Paper Title
SYNERGETIC ALLOYING EFFECTS OF Mo, V AND Zr ON THE MICRO STRUCTURE AND PROPERTIES OF Nb-Si BASED ALLOYS

Abstract
Nb-Si based ultrahigh temperature alloys are one kind of the potential alternatives for Ni based super alloys to operate at 1200~1400 ℃ due to their lower densities and excellent high temperature strength. However, their insufficient room temperature fracture toughness and poor high temperature oxidation resistance are the bottlenecks for their practical applications. Alloying is an effective way to improve the comprehensive properties of these alloys. It has been reported that alloying with V or Zr in the binary Nb-Si alloy improves the room temperature fracture toughness and high temperature oxidation resistance, while lowers high temperature strength. Considering that Mo element has higher melting point and thus is beneficial for high temperature strength, the present work aims to add Mo, V and Zr simultaneously to improve the comprehensive properties of Nb-Si based ultrahigh temperature alloys. Additionally, with the development of alloying, Nb-Si based ultrahigh temperature alloys have been developed to multi-elemental systems with more than six elements. In multi-elemental Nb-Si based ultrahigh temperature alloy systems, the composite alloying effects become more and more complex and is worthy of exploring. It is essential to systematically investigate the effects, especially the composite effects of Mo, V and Zr on the comprehensive properties of multi-elemental Nb-Si based ultrahigh temperature alloys for further compositional optimization. Fifteen multi-elemental Nb-Si based ultrahigh temperature alloys with nominal compositions of Nb-22Ti-15Si-5Cr-xMo-yV-zZr (at.%) were prepared by vacuum non-consumable arc melting. The alloying effects of Mo, V and Zr on the arc-melted and heat-treated microstructures have been investigated. The phase constituents of the multi-elemental Nb-Si based ultrahigh temperature alloys are Nbss (Niobium solid solution), α/γ(Nb,X)5Si3 and Laves Cr2Nb phases. Alloying with V or Zr promotes the formation of hypereutectic structure, and cooperatively alloying with V and Zr further increases the content of primary silicides. While, in the presence of V or Zr, further adding Mo decreases the content of primary silicides. The additions of Mo, V and Zr can suppress the formation of α(Nb,X)5Si3, whilst promote the formation of γ(Nb,X)5Si3. During heat treatment at 1450 ℃ for 50 h, some γ(Nb,X)5Si3 phases in 0Mo-0V-0Zr and 0Mo-3V-0Zr alloys have transformed to α(Nb,X)5Si3 phases, but this phase transformation is not observed in Mo- or Zr-containing alloys.The oxidation performances of the fifteen arc-melted multi-elemental Nb-Si based ultrahigh temperature alloys were evaluated at 1250 °C for 1 and 20 h, respectively. The effects of Mo, V and Zr additions on the oxidation resistance of multi-elemental Nb-Si based ultrahigh temperature alloys have been investigated. The microhardness, three points bending and compression at 1250°C measurements were conducted for revealing the effects of Mo, V and Zr additions on the room and high temperature mechanical properties. Keywords - Nb-Si Based Ultrahigh Temperature Alloy; Phase Constituents; Alloying; Molybdenum, Vanadium and Zirconium; Properties.