Study On Preparation And Passivation Of High Entropy Alloys By Mechanical Alloying

MA（Mechanical Alloying）is a common method for preparing high entropy alloys,but the high entropy alloy powders prepared have high system energy and poor stability,so the whole preparation process needs to be carried out in the atmosphere of inert gas,the operation is more complicated,and it is difficult to apply to industrial mass production.Therefore,this topic through the rapid cooling of MA powders and filling gas passivation treatment,to achieve the purpose of preparing high entropy alloy in the air and does not affect the performance of the sintered body,convenient for industrial production.The high entropy alloys are treated by rapid cooling method.By reducing the system energy of the powders after MA,the energy of the powders in the air are not enough to react violently with the oxygen in the air,to achieve the purpose of passivation.The effects of cooling method,standing time,cooling time and cooling temperature on CoCrNiCuFe high entropy alloy were studied.The study found that,under the condition of constant cooling time,the powder of 1h after ball milling will spontaneously ignite when taking material in the air,the other CoCrNiCuFe high entropy alloy powders prepared by the cooling process did not occur spontaneous combustion.Rapid cooling can achieve the purpose of passivation.The optimum passivation process was as follows:MA for 2h,water cooling for4h.After SPS（Spark Plasma Sintering）sintering,the density and hardness of CoCrNiCuFe high entropy alloy were 94.6%and 460HV_0.5,respectively,and had good oxidation resistance.Compared to CoCrNiCuFe high entropy alloy prepared in Ar protected glove box,the phase and microstructure had no obvious change,and the properties were similar.The passivation of Co Ni Cu Fe Mn was carried out with the best technology,and the powders in the air did not occur spontaneous combustion and the properties did not change significantly,which also achieved the purpose of passivation.The method of gas passivation is used to passivate the high entropy alloy.In the atmosphere of low oxygen concentration,the surface of the high entropy alloy powders react with oxygen to produce low active oxide,which isolated the air and prevented the powder with high internal system from contacting with the air to spontaneously ignite,to achieve the purpose of passivation.The high entropy alloy was passivated by gas passivating agent,the effect of gas filling time and the type of gas passivator on CoCrNiCuFe high entropy alloy was studied.The results showed that Ar and N₂:x vol.%O₂（x=0,5,10）were charged after the end of MA,and no spontaneous combustion occurred in the air,so the passivation effect could be achieved.The optimal process of gas passivation was as follows:filling N₂ for 2h after MA30h.After SPS sintering,the density,hardness and compressive strength were 95.2%,465HV_0.5 and 1759MPa,respectively,the oxidation resistance and corrosion resistance was good.The properties of CoCrNiCuFe high entropy alloy prepared in Ar protected glove box were similar.Co Ni Cu Fe Mn high entropy alloy was passivated by the best gas passivating process.No spontaneous combustion occurred in the air,and the performance was not different from that of conventional preparation,so the purpose of passivation could be achieved.CoCrNiCuFe,which has relatively good performance after passivation,was used as the binder and WC composite sintering to prepared WC-10wt.%CoCrNiCuFe cemented carbide,and the influence of CoCrNiCuFe on WC-CoCrNiCuFe cemented carbide after passivation was observed.The results showed that N₂ passivated cemented carbide had the best properties,and the density,hardness and fracture toughness can reach 97.9%,1752HV₃₀ and 9.42MPa×m^1/2,respectively,which was similar to the conventional preparation properties.