Generation of extremely high stress levels in a TiAl based alloy

The aim of this work is to use ultrasonic shot peening, a mechanical surface treatment derived from conventional shot peening, in order to increase the fatigue life of TiAl alloys. The goal of this treatment is to generate compressive superficial residual stresses which are aimed at enhancing to fatigue crack initiation and growth resistance. For this purpose, different ultrasonic shot peening tests have been carried out on Ti-48Al-2Cr-2Nb samples in order to optimize treatment conditions. The stress profile as well as the treated surface characterization (roughness, hardness, microcracks...) have been determined for each sample. The first results reveal that it is possible to generate very high stress levels (≈1000 MPa) beneath the surface, far much higher than the tensile yield stress of the material which is in the range 350-600 MPa. Such a phenomenon was also observed in ultrasonic shot-peened iron or stainless steel and seems to be associated to the creation of a new homogeneous and nanometric structure below the surface of the alloy. Further experiments are actually carried out in this direction.