Properties that state the suitability of superalloys
At the normal temperatures, the strengths of several metals are assessed in terms of short time properties like yield strength or ultimate strength. Although when temperature increases, specifically up to 50% of the melting temperature for an alloy, strengths should be reckoned in terms of time at which these quantities are measured. Hence, if a metal is exposed to a load significantly lower than the load that would crack it at room temperature, however is at the high temperature, then stress will start to develop on the metal with time.
The time based extension is known as creep and if permitted to maintain sufficiently longer, will cause fracture known as rupture, hence the creep strength of a metal or rupture strength or both are essential in recognizing its mechanical nature as much as are the essential yield and ultimate strengths. Likewise, the fatigue tendency will be decreased. Hence to completely confirm the tendency of metal alloy, based on the application temperature and load, it may be essential to offer yield and ultimate strengths, creep strengths, stress rupture strengths and suitable fatigue strengths.
The melting temperatures of pure elements are stated following:
Nickel : 2647oF or 1453oC
Cobalt : 2723oF or 1495oC
Iron : 2798oF or 1537oC
The incipient melting points and meting limits of superalloys are functions of composition and early treatment. Normally, incipient melting temperatures are more for cobalt base as compare to nickel or iron-nickel base superalloys. Nickel base superalloys may offer incipient melting at temperatures low down to 2200oF or 1204oC. Modern Nickel base single crystal superalloys have limited extents of melting point depressants causing incipient melting temperatures equal to or in abundance of those of cobalt based superalloys.
Pure Iron has density of 0.284 lb/in3 or 7.87 g/cm3 and pure nickel and cobalt have densities about 0.322 ln/in3 or 8.9g/cm3. Iron nickel based superalloys show densities up to 0.285 to 0.300 lb/im3 or 7.9 to 8.3 g/cm3. Cobalt based superalloys have density about 0.30 lb/in3 to 0.340 lb/im3 or 8.3to 9.4 g/cm3. Nickel based superalloys offer 0.282 to 0.322 lb/im3 or 7.8 to 8.9 g/cm3. The density of superalloy is influenced by inclusions of aluminum,chromium and titanium, decrease density, however tungsten, tantalum and rhenium increase the density. The corrosion resistance of superalloys is based basically on the compositional elements such as chromium and aluminum and conditions used.
Superalloys generally show moduli of elasticity in the limit of 30x 10(6) or 207 GPa, however moduli of certain polycrystalline alloys may change from 25 to 35 x 10(6) psi or 172 to 241 GPa at room temperature based on the alloy system. The processing for directional grain or crystalline orientation may provide elastic moduli up to 18 x 10(6) psi to 45 x 10(6) or 124 to 310 Gpa based on the relation of grain or crystal orientation to analyze the direction. The decreased moduli are received after directional solidification.