Materials Science and Engineering A 486 (2008) 273–282 The effect of grain size and composition on the fracture toughness of Ti–Al–Nb alloys undergoing stress-induced martensitic transformation Archana Paradkar a,∗ , Vikas Kumar a , S.V. Kamat a , A.K. Gogia b , B.P. Kashyap c a Defence Metallurgical Research Laboratory, Hyderabad 500058, India b Project Office (Materials), Kaveri Engine Programme, Hyderabad 500058, India c Indian Institute of Technology, Department of Metallurgical Engineering and Material Science, Mumbai 400076, India Received 22 March 2007; received in revised form 31 August 2007; accepted 3 October 2007 Abstract The effect of grain size and composition on the fracture toughness of Ti–Al–Nb alloys in  solution-treated condition was investigated. The fracture toughness of the alloys was found to increase with an increase in grain size initially, reach a maximum and subsequently decrease with further increase in grain size. This trend was attributed primarily to the effect of grain size on the enhancement of fracture toughness due to stress-induced martensitic transformation (SIMT) at the crack tip, which in turn can be related to the effect of grain size on trigger stress for SIMT. Alloys containing higher Al and Nb showed a higher toughness for the same grain size, which was also explained in terms of effect of composition on the trigger stress. © 2007 Elsevier B.V. All rights reserved. Keywords: Ti–Al–Nb alloys; Stress-induced martensitic transformation (SIMT); Trigger stress; Inherent fracture toughness; Fracture toughness enhancement due to SIMT 1. Introduction Ti–Al–Nb alloys are new generation alloys developed for high temperature applications in aero engines. It is now estab- lished that adequate engineering plasticity can be realized in alloys based on intermetallic Ti 3 Al ( 2 ) when Nb is present in concentrations large enough to stabilise some amount of  phase in the structure. Increasing Nb content in these alloys results in greater amount of /B2 phase. In two phase (/ 2 ) and (/B2) mixture, it is established that the enhancement of fracture tough- ness arises from the effect of /B2 phase in accommodating plastic incompatibility at grain/phase boundaries, blunting of deformation-induced micro-cracks and bridging of crack faces [1–3]. The fracture toughness of such alloys thus increases with increase in volume fraction of /B2 and hence with an increase in Nb content [4]. Increase in the Al content on the other hand is detrimental to the fracture toughness [4]. The grain size is yet another factor which plays a sig- nificant role in controlling the fracture toughness of alloys. ∗ Corresponding author. E-mail address: agparadkar@rediffmail.com (A. Paradkar). While some work has been reported in  titanium alloys, the results do not reveal a clear picture on the grain size effect on fracture toughness. While beta grain refinement reduces the fracture toughness of Ti–15V–3Cr–3Sn–3Al alloy in  water-quenched and aged condition [5,6]; fracture toughness of Ti–10V–2Fe–3Al alloy increases with a decrease in grain size in  water-quenched condition [7]. Some studies, on the other hand, report that the fracture toughness is independent of grain size in Ti–10V–2Fe–3Al in  water-quenched and aged condition and Beta C alloys [5,8]. In case of Ti–Al–Nb alloys, the issue has hardly received any attention. Moreover, in the range of compositions in the present study, Ti–Al–Nb alloys were found to undergo stress-induced transformation (SIMT) [9]. The stress-induced transformation being a combined process of deformation and phase transfor- mation [10], has a significant effect on the fracture toughness. Niinomi et al. [11] have reported that the deformation-induced transformation was the most effective mode for improving toughness in the Ti–6Al–4V alloy. Some limited work [12] on Ti–10V–2Fe–3Al suggested that the same trend is followed in this alloy. Stress-induced transformations have also been found to result in an increase in the fracture toughness of other mate- rials like TRIP steels [13], as well as certain ceramics [14]. 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.10.005