Phase equilibrium and mechanical properties of Cr-Mo-Nb-Si-B alloys Composed of BCC and T 2 -silicide phase
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.110
Phase equilibrium and mechanical properties of Cr-Mo-Nb-Si-B alloys Composed of BCC and T2-silicide phase Daisuke Goto*1, Ken-ichi Ikeda2, and Seiji Miura2 1 Graduate student, Department of Materials Science and Engineering, Hokkaido University, Kita13 Nishi 8, Kita-ku, Sapporo 060-8628, Japan
2 Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University, Kita13 Nishi 8, Kita-ku, Sapporo 060-8628, Japan.
*Corresponding author: [email protected]
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.
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INTRODUCTION Currently, Ni-base superalloys are used as turbine blades for thermal power plants because these alloys are superior in heat and oxidation resistance. The operation temperature of a gas turbine reaches approximately 1150 °C, which is close to the melting point of the Ni-base superalloy (ca. 1350 °C). Therefore, new heat-resistant materials are necessary to further improve the thermal efficiency of gas turbine systems. From this viewpoint, refractory metals such as Nb, Mo, Ta, and W are considered to be promising alternative materials to the Ni-base superalloy. This is because the melting points of these alloys are over 2000 °C. Among these alloys, and to suppress centrifugal forces subjected to the turbine blade, low-density materials such as Nb and Mo would be suitable. Much research work has been conducted mainly on Nb-Si and Mo-Si-B systems composed of ductile BCC phases (Nb or Mo) with high-strength T2 intermetallic compounds (Nb5Si3 or Mo5SiB
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