© 2005 The Royal Microscopical Society Journal of Microscopy, Vol. 217, Pt 2 February 2005, pp. 118–121 Received 1 June 2004; accepted 24 June 2004 Blackwell Publishing, Ltd. Determination of the fatigue fracture planes of Co–Cr–Mo biomedical alloys using electron backscatter diffraction S. C. WANG* 1 , M. BROWNE†, H. S. UBHI‡ & M. J. STARINK* *Materials Research Group, and †Bioengineering Science Group, School of Engineering Science, The University of Southampton, Southampton SO17 1BJ, U.K. ‡ QinetiQ Ltd, Farnborough, Hampshire GU14 0LX, U.K. Key words. Co –Cr–Mo alloys, EBSD, facet fatigue planes. Summary Electron backscatter diffraction on a scanning electron microscope has been utilized to acquire crystal orientation information around faceted fatigue cracks in a Co –Cr–Mo alloy for medical implants. The faceted fracture planes are unambiguously determined as {111} planes. Received 1 June 2004; accepted 24 June 2004 Introduction The misorientation between two grains can be obtained by electron backscatter diffraction (EBSD) in a scanning electron microscope (SEM) or convergent beam electron diffraction (CBED) in a transmission electron microscope (TEM) (Randle, 2001; Wang & Starink, 2003; Wang et al., 2003). However, the orientation of two-dimensional planar defects, such as grain boundary (GB) planes and fracture planes, can only be deter- mined by knowledge of the correlation between the plane morphology and diffraction. There are a few methods to deter- mine GB planes in TEM: for example by tilting the GB plane to the edge-on condition and determining the diffraction vector perpendicular to the GB trace line; and, in deformed alloys, the GB plane normal can be calculated from the vector product of line directions of dislocations (Wang & Aindow, 2000). Because it is difficult to keep the facture cracks intact during electro- polishing, it is very difficult to determine the fracture planes in TEM. By contrast, EBSD on an SEM is an ideal technique for identifying local crystal orientations and correlation with the GB/fracture plane can be obtained from SEM images in bulk samples (see, for example, Randle & Davies, 2002). This short paper describes the determination of fracture faceted planes in Co–Cr alloys for medical implants, based on the crystal information provided by EBSD. Load-bearing medical implants such as the hip and knee joints undergo highly variable loading in an aggressive physi- ological environment. The implant materials are required to have appropriate mechanical, physical, chemical and biological properties. Co–Cr alloys, which have excellent corrosion resist- ance, wear behaviour and biocompatibility, are among the few materials that can fulfil these requirements (Browne & Gregson, 2001; Chen et al., 2004). The quality of Co –Cr cast- ings has been improved thanks to advances in manufacturing techniques. However, rare incidences of fracture still occur in service. Scanning electron microscopy has revealed extensive facets on the fatigue fracture surface and zig-zag-shaped secondary cracking on the adjacent fracture surface. The matrix of the Co –Cr alloy has a face centred cubic (fcc) struc- ture and it would be expected that crack propagation occurs along {111} planes; for example, in Ni-based alloys, such planes are the main slip planes for deformation at low temper- ature (Reed et al., 2000). In studies of failure in Co –Cr alloys, crack propagation along these planes has been suggested to occur (Zhuang & Langer, 1989, 1990), but crystallo- graphic evidence for this is lacking. Experimental procedure Cast keel bars of Co-28.6 wt% Cr-5.8 wt% Mo were heat treated at 1200 °C for 4 h in a vacuum, in order to dissolve any interdendritic particles and to homogenize the micro- structure. The microstructure consisted of coarse grains (a few millimetres in diameter) with a cobalt-rich fcc structure and evidence of alloy carbides both within the grains and at grain boundaries. The alloy had an average ultimate tensile strength (UTS) of 907 MPa, yield strength of 515 MPa and elongation of 19.8%. Fatigue testing was performed in air on an Instron 8502 servohydraulic machine using four-point bend bar test-pieces, as shown in Fig. 1. The EBSD specimens Correspondence: Dr Shuncai Wang. Fax: +44 (0)23 8059 3016; e-mail: wangs@soton.ac.uk 1 Also at: Electron Microscopy Centre, Faculty of Engineering and Science, The University of Southampton, Southampton SO17 1BJ, U.K.