Effect Of W And Al On The Structure And Properties Of MoSi₂ Materials

With the development of the aerospace industry,the modern industry has put forward higher requirements for the use of high temperature structural materials: higher use temperature and better stability in high temperature environment.MoSi2 material with high melting point,good high temperature oxidation resistance characteristics are widely attention by researchers,have hope to become a new type of high temperature structural materials.However,due to the large brittleness of MoSi2 material at room temperature,the fracture toughness is only 2.5-3.0 MPa·m1/2,which restricts the application of this material in practice.In this paper,the metal elements W and Al were added to MoSi2 by the alloying method,to improve the room temperature brittleness of MoSi2 materials.In the first step,two kinds of composite powders(MoSi2/WSi2 and Mo(Al,Si)2)were prepared by self-propagating high-temperature synthesis(SHS)technique,the characteristics of SHS and phases constitute of products were analyzed.The second step,the effect of the addition of W and Al on the microstructure and mechanical properties of MoSi2 was studied,which was prepared by vacuum hot pressing sintering.(1-x)MoSi2/xWSi2 composite powders were prepared via SHS technique using Si,Mo and W elements as raw materials,where x represents the molar content of W powder,5%,10%,15%,20%,respectively.Thermodynamic analysis indicated that the incorporation of W in the starting materials decreased the adiabatic temperature during the SHS reaction,and the theoretical adiabatic temperature of the system is 1859 K when the content of W reached up to 20 at%.The result of peakdifferentiating and imitating of XRD patterns indicated the formation of MoSi2 and WSi2 composite phase in all samples.The morphology of combustion products is similar to the initial morphology of Mo powder,which is approximately equiaxed and aggregates.(1-x)MoSi2/x WSi2 composites were obtained by vacuum hot pressing sintering,the temperature is 1400 oC,holding time for 90 min and maximum pressure is 25 Mpa,all the density of the composites reached 97%.With the increase of W content,the hardness and bending strength of(1-x)MoSi2/xWSi2 composites increased,and when x=0.20,the hardness reached 14.18 GPa,the bending strength was 300.21 MPa,increased by 17.6% and 6.0% respectively,compared with pure MoSi2,but the fracture toughness decreased significantly,reduced to 2.01 MPa·m1/2.The addition of W to form the WSi2 second phase particles in the MoSi2 matrix improved the hardness and bending strength,but at the same time leads to greater brittleness of the composites.Mo(Aly,Si1-y)2 composite powders were prepared via SHS technique using Si,Mo and Al elements as raw materials,where y represents the molar content of Al powder,5%,10%,15%,20%,respectively.The results of XRD patterns showed that the addition of Al to replace the Si in the MoSi2 structure formed C11 b Mo(Al,Si)2.Since the atomic radius of Al is larger than Si,the lattice was distorted.It is found that there is a phase transition from C11 b to C40 when the Al content reached a certain value.Mo(Aly,Si1-y)2 composites were obtained by vacuum hot pressing sintering,the result show that: with the increase of Al content,the density and hardness of Mo(Aly,Si1-y)2 alloy decreased,the bending strength decreased and the fracture toughness increased first and then decreased.When y=0.10,the performance was the best,the density was 93.6%,the hardness was 9.97 GPa,the bending strength was 238.55 MPa,and the fracture toughness was 7.38 MPa·m1/2,increased 113.3% compared with pure MoSi2.The addition of a small amount of Al(y?0.10)formed C11 b Mo(Al,Si)2,which can improve the fracture toughness of the alloy material effectively.However,when the Al content is increased,the mechanical properties of the alloy were obviously reduced due to the crystal transition.In this paper,it is expected that the mechanical properties of MoSi2 can be improved by alloying method.However,it is necessary to do further work on the selection of alloying elements,the synergistic effect of various elements and the toughening mechanism of the alloy.