Reactive melt infiltration of copper in Al-Cr preforms produced through combustion synthesis

Combustion synthesis of Al-Cr preforms used for infiltration and reinforcing of composite materials was developed. Compacts of powdered Al and Cr with stoichiometric ratio Al/Cr equal to 2/1 were synthesized in a microwave reactor furnished with a pyrometer for controlling phase transformations. Due to low enthalpy of the reaction, green compacts were preheated and ignition occurred together with partial melting of Al at the interface with Cr particles. The synthesis proceeded by peritectic transformations L + Al7Cr → L + Al11Cr2 → L + Al4Cr, reaching maximum temperature of ca. 1000 °C. Porous structures including residual unprocessed Cr particles were soaked to homogenize them and to transform the phases into the stable intermetallic compound Al9Cr4. Reactive infiltration of the preforms with molten Cu proceeds along with interfacial diffusion of Al that, released from a preform, infiltrates into the matrix changing its composition to Cu9Al4(Cr). At the same time, the preform is decomposed and converted into a mixture of globular precipitates of Cr52Al35Cu13 embedded in the Cu47Al41Cr12 phase. The produced composite materials exhibit significant hardness and oxidation resistance at elevated temperatures. The protective layer is composed of oxides Al2O3 and (AlCu)2O3 created at parabolic constant oxidation rate (kp) equal to 1.9 × 10−6 g2 m−4 s−1.