Influence of thermo-mechanical processing on ordering kinetics in 18 karat red gold alloy

The aim of this project is to understand the correlation between alloy composition and the build-up of internal stresses in 18 carat Au-Ag-Cu alloys under thermo-mechanical loads. The copper rich end of this ternary system exhibits order-hardening of the equiatomic AuCu phase. There is very limited information in literature on the ordering transformation behavior of the copper-rich alloys under anisothermal annealing and in a pre-deformed state; and more importantly, how does the misfit strain induced by ordered nano- precipitates affect internal stresses of the material. The link between microstructural evolution and residual stresses is lacking. This project aims to quantify the coupling between the morphology of ordered precipitates and the evolution of residual stresses in pre-deformed samples, via in-situ temperature experiments with synchrotron x-ray and neutron diffraction and scattering methods. Electron microscopy will complement the microstructural investigations. This study will shed light on how thermo-mechanical loads impact internal stresses on 18 carat Au-Ag-Cu alloys subject to order-hardening of equiatomic AuCu.

Measurements and modelling of residual stresses during quenching of thick heat treatable aluminium components in relation to their precipitation state

Nicolas Paul Henry Chobaut

In the fabrication of heat treatable aluminium parts, solutionising and quenching are key steps in order to obtain the required mechanical characteristics. Fast quenching is necessary to avoid coarse precipitation as this one reduces the mechanical properties after heat treatment. However fast quenching gives birth to high residual stresses, which cause unacceptable distortions during machining and can reduce service life drastically. For this reason, plates and forgings such as those considered in this thesis are subjected to a stress relieving operation, removing most of the residual stresses after quenching. For thick products, fast quenching throughout the thickness is no longer possible owing to the high but still limited thermal diffusivity of aluminium alloys, but residual stress occurrence remains problematic. The objective of this work is to develop a comprehensive model of quenching including precipitation effects for understanding the development of residual stresses in thick heat treatable aluminium components. Two industrial contexts are studied in this work: - the cold-water quenching of hot-rolled AA7040 and AA7449 thick plates for aerospace applications, - the boiling-water quenching of large AA2618 compressor wheel forgings used in turbochargers. Besides an accurate knowledge of the thermal field in the components during quenching, this work has involved an extensive thermo-mechanical characterisation of the three alloys in different precipitation states by interrupted quench-tests achieved in a Gleeble machine. Plastic strain recovery at high temperature is considered in a simple way in the description of the material behaviour and the Bauschinger effect is neglected as suggested by dedicated experiments. Three numerical models are developed to predict as-quenched residual stresses: - a thermo-mechanical model ignoring precipitation, - a physically-based thermo-metallurgical-mechanical model where a yield strength model is coupled with a precipitation model, - a thermo-mechanical model accounting for precipitation in a simple but realistic way. Instead of modelling precipitation that occurs during quenching, the model parameters are identified using a limited number of tensile tests achieved after representative interrupted cooling paths in a Gleeble machine. The results are compared to as-quenched residual stress measurements achieved by neutron diffraction and layer removal in thick AA7449 and AA7040 plates and in large AA2618 forgings. The thermo-mechanical model ignoring precipitation is found to be sufficient for relatively thin products. For thicker products, precipitation hardening by small precipitates is found to increase as-quenched residual stresses. Since the thermo-mechanical model ignoring precipitation underestimates these residual stresses, precipitation must thus be accounted for. The second model gives an excellent agreement between measurements and simulations provided that the precipitation model is well calibrated. This physically-based model is necessary for modelling complex parts with different thicknesses. Nevertheless, a lot of effort has to be dedicated to the characterisation and modelling of the precipitation of metastable non-stoichiometric phases and GP zones during quenching. This tedious work is not needed for the third model which requires only a few interrupted quench-tests while giving an excellent agreement between measurements and simulations for all the investigated components. In particular, it is verified that the two models taking into account precipitation give identical residual stress profiles in thick AA7449 plates.

EPFL2015

Analysis of nano-sized irradiation-induced defects in Fe-base materials by means of small angle neutron scattering and molecular dynamics simulations

Gang Yu

Thermonuclear fusion of light atoms is considered since decades as an unlimited, safe and reliable source of energy that could eventually replace classical sources based on fossile fuel or nuclear fuel. Fusion reactor technology and materials studies are important parts of the fusion energy development program. For the time being, the most promising materials for structural applications in the future fusion power reactors are the Reduced Activation Ferritic/Martensitic (RAFM) steels for which the greatest technology maturity has been achieved, i.e., qualified fabrication routes, welding technology and a general industrial experience are almost available. The most important issues concerning the future use of RAFM steels in fusion power reactors are derived from their irradiation by 14 MeV neutrons that are the product, together with 3.5 MeV helium ions, of the envisaged fusion reactions between deuterium and tritium nuclei. Indeed, exposure of metallic materials to intense fluxes of 14 MeV neutrons will result in the formation of severe displacement damage (about 20-30 dpa per year) and high amounts of helium, which are at the origin of significant changes in the physical and mechanical of materials, such as hardening and embrittlement effects, for instance. This PhD Thesis work was aimed at investigating how far the Small Angle Neutron Scattering (SANS) technique could be used for detecting and characterizing nano-sized irradiation-induced defects in RAFM steels. Indeed, the resolution limit of Transmission Electron Microscopy (TEM) is about 1 nm in weak beam TEM imaging, and it is usually thought that a large number of irradiation-induced effects have a size below 1 nm in RAFM steels and that these very small defects actually contribute to the irradiation-induced hardening and embrittlement of RAFM steels occurring at irradiation temperatures below about 400°C. The aim of this work was achieved by combing SANS experiments on unirradiated and irradiated specimens of RAFM steels with Molecular Dynamics (MD) simulations of main expected nano-sized defects in irradiated pure Fe and Fe-He alloys, as model materials for RAFM steels, and simulations of their corresponding TEM images and SANS signals. In particular, the SANS signal of various types of defects was simulated for the first time. The methodology used in this work was the following: SANS experiments were performed by applying a strong saturating magnetic field to unirradiated and irradiated specimens of three types of RAFM steels, namely the European EUROFER 97, the Japanese F82H and the Swiss OPTIMAX A steels. The available irradiated specimens included specimens which had been irradiated with 590 MeV protons in the Proton IRradiation EXperiment (PIREX) facility at the Paul Scherrer Institute (PSI) at temperatures in the range of 50-350°C to doses in the range of 0.3-2.0 dpa. SANS spectra as well as values of the so-called A ratio, which represents the ratio of the total scattered intensity to the nuclear scattered intensity, were obtained for the various irradiation doses and temperatures investigated. MD simulations of atomic displacement cascades in pure Fe and in Fe-He alloys were performed using Embedded Atom Method (EAM) many-body interatomic potentials. The main nano-sized defects that should be produced in RAFM steels under irradiation were created by means of MD in pure Fe. These included dislocation loops of various types, voids, helium bubbles with various He concentration and Cr precipitates. TEM images of cascade damage and all the defects created by MD were simulated in the dark field/weak beam imaging modes by using the Electron Microscopy Software (EMS) developed by P.A. Stadelmann (EPFL) and analyzed in terms of variations of contrast intensities versus depth inside the specimen. The SANS signal provided by cascade damage and all the defects created by MD was simulated by using a slightly modified version of EMS, accounting for neutrons instead of electrons. The SANS technique has been proven in this work to be a very powerful tool for detecting nano-sized irradiation-induced defects and a tool well complementary to TEM for characterizing such very small irradiation-induced defects. Indeed, TEM appears most adapted to investigate structural defects, such as dislocation loops and helium bubbles with high helium concentration, which yield significant lattice deformation of the surrounding matrix, while SANS is most adapted to investigate phase defects, such as voids, helium bubbles with low helium concentration and Cr precipitates. By combining the results of SANS experiments with those of MD simulations, TEM image simulations and SANS signal simulations, the nano-sized irradiation-induced defects were tentatively identified as small helium bubbles. While the radiation hardening measured for RAFM steels cannot be explained by accounting only for the defects observed in TEM, it could be successfully modeled by accounting also for a reasonable number density of the nano-sized defects evidenced using the SANS technique.

EPFL2008

Microstructure and embrittlement of leaded copper alloys

Laurent Felberbaum

Mechanisms responsible for the embrittlement of leaded "free-machining" copper alloys at intermediate temperatures (i.e. around 400 °C) are examined using a combination of microstructural and mechanical characterization. Tensile tests are conducted on pure copper and on industrial high-strength Cu-Ni-Sn alloys, comparing leaded and unleaded versions, at strain rates ranging from 10-4 to 10 s-1, and various temperatures between 25 °C and 800 °C. It is found that, in the leaded samples, both liquid metal embrittlement and grain boundary embrittlement operate at intermediate temperature,i.e., near the melting point of lead. Their respective importance depends mainly on the strain rate: fracture at high strain rate (10 s-1) displays characteristic signatures of liquid metal embrittlement, while at low strain rate (10-4 s-1) the ductility trough behaviour can be attributed to grain boundary embrittlement. In leaded copper and copper alloys, lead is mostly present as discrete inclusions, frequently located along grain boundaries. The dihedral angle of intergranular lead inclusions is then the main microstructural parameter besides their size. A new technique is proposed and demonstrated for the measurement of inclusion dihedral angle in a high-purity Cu-1 wt.% Pb alloy, based on room temperature characterization of samples that were held at elevated temperature at times sufficiently long for shape equilibration of the inclusions and then rapidly quenched. Quantitative scanning electron microscopic analysis of metallographic surfaces along which individual inclusions have been dissolved is used to deduce the dihedral angle using a mathematical fit of their solid/liquid interface. For a specific temperature, we show that the dihedral angle is not unique; this reflects the fact that high-angle grain boundary energies are not constant in copper. Existing micromechanical analyses of shape equilibrium for intergranular inclusions in the presence of external stress are adapted to produce an improved description at low stress and to take into account the inclusion bulk compressibility. The derivation is based on minimization of the global capillary and elastic strain energy associated with such inclusions. An adimensional parameter that depends on the applied stress, the inclusion volume, the elastic properties of the solid, and the relevant interfacial energies emerges from the analysis as the sole factor governing the shape of the inclusion. If void nucleation occurs easily within the inclusion, such that its apparent bulk modulus is nil, the inclusion become unstable, degenerating to a crack when this adimensional parameter exceeds a critical value. Measurements of the dihedral angle on samples previously subjected to a remote stress at 400 °C show that applied stress causes a reduction in the apparent dihedral angle of liquid lead inclusions. Calculated critical stresses for both leaded pure copper and leaded copper-nickel-tin alloys that fail at high strain rate by liquid metal embrittlement are calculated for individual inclusions in the metal. These calculated critical stresses correlate relatively well with the measured fracture stress, suggesting that shape instability of the inclusions cause fracture of the alloys. This result is applied towards the suggestion of general strategies for reduction of the susceptibility of leaded copper alloys to liquid metal embrittlement. A case study addressing the problem of quench cracking of leaded Cu-Ni-Sn alloys on the basis of experimentation and finite element thermomechanical simulation, showing how the problem can be alleviated in production, concludes the thesis.

EPFL2005