Thomas Dorin

Quantitative description of the T1formation kinetics in an Al–Cu–Li alloy using differential scanning calorimetry, small-angle X-ray scattering and transmission electron microscopy

The object of the present study is to design a methodology to follow the kinetics of T1 precipitation, in an AA2198 alloy, in terms of precipitate size, morphology (thickness, diameter) and volume fraction, during a two-temperature isothermal heat treatment. We used in situ small-angle X-ray scattering (SAXS) as a way to measure the evolution of the T1 mean thickness and diameter during the heat treatment. Transmission electron microscopy (TEM) was then performed in order to calibrate these evolutions. Furthermore, we demonstrate that the volume fraction evolution can be described successfully using a simple analysis of the differential scanning calorimetry (DSC) thermograms. The latter was calibrated by selected observations in high angular annular dark field scanning transmission electron microscopy (HAADF-STEM). Microstructure evolution during DSC heating ramps was analysed using in situ SAXS: the T1 phase transformation is found to consist in a two-step thickening process explained by two consecutive diffusion stages. The enthalpy of formation of the T1 phase is deduced from the DSC measurements.

Size distribution and volume fraction of T1 phase precipitates from TEM images: Direct measurements and related correction

Transmission Electron Microscopy (TEM) can be used to measure the size distribution and volume fraction of fine scale precipitates in metallic systems. However, such measurements suffer from a number of artefacts that need to be accounted for, related to the finite thickness of the TEM foil and to the projected observation in two dimensions of the microstructure. We present a correction procedure to describe the 3D distribution of disc-like particles and apply this method to the plate-like T1 precipitates in an Al-Li-Cu alloy in two ageing conditions showing different particle morphologies. The precipitates were imaged in a High-Angular Annular Dark Field Microscope (HAADF-STEM). The corrected size distribution is further used to determine the precipitate volume fraction. Atom probe tomography (APT) is finally utilised as an alternative way to measure the precipitate volume fraction and test the validity of the electron microscopy results.

Effect of Cooling Rate on Phase Transformations in a High-Strength Low-Alloy Steel Studied from the Liquid Phase

The phase transformation and precipitation in a high-strength low-alloy steel have been studied over a large range of cooling rates, and a continuous cooling transformation (CCT) diagram has been produced. These experiments are unique because the measurements were made from samples cooled directly from the melt, rather than in homogenized and re-heated billets. The purpose of this experimental design was to examine conditions pertinent to direct strip casting. At the highest cooling rates which simulate strip casting, the microstructure was fully bainitic with small regions of pearlite. At lower cooling rates, the fraction of polygonal ferrite increased and the pearlite regions became larger. The CCT diagram and the microstructural analysis showed that the precipitation of NbC is suppressed at high cooling rates, and is likely to be incomplete at intermediate cooling rates.

Complementarity of Atom Probe, Small Angle Scattering and Differential Scanning Calorimetry for the Study of Precipitation in Aluminium Alloys

Two examples of precipitation studies (in Al-Li-Cu and Al-Li-Mg alloys) are shown to demonstrate the complementarity of atom probe tomography, small-angle-scattering and differential scanning calorimetry for precipitation studies. It will be used to unravel an unexpected two-step precipitation behavior of T1 in Al-Li-Cu and to ascertain precipitates size in Al-Li-Mg. through a model free comparison between atom probe and SAXS.

Complex precipitation phenomena in strip cast steels with high sulfur and copper contents

A series of three steel alloys with increasing Cu and S concentrations has been prepared by simulated direct strip casting. It was found that the rapid solidification that occurs during direct strip casting results in the formation of a high number density of fine MnS precipitates, while Cu was retained in solid solution above equilibrium concentration. Upon ageing the MnS particles were found to coarsen and increase in volume fraction, indicating that some S was retained in solid solution in the as-cast condition. Ageing also resulted in the precipitation of Cu-rich precipitates. A new method to determine precipitate composition from small-angle neutron scattering is presented. This methodology, in conjunction with atom-probe tomography, has been used to show that the composition of the Cu-rich precipitates depends on the alloys bulk Cu content.

New Research Techniques in Aluminium Alloy Development

Abstract This chapter reviews new research techniques and recent progress in the development and understanding of aluminium alloys by the application of advanced characterization for composition and structure determination. In particular, this chapter will focus on high-resolution transmission electron microscopy, atom probe tomography, and small-angle X-ray scattering, which all allow atomic-scale characterization. Furthermore, the complementary use of these techniques as well as correlation with other microscopy and microanalysis techniques, provide opportunities to overcome the inherent limitations of the individual methods and to capitalize on their unique advantages. This multitechnique approach will also be discussed in the context of both combinatorial studies and direct correlation with atomic-scale, first-principles modelling, namely density functional theory simulations, in the process of aluminium alloy development by investigating structure–property relationships.

The Effect of Scandium and Zirconium on the Microstructure, Mechanical Properties and Formability of a Model Al–Cu Alloy

The addition of zirconium (Zr) together with scandium (Sc) is known to increase the strength of Al alloys. This work explores the impact of Sc and Zr on the extrudability of a model extruded Al-4 wt%Cu alloy. A traditional material’s property which can be directly related to the energy required to form a certain alloy is an alloy’s flow stress. The flow stress behaviour is evaluated from the stress displacement curves recorded during extrusion. The addition of Sc and Zr is found to result in an increase of the flow stress of 14% when the yield strength in the T8 temper increases by 100%. This increase in flow stress is compared to other heat treatable Al alloys and it is found that Sc and Zr result only a minor increase in the flow stress.

Understanding the Co-precipitation Mechanisms of Al 3 (Sc, Zr) with Strengthening Phases in Al–Cu–Li Model Alloys

The addition of scandium (Sc) is known to be beneficial to the properties of aluminium alloys. Sc has a significant impact on the strength, especially when combined with zirconium (Zr) where nano-size Al3(Sc, Zr) dispersoids are formed. However, to date the extraction of Sc from its oxide has been too expensive to promote its use on the industrial scale. New extraction methods have made the process more cost-effective. So far, there has been only limited research done concerning the effect of Sc in Al–Cu–Li alloys. Interaction between the Al3(Sc, Zr) dispersoids and other precipitate phases containing copper (Cu) and/or lithium (Li) have been rarely studied. In this study, the impact of Sc and Zr on precipitation and further the properties of Al–Cu–Li alloys is investigated. Mechanical properties were examined by quasi-static tensile tests. Using TEM, the morphology and size of dispersoids and precipitates were examined.

Influence of the Al 3 (Sc,Zr) Dispersoids and the Stretching on the Natural Ageing Behavior of a Binary Al-4 wt%Cu Alloys

Aluminum alloys are extensively used in the aerospace industry due to their high specific strength. Combined scandium (Sc) and zirconium (Zr) additions result in the formation of nano-size Al3(Sc,Zr) dispersoids which considerably increase the mechanical properties of these alloys. Recent research on Al–Cu–Sc–Zr model alloys showed that these dispersoids accelerate the formation kinetics of the copper (Cu) rich precipitates and modify their precipitation sequence. In this work, the effect of Sc and Zr addition and of the stretching on the precipitation during natural ageing of an Al-4 wt%Cu is investigated. Hardness and tensile measurements are used to quantify the hardening kinetics from precipitate formation during natural ageing. Transmission electron microscopy and differential scanning calorimetry are used to characterize the formed precipitates. Although the work hardening experienced during the stretching was significantly higher in the Al–Cu–Sc–Zr alloy, the strength increment during natural ageing was higher in the binary Al–Cu alloy. The presence of Sc and Zr inhibits the formation of GP zones that can explain the lack of mechanical response during the natural ageing.

Aluminium Lithium Alloys

Abstract In recent years, the utilization of aluminium–lithium alloys has increased as aerospace manufacturers have aimed to improve both performance and fuel efficiency. While aluminium–lithium alloys (first- and second-generation alloys) have been studied for over 50 years, it was only recently that the third generation of these alloys found widespread use in the aerospace industry. Historically, first- and second-generation alloys encounter many issues, which will be discussed. Through extensive research efforts the third generation of aluminium–lithium alloys were able to overcome many of the systemic issues that plagued early aluminium–lithium alloys. This was accomplished by not only optimizing compositional and processing controls, but through a better understanding of key metallurgical concepts, such as precipitate characterization. In this section, the key metallurgical concepts pertinent to manufacturing aluminium–lithium alloys, including the physical metallurgy (precipitates, effects of minor alloying elements, texture effects, etc.), important processing steps (casting, hot forming, etc.), and the main strengthening mechanisms in these alloys will be reviewed.