The role of surface condition, residual stress and microstructure on pre-yield cracking in Ti44Al8Nb1B Xinhua Wu a, *, D. Hu a , M. Preuss b , P.J. Withers b , M.H. Loretto a a IRC in Materials, The University of Birmingham, Edgbaston B15 2TT, UK b Manchester Materials Science Centre, University of Manchester & UMIST, Grosvenor St., Manchester M1 7HS, UK Received 1 July 2003; received in revised form 20 October 2003; accepted 21 October 2003 Abstract Fully lamellar and duplex samples of Ti44Al8Nb1B, with a range of surface treatments, have been tested in tension using acoustic emission to detect the onset of pre-yield cracking. The magnitudes and the senses of the stresses left in the surface regions of machined, polished and annealed samples have been measured using synchrotron radiation and these measurements correlated with the minimum level of the applied stress at which cracking was detected. The observations have shown that the surface condi- tion is far less significant than the microstructure in determining when cracking is first observed and that, whatever the surface condition, the duplex structure never shows detectable pre-yield cracking. These results are discussed in terms of the mechanism which leads to pre-yield cracking and its role in limiting fatigue life. # 2003 Elsevier Ltd. All rights reserved. Keywords: A. Titanium aluminides based on TiAl; B. Brittleness and ductility; D. Microstructure; F. Residual stress movement 1. Introduction The intensive research and development which has been carried out over the last 10–20 years on TiAl-based alloys has led to the development of alloys which appear to offer significant advantages in terms of their strength to weight ratios for operating temperatures of about 750  C [1,2]. This work has established that the best balance of properties is obtained in samples with a near fully lamellar microstructure. In such a microstructure the a 2 and g phases form narrow lamellae (with the {111} planes in the TiAl (tetragonal phase) parallel to the (0001) plane of the a 2 ) as the high temperature alpha phase cools through the two phase region. Yield stresses of over 800 MPa can be obtained in suitably processed alloys such as Ti44Al8Nb1B, the high strength coming from the low Al content (which increases the amount of alpha, the stronger phase) with a small solid solution strengthening contribution from the Nb. A duplex structure in which small lamellar grains and g grains co-exist can be formed by working the alloys in the two phase region. Such a structure can have better ductility but the creep strength is reduced significantly [3]. Recent work [4,5] using acoustic emission has shown that pre-yield cracking can be observed in fully lamellar, electropolished, flat, dog-boned shaped samples of Ti44AlNb1B during testing in tension and that the presence of such cracks can degrade the fatigue per- formance. No such pre-yield cracking was observed in duplex samples. Optical microscopy confirmed that the acoustic events, detected typically at stresses of about 60% of the 0.2% proof stress, corresponded to the generation of surface cracks, which were not observed before mechanical loading. It has been suggested that fully lamellar samples show pre-yield cracking because of the large variation of yield stress of the differently oriented lamellar grains coupled with the difficulty of transmitting deformation across lamellae boundaries and therefore from grain to grain [4]. It is argued that these two factors lead to the development of pile-up stresses and that these in turn lead to interlamellar fail- ure. The absence of pre-yield cracking in duplex samples is thus due both to the small grain size (which limits the magnitude of any pile-up stresses) and the relative ease of slip transfer from the small lamellar grains to adjacent soft gamma grains. 0966-9795/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2003.10.006 Intermetallics 12 (2004) 281–287 www.elsevier.com/locate/intermet * Corresponding author. Tel.: +44-121-414-7842; fax: +44-121- 414-7890. E-mail address: x.wu.1@bham.ac.uk (X. Wu).