Combustion synthesis of cubic Al3Ti alloys

Abstract

Combustion synthesis of elemental powder compacts has been used to produce alloys with compositions corresponding to Al₃Ti, Al₇₃Ti₂₄Cr₃, and Al₆₇Ti₂₅Cr₈. The binary composition exhibits a strong exothermic reaction at ∼665 °C, resulting in formation of single phase Al₃Ti with the tetragonal DO₂₂ structure. The heat of formation of the compound at 298 K, δH_f(298), was estimated from peak reaction temperature measurements to be −35.0 kJ mol⁻¹. Additions of Cr lowered the reaction initiation temperature and resulted in the formation of intermediate products attributed to Al–Cr reactions. The DO₂₂ structure remained the primary reaction product in Cr-containing alloys. The Cr particles did not completely dissolve during initial reaction; an additional heat treatment at 1200 °C or above was required to homogenize the material, resulting in partial or complete transformation of the DO₂₂ structure to the cubic L1₂ structure, depending upon Cr content. Homogenization at 1250 °C for 4 h produced material consisting approximately of a 50-50% mixture of the DO₂₂ and L1₂ structures for Al₇₃Ti₂₄Cr₃, and 100% L1₂ for Al₆₇Ti₂₅Cr₈. In all cases, reaction at ambient pressure was accompanied by swelling, resulting in final densities less than 60% of theoretical. Application of external pressure, either during or subsequent to combustion synthesis, produced near theoretical density materials exhibiting a homogeneous and equiaxed microstructure.