Rules for designing Nb silicide based alloys: The case for the solid solution phase
July 17-21, 2016
Nb silicide based alloys (or Nb in situ silicide composites) have the potential to replace Ni based superalloys in high temperature aero-engine applications. The two most important phases in the microstructure of these new alloys are the bcc Nb solid solution (Nbss) and the tetragonal Nb5Si3 silicide. Different types of solid solution can form in the new alloys. The Nbss is the key phase for achieving the mechanical property and oxidation goals.
Alloying with transition metals, including refractory metals, and with simple metals and metalloids is essential for improving the mechanical behaviour and oxidation of the Nb silicide based alloys. Two questions (among many others) that are critical for the design of the new ultra-high temperature alloys are: (a) what are the rules that govern the formation of the bcc Nbss in Nb silicide based alloys and (b) can these rules discriminate between alloying additions in the bcc Nbss that are essential for improving the oxidation behaviour and creep resistance of the new alloys?
The Nbss formed in different families of Nb silicide based alloys has been studied using thermo-physical parameters and properties that include enthalpies and entropies of mixing, atomic size and electronegativity and electronic structures. The presentation (i) will discuss the different types of Nbss that can form in the new alloys, and (ii) will use maps based on thermo-physical parameters and properties to discuss the rules that favour the stability of different forms of the Nbss.
Panos Tsakiropoulos, "Rules for designing Nb silicide based alloys: The case for the solid solution phase" in "Beyond Nickel-Based Superalloys II", Chair: Dr Howard J. Stone, University of Cambridge, United Kingdom Co-Chairs: Prof Bernard P. Bewlay, General Electric Global Research, USA Prof Lesley A. Cornish, University of the Witwatersrand, South Africa Eds, ECI Symposium Series, (2016). http://dc.engconfintl.org/superalloys_ii/45