Microstructure

The initial microstructure of the ST Ti15Mo alloy is shown in Figure 1. A coarse-grained (CG) structure consisting of grains of the average size of ~50 μm is well visible due to the channeling contrast. In addition, brighter and darker and areas are also visible in the material due to the chemical contrast (marked by yellow arrows in Figure 1). These chemical inhomogeneities were investigated by EDS. Table 1 summarizes the results of the EDS point analysis. Note that results were obtained by standardless EDS in which the measured spectra are compared to the data collected from standards by the EDS manufacturer under different conditions (namely different beam conditions). Such data are therefore subjected to systematic error and as such they are not fully reliable in terms of exact

quantitative Mo content determination. However, the relative difference in chemical composition between different areas is accurate and unambiguous.

**Figure 1.** Scanning electron microscopy–back-scattered electrons (SEM–BSE) micrograph of the solution-treated (ST) Ti15Mo alloy (the yellow arrows indicate chemical inhomogeneities in the material).

**Table 1.** Chemical composition of the darker and brighter bands (marked by yellow arrows in Figure 1) as determined by energy dispersive spectroscopy (EDS) point analysis.


Local chemical inhomogeneities in the ST material were also investigated by EDS mapping. In Figure 2a, several β-grains and darker and lighter areas (visible especially in the top left corner of the image) are visible due to channeling contrast and chemical contrast (*Z*-contrast), respectively. EDS mapping confirms the chemical inhomogeneity—darker areas in Figure 2a contain less Mo as shown in Figure 2b. Variations in the local content of Mo (β stabilizing element) may affect the phase stability of the β phase matrix.

**Figure 2.** Local chemical inhomogeneities in ST Ti15Mo alloy: (**a**) scanning electron microscopy– secondary electrons (SEM–SE) micrograph of the area of interest, (**b**) corresponding element map of Mo using EDS.

The chemical inhomogeneities were also studied in the HPT-deformed sample. The SEM–BSE micrograph in Figure 3 clearly shows lighter and darker bands corresponding to the chemical composition differences, which were also confirmed by EDS. Darker areas with lower Mo were formed from curly bands in the non-deformed material (Figure 1). In HPT deformed material, they are elongated in the direction of the deformation (Figure 3).

**Figure 3.** SEM–BSE micrograph of Ti15Mo alloy after high-pressure torsion (HPT) processing. Surface of HPT disk with highlighted direction of deformation (azimuthal direction). Darker and brighter areas are caused by the difference in chemical composition (black dots are polishing artefacts).
