Non-isothermal tensile tests during solidification of Al–Mg–Si–Cu alloys: Mechanical properties in relation to the phenomenon of hot tearing

An original set-up has been used to study the mechanical properties of aluminium alloys in tension during solidification with a high cooling rate (70 K/s). The mechanical behaviour of 6056 aluminium alloy with and without grain refiner has been investigated as well as that of mixtures between AA6056 and AA4047. The results show that the alloys exhibit a viscoplastic behaviour in the mushy state. A transition is observed between fracture in the mushy state and fracture in the solid state as a function of the displacement rate. This displacement rate at the transition depends on the cooling rate and on the composition of the alloy. The displacement before fracture is observed to be independent of displacement rate but to depend on the composition and on the solidification rate. Based on the observations a criterion for fracture in the mushy state is proposed. A simple rheological law describing the mechanical behaviour of the alloys is coupled to a finite element calculation giving the thermal field during the tensile test. This simulation is able to reproduce the mechanical response of the solidifying alloy during a non-isothermal test

Résistance en fatigue en fonction de la température de feuilles d'aluminium brasées de fines épaisseur utilisées pour les échangeurs thermiques automobiles

Dans cette étude, une caractérisation détaillée des mécanismes d'endommagement cyclique à l'oeuvre dans les échangeurs thermiques automobiles a été effectuée dans le but d'améliorer leurs fiabilités. Le matériau étudié qui consiste en une plaque très fine (80% de la limite à rupture)

Development of Temperature-Controlled Shear Tests to Reproduce White-Etching-Layer Formation in Pearlitic Rail Steel

The formation of a white etching layer (WEL), a very hard and brittle phase on the rail surface, is associated with a progressive transformation of the pearlitic grain to very fragmented grains due to the cumulative passage of trains. Its formation is associated with a complex thermomechanical coupling. To predict the exact conditions of WEL formation, a thermomechanical model previously proposed by the authors needs to be validated. In this study, monotonic and cyclic shear tests using hat-shaped specimens were conducted in the temperature range of 20 C to 400 C to reproduce the WEL formation. The tests showed a strong sensitivity of the material to temperature, which does not necessarily favor WEL formation. For the monotonic tests, no WELs were produced; however, a localization of the plastic deformation was observed for tests performed at 200 C and 300 C. In this temperature range, the material was less ductile than at room temperature, leading to failure before WEL formation. At 400 C, the material exhibited a much more ductile behavior, and nanograins close to WEL stages were visible. For the cyclic tests, a WEL zone was successfully reproduced at room temperature only and confirmed the effect of shear in WEL formation. The same cyclic tests conducted at 200 C and 300 C yielded results consistent with those of the monotonic tests; the deformation was much more localized and did not lead to WEL formation.This work is part of the multi-disciplinary project MOPHAB, which aims to improve our knowledge and understanding of the mechanisms leading to the formation of the white etching layer in the materials used to construct railways and to develop corresponding numerical models. This project was supported by IRT Railenium and other industrial partners (RATP: Régie Autonome des Transports Parisiens, France, SNCF: Société Nationale des Chemins de Fer Francais, France, SAARSTAHL rail)

Impact of liquid metal embrittlement inner cracks on the mechanical behavior of 3 generation advanced high strength steel spot welds

International audienc

Investigation of the Failure of Advanced High Strength Steels Heterogeneous Spot Welds

Nowadays, environmental regulation encourages carmakers to reduce the global vehicle weight. Steelmakers develop grades with high performance (Advanced High Strength Steels, AHSS) and fine steel sheet assemblies are used in car body structures, with an optimized thickness in each part. However, unusual fracture modes are sometimes observed during the mechanical tests of heterogeneous AHSS welds, made of dissimilar steel grades and sheet thicknesses. Weld fractures can occur with a strength lower than expected. This study aims at understanding these fracture mechanisms and focuses on two common steel grades joined by Resistance Spot Welding (RSW): DP600 (a dual phase steel) and Usibor®1500 (a martensitic steel). The parameters affecting the failure modes and load bearing capacity are investigated during two common types of tests: the Cross Tension and Tensile Shear tests. The positive effects of heterogeneous welding with respect to the corresponding homogeneous configurations are discussed, as well as the consequences of a so-called Dome failure occurring at the weld nugget boundary

Finite element modeling of deformation and fracture of advanced high strength steels dissimilar spot welds

International audienc

Spherulitic growth process in Ti-based metallic glass: Microstructure, phase identification, and growth mechanism

International audienceTi40Cu36Zr10Pd14 metallic glass is often considered a promising candidate for biomedical applications, in particular for the manufacturing of small size dental implants. Nevertheless, its processing and limited glass-forming ability leads to the appearance of crystalline spheres called “spherulites”. These spherulites play a major role in the deformation behaviour of this alloy. It is therefore a critical issue for its future industrial use to identify their nature and better understand their growth process to enable control of their size distribution and localisation. This article presents a detailed description of the microstructure of these crystalline defects, from the sub millimetre to nanometric scales, using transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS)/electron backscatter diffraction (EBSD) coupled with scanning electron microscopy and X-ray diffraction characterisation. Several phases are identified thanks to the combination of EDS/EBSD and TEM information, in particular a CuTi B2 phase, analogous to the ZrCu B2 phase usually present in other composite metallic glasses. The presence of other phases such as Ti2Cu3 and CuTi3 phases is also detected. The spherulites are studied at different times during their growth process, and a growth scenario is proposed based on the characterisation observations. However, the nucleation step of the spherulites remains unclear despite the in-depth investigation. Thus, different hypotheses are discussed

An alternative method for the measurement of precipitate volume fractions in microalloyed steels by the means of atom probe tomography

International audienceKnowledge of the volume fraction of precipitates is crucial for estimating the impact of precipitation on microstructure evolution or mechanical properties. However, its experimental determination is often a difficult task. In this work, atom probe tomography was applied to an industrial TieNb microalloyed steel, to follow the evolution of austenite solute composition in titanium and niobium as a function of temperature in the austenitic domain. These composition measurements were used to calculate the volume fraction of (Ti x , Nb 1-x)C carbides in austenite, using mass balance. This type of measurement is made possible by the considerable evolutions experienced by tomographic atom probes over the past 20 years in terms of volume analyzed and mass resolution. Atom probe tomography is nowadays not only able to help determining volume fractions below 0.1%, but also provides unique information related to solubility limits as low as a few tens of ppm, most useful for developing phase diagrams, or assessing existing ones