Chemical composition and structure of interfacial boundaries in Cr3C2-Ti powder hard alloys after explosive pressing and subsequent heating

Abstract

The paper presents the results of studies of the fine structure, chemical and phase composition of boundaries between the components of the Cr 3 C 2 -Ti hard alloy containing 40 wt.% of titanium bond in the state after explosive pressing, as well as after heat treatment. The powder mixture was subjected to shock-wave loading at a heating temperature of 730 °C and pressure of 14 GPa to ensure the maximum compaction and consolidation of the powder mixture without sintering. Compact specimens were heat-treated by heating from 400 to 700 ° С and holding in the oven for 1 hour followed by still air cooling. The equilibrium phase composition was calculated by numerical thermodynamic modeling using Thermo-Calc software. The structure and elemental composition were studied using FEI Quanta 3D and Versa 3D electron microscopes with an integrated focused ion beam system for foil fabrication, as well as FEI Tecnai G2 20F and Titan 80-300 transmission electron microscopes with foil transmission scanning mode. The Bruker D8 Advance diffractometer was used for X-ray phase analysis. It was shown that the formation of strong interfacial boundaries under explosive pressing of titanium and chromium carbide powder mixtures is accompanied by chemical interaction between the components with the formation of boundary layers having a total thickness of about 90 nm. There is a continuous monotonic change in the Cr and Ti content within the transition layer at the almost constant carbon content. The phase composition of layers corresponds to the equilibrium one calculated at the shock-wave compression pressure but it is thermodynamically nonequilibrium under normal conditions. When heated to 400 °C, boundary layers dissolve with the transition of Cr 3 C 2 -Ti hard alloys into a two-phase state. When heated to 700 °C, alternating layers of carbon-depleted chromium carbides (Cr 7 C 3 , Cr 23 C 6 ) and titanium carbide (TiC) form along the interfacial boundaries by carbon diffusion from the original chromium carbide (Cr 3 C 2 ) to titanium.