Composition Design And Fabrication By Laser Cladding Of Mo-like High-entropy-alloy Coating

A series of Mo-like refractory high-entropy alloy(HEA)coatings are designed from the point of view of the average atomic radius and the average electron concentration,combined with the thermodynamics empirical criterion of the solid solution in HEA and the Miedema's calculation rule of the intermetallics'formation enthalpy.The MoFe_xCrTiWAl Nb_y HEA coatings were prepared on the M2 tool-steels by laser-cladding technique.Firstly,the influence of laser processing parameters on the microstructure and properties of the HEA coating were studied to realize the controllable fabrication of MoFeCrTiWAlNb HEA coating.Secondly,the formation mechanism of each phase in the coating and the microstructure evolutions were discussed.Subsequently,the effect of Fe and Nb on the microstructure and properties of MoFe_xCrTiWAl Nb_y HEA coating was demonstrated to obtain the optimal alloy composition of Mo-like HEA.Finally,the microstructure and performance evolution of Mo Fe_1.5Cr Ti WAl Nb₁(denoted as Fe_1.5Nb₁)HEA coating after various annealing temperature and long-time annealing were researched,and its high temperature thermal stability and oxidation resistance were investigated.The main conclusions are addressed as follows:(1)The laser-clad HEA coating fabricated using the rectangular-spot fiber laser with uniform energy distribution possesses the relatively low dilution,few microcracks and uniform microstructure.The optimal laser processing parameters are as follows:P=3.0 kw,v=4 mm/s.The HEA coating mainly consists of BCC solid solution,C14-Laves and MC carbides plus a few un-melted W-particles.Besides,the as-cladded coating shows high microhardness(?820 HV)and excellent abrasive wear behavior.(2)The C14-Laves phase particles are precipitated in the BCC matrix of MoFeCrTiWAlNb HEA coating above 600?.While the dendrites keep good thermal stability below 1000?.In the long-time annealing at 1000?,the dendrites gradually decompose.The un-melted W-particles also dissolve to form a diffusion layer.Besides,the HEA coating exhibits highest microhardness and best wear resistance after annealed at 700?for 4 h.(3)A series of Mo-like MoFe_xCrTiWAl Nb_y(x=1,1.5,2;y=1,1.5,2)HEA coatings through modifying the contents of Fe and Nb are systematically investigated.the phase structures have little altered with the variation of Fe and Nb.However,while Nb contents increasing,the contents of C14-Laves phases obviously increase and the microstructure transforms from cellular dendrites firstly to eutectic,and then to hyper-eutectic.The increasing content of C14-Laves phase could substantially improve cracking sensitivity.Among these coating,the Fe_1.5Nb₁ coating shows a uniform microstructure,microcrack-free and ultra-high average hardness(913.5 HV).(4)C14-Laves phase particles are precipitated in the BCC matrix of Fe_1.5Nb₁ HEA coating from 600?.while annealed at 650?for 4h,the dimension and volume of C14 precipitates reach the optimal combination.Gradually,the Fe_1.5Nb₁ coating shows excellent high-temperature softening resistance below 800?.(5)Moreover,the Fe_1.5Nb₁ coating exhibits nobler oxidation resistance at 800?,compared with M2 steel,because a fine and dense oxide film is produced on the coating's surface.In additional,that the C14 precipitate rapidly coarsening weakens the dispersion strengthening,resulting in the decreasing of microhardness and wear resistance.the wear mechanism changes from abrasive wear and oxidation wear to adhesive wear with the annealing temperature increasing.