Myriam Dumont

Use of small-angle X-ray scattering for the characterisation of precipitates in aluminium alloys

This paper presents two studies illustrating the possibilities of Small-Angle X-ray Scattering for characterising quantitatively the state of precipitation in aluminium alloys. In the first example, maps are presented, of precipitate size and volume fraction in the cross-section of friction stir welds of AA7449 alloy. It is shown that the influence of welding speed on the distribution of mechanical properties can be understood using this microstructural data. In the second example, the precipitation kinetics in an Al-Zr-Sc alloy is evaluated by in-situ small angle X-ray scattering. Evidence is given for the heterogeneous chemical structure of the Al3(Zr,Sc) precipitates, consisting of a Zr-rich shell surrounding a Sc-rich core. It is shown that this particular distribution results in a very good resistance to coarsening of the precipitate microstructure.

Dilatometry revealing Si precipitation in Al–Si-alloys

Abstract The precipitation kinetics of Si after different thermal treatments in AlSi-alloys have been studied by means of isothermal calorimetry, differential scanning calorimetry and dilatometry tests. The measured exothermic and endothermic effects produced by the precipitation and dissolution of Si are correlated with the increase and reduction in the coefficient of thermal expansion CTE(T), respectively. The deviation of the CTE(T) of the studied AlSi1.1 – 1.7 alloys with respect to pure Al correspond to the volume fraction of precipitating Si according to thermodynamic solubility limits. Two different kinetics in the growth of Si precipitates are distinguished: while Si precipitates in the range of 200 – 300 °C during heating of solution quenched samples supersaturated with vacancies, retardation into the temperature range of 300 – 420 °C is observed in samples slowly cooled after solution treatment. The presence of the eutectic Si increases the quench sensitivity of the precipitation kinetics of Si as observed in CTE(T) curves. The ripening of the eutectic Si during prolonged solution treatment retards the Si precipitation slightly.

Combined Effect of Heating Rate and Microalloying Elements on Recrystallization During Annealing of Dual-Phase Steels

Adjusting ferrite recrystallization kinetics during annealing is a way to control the final microstructure and thus the mechanical properties of advanced cold-rolled high-strength steels. Two strategies are commonly used for this purpose: adjusting heating rates and/or adding microalloying elements. The present work investigates the effect of heating rate and microalloying elements Ti, Nb, and Mo on recrystallization kinetics during annealing in various cold-rolled Dual-Phase steel grades. The use of combined experimental and modeling approaches allows a deeper understanding of the separate influence of heating rate and the addition of microalloying elements. The comparative effect of Ti, Nb, and Mo as solute elements and as precipitates on ferrite recrystallization is also clarified. It is shown that solute drag has the largest delaying effect on recrystallization in the present case and that the order of solute drag effectiveness of microalloying elements is Nb > Mo > Ti.

Influence of Microalloying Elements Ti and Nb on Recrystallization during Annealing of Advanced High-Strength Steels

Microalloying elements Ti and Nb are commonly added to high-strength Dual Phase steels as they can provide efficient means for additional strengthening due to grain refinement and precipitation strengthening mechanisms. In the form of solute elements or as fine carbonitride precipitates, Ti and Nb are also expected to have a significant effect on the microstructural changes during annealing and especially on recrystallization kinetics. The present work investigates the influence of microalloying elements Ti and Nb on recrystallization in various cold-rolled Dual Phase steel grades with the same initial microstructure but different microalloying contents. Using complementary experimental and modeling approaches makes it possible to give some clarifications regarding both the nature of this effect and the comparative efficiency of Ti and Nb on delaying recrystallization. It is shown that niobium is the most efficient micro-alloying element to impede recrystallization and that the predominant effect is solute drag.

Influence of Heating Rate on Ferrite Recrystallization and Austenite Formation in Cold-Rolled Microalloyed Dual-Phase Steels

Compared with other dual-phase (DP) steels, initial microstructures of cold-rolled martensite-ferrite have scarcely been investigated, even though they represent a promising industrial alternative to conventional ferrite-pearlite cold-rolled microstructures. In this study, the influence of the heating rate (over the range of 1 to 10 K/s) on the development of microstructures in a microalloyed DP steel is investigated; this includes the tempering of martensite, precipitation of microalloying elements, recrystallization, and austenite formation. This study points out the influence of the degree of ferrite recrystallization prior to the austenite formation, as well as the importance of the cementite distribution. A low heating rate giving a high degree of recrystallization, leads to the formation of coarse austenite grains that are homogenously distributed in the ferrite matrix. However, a high heating rate leading to a low recrystallization degree, results in a banded-like structure with small austenite grains surrounded by large ferrite grains. A combined approach, involving relevant multiscale microstructural characterization and modeling to rationalize the effect of the coupled processes, highlights the role of the cold-worked initial microstructure, here a martensite-ferrite mixture: recrystallization and austenite formation commence in the former martensite islands before extending in the rest of the material.