Turbine blades repair

Performance in fatigue and failure

As for all metallic materials, the single-crystal superalloys are prone to fatigue failure due to oscillating loads, at stresses which are nominally elastic. This is because plastic deformation can occur in a very localised sense, typically near sites of stress concentration; this fatigue damage is most often in the form of intense, localised slip with the dislocation activity restricted to a small number of lattice planes. This phenomenon is often referred to as 'cyclic slip localisation'; particularly at temperatures of around 700C and when s stresses are high this encourages the formation of 'persistent slip bands' (PSBs) which shear the y' particles. Fatigue initiation can always be traced to sites of stress concentration, for example casting pores or machining marks. At higher temperatures and when stresses are low, dislocation activity is restricted to the y channels and cyclic slip localisation is less intense: thus deformation is more homogeneous, but nonetheless limited to the most highly stressed planes. Under these conditions, fatigue initiation occurs at surface pits or cracks in the oxide films.

Protective coatings 

Life is improved markedly if protective coatings (for example aluminides) are used.

In practice, the environment experienced by the blading in service is very difficult to replicate exactly in the laboratory. Consequently, reliance is placed on the characterisation of fatigue performance under conditions which can be easily controlled.