Microstructure Control of Nb-Si Based Alloys with Cr, W, Ta and Zr by Using Nb 3 Si Phase Stability Control
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Microstructure Control of Nb-Si Based Alloys with Cr, W, Ta and Zr by Using Nb3Si Phase Stability Control Yuting WANG1, Seiji MIURA2, Akira YOSHINARI1 1
Materials Research Centre, Hitachi Research Laboratory, 1-1, Omika-cho 7-chome, Hitachi, Ibaraki, 319-1292, Japan 2 Faculty of Engineering, Hokkaido University, N13 W8, Sapporo, Hokkaido, 060-8628, Japan ABSTRACT Recently, Nb-Si based alloys have attracted considerable attention as potential candidate materials for ultra-high temperature applications, because of their low densities and high melting points. However, it is still very difficult to obtain materials with a good balance of hightemperature strength and room-temperature toughness. To address this issue, microstructure control is considered to be a promising method. In applying microstructure control to Nb-Si based alloys with a eutectic reaction (L → Nbss + Nb3Si) and a eutectoid reaction (Nb3Si → Nbss + Nb5Si3), the key is the control of Nb3Si phase stability. Nbss (Nb solid solution) is considered as a ductile phase. In previous reports, it was revealed that different elements had different effects on the stability of Nb3Si. In particular, Mo and W (>3 at %) destabilize the Nb3Si phase, while Ti and Ta stabilize it, and Zr acts as an accelerator for decomposition of Nb3Si. On the other hand, Cr is known to enhance the formation of the ductile Nbss phase. In the present study, we investigated the effects of adding combinations of stabilizing, destabilizing, and accelerating elements with Cr, such as Cr and W, Cr and Ta, Cr and Zr. According to SEM observation, different microstructures were obtained with different combination of additives, and the fracture toughness at room temperature of these samples were also evaluated to reveal the effects of the microstructure on the mechanical properties of Nb-Si based alloys. INTRODUCTION In order to improve the performance and achieve higher efficiency of thermal systems, materials with higher thermal functioning than current materials, nickel (Ni)-based superalloys have been receiving considerable attention [1-3]. Recent studies have identified alloys based on Nb-Si as having great potential for application to turbine engines and land-based gas turbines because of their high melting points [1-5]. The lower density of a Nb-Si system is also considered to be a significant advantage over Ni-based superalloys. However, the lack of balance between low-temperature damage tolerance and high-temperature strength is still one of the major issues for practical application. To improve the mechanical properties of Nb-Si based alloys, microstructure control is considered to be a promising method. Microstructure control involves two kinds of phase reactions: eutectic solidification (Eq. 1), followed by a eutectoid composition reaction (Eq. 2) (1) L → Nbss + Nb3Si
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Nb3Si→ Nbss + Nb5Si3 (2) The primary Nbss dendrite phase and the Nb3Si phase form through solidification (Eq. 1). The Nbss phase is considered as a ductile phase, as the larger size of Nbss grains is helpful in
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