Mechanical properties and deformation mechanisms of Ti-3Al-5Mo-4.5 V alloy with varied β phase stability

doi:10.1016/j.jmst.2018.04.004

Abstract

Abstract:

Evolution of deformation mechanisms and mechanical properties of Ti-3Al-5Mo-4.5 V alloy with different β phase stability have been systematically investigated. β phase stability alteration is achieved through quenching temperature variation from dual α + β field (700 °C) to single β field (880 °C). Tensile tests at ambient temperature show that apparent yield strength of the alloy experiences an abrupt decrease followed by a significant increase from 700 °C to 880 °C. Work hardening behavior is characterized by transition from the initial two-regime feature to the three-stage outlook. Concurrently, the maximum working hardening rate drops from 14000 MPa to 3000 MPa, which is concurrent with the shrinking volume fraction of primary α phase. Detailed discussion about the relationship between deformation mechanisms and β phase stability has been outlined.

Key words: Dual-phase titanium alloy, β phase stability;, Work hardening behavior, Deformation mechanisms

Q. Xue, Y.J. Ma, J.F. Lei, R. Yang, C. Wang. Mechanical properties and deformation mechanisms of Ti-3Al-5Mo-4.5 V alloy with varied β phase stability[J]. J. Mater. Sci. Technol., 2018, 34(12): 2507-2514.

Figures/Tables 10

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

Al	Mo	V	Fe	O	N	H	Ti
2.92	5.00	4.47	0.034	0.090	0.007	0.005	Bal.

Fig. 1. Ambient temperature tensile true strain/true stress curves (solid black lines) with different heat treatment schemes: (a) ST700, (b) ST750, (c) ST800 and (d) ST880. Corresponding strain-hardening rate profiles, dσ/dε, are plotted in red dots, respectively. Work hardening rates are divided into three stages during the deformation process. Region I: work hardening rate decreases (a, b, c and d) in the elastic and plastic transition regimes. Region II: work hardening rate increases (b, c and d) or stay constant (a). Region III: work hardening rate keeps decreasing from maximum value to 0 (b, c and d).

Table 2 Volume fraction of primary α phase and mechanical properties (YS, UTS and elongation at UTS) of Ti-3Al-5Mo-4.5 V alloy with different heat treatment schemes.

Heat Treatments	Volume fraction of primary α phase	YS (MPa)	UTS (MPa)	Elongation at UTS (%)
ST700	51.6%	673	773	6.8
ST750	38.8%	463	807	8.1
ST800	22.7%	343	843	11.0
ST880	0	607	910	14.8

Fig. 2. Typical SEM images for samples after tensile deformation: (a) ST700 exhibits thin and river-like lines; (b) ST750 shows parallel thin lines; (c) the intersecting laths with different orientation in β matrix of ST750; (d) the thick and parallel laths formed after plastic deformation in ST750; (e) the thick laths in β matrix of ST800 indicated by white arrows.

Fig. 3. TEM analysis of ST700 and ST750 samples after tensile deformation: (a) BF image of dislocations in ST700; (b, c) BF and DF images of SIM in ST750; (d) SAED pattern along the [1-1-1]β zone axis taken from the region of Fig. 3(c); (e) morphology of {332} <113> β twins and SIM α” in ST750; (f) DF image of {332} <113> β twins in ST750(where T indicates Twinning); (g) DF image of SIM α” in ST750; (h) SAED patterns of [-110]β∥[-110]βT and [-110]β∥[-12-3] α” zone axis in ST750.

Fig. 4. {112} <111> β twins in ST750 sample after tensile deformation: (a) BF and (b) DF images of nano-sized {112} <111> β twins (where T indicates twinning); (c) SAED pattern along [-110]β zone axis showing both twin and matrix lattices.

Fig. 5. TEM observations of ST800 sample after tensile deformation: (a) BF image of SIM and athermal α” martensite; (b) DF image of SIM α”1; (c) DF image of athermal martensite α”2; (d) SAED pattern of β[1-1-1]∥α”1[001] and β[1-1-1]∥α”2[-101] zone axis.

Table 3 The chemical composition and [Mo]eq. of transformed β in the Ti-3Al-5Mo-4.5 V alloy obtained by quantitative EPMA analysis[21].

Samples	Elements(wt.%)
Samples	Ti	Al	Mo	V	[Mo]_eq.
ST700	82.63	2.04	8.73	6.66	13.19	2.3787	2.7896
ST750	84.08	2.15	7.70	6.26	11.89	2.3824	2.7875
ST800	86.55	2.44	6.21	5.20	9.69	2.3923	2.7830
ST880	88.42	2.86	4.90	4.28	7.77	-	-

Fig. 6. The primary deformation mechanisms of β phase in Ti-3Al-5Mo-4.5 V alloy as a function of β phase stability [Mo]eq.. Point 1, 2, 3 and 4 represent ST880, ST800, ST750 and ST700, respectively.

Fig. 7. Phase stability index diagram based on M?dˉ and B?oˉ parameters. Point 1, 2 and 3 represent ST700, ST750 and ST800, respectively.

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