Characterization of α2 Precipitates in Ti-6Al and Ti-8Al Binary Alloys: A Comparative Investigation

doi:10.1007/s40195-020-01149-2

Abstract

Abstract:

Transmission electron microscopy (TEM) and atom probe tomography (APT) techniques were used to investigate the nano-scale ordered α₂ (Ti₃Al) precipitates in Ti-Al binary alloys. Ti-6Al and Ti-8Al binary alloys were solution treated and aged to obtain Widmanstatten microstructure and promote α₂ precipitates. The TEM results displayed strong short-range ordering of α₂ precipitates in Ti-8Al alloy, while no evidence of the superlattice reflections of α₂ in Ti-6Al alloy. The results acquired from APT showed the α₂ clusters and atoms distribution at the interface between the matrix and α₂ precipitates. The size and morphology of α₂ particles in Ti-8Al alloy, respectively, obtained by TEM and APT are closely consistent. Meanwhile, the APT results displayed tiny size clusters in Ti-6Al alloy, which supposed to give evidence of the initial ordering process of α₂ precipitates in the absence of correlative results from TEM.

Key words: Ti-Al binary alloys, Ti₃Al precipitates, Transmission electron microscopy, Atom probe tomography

Sabry S. Youssef, Xiaodong Zheng, Yingjie Ma, Sensen Huang, Min Qi, Jianke Qiu, Ruixue Zhang, Peitao Hua, Shijian Zheng, Jiafeng Lei, Rui Yang. Characterization of α₂ Precipitates in Ti-6Al and Ti-8Al Binary Alloys: A Comparative Investigation[J]. Acta Metallurgica Sinica (English Letters), 2021, 34(5): 710-718.

Figures/Tables 8

Table 1 Chemical composition of Ti-Al binary alloys employed in this study (wt%)

Material	Al	O	N	H	C	Ti
Ti-6Al	5.8	0.048	0.0023	0.0024	0.016	Bal.
Ti-8Al	7.8	0.051	0.0035	0.003	0.013	Bal.

Table 2 Heat treatment procedure of Ti-Al alloys

Alloy	Thermal treatment: Solution treatment + Aging
Ti-6Al	1070°C, 1 h, F.C. + 650°C, 168 h, A.C.
Ti-8Al	1100°C, 1 h, F.C. + 550°C, 100 h, A.C.

Fig. 1 SEM morphologies of Widmanstatten microstructure of Ti-6Al alloy aged at 650 °C/168 h a, Ti-8Al alloy aged at 550°C/100 h b; the blue dashed line indicates the boundary for one of the prior β grains and the red line for the boundary of one of α colonies, c XRD patterns of the two alloys

Fig. 2 a Selected area [11-20] α diffraction pattern of Ti-6Al aged at 650°C/168 h showing no obvious reflections related to α2 precipitates, b superlattice reflections of α2 in SADP pattern of Ti-8Al alloy aged at 550°C/100 h, c dark filed image of the ordering regions in Ti-8Al alloy

Fig. 3 a Illustration of α2 phase observation under high resolution HAADF-STEM, b corresponding IFFT image of the α2 phase in Ti-8Al aged at 550°C/100 h, wherein the red circles and the arrows point to the α2 phase, and the inset shows FFT image matching to α2

Fig. 4 3D reconstruction maps and Al clusters morphology of Ti-8Al a and Ti-6Al c sample at 16 at.% and 14 at.% isosurface, respectively, proxigrams across the boundary between the aged matrix and several α2 precipitates in Ti-8Al b, Ti-6Al d sample, respectively, while the dotted vertical line corresponds to the concentration isosurface

Table 3 Average composition (at.% and wt%) of the matrix and Al-rich precipitates of Ti-Al alloys as measured by APT at two different isosurface values

Alloy	Isosurface value		Ti	Al
Ti-6Al	14 at.%	Matrix	87.1 at.% 93.7 wt%	10.3 at.% 6.2 wt%
Ti-6Al	14 at.%	Precipitates	73.9 at.% 85.2 wt%	22.6 at.% 14.7 wt%
Ti-8Al	16 at.%	Matrix	82.9 at.% 92.1 wt%	12.6 at.% 7.8 wt%
Ti-8Al	16 at.%	Precipitates	71.3 at.% 83.6 wt%	24.8 at.% 16.3 wt%

Fig. 5 3D reconstruction maps of one dimension (1D) analysis and composition profiles: a 1D cylinder and b atomic concentration for Ti-8Al sample, c, d for Ti-6Al sample

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Characterization of α₂ Precipitates in Ti-6Al and Ti-8Al Binary Alloys: A Comparative Investigation

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