Heat-resistant Alloys

alloys that have high creep resistance and strength at high temperatures. They are used as the construction material for parts of internal-combustion engines, steam and gas turbines, jet engines, and atomic-power installations. The great heat resistance of alloys is determined by two basic physical factors: the strength of interatomic bonds in the alloy and the alloy’s structure.

The structure necessary for high strength is usually produced by heat treatment that leads to heterogenization of the microstructure—most often precipitation hardening. In this case, the strengthening is caused mainly by the appearance within the alloy of evenly distributed very small particles of chemical compounds (intermetallides, carbides, and other compounds) and by the microscopic distortions of the crystal lattice of the alloy base generated by the presence of the particles. The corresponding structure of heat-resistant alloys retards the formation and movement of dislocations and also increases the number of bonds between atoms, which simultaneously participate in the resistance to deformation. On the other hand, a large number of interatomic bonds makes possible the retention of the required structure for long periods at high temperature.

According to the conditions of use, heat-resistant alloys may be divided into three groups: alloys subjected to significant but short-term mechanical stress (seconds or hours) at high temperatures, alloys subjected to loads at high temperatures for dozens and hundreds of hours, and alloys designed to perform under conditions of high loads and high temperatures for thousands of hours. The structural requirements for the alloys vary substantially in accordance with these requirements. For example, any factor causing instability of the alloy structure under operating conditions leads to acceleration of processes of buckling and failure. Therefore, alloys designed for prolonged service undergo stabilizing treatment, which renders them more resistant to prolonged action of loading, although it may lead to a certain decrease in resistance to short-term loads.

Heat-resistant alloys are classified according to their base, which may be nickel, iron, titanium, beryllium, and other metals. Classification according to the base gives a representation of the range of working temperatures, which is 0.4–0.8 of the melting point of the base, depending on the load applied and the duration of its application.

Composite materials (alloys strengthened by disperse particles of refractory oxides or by high-strength fibers) are a variety of heat-resistant alloys. Such materials are characterized by extremely high stability of their properties, which are not highly dependent on the residence time at high temperatures. Depending on their purpose, heat-resistant alloys are made with increased resistance to fatigue and erosion and low notch sensitivity, as well as with high thermal stability and short-term high-load resistance. For example, heat-resistant alloys for use in space technology must have low evaporability.