Characterization of Complex Microstructure in the Selective Laser Melted Near-α Ti-6Al-2Sn-4Zr-2Mo Alloy Using Correlative Scanning Electron Microscopy and Scanning Transmission Electron Microscopy

Due to improved creep properties compared with Ti-6Al-4V (wt.%) alloy, near-α Ti-6Al-2Sn-4Zr-2Mo (wt.%, Ti-6242) has been widely used in the aerospace industry for high temperature application [1]. Recently, additive manufacturing, e.g., selective laser melting (SLM) technology, has gained drastic acceptance in manufacturing different titanium alloys, to overcome the fabrication difficulties of these alloys [2]. In order to optimize the SLM process of titanium alloys, it is of critical importance to study the complex microstructure in these SLMed titanium alloy under the complicated rapid heating and cooling processes. Unlike widely studied SLMed Ti-6Al-4V alloy, there still lacks detailed study on microstructure in the SLMed near- α Ti-6242 alloy. Especially, prior research could not unambiguously differentiate the hexagonal closed packed (hcp) α’ martensite from the hcp α phase formed by diffusional phase transformation in the as-printed microstructure. Because both α’ martensite and α phase share the identical hcp crystal structure and nearly the same lattice parameters. Thus, in this presentation, we will introduce our latest study of the microstructure in the SLMed Ti-6242 using correlative scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM), to identify the α’ martensite and/or α phase. In the first part of this work, the microstructure in the SLMed Ti–6242 along all three directions, e.g., scanning, transverse and build directions, was studied via SEM backscattered electron (BSE) imaging, electron backscatter diffraction (EBSD) and X-ray energy dispersive spectroscopy (XEDS) using the ThermoFisher Scientific Scios 2 dual-beam focused ion beam- scanning electron microscope (DB FIB-SEM). The EBSD inverse pole figure map(IPF) in Fig. 1(a) shows that coarse columnar grains are formed along the building direction. The BSE SEM images in Fig. 1(b-c) reveal the presence of complex hierarchical precipitates microstructure across the prior β grains. Additionally, the SEM BSE image in Fig. 1(c) show the presence of some nanotwins across the laths, indicating the formation of α’ martensite. The SEM XEDS mappings in Fig. 1(d) show all elements are almost uniformly distributed in the sample, supporting that the observed precipitates belong to α’ phase. The second part of this work focuses on the study of observed precipitates via TEM and STEM using FEI Talos F200s scanning transmission electron microscope (S/TEM). The selected area diffraction pattern (SADP) and the corresponding dark field image in Fig. 2(a) verify that the observed laths belong to the hcp structure. The STEM XEDS mappings in Fig. 2(b-c) revealed small amount of Ti and Al enrichment in some of the laths observed. Thus, the SEM, TEM and STEM results indicate that there are small number of α laths formed compared with many martensitic α’ laths formed in the SLMed Ti-6242 [3]. Characterization of SLMed Ti-6242 using SEM: (a) SEM EBSD IPF showing the coarse columnar grain; (b-c) SEM BSE images showing the complex precipitates microstructure; (d) SEM XEDS mapping showing the uniform distribution of elements. Characterization of SLMed Ti-6242 using TEM and STEM: (a)TEM SADP and corresponding dark field image showing the formation of hcp structure; (b-c) STEM XEDS mapping showing the formation of α and α’ laths.