Fatigue properties of TiAl alloys
Introduction
TiAl alloys now appear as potential competitors to steels and superalloys for components in jet engines and turbines. Depending on the processing route selected to manufacture a given component, a broad range of microstructures (equiaxed γ, two-phase nearly fully lamellar, duplex) can be achieved. Now it is well established that differences in microstructural features in these alloys lead to very different balances of mechanical properties (ductility, fracture strength, fracture toughness, creep resistance, etc.) Besides, in most foreseen applications, the high-cycle and/or low-cycle fatigue resistance, as well as the crack growth resistance of the material, might be an issue. Hence, there is a need for both metallurgists and designers to get insights into the mechanisms governing the fatigue properties in TiAl alloys, in order to optimise microstructures on the one hand, and to develop relevant lifing methods on the other hand. Now, it has also to be recognised that these properties might depend on different extrinsic factors such as temperature and environment. Therefore, the development of relevant lifing methods implies an interdisciplinary research effort in order to fill the gap between the basic characterisation of cyclic properties and an in-depth understanding of the fundamental mechanisms governing these properties. The present paper presents an overview of the current understanding of the community on these issues. The different fatigue properties of TiAl alloys, namely high-cycle fatigue (HCF), cyclic stress–strain (CSS) behaviour and fatigue crack growth (FCG) resistance are reviewed, with a special attention paid to the influence of parameters such as microstructure, defects, environment and temperature.
Section snippets
General features on fatigue resistance of TiAl alloys
The fatigue behaviour of TiAl-based alloys was early investigated by Sastry and Lipsitt [1]. The S–N curves determined for a binary Ti–36.5Al (wt%) alloy with a lamellar microstructure at various temperatures are shown in Fig. 1. First, it can be noticed that, regardless of temperature, the S–N curves are extremely flat. This means that a slight variation in the applied stress amplitude can lead to significant differences in the number of cycles to failure. The second point is that the fatigue
Low-cycle fatigue resistance
Recina et al. [22], [23], [24], [25] have examined the low cycle fatigue (LCF) behaviour of different alloys at 600 °C. Some of their results are presented in Fig. 9. The alloys considered are the XD45 alloy (Ti–45Al–2Mn–2Nb+0.8% vol TiB2), solidified via a β path and characterised by a randomised lamellar microstructure, a XD47 alloy (Ti–47Al–2Mn–2Nb+0.8% vol. TiB2), with a duplex microstructure where the lamellar colonies are elongated along the loading axis, a ABB alloy (Ti–48Al–2W–0.5Si)
Fatigue crack propagation
Depending on the selected processing route, different types of surface and internal defects can be present in a component at the very beginning of the service life. In addition, more severe surface defects can be introduced during service by impacts for instance. Therefore, the residual fatigue strength in presence of such defects has to be evaluated, which implies a sound description of the fatigue crack growth process in these low-fracture toughness materials. As a matter of fact, the Fatigue
Microstructural design for reliability in long-term service
The data presented above allow the definition of a fatigue and damage tolerance design methodology for this class of alloys.
Fig. 30 presents critical defect sizes for FCG and fracture determined for two characteristic microstructures as a function of the maximum stress in the case of a surface defect in a round bar fatigued at R=0.1with the stress intensity factor estimated from [73]. The critical size for propagation is derived for the effective threshold value ΔKeff,th, which is supposed to
Summary and conclusions
The different fatigue properties of TiAl alloys have been reviewed in the present paper. It has been shown that TiAl alloys, in the absence of sharp defects, exhibit an excellent fatigue resistance as characterised by an endurance ratio higher than 80% of the ultimate tensile strength. This fatigue resistance can be altered by different factors such as surface preparation, as commonly observed in conventional alloys. The low-cycle fatigue and cyclic stress–strain behaviour have received less
Acknowledgements
A part of this work was conducted within the framework of a national project (CPR ‘TiAl intermetallic’) in collaboration with LTPCM (Grenoble), CECM (Vitry), LSG2M (Nancy), GMP (Rouen), CEMES (Toulouse), LMS (Palaiseau), LMPM (Poitiers), the companies SNECMA MOTEURS and TURBOMECA, with the financial support of CNRS and DGA.
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2021, Acta MaterialiaCitation Excerpt :The last two deformation modes are supposed to need a higher critical shear stress to get activated if compared to slip of the ordinary dislocations [67]. Therefore, the dislocation structure in the Ti-48Al based alloys [1,2,4-12,64] is largely dominated by unit ½ <110] dislocations even in the grains with lower Schmid factors, whereas <101] superdislocation slip occurs also in favorably orientated grains. However, even the slip of ordinary ½ <110] dislocations does not occur smoothly.