Study On The Synthesis, Microstructure And Roperties Of MoSi₂Alloyed With Nb, Al

Nb,Al alloyed MoSi₂was synthesized by combustion synthesis in this work. Thecombustion mode, combustion temperature, reaction mechanism, flame-frontpropagation velocity and product structure were investigated. Then dense materialwas prepared by vacuum hot pressing sintering and sparks plasma sintering usingcombustion synthesis products as raw materials. The organizations and propertieswere studied, and the action mechanisms of alloys were clarified.From the element powders,(Mo_1-xNb_x)Si₂, Mo(Si_1-yAl_y)₂and(Mo_1-xNb_x)(Si_1-yAl_y)₂alloys were successfully synthesized by the mode ofself-propagating high-temperature synthesis. And the combustion products are formedthrough melting, dissolution and precipitation mechanism in combustion synthesis. Simelts first in alloys without Al, while Al-Si eutectic melt firstly forms after whichSi-rich melt forms in alloys containing Al. Solid Nb and Mo dissolute into the meltwhen contacted with it, and then reaction products precipitate from the supersaturatedsolution.Using combustion synthesis products as raw materials, dense (Mo_1-xNb_x)Si₂and(Mo_1-xNb_x)(Si_1-yAl_y)₂alloy were prepared by vacuum hot pressing sintering and sparkplasma sintering. The precipitation reaction occurs during the sintering process andC40phase forms by Nb/Al precipitate from supersaturated C11_b phase. The phasecomposition of HP alloy is close to the equilibrium state, while saturated C11_bphasecould also be found in the SPS alloys together with tiny grains, because thetemperature is low and time is short in spark plasma sintering process.For HP alloys, a little Nb solid solution raises the bending strength from216Mpato284MPa, a small amount of Nb and Al co-alloying could significantly increase thefracture toughness at room temperature, up to10.7MPa/m^1/2, which is74%higherthan pure MoSi₂; but massive alloys promote the formation of C40phase and result inthe decreasing of fracture toughness and increasing of hardness. For SPS alloys,（Mo0.85Nb0.15）(Si_0.97Al_0.03)₂alloy shows the best combination of high-temperaturecompression strength and best room-temperature mechanical properties, and itsstrength at1400℃is252MPa, which is115%higher than pure MoSi₂.No pesting phenomena were observed in HP alloys when oxidized at at500℃.Nb alloying accelerates the low-temperature oxidation of MoSi₂, while Al isadvantageous for inhibiting oxidation at500℃.(Mo_0.97Nb_0.03(Si_0.97Al_0.03)₂shows thebest oxidation performance in all alloys. An approach for avoiding low-temperature oxidation was put forward. A glass protective scale can be formed on the surface of(Mo_1-xNb_x)Si₂materials by high-temperature pre-oxidation treatment, resulting in areduced rate of low temperature oxidation. For the alloys of (Mo_1-xNb_x)Si₂and(Mo_1-xNb_x)(Si_1-yAl_y)₂, the formation mechanisms of oxidation scale duringhigh-temperature oxidation were established.Based on the two sublattice model, the theoretical adiabatic temperature of(Mo_1-xNb_x)Si₂and Mo(Si_1-yAl_y)₂were simulated, and the results basically tally withthe change trend of the tested highest combustion temperature. Based on the empiricalelectron theory of solids and molecules, the valence electron structure of C11_b typeMeS₂（Me=Mo,Nbï¼›S=Si,Al）was calculated and the changing of properties wasanalyzed. The results indicates that Nb/Al alloying changed the hybridization state ofatoms in C11_b phase, which correspondingly reduces the bond energy of the strongestbond B and the second strongest bond C in C11_b phase, results in the reduction ofhardness. The ratio of covalent valence electron number to total number ofvalence electron of C11_b phase was elevated when alloying with Nb, which isfavorable for improving the strength of C11_b phase, but declined when alloyedwith Al or trace amount of Nb-Al, resulting the decrease of strength.. And most resultsaccords with the properties changing trend of HP alloys.