Research into the Possibility of Producing Single-Phase Tantalum–Hafnium Carbide by SHS

Resumo

The influence of mechanical-activation (MA) conditions on the microstructure and phase composition of Ta–Hf–C reaction mixtures and products fabricated from them by self-propagating high-temperature synthesis (SHS) is investigated. The Ta–Hf–C reaction mixtures are mechanically activated in centrifugal planetary mills with various drum revolution rates. It is revealed that an increase in the drum revolution rate from 250 to 900 rpm lowers the heterogeneity scale of the reaction charge, decreases the size of coherent scattering regions of tantalum and hafnium by an order of magnitude, and leads to a rise in the degree of microdeformation of their crystal lattices by a factor of 1.5–2.0. It is established experimentally that it seems impossible to initiate an SHS reaction in the Ta–Hf–C activated mixture at initial temperature T₀ < 550 K. The combustion process is implemented only at T₀ = 800 K, when the adiabatic combustion temperature reaches 3274 K in mixtures treated with a revolution rate higher than 678 rpm. Single-phase carbide (Ta,Hf)C with lattice parameter a = 0.4487 nm, which corresponds to 18.0 at % dissolved HfC in TaC, is formed from reaction mixtures activated according to optimal regimes. The hafnium oxide content in products does not exceed 1%. The sample structure has high porosity (larger than 30%) and small carbide grain size (smaller than 10 μm), which makes it possible to produce the (Ta,Hf)C powder by milling the SHS product in a ball rotary mill.