Synthesis,Microstructure And Properties Of Molybdenum-Aluminum Alloys

The alloys with the nominal composition of Mo₃Al₈、Mo₃Al_7.5Si_0.5、Mo₃Al₇Si₁、Mo₃Al_6.5Si_1.5 and Mo₃Al₆Si₂ were prepared by combustion synthesis method, and Mo,Al, Si powders were used as the starting materials. The combustion mode, combustion temperature and combustion products were investigated. The quenching of combustion front was adopted to understand the reaction of Mo₃Al₈. The results indicate that the combustion mode of the five components is an unstable combustion.The X-Ray diffraction patterns show that the main phase of combustion products of the five components is Mo₃Al₈. when Si is added, Mo（Si,Al）₂ and MoAl₄ are found and with the increase of Si, the peaks of Mo（Si,Al）₂ and MoAl₄ become stronger. The peaks of Mo₃Al₈ shift to high angles with the increase of Si and reach the highest angles at Mo₃Al₇Si₁. The SEM photographs show that the combustion products are porous structure materials and the products particle grows form sphere to strip. The quenching shows that the reaction of Mo-Al is solid-liquid reaction.Combustion synthesized Mo₃Al_8-xSi_x alloys were used as raw materials, dense Mo₃Al_8-xSi_x bulk were prepared by hot pressing. Systematic research on mechanical properties of hot-pressed materials at room temperature was carried out, the results showed that the main phase of hot-pressed Mo₃Al_8-xSi_x alloys is Mo₃Al₈, few MoAl₄,Mo（Si,Al）₂, MoAl and Al₂O₃ are detected. With the increase of Si, MoAl fades away and peaks of MoAl₄ and Mo（Si,Al）₂ become stronger. The relative density of hot-pressed Mo₃Al_8-xSi_x alloys are over 96% and reach the highest density, about98.2% when x = 1.5. With the increase of Si, the bending strength, fracture toughness and hardness show continuous increase, while the electrical resistance decreases gradually. When x = 2, the bending strength, fracture toughness and hardness reach the peak value, are 523 MPa、4.56 MPa·m1/2 and 7.88 GPa, respectively.The pre-oxidation of hot-pressed Mo₃Al_8-x Si_x bulk was done at 1423 K. The oxidation behavior of pre-oxidized and original Mo₃Al_8-xSi_x bulk were investigated at923 K in air for 96 h. The pre-oxidation shows that a dense Al₂O₃ scale was formed on the surface of samples with 5 components, and the oxide scale of Si-added samples was thicker than the pure Mo₃Al₈. The kinetics at 923 K shows the mass loss of both the original and pre-oxidized samples, and the mass loss decreased with increase of Si,and reaches the lowest value,-1.7 × 10-3 Kg m-2 and-0.9 × 10-3 Kg m-2 at Mo₃Al₆Si₂,respectively. The mass change of pre-oxidized samples is much mild. The oxidationproduct of original samples at 923 K is Al₂O₃. A 0.6 μm Al₂O₃ scale was formed on the surface of Mo₃Al₈, and only small amount of Al₂O₃ was formed on the other components. The surface structure of pre-oxidized sample showed no obvious change at 923 K. No samples experienced pest oxidation at 923 K.