Refractory Superalloys Team

Our aim is to develop new generation superalloys, which can be used above 1473 K. It is well known that FCC L12 coherent two-phase structure plays an important role in strengthening Ni-base superalloys, which are used in many application fields, such as gas turbines of generators in power stations and jet engines in airplanes operating at high temperatures. To develop new-generation superalloys which can be used above 1473 K, we have designed alloys based on platinum group metals, mainly Ir with an FCC L12 coherent two-phase structure, and called them "refractory superalloys." Iridium has high melting temperature (2720 K), and the highest elastic modulus (570GPa) at room temperature in addition to being one of the most stable elements against corrosion. The strengthening behavior, creep property, ductility, and fracture mode of the Ir-based alloys are investigated in terms of microstructure, deformation structure, and lattice misfit under the High-Temperature Materials 21 Project. Our final target is the development of an alloy whose creep life at 2073 K under 137 MPa is over 1000 hours.

To reach our final target, we are investigating the following topics.

1. Strength behavior, creep properties, and deformation mechanism of binary Ir-base alloys.
   ex. Ir-Nb, Ir-Zr, Ir-Ti, Ir-Ta, Ir-Hf, Ir-V
2. Third element addition effect on strength, creep properties, fracture mode, and deformation mechanism of Ir-base alloys.
   ex. addition of Mo, Ta, W, B, C, Ni, Pt, Zr to Ir-Nb
3. Phase constitution, microstructure, strengthening behavior, fracture mode, and creep properties of Quaternary alloys
   ex. combination of Ir-Nb and Ni-Al, Ir-Nb-Ni and Rh-Nb-Ni
   These alloy can be called an Ir-Nb- or Ir-Rh- base alloys containing some FCC or L1₂-phase forming elements.
4. Development of processing technique ex. Pulse electric current sintering

Project report in 2001:Mat. Sci. Eng. A accepted.

In addition to HTM21 project, the project 'Development of refractory superalloys for propulsive engine of space plane' funded by NEDO (New Energy and Industrial Technology Development Organization) was launched between November 1, 2000 and March 31, 2003.

A space plane which is a reusable system that produces little air pollution using liquid hydrogen as a propellant is very promising for reducting the cost of transport and to protect environment. To develop the space plane, the properties of the propulsive engine need to be improved. It is necessary to produce a heat exchanger by high temperature materials. The heat exchanger is generally exposed to high temperatures but not subjected to high stresses. This is a great demand for a material, which has long creep life under low stress at high temperature that has a good fabricability. To offer high temperature material for the heat exchanger, we have developed materials based on Ir with high melting temperature. Our target is the development of an alloy with long creep life of more than 160 hours at 1773 K at 100MPa, that offers good ductility at room temperature and good fabricability.

Project report in 2001