Combustion synthesis of Cr–Al and Cr–Si intermetallics with Al2O3 additions from Cr2O3–Al and Cr2O3–Al–Si reaction systems

Abstract

Formation of chromium aluminides (Cr2Al, Cr5Al8, and Cr4Al9) and silicides (Cr3Si, Cr5Si3, CrSi, and CrSi2) was achieved by self-propagating high-temperature synthesis (SHS) involving the thermite reaction of Cr2O3 with Al. Compressed samples were prepared from the powder mixtures of Cr2O3–xAl (with x = 3.0–7.0) and Cr2O3–2Al–ySi (with y = 0.67–4.0). The combustion process takes advantage of the reaction heat released from aluminothermic reduction of Cr2O3 to reach self-sustaining and to facilitate the formation of Cr–Al and Cr–Si compounds. Due to a decrease in the reaction exothermicity, the flame-front propagation velocity and combustion temperature of the sample compact decreased with increasing Al and Si contents in the reactant mixtures. Based upon the XRD analysis, three aluminides, Cr2Al, Cr5Al8, and Cr4Al9, were identified along with Al2O3 in the products of the powder compacts with corresponding stoichiometries. Aluminide phases highly rich in Al, like CrAl4, Cr2Al11, and CrAl7, were not produced, because reaction exothermicity of the sample with Al/Cr2O3 ≥ 8.0 was insufficient for self-sustaining combustion to occur. For the Cr–Si compounds, Cr3Si and CrSi2 were produced in a single-phase form with Al2O3 from the stoichiometric samples. The other two silicides, Cr5Si3 and CrSi, were detected in the products containing multiple silicides. In addition, the fracture toughness of the as-synthesized composite was determined.