Alexis Deschamps

Hardening precipitation in a Mg–4Y–3RE alloy

Abstract Early stages of precipitation in a Mg–Y–Nd based alloy aged at 150 °C have been studied using TEM and small angle X-ray scattering (SAXS). The former brings information concerning nature, morphology and size of precipitates, and the latter adds qualitative and quantitative information concerning populations of precipitates in terms of size and volume fraction. Precipitation at 150 °C involves formation of DO19 monoplanar precipitates, which further develop into the β″ and β′ phases having platelet and globular morphologies, respectively. TEM observations on samples aged at 150 °C reveal the formation mechanism of the bco-β′ structure by the ordering of monoplanar DO19-β″ precipitates. Additional examinations at 250 °C revealed the DO19-β″→bco-β′ transformation, as well as β1 precipitates. Estimation of the volume fraction deduced from SAXS is discussed on the basis of the TEM results.

Influence of predeformation on ageing in an Al–Zn–Mg alloy—I. Microstructure evolution and mechanical properties

Abstract We investigate the ageing kinetics of a ternary Al–Zn–Mg alloy, and the coupling between prestraining and subsequent ageing. The precipitation sequence is investigated using DSC. The microstructure is followed using TEM and Small-Angle X-ray Scattering, and the hardening resulting from precipitation studied by hardness measurements. The effect of prestraining depends significantly on the parameters of the thermomechanical treatment, and has to be considered in terms of competition between homogeneous precipitation and heterogeneous precipitation on dislocations. The effect of prestraining on peak hardening is controlled by the nucleation of the hardening η′ phase, and therefore depends critically on the heating rate to the ageing temperature. The effect of prestraining on overageing is an increase in overall coarsening kinetics due to the combination of faster precipitate coarsening on dislocations and dissolution of homogeneous precipitates followed by solute diffusion towards dislocations.

On the relationship between microstructure, strength and toughness in AA7050 aluminum alloy

The effect of process parameters such as quench rate and precipitation heat treatment on the compromise between the toughness and the yield strength of AA7050 aluminum alloy (AlZnMgCu) are investigated, as well as the anisotropy of this compromise in the rolling plane. Fracture toughness is experimentally approached by the Kahn tear test. The microstructure is studied quantitatively in detail by a combination of scanning electron microscopy, transmission electron microscopy and small-angle X-ray scattering, and the relative fractions of the various fracture modes as a function of microstructural state are quantitatively determined on scanning electron microscopy images. Toughness is confirmed to be minimum at peak strength, and lower for an overaged material than for an underaged material of the same yield strength. A lower quench rate is shown to result in an overall reduction of toughness, and in a reduced evolution of this toughness during the aging heat treatment. The overall toughness is also lowered when the main crack propagation direction is parallel to the preferential elongation direction of the coarse constituent particles (rolling direction). The competition between intergranular and transgranular fracture is explained in terms of the modifications of the work hardening rate, and of grain boundary precipitation. The evolution of fracture toughness is qualitatively explained in terms of evolution of yield stress, strain hardening rate, grain boundary precipitation and intragranular quench-induced precipitates.

Characterisation and modelling of precipitate evolution in an Al–Zn–Mg alloy during non-isothermal heat treatments

Abstract This paper describes the response of a precipitate microstructure to various types of non-isothermal temperature changes, namely reversion, ramp heating and thermal cycles, met in heat-affected zones (HAZs) of arc-welds, in an Al–Zn–Mg alloy in the T6 and T7 states. During these thermal histories, the precipitate size and volume fraction are quantitatively measured by small-angle X-ray scattering (SAXS). Reversion experiments are characterised by a fast dissolution stage at almost constant average precipitate size, followed by a coarsening stage that affects the surviving precipitates. Ramp heating experiments show a dissolution behaviour, which is temporarily interrupted for low heating rates, either due to phase transition when the initial precipitates are of a metastable nature (the case of T6 initial state), or due to a dynamic competition between the average and critical radii during the temperature increase (the case of the T7 material). The HAZ of arc-welds is characterised by a gradual increase of the dissolved precipitate fraction as one gets closer to the weld line. In the zone immediately before complete dissolution, precipitate coarsening is observed. A simple model, based on the growth/dissolution of precipitates distributed in size classes, has been adapted to the present ternary alloy. This model, calibrated using the reversion experiments, has proven its predictive nature in all the other thermal cycles investigated (ramp heating and welding). The predictions of the model are used in order to improve the physical insight into the range of material behaviour observed in the experiments.

Characterization and modeling of precipitation kinetics in an Al-Zn-Mg alloy

In this article, we present a systematic study of precipitation kinetics in a Fe-Si-Ti alloy in the temperature range 723 K to 853 K (450 °C to 580 °C), combining complementary tools (transmission electron microscopy (TEM), atom probe tomography (APT), and small-angle neutron scattering (SANS)). We show that the Heusler phase Fe2SiTi dominates the precipitation process in the investigated time and temperature range, regardless of the details of the initial temperature history. A numerical model based on the evolution of precipitate size classes gives very good agreement with the experimental results, and its application to different alloy compositions provides directions for future alloy optimization.

Low-temperature solubility of copper in iron: experimental study using thermoelectric power, small angle X-ray scattering and tomographic atom probe

Measuring the solubility limit of copper in iron at temperature lower than 700°C is problematic because copper diffusion is too slow in this temperature range. To overcome this difficulty, fine precipitation of copper is studied. The solubility limit of copper is measured after complete precipitation using two complementary techniques: thermoelectric power and small angle X-ray scattering. Values obtained are confirmed by tomographic atom probe and give results much higher than what is usually extrapolated from high-temperature experiments.

Characterisation of precipitation microstructures in aluminium alloys 7040 and 7050 and their relationship to mechanical behaviour

Abstract This paper investigates the precipitation microstructures in aluminium alloys 7040 and 7050 (Al - Zn - Mg - Cu variants) as a function of quench rate and aging treatment, and the associated compromise between yield strength and fracture toughness. The precipitate microstructures are quantitatively characterised by a combination of techniques covering the different scales involved: transmission electron microscopy (TEM), field emission gun scanning electron microscopy (FEG-SEM) and small angle X-ray scattering (SAXS), and fracture toughness is estimated using the Kahn tear test. Yield strength and strain hardening behaviour are determined by conventional tensile tests. It is shown that the composition modification from alloy 7040 to alloy 7050 results in a better compromise between yield strength and toughness. The strength decrease in alloy 7040 following a slow quench is much reduced compared to alloy 7050 because of a lower sensitivity to quench-induced precipitation on dispersoids. Both alloys show a large decrease in toughness upon slow cooling, however this decrease is much more pronounced in the case of alloy 7050 because of the presence of bands of dispersoid nucleated quench-induced precipitates, promoting low energy transgranular fracture. Further improvement of toughness values in the slowly quenched materials would require a decrease in the quench sensitivity of grain boundary precipitation.

The influence of plastic instabilities on the mechanical properties of a high-manganese austenitic FeMnC steel

Abstract The serrations observed on the stress–strain curves of a Fe-22Mn-0.6C wt.% steel present all the phenomenological features associated with dynamic strain ageing, including negative strain rate sensitivity and thermal activation. We show that the activation energy is, however, inexplicably low and that the dynamic strain ageing contribution to work-hardening is limited, i. e. mechanical twinning is the dominant strain hardening mechanism. Finally, novel in-situ experiments are used to study the dynamic behaviour of intense strain localisation bands which form on tensile samples. In spite of the localisation behaviour seen in tensile tests, stamped parts do not present any surface defects.

On the validity of simple precipitate size measurements by small-angle scattering in metallic systems

This paper assesses how simple small-angle scattering particle size evaluation models, such as Porod or Guinier radii, which have a normally limited validity range, may see this range extended to larger q values. This is shown to be particularly true for metallic systems, where the dispersion in particle size is always large. Because of the size dispersion, the relationship between the average particle size and the Guinier radius is shown to change. For systems with relatively large size dispersion, the paper shows that the Porod and Guinier radii, and simple extensions thereof, give valuable information on particle size and particle size distribution. This is demonstrated to be valid for particles with moderate aspect ratios. These simple evaluations are quick and very well adapted to large data sets, such as those originating from time-resolved or scanning small-angle experiments.

Influence of copper addition on precipitation kinetics and hardening in Al–Zn–Mg alloy

AbstractThe influence of the addition of copper on the precipitation behaviour of an Al–Zn–Mg alloy at 160°C has been investigated. Phase transformation has been investigated by differential scanni...