Phase equilibrium and mechanical properties of Cr-Mo-Nb-Si-B alloys Composed of BCC and T2-silicide phase

Abstract

A new class of high-temperature materials based on refractory elements was investigated with an aim to improve the energy efficiency of thermal power plants. Alloys based on Nb and Mo composed of BCC solid solution (BCCss) (Nb-Mo) and T2-silicide (Nb,Mo)5(Si,B)3 are promising candidates as high-temperature materials. Further investigation on the alloy phase equilibria of this system is required to improve the mechanical properties and oxidation resistance through optimization of the phase compositions. Cr is one candidate to modify the properties of the alloy because Cr is expected to stabilize the T2 compound phase along with B. Here, the phase equilibria among BCCss and the T2 compound are widely investigated in the Cr-Mo-Nb-Si-B system, and a BCCss-T2 two-phase microstructure is found in Mo-rich alloys. The B/Si ratio in the T2 phase increases with the Cr content, while almost no B solubility was found in BCCss. As the Si content increases in alloys, the A15 silicide phase ((Cr, Mo, Nb)3Si) and/or Laves phase appear. Nanoindentation tests were conducted to investigate the mechanical properties of the BCCss phase of the alloys in the Cr-Mo-Nb-Si-B system. The nanohardness and reduced elastic modulus of these alloys tended to be higher with an increase in Cr.