Effect of processing parameters on solid-liquid interface of Nb-Si base alloy fabricated by electromagnetic cold crucible directional solidification

Nb-Si base alloys have attracted considerable attentions as the potential high temperature structural materials working in the service temperature range of 1200~1400 ℃ because of their high melting points (> 1750 ℃), moderate densities (6.6~7.2 g/cm³) and excellent high temperature strength. However, the mismatching between room temperature fracture toughness and high temperature strength has limited their practical applications. Directional solidification (DS) and alloying have been proved to be the effective methods to overcome this critical issue. The DS processes used to prepare Nb-Si base alloys included Czochralski directional solidification in a copper crucible, electron beam directional solidification, optical floating zone melting, integrally directional solidification and electromagnetic cold crucible directional solidification (ECCDS). The previous studies focused on the effect of process parameters on microstructure and mechanical properties in the steady- state growth region (SS- GR). However, the microstructure in the SSGR was controlled by the solid-liquid interface, and the solid-liquid interface was controlled by process parameters. Therefore, the study about the effect of process parameters on solidliquid interface was very important. In this work, the master alloy with the nominal composition of Nb-22Ti-16Si-3Cr-3Al-2Hf (atomic fraction, %) was prepared by vaccum non-consumable arc-melting first, and then induction skull melting. The DS experiments were performed in the ECCDS device equipped with a square water cooled copper crucible (internal dimension: 26 mm×26 mm×120 mm) and a Ga-In alloy pool. There were three processing parameters in ECCDS including heating power of power supply, withdrawal rate and holding time. The DS ingots were prepared according to the orthogonal test (L₉ (3³)). Instability degree was defined as the ratio of the height of solid-liquid interface to the width of the DS ingot. The results showed that there were three macroscopic morphologies of solid- liquid interfaces; the increase of holding time, decrease of withdrawal rate and elevation of heating power were conducive to keeping the solid-liquid interface macroscopic morphology planar. With the increase of withdrawal rate, primary dendrite arm spacing (d₁) and secondary dendrite arm spacing (d₂) decreased gradually; with the increase of heating power, d₁ and d₂ increased gradually; with the increase of holding time, d₁ and d₂ increased first and then decreased. The higher withdrawal rate, lower heating power and less holding time were beneficial to refining the d₁ and d₂.