Other Materials For Elevated Temperatures
High temperature creep is vital phenomenon to be guarded in many machines like gas turbines, missiles, propulsion engines, and reentry vehicles. Ferritic & other all stainless steel are good against creep but only to restricted extent. Refractory metals & their alloys are under close investigation for this cause. The goal is not yet attained and several developments are still proprietory. It will be worthwhile to note down some developments that have already taken place.
Iron base super alloys are modification of austenitic stainless steel. Yet they are proprietory and manufactured by only one company. Austenitic steel is further alloyed with some additional metals to generate super alloys. These alloying elements include W, Mo, Co, V, Nb, Ti, Zr, Al, B and Cu. These super alloys are strengthened by precipitation hardening or by strain hardening. These alloys are frequently used between 650oC & 760oC, the range in which strain hardening effects are retained because addition of alloying elements increasing recrystallization temperature to 925oC. The precipitation hardening is attained due to precipitation of hard particles of carbides nitrides or boride which are stable above 750oC and therefore super alloys are conveniently utilized in the range of 650oC-750oC.
Cobalt and nickel based super alloys contains most of iron replaced by cobalt or Ni. These alloys might also be hardened by strain hardening or precipitation hardening. In precipitation large size particles (0.5 μ diameter) built up of Ni3, Al or Ni3 Ti or their mixture are precipitated that are stable up to very high temperature. The usable temperature limit of these alloys is as high as 925oC.
Dispersion hardened alloys attain their high temperature strength form finely dispersed ultrafine ceramic particles in the matrix of nickel. T. D. Nickel with fine thoria (ThO2) dispersed in the matrix is possibly the only example of this type. The material is extremely hard at room temperature and may not be deformed plastically. Though, it is very costly.