Improvement of the Strength of Ternary Eutectic V-Si-B Alloys at Ambient and High Temperatures Due to Cr Additions

The V-Si-B system has gained scientific interest as a new low-density, refractory metal-based structural intermetallic alloy system. The alloy design is strongly influenced and driven by the developments in the field of Mo-Si-B alloys and shares some interesting structural and microstructural features. Very recently, the formations of ternary eutectic V_SS-V₃Si-V₅SiB₂ microstructure has been reported which contains the same isomorphous phases as the ternary eutectic in the well-studied Mo-Si-B system: a refractory metal-based solid-solution phase (Mo_SS or V_SS) and the two intermetallic phases with either an A15 (Mo₃Si and V₃Si) or a D8_l (Mo₅SiB₂ and V₅SiB₂) structure. However, while the Mo-Si-B-based ternary eutectic shows some oxidation resistance due to its intermetallic character, oxidation of the V-based eutectic is an even more serious issue. To address this problem, different amounts of Cr were added to a eutectic V-Si-B alloy to study the microstructural influence on the ternary eutectic reaction, the phase stability as well as the mechanical and oxidation properties as a function of Cr concentration. Alloys with Cr additions between 5 - 30 at.% were fabricated by conventionally arc-melting and were analyzed in the as-cast state or heat-treated at 1400 °C for 100 hrs.<br/>Prior to the mechanical compression tests the microstructures of the Cr-added ternary eutectic alloys V-9Si-6.5B were investigated. Even at high Cr-additions of 30 at.% the eutectic V_SS-V₃Si-V₅SiB₂ microstructure could be maintained. Thus, Cr has almost no influence on the solidification behavior in this part of the V-Si-B system, which seems to be plausible since Cr stabilizes all three eutectic phases.<br/>The ternary eutectic alloy V-9Si-6.5B features the ductile V_SS phase as the major phase. Thus, first compression test revealed a deformability even at room temperature. However, the yield strength decreases quickly with increasing test temperatures and is even more pronounces in the heat-treated state. In order to develop a comparably low-density materiel for high temperature structural applications, the high-temperature strength needs to be further improved, i.e. as shown in this study by Cr-additions. Cr dissolves in all the ternary eutectic phases and, as mentioned before, does not influence the eutectic formation. This fact makes Cr an ideal candidate to study the strengthening behavior of the alloys by subsequently increasing it’s Cr content. With increasing Cr-additions the compressive yield strength increased, too. Accompanied with the hardening effect, Cr leads to brittle failure during the compression tests. The strengthening effects is mainly attributed to solid-solution strengthening of the V_SS phase, which forms the major phase (about 50 - 60% phase fraction) in the Cr-free and Cr-added alloys. Since Cr is also dissolved in the intermetallic phase, a strengthening effect may also occur in these phases.