| As a new type of alloy,high entropy alloy has excellent comprehensive properties,which is very suitable for surface modification.In order to obtain the coatings with good hardness and high temperature wear properties,the multi-phase high entropy alloy coating was prepared by introducing the second phase into the single-phase solid solution,which further improved the hardness and high temperature wear properties.In this paper,the composition design was used to realize the second phase precipitation of CoCrFeNi high entropy alloy to improve the comprehensive properties of CoCrFeNi high entropy alloy.The microstructure,microhardness,high temperature stability and high temperature wear resistance of the CoCrFeNi(M=Ti,Nb,W;x=0,0.25,0.5,0.75,1.0)(Mx for short)multi-phase high entropy alloy coating were investigated.The details are as follows:The addition of Ti,Nb and W elements in CoCrFeNi high entropy alloy as single-phase FCC phase promotes the generation of BCC phase and Ti O2 phase,Laves phase,?phase and W phase of unfused particles,respectively,and promotes the grain size refinement of the microstructure.In Tixcoating,the intergranular precipitates continue to precipitate with the addition of Ti element.In the Nbx coating,the lamellar eutectic structure(FCC phase+Laves phase)first increases and then decreases with the increase of Nb content,and the Nb0.5coating is completely eutectic.In the Wx coating,the coating is subeutectic,and the intercrystalline eutectic structure(FCC phase+?phase)is formed.Compared with CoCrFeNi base alloy,the average microhardness of Ti1.0,Nb0.75 and W1.0 coatings is increased by 150%,210%and 105%,respectively.It can be seen that the doping of elements significantly improves the microhardness of the coating.The internal strengthening mechanisms of different coatings include solution strengthening,grain refining(boundary strengthening)and second phase strengthening.The high temperature stability of the coating shows that Nb0.75 coating has the best high temperature stability.The Ti0.75 coating precipitates petal-like phase at 500?,which increases the hardness of the coating to 892.9 HV,which is about twice that of that before annealing.The Nb0.75 coating has good phase stability.The microstructure of Nb0.75coating does not change significantly at 500??900?,and it is still typical dendritic crystal.After annealing at 1100?,the microhardness of the coating decreases obviously due to the increase of grain size.The W1.0 coated FCC solid solution experienced continuous inner-grain desolution at 500?,and showed complete desolution phase after annealing at 900?.The coating has good wear resistance at high temperature,and the precipitation of the second phase is beneficial to improve the anti-softening behavior of the coating,which plays a supporting role for the friction pair.With the increase of test temperature,the adhesion wear of Ti0.75 coating gradually changed to delamation wear,the oxidation wear of Nb0.75 coating changed to oxidation wear from oxidation wear and slight abrasive wear,and the oxidation wear of W1.0coating changed to oxidation wear from oxidation wear with slight abrasive wear and adhesive wear.It can be seen that the oxidation wear gradually occupies the dominant position with the increase of test temperature.At RT and 800?,the best wear resistance of Nb0.75 coating is 1.3×10-5mm3/(N·m)and 5.05×10-5 mm3/(N·m),respectively.At 400?and 600?,the wear rate of Ti0.75 coating is 5.7×10-5 mm3/(N·m)and 1.8×10-5 mm3/(N·m),respectively.The wear resistance of Ti0.75 coating is good.At 200?,the wear resistance of W1.0coating is slightly better than that of Nb0.75 coating.There are 64 figures,15 tables,and 98 references in this paper... |
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