Reactions and Phase Relations in the System Cr-B-N at 1600°C under 1 atm N₂

Abstract

Reactions and phase relations in the system Cr-B-N at 1600°C under 1atm N₂ have been studied. In the region of Cr₅B₃-B-N, Cr or Cr₂N reacted with BN to form Cr₅B₃, while CrB or CrB₂ reacted with N₂ finally to form Cr₅B₃+BN. The reaction processes seem to be by steps as CrB₂→Cr₃B₄→CrB→Cr₅B₃. It can be concluded that Cr₅B₃ is an equilibrium phase at 1600°C under 1atm N₂. Lattice parameters of Cr₅B₃ obtained from the initial composition of Cr+BN were a=5.467±0.007Å, c=10.081±0.007Å, which were equal to those from the step reactions of CrB with N₂ or CrB₂ with N₂.
Cr₂B+Cr₅B₃ or Cr₂N+Cr₂B was produced as the final solid phases in the region of Cr₂B-Cr₅B₃-N, or Cr-Cr₂B-N respectively.
Cr₂N phase in the equilibrium state was obtained both from the nitridation of Cr and the decomposition of CrN.
Their lattice parameters were a=4.765±0.002Å, c=4.441±0.003Å.
The isothermal section of the system Cr-B-N at 1600°C under the above conditions was divided into 3 three-phase triangles; Cr₂N+Cr₂B+N₂, Cr₅B₃+Cr₂B+N₂ and Cr₅B₃+BN+N₂. No homogeneity ranges can be detected on the tie lines Cr₂N-Cr₂B, Cr₂B-Cr₅B₃ and Cr₅B₃-BN.