Unidirectional crystallization and 3D structure of ternary four-component eutectic of B4C-NbB2-SiC system

Purpose. To investigate the laws of crystallization and formation of 3D morphology of ternary eutectics in system B4C-NbB2-SiC. Research methods. SEM (SE, BSE), РСМА (EDS, WDS), XRD. Results. The micro- and macro-morphology of eutectic colonies in alloys of the system NbB2-SiC-В4С formed during directional crystallization have been investigated. The obtained patterns are embodied in a 3D structural model of (B4C+NbB2+SiC) eutectic cell and from these result, a microscopic kinetics of crystallization of this cell was proposed. The continuity of eutectic phase dendrites from nucleation to the end of growth and inadequacy of ideas about eutectic as a mechanical mixture of small crystals of eutectic phases were shown. Scientific novelty. For the first time a 3D model of a three-phase 4-component eutectic cell of eutectic colony was built. For the first time the possibility of combined microscopic crystallization kinetics of a three-phase eutectics was revealed, including both paired cooperative microscopic kinetics of (SіC+ NbB2) growth and the kinetics of autonomous growth of the third eutectic phase B4C. Practical value. Revealing the pattern of micro and macrostructure formation of a three-phase eutectics, which was formed in the course of directional crystallization, opens up technological prospects for purposeful control of the structure and properties of eutectic alloy due to the change of micro and macro morphological constituents, including the method of modification.

A Study of the Effects of Hf and Sn on the Microstructure, Hardness and Oxidation of Nb-18Si Silicide Based Alloys without Ti Addition

The paper presents the results of an experimental study of large (≈0.6 kg) arc melted buttons of four Ti free Nb-silicide based alloys with Sn addition with nominal compositions (at.%) Nb-18Si-5Hf-5Sn (EZ1), Nb-18Si-5Al-5Sn (EZ7), Nb-18Si-5Cr-5Hf-5Sn (EZ3) and Nb-18Si-5Al-5Hf-5Sn (EZ4). The alloys were studied in the as-cast and heat treated conditions. In all the alloys there was macrosegregation of Si (MACSi). Among the single element additions Hf had the weakest and Sn the strongest effect on MACSi. The simultaneous presence of Cr and Hf in the alloy EZ3 had the strongest effect on MACSi. In all the alloys the βNb5Si3 was the primary phase and was present after the heat treatment(s), the Nb3Si silicide was suppressed and the A15-Nb3Sn intermetallic was stable. The Nbss was not stable in the alloys EZ7 and EZ4 and the C14-NbCr2 Laves phase was stable in the alloy EZ3. Very Hf-rich Nb5Si3 was stable in the alloy EZ4 after prolonged heat treatments. Eutectics were observed in all the alloys. These were binary eutectics in the alloys EZ1 and EZ7, where respectively they consisted of the Nbss and βNb5Si3, and βNb5Si3 and A15-Nb3Sn phases. Most likely ternary eutectics consisting of the Nbss, C14-NbCr2 and βNb5Si3, and Nbss, βNb5Si3 and A15-Nb3Sn phases were observed, respectively in the alloys EZ3 and EZ4. The addition of Al increased the vol% of the Nb5Si3 and A15-Nb3Sn phases, particularly after the heat treatment(s). The lattice parameter of Nb respectively increased and decreased with the addition of Hf, and Al or Cr and the latter element had the stronger negative effect. Pest oxidation was not suppressed in the alloys of this study.

Effects of the Chemical Compositions of KF-CsF-AlF3 Middle Temperature Aluminum Flux on Melting Characteristics

According to the KF-CsF-AlF3 ternary phase diagram, 25 samples (named group A) near the region around the E4 and E5 (ternary eutectics points), and 22 samples (named group B) near another region around the e5 (binary eutectic point) and m2 (ternary eutectics point) were prepared by the wet synthesis method. The melting curves of these samples were measured by differential scanning calorimetry (DSC). Based on the DSC data, the effects of the fluxs compositions on the melting characteristics were analyzed and summarized. The results indicated that the lowest solidus of group A samples was about 480°C, meanwhile, their liquidus was about 520~530°C, and the melting range was approximately 40°C. Furthermore, when the content of CsF was lower than 18mol%, the solidus of group A samples apparently rose to about 510°C, but the liquidus did not change greatly. The lowest solidus of group B samples was about 420°C, meanwhile, their liquidus was below 450°C, and the melting range was approximately 20~30°C. So, with this profile of melting characteristics, some of the samples of group B flux would be good substrates for aluminum alloy middle temperature brazing, with good prospects for application.