Research On Design And Preparation Of High Hardness Ternary Boride-based Cermets

WC-Co cemented carbide is known as the "industrial tooth" and is one of the most widely used cermet materials.However,one of its primary components,W,is scarce and also has a high density.Searching for tungsten-free,low-density cermets has always been a research hotspot.Ternary borides have high hardness,high melting point,low density,and good compatibility with metals,which made them potential candidates for tungsten-free cermets.In this work,first-principles calculations were used to screen out MoCoB as the hard phase firstly.Then,MoCoB-Co cermet was synthesised by in-situ sintering,and phase transformation of the cermets was investigated.Finally,the effects of chemical composition and doping transition metal elements on the structure and properties of the cermets were studied.This work will provide new ideas for the exploration of tungsten-free cermets.The research content and main conclusions of the paper are as follows:(1)Evaluation of the comprehensive performance of 26 ternary borides.Analyzing the crystal structure characteristics of various superhard and hard materials,it was determined that M?xM?Bx(x=1 or 2)type ternary boride as the hard phase.First-principles calculations were used to investigate the structure,mechanical,electronic,and thermal properties of 26 M?xM?Bx(x=1or 2).MoCoB is the ternary boride with the highest hardness,and has higher toughness and Deby temperature than WC.The presence of a MoCoB+Co phase region in the Mo-Co-B diagram indicates that the thermodynamic conditions for the synthesis of MoCoB-Co cermets are available.Therefore,MoCoB-Co cermets were determined as the research object.(2)Phase transformation and microstructure evolution of 25Mo-50Co-25B(at.%)alloy during sintering.The mixed powders experienced three solid-phase reactions,Co+B?Co2B,11Mo+15Co2B?Mo2Co21B6+9MoCoB,and 4Mo+Mo2Co21B6?6MoCoB+15Co,and underwent two liquid-phase reactions.Nanosized,tremella-like Co structures were formed on the surface of the particles between 1100 and 1200?.The two liquid phases L1 and L2 were eutectic liquid phases formed by the reaction of nanoporous Co and non-nanoporous Co with MoCoB respectively.L1 made the sintered body initially dense,and L2 made the sintered body further fully dense.(3)Effect of Mo/B atomic ratio(ARmo/B)and Co content on the structure and properties of MoCoB-Co cermets.The MoCoB hard phase exhibited the maximum grain size and anisotropy when the ARMo/B was 0.84,which diminished mechanical properties.For ARMo/B?1.08,? phase and three-dimensional networks densely composed of stacking faults and ?-martensite plates were observed;these three-dimensional networks limited the dislocation slip in ? and ?-martensite,improving the hardness of the cermet and decreasing the transverse fracture strength.When the ARMo/B was 0.92 and the Co content was 45 at.%,the cermet displayed the best overall performance,with hardness,TRS,and fracture toughness values of HRA 89?1720 MPa and 13.2 MPa·m1/2,respectively.(4)First-principles investigation of MoCoB doped with transition metal elements in the fourth period.Sc showed a strong Mo-site preference in Mo8Co8B8,Ti,V and Cr showed a weak Mo-site preference,Mn,Fe,Cu and Zn showed a weak Co-site preference,and Ni showed a strong Co-site preference.Except for Mo7CrCo8B8,the Deby temperature of all doped models were lower than that of Mo8Co8B8,and the hardness of all doped models were less than that of MosCo8Bs.Doping Sc,Mn,Ni,Cu and Zn can increase the stress required for the dislocation in the hard phase to overcome the long-range internal stress field and slip.(5)Effect of Cr,Ni and Mn on the microstructure and mechanical properties of MoCoB-Co cermets.The addition of Cr will cause a brittle phase ? phase to appear in the cermet,which deteriorated the performance of the cermets.Adding Ni will reduce the hardness and wear resistance of the cermets.Adding an appropriate amount of Mn can increase the density of the cermets and refine the hard phase grains.With the increase of Mn content,the hardness of the cermet decreased slightly and then gradually increased and the TRS gradually decreased.When the Mn content was 6 wt.%,the cermet has the smallest hard phase grain size and lower porosity.The hardness,TRS,and fracture toughness values of HRA 91.7,1200 MPa and 11.4 MPa·m1/2,respectively.When the Mn content was 10 wt.%,the cermet displayed the highest hardness HRA 92.1.