Preparation And Toughening Of Mg And Al Co-alloyed MoSi₂

MoSi₂ has become the potential candidate material for high temperature applications because of its excellent properties,such as high-melting,suitable density and high-temperature oxidation resistance.However,its low fracture toughness(2-2.5MPam^1/2)and low bending strength?200-250MPa?in room temperature limit its application.In this paper,based on Mg and Al co-alloying MoSi₂,Mo[Si_0.9(Al_xMg_0.1-x)]₂ composite powders were prepared by self propagating high temperature synthesis technology,and compact MoSi₂ composite ceramics were prepared by vacuum hot pressing sintering,and then annealed in different processes,and their phase composition and mechanical properties were studied.Using Mg,Al,Mo and Si powders as raw materials,Mo[Si_0.9(Al_xMg_0.1-x)]₂composite powders were successfully prepared by self propagating high-temperature synthesis,where x represents the molar content of Al powder,taking 0,0.02,0.04,0.06,0.08,0.10,respectively.With the increase of Mg content,the swelling degree of the synthesized products increases,and the particles of the synthesized products become smaller and the grains are refined,the residual Mo remains in the phase.With the increase of Al content,the synthesis products are reunion,the grain coarsening,the residual Mo phase decreases gradually,the Mo?Si,Al?₂ phase with C40 structure appears when x exceeded 0.04.Mo[Si_0.9(Al_xMg_0.1-x)]₂ composite ceramics with density above 95%were prepared by vacuum hot pressing sintering with composite powders synthesized by high temperature self propagating.After sintering,the C11_b phase MoSi₂ is the main phase of ceramic samples,and the residual Mo of each group is disappeared,and the C40 structure Mo?Si,Al?₂ is reduced.The fracture toughness is the highest when the Mg content is 6at%,at this time,the fracture toughness is 7.28MPa.m^1/2,which is62.5%higher than that of pure MoSi₂ and the hardness is reduced by 49.8%.To further improve the mechanical properties of Mo[Si_0.9(Al_0.04Mg_0.06)]₂ceramics,the annealing temperature was first selected for annealing treatment,the annealing temperature was set to 1300?,1400?,1500?and 1600?,and the annealing time was 6h.The C40 structure Mo?Si,Al?₂ phase gradually disappeared when the annealing temperature was higher than 1300?.It was found that the hardness and fracture toughness of Mo[Si_0.9(Al_0.04Mg_0.06)]₂ ceramics increased with the increase of annealing temperature.In addition,When the annealing temperature was 1600?,the fracture toughness reached 7.86MPa.m^1/2,which was 18.6%higher than that before annealing,and the hardness increased to 9.22 GPa,which was 39.1%higher than before annealing.In the next place,the Mo[Si_0.9(Al_0.04Mg_0.06)]₂ ceramics were annealed at 1600?,but the annealing time was 3h,6h,9h and 12h respectively.It is found that with the prolonging of annealing time,the densification degree of samples increases,and the hardness and fracture toughness increase first and then decrease.When the annealing time is 12h,the density is the highest,reaching 98.61%.The mechanical properties are best when annealing time is 9h.The hardness is up to 10.40GPa,which is 54.76%higher than that before annealing,and the fracture toughness is up to 8.50MPa.m^1/2,which is 23.37%higher than that before annealing.The fracture toughness of MoSi₂ was improved by the co-alloying of Mg and Al,but it was difficult to avoid the decrease of the hardness value at the same time,while the later annealing treatment improved the hardness of the composite ceramics.A dual enhancement of fracture toughness and hardness of ceramic materials has been achieved,and an ideal MoSi₂ material has been prepared.