Abstract
In situ formation of intermetallic/ceramic composites composed of molybdenum silicides (Mo5Si3 and Mo3Si) and magnesium aluminate spinel (MgAl2O4) was conducted by combustion synthesis with reducing stages in the mode of self-propagating high-temperature synthesis (SHS). The SHS process combined intermetallic combustion between Mo and Si with metallothermic reduction of MoO3 by Al in the presence of MgO. Experimental evidence showed that combustion velocity and temperature decreased with increasing molar content of Mo5Si3 and Mo3Si, and therefore, the flammability limit determined for the reaction at Mo5Si3 or Mo3Si/MgAl2O4 = 2.0. Based upon combustion wave kinetics, the activation energies, Ea = 68.8 and 63.8 kJ/mol, were deduced for the solid-state SHS reactions producing Mo5Si3– and Mo3Si–MgAl2O4 composites, respectively. Phase conversion was almost complete after combustion, with the exception of trivial unreacted Mo existing in both composites and a minor amount of Mo3Si in the Mo5Si3–MgAl2O4 composite. Both composites display a dense morphology formed by connecting MgAl2O4 crystals, within which micro-sized molybdenum silicide grains were embedded. For equimolar Mo5Si3– and Mo3Si–MgAl2O4 composites, the hardness and fracture toughness are 14.6 GPa and 6.28 MPa m1/2, and 13.9 GPa and 5.98 MPa m1/2, respectively.
Highlights
Transition metal silicides are considered as a unique class of intermetallic compounds for ultra-high-temperature applications
On in situ formation of MgAl2 O4 and Mo5 Si3 or Mo3 Si composites, this study explores the influence of the stoichiometry of reactants on the flammability limit and combustion wave kinetics of the SHS
In situ formation of Mo5Si3– and Mo3Si–MgAl2O4 composites1/2 was investigated by combustion synthesis the pure
Summary
Transition metal silicides are considered as a unique class of intermetallic compounds for ultra-high-temperature applications. In the Mo–Si binary system, there exist three silicides—MoSi2 , Mo5 Si3 , and Mo3 Si. Among them, Mo5 Si3 with a melting point of 2180 ◦ C is the most refractory and MoSi2 is the most studied phase. Mo5 Si3 has attracted growing interests due to its high melting point, and because of its large alloying potential, wide homogeneity range, high strength, and creep resistance superior to MoSi2 [3,4]. Because silicides and ceramics are two sorts of high-temperature materials highly synergistic with each other, mechanical properties of silicide intermetallics can be effectively improved by the addition of ceramic phases [7,8,9]
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