Turbine blades need to operate at high temperatures and maintain their shape as well as their mechanical properties. Many metals have reduced yield strength at high temperature, meaning it takes less stress to make them change shape. Interestingly, shape changes can also take place at stresses below the yield strength if a component is at high temperature for a long time. This phenomenon is called creep, and it is facilitated by stress-induced, thermally-aided diffusion.
Diffusion is the movement of mass within a medium, and in the case of creep, the atoms of a metal diffuse into gaps called vacancies, which appear along the direction of stress as the metal atoms are pulled away from one another. At high temperatures, diffusion rates are high enough that the part changes shape, lengthening along the direction of applied stress, in a matter of hours to days.
In their service life, turbine blades see hundreds to thousands of hours at temperatures upwards of 600 degrees Celsius, conditions ripe for diffusion creep. The fastest diffusion pathways are crystalline grain boundaries, meaning polycrystalline materials are especially susceptible to creep.
Turbine blades are made as single crystals in order to eliminate grain boundary diffusion paths and reduce the creep rate so that the component can spend more time at high temperature.
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