C (ST800) and 880 °C (ST880). All figures have the same bar.

Table 3 The chemical composition and Mo equivalent of transformed β phase in the alloy (wt.%, average five measurements) obtained by quantitative EPMA analysis.

Samples	Elements (wt.%)
Samples	Ti	Al	Mo	V	[Mo]_eq.
ST700	82.629	2.043	8.727	6.659	13.19
ST750	84.082	2.145	7.701	6.260	11.89
ST800	86.548	2.439	6.205	5.202	9.69
ST880	88.417	2.857	4.904	4.281	7.77

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