Crack growth studies on P/M-Udimet 700 using acoustic microscopy

The operation principles of the acoustic scanning reflection microscope and information regarding its capabilities are presented, and demonstrated with the aid of a number of examples. The use of the microscope is illustrated in detail with reference to investigations carried out on the super alloy Udimet 700. The service life of P/M-turbine disc materials is dependent upon technologically unavoidable ceramic inclusions (≤150 μm), as well as from the mechanisms of formation and growth of the small cracks which form at them. In order to investigate the formation and growth behaviour, the ASRM microscope was used which supplemented the investigations carried out in both light and scanning electron microscopes. On fatigue specimens, even after only a few cycles, delamination at the inclusion/matrix interface was observed. From this, after higher numbers of cycles, cracks form at right angles to the direction of the main stresses. It was shown that the growth of such cracks could be hindered by the presence of coarse γ'-particles. Using the ASRM, it was possible to follow small surface cracks to depths within the body of the material, and to discover the actual particles responsible for initiating them. (orig.)