Precipitates and alloying elements distribution in near α titanium alloy Ti65

doi:10.1016/j.jmst.2019.03.018

Abstract

Abstract:

Precipitates, including silicides and Ti₃Al (α₂) phase, and alloying elements distribution in a near α titanium alloy Ti65 (Ti-5.8Al-4.0Sn-3.5Zr-0.5Mo-0.3Nb- 1.0Ta-0.4Si-0.8W-0.05C) after solution treatment and aging process were characterized by using transmission electron microscopy (TEM) and atom probe tomography (APT). Quantitative composition analysis and TEM observation indicate that the silicides fit to (Ti, Zr)₆(Si, Sn)₃. Zr exhibits a β-stabilizing effect in near α titanium alloys but is weaker than other β stabilizing elements. The enriching tendency of the alloying elements in the retained β phase is in the order of Zr < Nb < Ta < Mo < W. The experimental results are rationalized by the relative stability of alloying elements in the α and β phases and the mobility of these atoms in the matrix. An enrichment of Si in the α₂ phase over the α matrix phase is noticed, which is attributed to the lower formation energy of Si in the α₂ phase.

Key words: Atom probe tomography, Titanium alloys, Precipitation, First-principles calculation, Alloying elements distribution

Ke Yue, Jianrong Liu, Haijun Zhang, Hui Yu, Yuanyuan Song, Qingmiao Hu, Qingjiang Wang, Rui Yang. Precipitates and alloying elements distribution in near α titanium alloy Ti65[J]. J. Mater. Sci. Technol., 2020, 36: 91-96.

Figures/Tables 7

Fig. 1. (a) Bimodal microstructure for Ti65 alloy after solution treatment and aging process; (b) the morphology of silicides at α/β interfaces with indexed diffraction spots; (c) the dark field image of α2 phase with the corresponding SAD pattern containing matrix and α2 phase.

Fig. 2. (a) 3D reconstruction of Si atom map with 5 at.% Si isosurface demarcated; (b) one dimension composition profile across the boundary between matrix and silicides, containing Al, Zr, Si, Ti, and Sn elements.

Table 1 Atomic percent of alloying elements in silicides (at.%).

Elements	Ti	Zr	Si	Sn
Composition (at.%)	36.51	28.53	28.95	2.75

Fig. 3. (a) 3D reconstruction of alloying elements atom map with 5 at.% isosurface Si map; (b) one dimension composition profile along the cylinder of selected region across the α/β interface. Al, Zr, Sn and W refer to the left Y-axis, other elements to the right.

Table 2 Compositions of alloying elements for selected regions in Ti65 alloy.

Composition (at.%)	Al	Zr	Si	Sn	Mo	Nb	Ta	W	C
α_p	10.31	1.26	0.46	1.45	0.09	0.18	0.02	0.21	0.21
α_l	7.19	1.05	0.21	2.03	0.21	0.16	0.06	0.21	0.091
β	3.12	2.75	0.30	2.13	2.68	0.82	0.48	4.73	0.037
S₂	0.74	27.76	28.72	2.33	0.36	0.42	0.09	0.31	0.015
α₂	14.18	1.29	0.85	1.30	0.13	0.08	0.03	0.2	0.16
Nominal composition	10.28	1.86	0.69	1.63	0.25	0.16	0.27	0.21	0.22

Table 3 Concentration ratio (Cβ/Cα) of alloying elements in the α and β phases of Ti65 and the slop of the formation energy difference of alloying elements against the concentration x for binary Ti-X alloys (X=Zr, Nb, Ta, Mo, and W), d(ΔE)/dx.

Number of valence electrons, N_d	N_d = 2	N_d = 3		N_d = 4
Alloying elements	Zr	Nb	Ta	Mo	W
C_β/C_α	2.62	5.03	7.70	13.03	22.4
d(ΔE)/dx*	0.11	0.35	0.27	0.68	0.63

Fig. 4. (a) 3D reconstruction of Al clusters map; (b) 3D reconstruction of 15 at.% Al isosurface; (c) one dimension composition profile along the cylinder in (b). Al and Ti refer to the left Y-axis, other elements to the right.

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