G. Hénaff

Environmental influence on the near-threshold fatigue crack propagation behaviour of a high- strength steel

Abstract The near-threshold fatigue crack propagation behaviour of a high-strength low-alloy steel has been investigated in ambient air and in vacuum so as to determine the role of the environment precisely. The analysis of the results is conducted by taking crack closure effects into account. It is concluded that fatigue crack growth rates measured in ambient air depend upon three processes: intrinsic fatigue crack propagation as observed in vacuum, adsorption of water vapour molecules on freshly created rupture surfaces, which enhances crack propagation, and a subsequent step of hydrogen-assisted cracking. The appearance of intergranular ruptures and oxide layers on rupture surfaces in ambient air is also discussed.

Influence of temperature and environment on fatigue crack propagation in a TiAl-based alloy

Abstract The fatigue crack propagation behaviour of a quaternary γ -TiAl alloy has been investigated under various test conditions in order to discern the specific role of temperature and environment. The results obtained in ambient air at room temperature and in the range 700–800°C reveal that temperature has only a slight influence on the global fatigue crack growth resistance. The analysis is then conducted by considering the influence of temperature on the intrinsic resistance and on extrinsic factors like environmental effects. Thus, it is shown that an increase in temperature improves the intrinsic fatigue crack growth resistance in inert environment only, in the high crack growth rates regime. An intrinsic propagation rate law is proposed to describe this effect related to the fracture toughness. A marked environmental effect is highlighted both at room temperature as well as at elevated temperature. However, this influence turns out to be temperature-independent. The nature of mechanisms involved and their dependence with temperature are discussed.

Impact of microstructure, temperature and strain ratio on energy-based low-cycle fatigue life prediction models for TiAl alloys

In this paper, two fatigue lifetime prediction models are tested on TiAl intermetallic using results from uniaxial low-cycle fatigue tests. Both assessments are based on dissipated energy but one of them considers a hydrostatic pressure correction. This work allows to confirm, on this kind of material, the linear nature, already noticed on silicon molybdenum cast iron, TiNi shape memory alloy and 304 L stainless steel, of dissipated energy, corrected or not with hydrostatic pressure, according to the number of cycles to failure. This study also highlights that, firstly, the dissipated energy model is here more adequate to estimate low-cycle fatigue life and that, secondly, intrinsic parameters like microstructure as well as extrinsic parameters like temperature or strain ratio have an impact on prediction results.

Fatigue crack growth behaviour of a gamma-titanium-aluminide alloy prepared by casting and powder metallurgy

Abstract The fatigue crack growth behaviour of a Ti–48Al–2Cr–2Nb alloy prepared by two different processing routes, namely casting and powder metallurgy (PM), is examined. The lamellar microstructure of the cast alloy exhibits a higher resistance than the fine equiaxed γ grain microstructure of the PM alloy when tested at R =0.1. Crack closure effects are lower in the PM alloy than in the cast alloy, due to smoother fracture surfaces. However, after crack closure correction, this alloy still presents the lower resistance, including in the near-threshold regime.

Fatigue crack propagation resistance of a Ti48Al2Mn2Nb alloy in the as-cast condition

Abstract In the present study the fatigue crack growth behaviour of a quaternary Ti 48Al 2Mn 2Nb based alloy in the as-received condition was investigated in air and vacuum at room temperature. The respective influence of crack closure and environment was more specifically examined. It was shown that closure induces a significant shielding resulting in a decrease in crack growth rates. The interactions of closure behaviour with test procedure is also discussed. Besides, ambient air was shown to severely enhance the crack growth. The role of moisture present in the test environment is examined. Finally a detailed examination of the crack path and the rupture surface morphology has established features of the damage process.

Environmentally-assisted fatigue crack propagation in aluminides at room temperature

Alloys based on ordered intermetallics, and among them FeAl and TiAl, are now very close to meet design requirements for a wide use in structural applications where there is a need for stiff and light materials. However the damage tolerance of these compounds is still under question and the relevant Fatigue Crack Propagation (FCP) mechanisms are ill-defined. In particular, it is namely now well established that, as in conventional alloys, fatigue crack growth rates in moist atmospheres like ambient air are significantly higher than in vacuum, although the underlying processes are not fully understood. The methodology used here aims precisely in determining the specific contribution of environmental enhancement in the FCP, so that only effective data, that means data obtained after crack-closure correction or under closure-free conditions, are considered. Two alloys based on different aluminides, namely a B2-FeAl and a {gamma}-TiAl alloy, have been investigated. Comparisons between these two aluminides are expected to provide information, especially on surface reactions. Fatigue crack growth testings have been carried out at room temperature under various controlled atmospheres which differ by the respective amount of oxygen and water vapor.

Fatigue crack propagation resistance of a FeAl-based alloy

The fatigue crack growth behavior of a powder metallurgically produced iron aluminide of composition Fe-40 at.%Al is investigated. The material is processed by mechanical alloying followed either by extrusion or by hot isostatic pressing. The resulting microstructures are examined by transmission electron microscopy. The test results indicate that the processing route has almost no influence on the resistance to fatigue crack propagation. Special emphasis is given to the roles of crack closure and environment. The fatigue crack growth resistance is shown to be significantly reduced in ambient air. The results, including closure measurements, indicate that this is due to an environmental enhancement of the growth process in conjunction with a lack of complementary shielding from oxide-induced closure. This enhancement appears to be mainly governed by the moisture content of the test atmosphere. Oxygen adsorption is proved to be efficient in preventing this embrittling effect. The mechanisms of embrittlement are discussed and a limited influence of hydrogen on the fracture process is suggested.

Environmental influence on fatigue crack propagation in TiAl alloys

Abstract The fatigue crack growth behaviour of a quaternary Ti-48Al-2Mn-2Nb is investigated with a special emphasis on the role of environment. The fatigue crack-growth resistance is shown to be significantly lessened in ambient air. This is due to a lack of complementary shielding from oxide-induced closure in conjunction with an environmental enhancement of fatigue damage at the crack tip. This enhancement is proved to be related to the moisture content of the test atmosphere regardless of the oxygen content. Finally, the nature of this fatigue crack growth enhancement is discussed.

THE ROLE OF CRACK CLOSURE IN FATIGUE CRACK PROPAGATION BEHAVIOUR OF A TiAl- BASED ALLOY

The fatigue crack growth rates measured in the case of the TiAl-based alloy investigated in the present study are highly sensitive to the {Delta}K value. The fatigue crack propagation is deeply affected by the crack closure phenomenon. As a consequence the crack growth rates are severely lessened as soon as closure develops. The role of crack closure is confirmed by the influence of the R load ratio: indeed the crack growth rate measured under closure-free conditions at high R ratios is equal to that obtained at low R values after crack closure correction for a given {Delta}K{sub eff} value.

Low Cycle Fatigue and Cyclic Deformation of TiAl Alloys

The relation between microstructural parameters, microscopic cyclic deformation mechanisms and macroscopic cyclic strain hardening is investigated in the case of a heat treated PM Ti-48Al-2Cr-2Nb alloy at 20°C and at 500°C. In particular it is shown that, for the different microstructures considered, a moderate hardening is related to the formation of highly tangled dislocation structures which is in turn controlled by the dislocation glide mean free path. At 500°C this structures evolves towards a braid like structure for the fine fully lamellar microstructure.