Fig. 1(a–f) shows the cross-sectional morphology and elemental distributions at the interface of the FeCoCrAlCu-xTiC HEA coatings. As can be seen obviously, a narrow zone of element interdiffusion showed in the transition zone (TZ), which presented good metallurgical bonds between the substrate and coating were achieved. For Fig. 1(e) and (f), the contents of element changed significantly from the coating to the inside of the substrate, especially in the TZ. It was believed that the elements did not have sufficient time to diffuse inside the molten pool during the LSA process. Moreover, the rapid directional solidification structure can be observed around the bonding area of the coating, and the growth direction of the columnar crystal was perpendicular to the interface. It was important to note that a thin layer of plane crystal was formed near the bonding zone, which was related to the solidification parameters of the degree of constitutional undercooling such as the thermal gradient (G) and the solidification rate (R). The G at the bonding zone of the coating and the substrate was very large, and the R was very small. Therefore, the crystal grew in the form of plane crystal from the substrate due to the G/R tended to infinity. Based on the above analysis results, it was concluded that the FeCoCrAlCu-xTiC HEA coatings fabricated by the laser surface alloying had the good formability.
Fig. 1. Morphology images and line scans of element near the fusion lines: (a) and (e) FeCoCrAlCu HEA coating, (b) FeCoCrAlCu-10 wt%TiC HEA coating, (c) FeCoCrAlCu-30 wt%TiC HEA coating, (d) and (f) FeCoCrAlCu-50 wt%TiC HEA coating.
Fig. 2 presents the SEM morphology of cross-section for FeCoCrAlCu-xTiC HEA coatings. For the FeCoCrAlCu HEA coating, it was mainly composed of banded microstructure, as shown in Fig. 2(a). Fig. 4(b) shows the SEM images of FeCoCrAlCu-10 wt%TiC composite coating. Obviously, the microstructures of HEA coating had been changed after adding 10 wt% TiC powders. Magnification image and EDS line scanning of the FeCoCrAlCu-10 wt%TiC composite coating are shown in Fig. 2 (b1) and (b2). It was interesting to note that some particles appeared in the matrix. The measured particles size was 2–4 μm, approximately (Fig.2(b1)). The results of EDS (Fig.2(b2)) shows that the content of Ti and C elements were significantly higher than that of other elements. Based on the analysis of the XRD and EDS, it can be reasonably speculated that the particles was TiC.
Fig. 2. Microstructure of cross-section for FeCoCrAlCu-xTiC high entropy alloy coatings: (a) FeCoCrAlCu HEA coating, (b) FeCoCrAlCu-10 wt%TiC HEA coating, (b1) the high magnification SEM images, (b2) the EDS line scanning, (c) FeCoCrAlCu-30 wt%TiC HEA coating, (d) FeCoCrAlCu-50 wt%TiC HEA coating.
For the HEA containing 30 wt% TiC, a small amount of TiC particles appeared. Moreover, TiC mainly presented in the form of equiaxed spherical or nearly spherical fine particles. The typical structure formed could be attributed to that the TiC unit cell was completely symmetrical in both geometric and chemical bonds and it was no preferentially grown crystal face. On the one hand, when the crystal structure of a compound was a symmetrical structure, its bound energy and atom binding energy were highly isotropic, which made the compound to grow in an isotropic manner. On the other hand, when the TiC was nucleated, the growth rate of the symmetric crystal faces was same, and it was easy to form equiaxed spherical particles structure.
When it grew into a spherical shape, it had the lowest surface energy that was favorable for nucleation. Therefore, TiC generally presented in the coatings in equiaxed spherical or nearly spherical particles when the volume of TiC reached a certain content, as shown in Fig. 2(c). When the content of TiC reached 50 wt%, plenty of TiC paticles appeared in the microstructure of HEA coating, and the average particle size was about 5 μm, as shown in Fig. 2(d). Compared with the morphology of TiC in HEA containing 30 wt% TiC powders, the shape of precipitated TiC transformed into polygonal morphology. According to the report in the literature, the introduction of TiC into high entropy alloy can play a role in grain refinement, which is also observed in present study. The phenomenon can be explained as follows: the TiC particles can be regarded as heterogeneous nucleation core during laser irradiation. The heterogeneous nucleation core in the melt increased with the content of TiC ceramic phase increasing, resulting in the nucleation rate increasing. Additionally, the TiC paticles preferred to form at grain boundaries. Therefore, the pinning effect resulting from the TiC recipitated around the grain boundary caused the limited grain growth process and then the microstructure of the high-entropy alloying layer was refined.
In order to study the morphology, preferred growing orientation and related crystallographic structure of grains, EBSD characterization was further carried out of the FeCoCrAlCu and FeCoCrAlCu-50 wt%TiC HEA coatings. Fig. 3(a) and (b) shows the EBSD orientation maps of FeCoCrAlCu and FeCoCrAlCu-50 wt%TiC HEA coatings. The red, green, and blue regions presented the grain orientation of <001>, <101>, and <111>, respectively. For crystals with a cubic structure, grains tend to grow in the <001> direction. However, the EBSD result of two specimens exhibited anisotropy, certain grains had <001> orientation, others had <101> orientation and some between <101> and <001>. The reason for this phenomenon was the complex heat flow direction during the LSA process weakened the formation of texture. Fig. 3(c) and (d) shows inverse pole figures of FeCoCrAlCu and FeCoCrAlCu-50 wt%TiC HEA coatings. The different colors represented different grain orientation trends. It can be seen no preferential orientation of the crystal grains in two HEA coatings. However, the texture intensity of FeCoCrAlCu-50 wt%TiC HEA coating was increased to 7.17 compared with FeCoCrAlCu HEA coating (2.90).
Fig. 3. EBSD orientation map and inverse pole figures of the FeCoCrAlCu-xTiC high entropy alloy coatings: (a) and (c) FeCoCrAlCu HEA coating, (b) and (d) FeCoCrAlCu-50 wt%TiC HEA coating.
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