*3.2. Phase Transformation*

X-ray Diffraction (XRD) is a commonly used phase analysis method [31,32]. The XRD patterns of the pre-placed powders and the FeCoCrNi coatings are shown in Figure 5, which shows that the HEA coatings are mainly composed of the FCC phase and BCC phase, precipitating a small amount of Fe-Cr phase and Laves phase, while the raw powders possess the single-phase FCC crystal structure only. The addition of the Ti element influences the valence electron concentration (VEC) inside the FeCoCrNi high entropy alloy, thus inhibiting the formation of the FCC phase in the cladding layer. Meanwhile, the atomic size of the Ti element is quite different from that of Fe, Co, Cr, Ni, which is beneficial to the formation of the BCC phase. It is also can be found from Figure 5 that the addition of the Ti element causes the change of the lattice constant of the FCC phase. This is also due to the above-mentioned atomic size. Compared with other elements, Ti has a larger atomic radius. Therefore, its addition led to severe lattice distortion, thereby increasing the lattice constant of the FCC phase.

**Figure 5.** XRD patterns of as-alloyed powder and high entropy alloy (HEA) coatings.

Besides, Ti and Co\Cr elements have a low composite enthalpy, hence, the excessively high temperature inside the molten pool provides an opportunity for the reaction between the elements. There is a grea<sup>t</sup> possibility for the Ti element in Case 3 to be combined with the Co element and Ni element to form (Co, Cr)2Ti Laves phase (as shown in Figure 5). However, the Ti element content in Case 1 is extremely small, and the XRD test result is the phase on the top of the cladding layer, so no detection results are consistent with the detection results of the high entropy alloy powder, and no obvious BCC phase and Fe-Cr phase are detected. As the specific energy increases, the Ti element content gradually increases, so the BCC characteristic gradually becomes obvious, and Fe-Cr can be detected. When the specific energy is too large, the Laves phase inside the cladding layer is significantly precipitated.
