Pre-yielding and pre-yield cracking in TiAl-based alloys

Abstract

Samples of Ti44Al8Nb1B (at.%) and Ti46Al8Nb (700 wt ppm oxygen) and of the alloy K5 (Ti-46.5Al-2Cr-3Nb-0.2W-0.15B-0.4C (800 wt ppm oxygen) have been heat treated to produce duplex, near fully lamellar and fully lamellar microstructures. These samples have been tested in tension, using detectors to detect acoustic events during pre-yielding. Many acoustic emission events and corresponding cracking, occur well below the 0.2% proof stress in the near fully lamellar samples of Ti44Al8Nb1B, in fully lamellar Ti46Al8Nb and fully lamellar K5. In fully lamellar Ti44Al8Nb1B alloy some samples show no acoustic events before yield and others show only single acoustic emission events. Duplex samples generate no signals until the applied stress exceeds the macroscopic yield point. The stress–strain curves from Ti448Nb1B, Ti46Al8Nb and from K5 show that in near fully lamellar and fully lamellar samples significant pre-yielding occurs, but none is obvious in the duplex samples. These observations are discussed in terms of the factors contributing to pre-yield plasticity and to pre-yield cracking in these alloys and with reference to earlier acoustic emission work where the reported behaviour is very different. It is concluded that yield, within sufficiently long lamellae or within gamma grains, can give rise to pre-yield cracking and thus to acoustic emission in lamellar samples.