As a general rule, creep starts to become significant when the homologous temperature is greater than 0.4. Most metals do not suffer from creep at room temperature, since they have much higher melting points than solder. However, creep can still be a major concern when designing metallic components that have to function at high temperatures.
An example of one such engineering challenge is in the design of turbine blades for use in jet engines. The blades in these engines can be exposed to hot gases at up to about 1400°C. They are also under stress, as a result of the high centrifugal forces. These blades must withstand this environment without excessive creep, which would cause them to strike the turbine enclosure.
Nickel-based Superalloys - an example of a creep-resistant material
The materials used to make aero-engine turbine blades are nickel-based superalloys. These not only have relatively high melting points, reducing the homologous temperature, but also have microstructures designed to impart high creep resistance at any given homologous temperature. Furthermore, relatively cold (by-pass) air is ducted through channels in the blades, to help keep them cool, and they are sometimes also coated with insulating ceramic layers (thermal barrier coatings). These measures ensure that the blades are kept below 1000°C, even when the turbine entry temperature is as high as 1400°C. It can be seen from the creep rate information in the figure that this is essential if the blades are to perform adequately.
Microstructure of Superalloys in Turbine Blades
In order to obtain creep rates as low as those shown in the figure, considerable
evolution towards an optimal microstructure has been necessary. For example,
as creep behaviour became better understood, it became clear that grains should
be elongated in the direction of the applied load - in fact single crystals
give the best creep resistance. The grain structure development shown in the
figure therefore took place during the 1960s and 1970s.
However, the microstructure within a grain is actually quite complex. Superalloys usually have two distinct phases, γ and γ', with coherent interfaces and an orientation relationship between them. This leads to lattice strain and resistance to dislocation motion (through the γ phase), particularly when the γ' precipitates are fine - see the figure.
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