A new ternary phase in the system TiB2-CrB2-WB2

Abstract

The quasi-ternary TiB2-CrB2-WB2 was studied by means of hot pressing and pressureless dilatometry in the temperature range between 1650 and 2300 °C. Long-term annealing up to 14 h was necessary for equilibration. Thus, samples were obtained first by densification and then by subsequent cooling to the respective temperature with continuous annealing for up to 16 h in some cases. A large homogeneity range of a (Ti,W,Cr)B2 solid solution has been noticed. A new ternary γ-phase emerges from a critical tie line between (W,Cr,Ti)2B4 and α-(Ti,W,Cr)B2 solid solution at 1950 °C, spreads slightly into both the WB2 and CrB2 direction and decomposes by the peritectic reaction P: γ ⇆ α-(Ti,Cr,W)B2 + (W,Cr,Ti)2B4 + Liquid at 2170 ± 30 °C. At 2050 °C γ reacts with the CrB2-WB2-rich liquid according to the transition equilibrium T: δ-(Ti,Cr,W)B2 + (W,Cr,Ti)2B4 ⇆ γ + Liquid. The γ-phase possesses the composition of (Ti0.31W0.63Cr0.06)B2 expanding slightly towards the TiB2 and WB2-corner depending on temperature. It is clearly characterized by broad 00l x-ray reflections and a small c/a-ratio of 1.005–1.010 which implies a layerwise accumulation and differentiation of Cr and W atoms in the basal plane of the AlB2-type structure. FESEM and TEM studies reveal spinodal decomposition products at nanometer scale as well as coherent platelet-shaped W2B4 and β-WB precipitates of nanometer thickness.