Combustion synthesis of mechanically activated powders in the Ti–Si system

Abstract

The effect of the mechanical activation of the reactants on the self-propagating high-temperature synthesis (SHS) of titanium silicides was investigated. SHS experiments were performed on reactant powders that were milled for different times. Mechanical activation was shown to have a large influence on the combustion characteristics, particularly on wave speed. A much weaker effect was observed on the products phase composition. Single-phase products were obtained only from Ti:Si = 1:2 and Ti:Si = 5:3 starting compositions. Observation of microstructural evolution in quenched reactions of Ti:Si = 1:2 mixtures milled for relatively long times revealed that the combustion reaction was primarily a solid-state process restricted to a surface layer of the large Ti grains. A secondary process involving a solid–liquid interaction between solid Ti and melted Si was dominant in the post front region. The mechanical activation in this case took the role of increasing the contact surface between the reactants. A single reaction coalescence mechanism involving only liquid phases was proposed for the Ti:Si = 5:3 composition. For this composition the apparent activation energy for the overall combustion process was determined (155 kJ mol⁻¹) and was shown to be independent on the degree of mechanical activation of the reactants.