Materials Science and Engineering A 400–401 (2005) 409–412 Cyclic deformation mechanisms in a cast gamma titanium aluminide alloy Mustapha Jouiad a , Anne-Lise Gloanec a , Marjolaine Grange b , Gilbert H´ enaff a,∗ a Laboratoire de M´ ecanique et Physique des Mat´ eriaux,UMR CNRS 6617, ENSMA BP 40109 86961, Futuroscope Cedex, France b SNECMA MOTEURS, D´ epartement Mat´ eriaux et Proc´ ed´ es, 77550 Moissy-Cramayel, France Received in revised form 23 November 2004; accepted 5 January 2005 Abstract The present study tackles the issue of the identification of the deformation mechanisms governing the cyclic stress–strain behaviour of a cast Ti–48Al–2Cr–2Nb (numbers indicate at.%) with a nearly fully lamellar microstructure. At room temperature, this behaviour and the corresponding deformation mechanisms are shown to be strongly dependent on the applied strain range. Indeed, at low strain range, where almost no hardening is noticed, deformation occurs by motion of long and straight ordinary dislocations. The moderate hardening observed at intermediate values of the strain range is associated with the formation of a vein-like structure due to the progressive tangling of ordinary dislocations. Finally, at higher strain-range values, twinning, by delaying the formation of this vein-like structure, induces a more pronounced cyclic strain hardening. At high temperature (750 ◦ C), the material exhibits a rapid saturation of the stress amplitude, regardless of the applied strain range. Transmission electron microscopy indicates that twinning is no longer operative at this temperature, but that dislocation climb is activated. © 2005 Elsevier B.V. All rights reserved. Keywords: Cyclic strain hardening; Twinning; Dislocation; Vein-like structure; Microstructure 1. Introduction Alloys based on TiAl have received considerable attention in recent years as potential competitors to steels and super- alloys for applications in jet engines and turbines, due their excellent combination of properties associated with a low density. Nevertheless, although the cyclic stress–strain (CSS) behaviour of these alloys is a design concern, there are only very few studies in the TiAl literature on low cycle fatigue of poly-crystalline materials [1–6]. Umakoshi and co-workers [7–10] have identified the deformation mechanisms govern- ing the CSS behaviour of poly-synthetically twinned (PST) crystals considered as model material for poly-grain, fully lamellar alloys of technical significance. Nevertheless, the question remains, whether this also holds for the behaviour of poly-grain materials. Additionally, since these materials will be used at elevated temperatures, one also needs to get ∗ Corresponding author. Tel.: +33 5 49 49 82 33; fax: +33 5 49 49 82 38. E-mail address: henaff@lmpm.ensma.fr (G. H´ enaff). insights into the influence of temperature on cyclic deforma- tion mechanisms. To this end, low cycle fatigue tests coupled with TEM observations were performed in the present study on a cast quaternary Ti–48Al–2Cr–2Nb alloy with a nearly fully lamellar microstructure at 25 ◦ C and at 750 ◦ C. 2. Experimental The material investigated is a quaternary Ti–48Al– 2Cr–2Nb (numbers indicate at.%) alloy received in the form of cast bars of 20 mm in diameter and 100 mm in length from SNECMA MOTEURS. The pieces were initially hipped and then heat-treated. The macrostructure is characterised by two zones. The first one, starting from the surface toward the cen- tre, is made of columnar grains, with the elongation axis ori- ented along the radial direction, with an average size of about 200–500 m. The second zone contains equiaxed grains with a grain size of about 50–100 m. The microstructure in both 0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.01.073