Metallothermal synthesis of the Laves phase TaCr2 from oxide raw materials

Abstract

The refractory Laves phase TaCr2 of two compositions (series) was synthesized by the metallothermal (hydride-calcium) method from oxide raw materials (Cr2O3 + Ta2O5). The synthesis temperature was 1200 oC, while the excess of the reducing agent CaH2 was varied relative to the theoretically required for the complete reduction of oxides. It is shown that regardless of the excess of the reducing agent or in its complete absence, the resulting  powders have the required chemical composition (composition 1: calculated Cr content — 37.38% (wt.), actual — 37.50±0.85% (wt.); composition 2: calculated Cr content — 35.30% (wt.), actual — 35.38±0.31% (wt.)). No significant losses of Cr and Ta were observed during the synthesis of the Laves phase under the conditions of hydride-calcium reduction of Cr2O3 and Ta2O5. All prepared powders contained ~ 0.25% (wt.) oxygen and 0.025% (wt.) calcium. This is a good result for experimental batches first obtained by this method. Powders of the first series (composition 1), regardless of the excess of CaH2, contained no less than 85% (wt.) of the TaCr2 phase of the С15 type and ~ 10% (wt.) of the Cr(Ta) solid solution. However, when the excess of the reducing agent was >50 wt%, a high-temperature modification of the TaCr2 phase with a C14-type hexagonal lattice came in sight. The same picture was observed in the second series of experiments (composition 2). In all cases, the amount of the C14 type TaCr2 phase did not exceed 5% (wt.). Powders of composition 2 contained no less than 90% (wt.) of the C15 type TaCr2 phase or 95% (wt.), if there were two modifications C15 + C14. Depending on the excess of the CaH2 reductant, the alloys contained small amounts of Cr(Ta) or Ta2H hydride. In both series, the powder particles had a finely dispersed spongy structure; large particles of regular shape were present in an insignificant amount. This work was carried out with financial support of the government of the Tula region (grant for work in the field of science and technology No. DS / 161 dated October 29, 2020).