| High entropy alloys have broken the restriction of traditional metals with a single element as the main element.The designability and adjustability of their composition facilitates the formation of a special microstructure,which in turn leads to ultra-high strength.The addition of a small amount of B in high entropy alloys can have a significant interstitial solid solution strengthening effect.Therefore,the study of boron-containing high entropy alloys is of great importance.However,when excessive amounts of B are added,brittle borides tend to form in the alloy which adversely affects its plasticity.To address these problems,this thesis uses mechanical alloying combined with spark plasma sintering(SPS)to prepare dense boron-containing high entropy alloys.The method is simple and efficient,in which the high energy ball mill is used to prepare high entropy alloy powder,which can improve the solid solution of B in high entropy alloy.Meanwhile,spark plasma sintering(SPS)can achieve a rapid sintering and effectively inhibit the formation of the boride phase.The advantages of powder metallurgy method for the preparation of high entropy alloys are fully demonstrated,which will provide useful reference for the industrial production of high entropy alloys in the future.In order to suppress the formation of a large number of the boride phase,this thesis starts from the constituent elements of boride.By varying the Fe content(x = 0.5,0.7,0.9,1.0,1.5)in the Fex Co Ni Al Cr B alloy,the solid solution of B in the alloy and the evolution of the boride phase were observed.The results show that as the Fe content increases,the boride phase first decreases and then increases,and then decreases again due to the formation of the FCC phase.When Fe content is 27.27 at.%(x=1.5),the alloy has better fracture toughness due to the presence of FCC phase,and the wear rate of the alloy is the lowest,W=0.76×10-5mm3/(N·m).Since Cr and B are also prone to the formation of coarse boride phases,the solid solution of B in the alloy and the evolution of the boride phase were observed by varying the Cr content(y=0.5,0.7,1.0,1.2)in the FeCoNiAlCryB alloy.The sintered bulk was annealed at high temperature to observe the changes of the boride phase in the alloy before and after annealing.The results show that as the Cr content increases,the boride phase tends to coarsen.When the Cr content was 23.08 at.%(y=1.2),the FCC phase solid solution appeared around the boride phase and gradually increased with the increase of the heat treatment temperature.The fracture toughness of the alloy was significantly improved to7.2±0.4 MPa·m1/2,and the hardness value of the alloy was only lost by 4.7%.Finally,to achieve pressureless sintering of high-density FeCoNiAlCrBz Ys high entropy alloys,the formation of the boride phase was suppressed by reducing the B content(z = 0,0.05,0.1,0.2,0.5,1.0)and adding trace amounts of Y(s =0,0.05,0.1,0.2).The results show that reducing the B content in the alloy can effectively reduce the boride phase content in the alloy.Instead,the addition of excess Y caused the alloy matrix structure to be damaged and the degree of densification was reduced. |
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