Microstructure And Properties Of Mo-Si-B Based Ultrahigh Temperature Alloys Prepared By Powder Metallurgy

Due to their high melting points,excellent high temperature strength and oxidation resistance,Mo-Si-B based alloys show great potential for being a new generation of ultrahigh temperature structural materials.However,their lower room temperature fracture toughness and relatively higher densities hinder their actural application.Alloying is an effective method to ameliorate the alloys'comprehensive properties.Among the numerous alloying elements,Zr is widely added to Mo-Si-B based alloys due to its positive effects not only on the toughness but also on the room and high temperature strength of the alloys.However,as a result of the phase transformation of Zr O₂ that triggers the breaking up of the protective scale,Zr-containing Mo-Si-B based alloys present catastrophic oxidation behavior beyond1200?.Inspired by the facts that Y can suppress the phase transformation of Zr O₂and facilitate the reaction between Zr O₂ and Si O₂ to form Zr Si O₄,Y addition maybe helps eliminate the above-mentioned negative effect of Zr alloying.Typical Mo-Si-B based alloys are composed of Moss(Mo solid solution),Mo₃Si and Mo₅Si B₂ phases,where Mo₃Si phase shows worse high temperature strength and oxidation resistance than Mo₅Si B₂.As a result,suppression of the formation of Mo₃Si without obviously changing the Si content in these alloys may result in better comprehensive properties.Sufficient dissolution of Nb in Mo₃Si can lead to its destabilization.Besides,Nb shows a lower density and higher toughness than Mo.As a result,it is possible to fabricate a lighter,stronger and tougher Mo₃Si-free Mo-Nb-Si-B alloy than conventional Mo-Si-B based alloy.In view of these,both Zr-Y and Nb alloyed Mo-Si-B based alloys have been designed respectively in the present dissertation.Powder metallurgy is widely used to fabricate Mo-Si-B based alloys due to their high melting points,where mechanical alloying(MA)followed by hot pressing(HP)draws much attention due to its low operation cost,simple process and good product quality.However,systematic studies on the mechanical alloying and hot pressing sintering behaviors of Mo-Si-B based alloys are not yet available in the open literatures.The MA mechanism of Mo-12Si-10B-3Zr-0.3Y(at.%)powders have been revealed,and the effects of ball milling parameters on the MA behavior have been investigated.The results show that with the progress of MA,the morphological evolution of powder particles can roughly be divided into five stages:individual particles,irregular blocky composite particles,flake-shaped particles,agglomerates and single particles,and the internal structure generally undergoes five stages:individual particles,coarse lamellar structure,fine lamellar structure,non-uniformly mixed structure and plum-pudding structure.Dissolution of B to Moss during milling can be negligible.On the contrary,Si and Zr are dissolved into Moss gradually with the progress of MA.In the cases,when the MA degree is high enough,all Si and Zr atoms can be dissolved into Mo to form supersaturated Moss.Generally,with increase in milling time,the grain size of Moss decreases and its microstrain increases continuously.In the cases,when the Moss grain size is small enough,their further refining is always accompanied by the sharp reduction of their microstrain.MA processes under different milling conditions follow the similar evolution route,while the evolution rate changes with the milling condition.Increasing the milling speed(300,400 and 500 rpm)can accelerate the MA rate significantly,while increasing milling ball size(10 and 20 mm)or decreasing the ball-to-powder ratio(15:1 and 10:1)postpones the MA rate slightly.With the addition of 2 wt.%stearic acid,the MA rate appears slowly first and fast afterwards.The effective input power from the milling sets to the powders plays a decisive role in the MA rate of different milling condition.In terms of MA rate,impurity Fe content and powder yield,the optimized milling condition presents a milling speed of 400 rpm,ball size of 10 mm,ball-to-powder ratio of 15:1,milling time of 20 h and without process control agent.Optimized MA followed by HP was used to fabricate five Zr-Y alloyed Mo-Si-B based alloys(Mo-12Si-10B-(0,1,2,3)Zr-(0,0.3)Y(at.%)(x Zr-y Y)).The effects of Zr-Y alloying on the microstructure,room temperature fracture toughness and oxidation resistance of alloys were studied.The results show that all the five alloys are mainly composed of the equilibrium Moss,Mo₃Si and Mo₅Si B₂ phases.Besides,Si O₂ or Zr O₂ can be found in the Zr-free or Zr-containing alloys respectively.All the five alloys show a microstructure of intermetallic matrixes with Moss islands,pores and oxide impurities dispersed in them.The addition of 1 at.%Zr to the alloy improves its room temperature fracture toughness(7.4 vs 8.4 MPa·m^1/2),but leads to catastrophic oxidation at 1250 ^oC.Further addition of 0.3 at.%Y to the alloy eleminates the detrimental effect of Zr alloying on the oxidation resistance under the premise of maintaining its beneficial effect on fracture toughness.The addition of Y facilitates the formation of monomorphic Zr Si O₄ instead of polymorphic Zr O₂ during oxidation.Besides,after longer oxidation(?20 h)a continuous and compact Y₂Si₂O₇layer forms on the scale surface that makes its oxidation resistance even better than that of 0Zr-0Y alloy.For 2Zr-0.3Y and 3Zr-0.3Y alloys,their relatively higher Zr contents increase both the content and size of Zr O₂ particles.The especially larger Zr O₂ particles in 3Zr-0.3Y alloy can act as crack sources to lower the fracture toughness of the alloy.Besides,the formation of the great amount of Zr Si O₄ during oxidation for these two alloys interrupts the continuity of their Si O₂ layer,which leads to the catastrophic oxidation of 2Zr-0.3Y and 3Zr-0.3Y alloys.The effects of Nb contents on the MA behaviors of Mo-x Nb-12Si-10B(at.%)(10?x?40)powders were studied.The milled powders were then cold pressed into thin disks and sintered to study the effects of Nb contents on the sintered microstructure.The critical Nb content for suppressing Mo₃Si has been determined.The results show that increasing Nb content from 10 to 30 at.%accelerates the MA process.However,further increasing Nb content to 40 at.%decreases this process due to excessive cold welding and high powders volume.For alloys prepared by 30 h milling and then sintering,a critical Nb content for suppressing Mo₃Si is found to be between 24 and26 at.%.Alloys with Nb content less than 24 at.%are composed of Moss,Mo₃Si and Mo₅Si B₂ phases,while alloys with Nb content between 26 and 40 at.%are composed of Moss,Mo₅Si B₂ and?Nb₅Si₃ phases.Prolongation of prior milling process facilitates the suppression of Mo₃Si and delays the formation of niobium silicides by promoting the composition uniformity of the milled powders.MA followed by HP was used to fabricate Mo-12Si-10B(at.%)(0Nb)and Mo-26Nb-12Si-10B(at.%)(26Nb)alloys.Comparative studies were carried out on the densification behavior,microstructure,room temperature fracture toughness,elevated temperature compression strength and oxidation resistance of these two alloys.The results show that 26Nb alloy exhibits better compactibility than 0Nb alloy.That is,0Nb alloy presents a relative density of 95.8%and a porosity of 2.5%,while 26Nb alloy presents an almost completely densified microstructure with a porosity as low as0.2%.0Nb alloy is composed of Moss,Mo₃Si and Mo₅Si B₂ phases,while 26Nb alloy is free of Mo₃Si,and has higher Moss content(35.7%vs 45.9%)and a littlegNb₅Si₃.Both alloys present intermetallic matrix,but Moss in 26Nb alloy shows higher continuity.Compared with 0Nb alloy,26Nb alloy shows a lighter density(8.96 vs 8.14 g/cm³),better room temperature fracture toughness(6.8 vs 8.8 MPa·m^1/2)and more excellent high temperature compression strength(644.2 vs 851.7 MPa),but worse oxidation resistance at 1300?due to the formation of non-protective Nb₂O₅ and its retardation effect on the complete coverage of SiO₂ scale.