Study On Microstructure And Properties Of Mo_0.5NbTiVCr_0.25 High Entropy Alloy Coating Prepared By Laser Cladding

The development of accident tolerance fuel（ATF）casing has become one of the key technologies to be overcome in the world of nuclear power safety.High-performance refractory high-entropy alloy developed in recent years was a potential excellent ATF casing material due to its excellent high temperature performance,oxidation resistance and irradiation resistance.However,due to high melting point and low plasticity,it was difficult to meet the forming of round thin-walled cladding shells.The deposition of coatings with excellent properties such as high temperature resistance,oxidation resistance,and corrosion resistance on the surface of Zr alloys has been the focus of ATF cladding research in recent years.Based on this,TC4 alloy,which has similar properties with zirconium alloy material,was chosen as the substrate for this project.By preparing Mo_0.5NbTiVCr_0.25 high entropy alloy coating on TC4 substrate,and then studying the evolution law of coating organization structure and properties to promote the application process of refractory high entropy alloy.The influence laws of different laser power,scanning speed and lap rate on the surface morphology,microstructure and mechanical properties were systematically investigated.The surface morphology of the coating was well formed under different parameters;the microstructure consisted of equiaxed crystals,dendrites,plane crystals and columnar crystals;the optimal laser power and scanning speed were determined by testing the hardness of different areas of a single coat;the denseness of different lap rates was analyzed.The optimal coating process parameters were determined as follows:laser power P=1.6 KW,scanning speed V=8 mm/s,and lap rate of 50%.Combining the service environment of the zirconium alloy cladding and improving the performance of the coating by heat treatment,three temperatures of 600℃,800℃and 1000℃were selected for the 24-hour aging heat treatment.The results by XRD analysis showed that the coating did not change its crystal structure after different heat treatment processes,and remained as a single BCC structure.The organization of the coating was equiaxed,dendritic and columnar crystals from the top,middle and bottom in order.By SEM and TEM characterization,it was found that the coating was heat-treated at 800°C and1000°C with precipitation of Ti（O,N）phase of FCC structure.Then,the properties of Mo_0.5NbTiVCr0.₂₅high-entropy alloy coatings were characterized in the molten and heat-treated states.The coating showed the highest surface hardness of 410 HV_0.2 at a heat treatment temperature of 600°C,and exhibited the best wear resistance,with smearing and abrasive wear.The coating exhibited the best corrosion resistance after heat treatment at 800°C.The corrosion may be a pitting corrosion or corrosion caused by surface porosity defects,which in turn causes intergranular corrosion and large area corrosion.