Properties of Nickel Based Superalloys required for industrial applications
Nickel based superalloys are widely utilized in the turbine engines of jet planes, marine and power plants. The application temperature of these alloys varies from 150oC to 1500oC. The high temperature strength of an alloy depends on the consistent austenitic matrix paired with solid solution hardening and / or precipitation strengthening. Essential properties of superalloys required by industries are:
Thermal stability of gamma phase, feasibility of solid solution hardening and precipitation strengthening, high elastic modulus of the matrix are the key aspects that describe the use of superalloys. High solubility of several elements like cobalt, iron, chromium, molybdenum and tungsten provides the feasibility of strength of austenitic matrix.
The inclusion of aluminum and titanium results in precipitation of an ordered compound depends on the formula Ni3(Al, Ti) that is consistent with austenitic gamma matrix. This phase is needed for elevated temperature strength and creeping resistance of an alloy.
Drawback while use of superalloys is small thermal conductivity that because of the high value of alloying elements. Quick heat removal provided by high thermal conductivity offers sufficient cooling that enhances the look of life limiting heat attacking region. Large thermal conductivity increases uniform heat distribution that decreases thermally caused stresses and hence enhances fatigue strength.
Other essential properties are creeping resistance at the elevated temperatures, high surface stability and corrosion and oxidation resistance. The most crucial characteristic is elevated temperature creeping resistance. Creep fracture is an ability of a solid material to slide slowly or damage completely on applying stress. The creep rupture occurs after a certain time period and is caused from the prolong exposure to stress levels and highly vigorous in the materials that are exposed to heat for the prolong periods at levels near the melting points of materials.
Creep resistant alloys have a special role in several applications such as jet engines, heat exchangers, nuclear power plants and incinerators. Another crucial characteristic is corrosion resistance provided by superalloys.
Thermal conductivity of superalloys is based on their chemical composition and service temperatures. Usually thermal conductivity increases with increase in temperature from 9.8 W/m/K to 29.5W/m/K at temperature 23oC to 1197oC for Inconel alloy 713LC. In many cases,thermal conductivity is determined as a function of specific heat, thermal diffusion-ability and density of an alloy.
Thermal conductivity is a crucial physical property of any material that enables to identify the significance of the metallic materials in the elevated temperature structural services.