Effects Of Heat Treatment On The Microstructure And Mechanical Properties Of Mo-12Si-8.5B Alloys With Different Lanthanum Oxide Contents

In this paper,the Mo-12Si-8.5B(at.%)alloys doped with different contents of lanthanum oxide(0wt%,0.3wt%,0.9wt%,1.5wt%and 2.5wt%)were fabricated by mechanical alloying(MA)and hot pressing sintering(HP)technology.Then,the alloys were subjected to farther heat treatment at different temperature(1400,1500,1600,1700 and 1800℃)for 1h,respectively.The effect of mechanical alloying time on alloy powders was researched,the influence of lanthanum oxide and heat treatment temperature on the microstructure,micro-hardness and density of alloys,were investigated as well.The relationship between microstructure and mechanical properties of the alloys was analyzed and obtained the influence of microstructure on the mechanical properties(at room temperature and high temperature)of the alloys.In addition,the regulation and mechanisms influence of the heat treatment process and lanthanum oxide on the material properties.The results show that:at the same milling rotational speeding and ball to powder ratio,there is no diffraction peaks of new phase.The diffraction peaks strength,of Mo were wakened and the peaks widened after long times of milling.The size of particles decreased and showed approxirmate normal distribution.The sintered alloys and the alloys after heat treatment mainly contain α-Mo,Mo3Si and Mo5SiB2 phases.The heat treatment and lanthanum oxide doping almost can’t change the phase composition of alloys and the content of each phase hardly.With the increasing content of lanthanum oxide,the structure of alloys is more uniform and the grain size is reduced.With the increasing temperature of heat treatment,the structure of alloys is more uniform and the grain size is increased.When the temperature of heat treatment over 1600℃,the grain sizes increase more significantly.After heat treatment at different temperatures,the hardness of the material is reduced.Micro Vickers hardness of the alloys after heat treatment at different temperatures is lower than the alloys after hot pressing sintering.With the increase of heat treatment temperature,the hardness of alloys increased firstly and then decreased.The maximum hardness of the alloys obtained at the heat treatment temperature of 1500℃.When the heat treatment temperature over 1600℃,the hardness of the alloys decrease rapidly.After the heat treatment at 1500℃,the strength at room temperature of the alloys reduced,the fracture toughness at room temperature increased slightly.The strength at high temperature of the alloys after heat treatment was significantly higher than that of the alloys after hot pressing sintering.The result shows that the heat treatment can weaken the mechanical properties at room temperature of the alloys,but it can significantly improve the high temperature strength of the alloys,which quite benefits for the Mo-Si-B alloys applying to high-temperature structural material superalloy.The strength and toughness at room temperature increase with the increase of content of lanthanum oxide,when the content of lanthanum oxide less than 0.9wt%.However,the mechanical properties at room temperature of the alloys decreased,when the content of lanthanum oxide more than 0.9wt%.The strength at high temperature of lanthanum oxide doped alloys decreased.