Olaf Engler

The concentration of alloying elements retained in solid solution is a key parameter controlling microstructure evolution and properties of wrought aluminium alloys. Quantitative determination of solute concentrations in multicomponent alloys remains challenging, however, because most available techniques either provide only local information or cannot separate the contributions of individual alloying elements. This paper presents a methodology combining low-temperature electrical resistivity and multi-temperature thermoelectric power (TEP) measurements for the quantitative determination of solute concentrations in wrought Al alloys. Element-specific temperature-dependent TEP coefficients were established using binary calibration alloys and incorporated into a numerical evaluation procedure based on the Nordheim–Gorter formalism. The resulting approach enables the simultaneous determination of multiple solute concentrations from a single set of resistivity and TEP measurements.Virtual validation experiments and sensitivity analyses confirm that the temperature dependence of the TEP response provides characteristic element-specific signatures that enable the separation of the contributions of individual alloying elements. Significant differences in sensitivity and information content were observed among the investigated elements, particularly for Si, and were found to govern the achievable accuracy and robustness of the calculated solute concentrations. Nevertheless, the principal alloying elements can generally be recovered with good accuracy. Application to experimental datasets confirms the robustness of the methodology and demonstrates its suitability for a wide range of non-heat-treatable Al alloys. The combined electrical-resistivity and multi-temperature TEP approach provides a rapid bulk-characterization technique for quantitative microchemistry analysis and a practical tool for alloy development, process optimization and microstructure modelling.

The effect of second-phase particles on the formation of recrystallisation textures in Al–Fe alloy sheet was analysed. Four commercial AA 8xxx series alloys were chosen on the basis of their Fe and Si contents, one alloy additionally comprised Mn. Main focus was laid on the efficiency of particle stimulated nucleation (PSN) at the large constituent phases. As known from other Al alloys, the recrystallisation textures of the present AA 8xxx alloys are composed of new grains forming at three distinct nucleation sites, viz. cube bands, pre-existing grain boundaries and coarse intermetallic particles. The efficiency of each of these nucleation mechanisms strongly depends on the details of the as-deformed microstructure, especially on the size distribution of the constituent particles.

Several key properties of aluminium semi-finished products, including formability, softening behaviour, strength and ductility, are influenced by the constitution of alloying elements in solid solution or in precipitated form, i.e. by the materials microchemistry. In the present paper, we study microchemistry reactions in two Al–Mg–Si automotive sheet alloys, AA 6005C and AA 6016, with different Mg:Si ratios. The formation of constituent phases and dispersoids during solidification and subsequent homogenisation is analysed by scanning electron microscopy and measurements of the specific electrical resistivity. Type, composition and volume fraction of the constituent phases in the as-cast state are assessed with the Alstruc solidification model. The changes in solute level and precipitation during homogenisation are traced with a statistical microchemistry simulation tool termed ClaNG.

An important aspect of the characterisation of non-heat-treatable Al alloys is the constitution of the material in terms of its alloying elements in solid solution and, in turn, volume, size, morphology and species of second-phase particles. These constitutional characteristics, conveniently summarised as micro-chemistry, have an impact on physical and mechanical materials properties of Al semi-finished products. In the present study, the evolution of micro-chemistry in the Al–Mn alloys of the AA 3xxx series during solidification and homogenisation is analysed. Especially, we focus on the impact of additional alloy elements, including Si, Mg and Cu, on the phase selection during solidification and the changes in solute level and volume, size and type of second-phase particles during subsequent homogenisation.