| Abstract: | The behavior of oxide scales on Ti-50Al andTi-50Al-2Nb (at.%) was investigated in constantstrain-rate tensile tests at 900°C in air. Thestrain rates ranged between 1 × 10-9sec-1 and 3 × 10-4sec-1. The tests were accompanied byacoustic-emission measurements in order to detectscale-cracking processes during deformation. Thecritical strains to scale cracking amounted to 0.12-0.5%for the scales on TiAl and 0.17-0.58% for Ti-50Al-2Nb. Thesevalues were found to depend strongly on the size of thepores in the scales and, by using a fracturemechanics-based model, the results for the criticalstrains could be condensed into a narrow scatterband,which is independent of the applied strain rate. Healingof scale cracks was found for strain rates below 1.5× 10-6 sec-1 (Ti-Al) and 1.9× 10-4 sec-1 (Ti-Al-Nb), respectively. It turned out that the healingprocess is dominated by TiO2 growth. In alater healing stage, the originalAl2O3 barrier is, however,restored in the scale on Ti-50Al. For Ti-50Al-2Nb, an Al2O3 layer is found onthe former scale-crack contours. The healing process isalso described by a quantitative model. As a generalconclusion from the investigations, it turned out thatcritical strains to oxide-scale cracking can be estimated fromjust simple oxidation experiments without sophisticatedmechanical testing if the microstructural parameters ofthe scale are determined quantitatively as a function of oxidation time by metallographicmeans. |
