Material Problems At Elevated Temperatures
It has been stated earlier that most materials lose their strength at higher temperatures. This can be conceived being due to greater mobility of atoms at high temperatures. The elastic constants also reduce at high temperature for the same reason and hence greater deformation results at elevated temperature. The elevated temperatures further deteriorate material behaviour by causing oxidation.
Present day technology could boast range of materials that are capable of working at very high temperatures. This has become possible for constant demand of higher technology involving applications in nuclear engineering, power generation steam and gas turbines and high pressure boilers, oil technology and transportation engineering. The modest temperature of 500oC in steam turbines has increased to 800oC in gas turbines and requirement of highly powerful engines for jet planes has pushed this limit to 1000oC.
The rocket engines, nose cones of ballistic-missile and re-entry vehicle face still higher temperatures. All these factors have been greatly responsible to accelerate the researches on high temperature resistant alloys and ceramic material that are being used presently.
While static properties of materials are not much affected by time-required for testing at room temperatures, the strain rate affects high temperature properties greatly. It is characteristics of high temperature properties that time element becomes important for them. This is of particular interest because a static load much below yield load of material could result into gradual deformation at elevated temperature.