The influence of microtexture on the fatigue and dwell-fatigue response of Ti-6Al-4V was investigated considering two regions of a part showing different degrees of microtexture. A high degree of microtexture resulted in a significant reduction in fatigue lifetime. No difference in crack initiation mechanism was observed, as main cracks formed along basal twist grain boundaries (BTGB) connected to the specimen surface. However, a key contribution of faster small crack growth in the strongly microtextured region is suggested. A 120 s dwell time resulted in a dwell-fatigue lifetime debit exceeding a factor of 10 in this region while it remained below a factor of 3 in the weakly microtextured region. In the dwell-sensitive region, internal nucleation of the main crack occurred at a BTGB and long range facetted growth was enabled by the adjacent microtextured region with a dominant [0001] orientation. In contrast, surface crack nucleation typical of fatigue failures dominated in the dwell-insensitive region. The limited growth of internal cracks suggests that the lack of path for rapid crack growth has a pivotal effect for the low dwell sensitivity. The competition between fatigue and dwell-fatigue damage highlights the importance of considering both in modeling approaches to obtain accurate lifetime predictions.

The Al-51 at% Zn alloy shows a total transformation of the eutectoid mixture at this concentration. It is therefore ideal for studying the evolution of the alpha and beta phases as a function of temperature. In the present work, we have clearly demonstrated a correlation between the evolution of the microstructure observed by scanning electron microscopy (SEM) and the results obtained by isothermal mechanical spectroscopy (IMS) [1]. IMS measurements on this alloy were carried out over a very wide frequency range (10(-5 )-50 Hz) and the results obtained show that it exhibits different relaxation peaks as a function of temperature between ambient and 540 K. Upon heating, two peaks P1 and P2 appear below the eutectoid transition temperature (550 K). Both peaks are thermally- activated; P1 decreases and disappears with increasing measurement temperature while P2 appears and increases continuously until the eutectoid transition temperature. These peaks have been associated with thermally induced diffusion of atoms across the alpha-beta interface; this could correspond, according to the literature, to a change in the shape of the lamellar interface between the alpha and beta phases [1]. The SEM observations were performed after several quenches at different temperatures (385 K, 433 K and 493 K after a holding time of 2 h 30 min) chosen in the temperature range of the internal friction peaks evolution. The results obtained allowed us to associate the P1 peak to the lamellar structure whose destruction leads to the collapse of P1. On the other hand, the P2 peak is associated to the globules coalescence and as this coalescence increases the amplitude of this peak becomes more important. The analysis of our SEM observations has clearly highlighted the transformation mechanisms of this eutectoid mixture by specifying without ambiguity the passage from the lamellar structure to the globular structure and then the collapse of the latter. (C) 2021 Elsevier B.V. All rights reserved.

This paper presents an investigation on the effects of ageing on the microstructure and the corresponding physical and mechanical properties of a 2024 aluminium alloy used in a civil transport aircraft wing structure in order to assess the residual resistance of the end of a service life. More precisely, heat treatments are applied in order to simulate thermal ageing actually endured by the structure during service. The results of characterisation of microstructural, physical and mechanical properties are compared not only to the data obtained on a pristine alloy, but also to the results obtained on coupons of a similar alloy coming from the teardown of an A320 aircraft using the same experimental procedure. The main findings are that, during a service life, no significant modification in fatigue resistance is noticed despite of changes in the precipitation structure.

This study evaluates the influence of residual stresses induced by the fabrication of surface anomalies on the fatigue crack growth in a nickel based superalloy. To separate the notch effect of the geometry from the residual stress field induced by fabrication of the surface flaws, two V-type anomalies are considered: scratches and dents with equivalent morphology and size. A specially designed heat treatment has been used to reduce the magnitude of residual stresses around these anomalies in order to highlight their effects on the different stages of the crack propagation, under low cycle fatigue conditions at 400 degrees C. The crack initiation life is short for both anomalies but in the presence of compressive residual stresses, a decrease of the fatigue crack growth rate has been observed during the first stages of the crack propagation. Furthermore, the results showed that without residual stresses, scratches and dents exhibit the same behaviour. Thus, the residual stress field below surface anomalies is the main parameter controlling the fatigue life from surface anomalies.

In this study, the effect of gaseous hydrogen on the fatigue crack growth behavior in a precipitation-hardened martensitic stainless steel is investigated. It is known that the degradation in fatigue crack growth behavior derives from a complex interaction between the fatigue damage and the amount of hydrogen enriching the crack tip, which is dependent on the hydrogen pressure, loading frequency, and stress intensity factor amplitude. Therefore, fatigue crack growth tests were performed in a range of 0.09 to 40 MPa under gaseous hydrogen at a frequency of 20 and 0.2 Hz. The fatigue data as well as fracture morphologies obtained so far indicate a sharp increase in crack growth rates in a narrow range of stress intensity factor amplitudes. Also, it is shown that by decreasing the loading frequency to 0.2 Hz at a given pressure of hydrogen the transition occurs at lower values of stress intensity factor amplitudes accompanied by a change in fracture mode. Scanning electron microscope (SEM) observations of the fracture surfaces are used to support the explanations proposed to account for the observed phenomena.

A novel technique, combining on the one hand creep-fatigue tests with an overheating and creep tests with thermal cycling in the other hand, performed on gamma/gamma' nickel base single crystal superalloy MC2 have led to an increased understanding of fine gamma' precipitation and its strengthening effect. Both creep and creep-fatigue tests were conducted at 1050 degrees C with 1200 degrees C overheating for creep-fatigue experiments and with repeated overheatings at 1100 degrees C and 1150 degrees C for creep. The resulting microstructures of these experiments were examined using both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It appears, both on creep or creep-fatigue, once an overheating is experienced a fine gamma' precipitation occurs in gamma matrix. These precipitates seem to have a transient strengthening effect on the mechanical properties. For the creep-fatigue experiments a decrease of the plastic strain rate was measured at once after the temperature peak. In the case of the creep tests under thermal cycling, no extra deformation induced by the overheating at 1100 degrees C was recorded. However, overheatings at 1150 degrees C lead to a plastic strain jump which progressively decreases upon thermal cycling, due to the formation of fine gamma' precipitates. Furthermore, the gamma' fine particles seem to have a hardening effect that vanishes once they dissolve. (C) 2010 Elsevier B.V. All rights reserved.