Shoichi Hirosawa

Classification of the role of microalloying elements in phase decomposition of Al based alloys

Abstract The fundamental role of microalloying elements in several aluminium alloys such as Al–Cu, Al–Li–Cu and Al–Cu–Mg has been investigated using a Monte Carlo computer simulation. All the utilized simulation parameters, e.g. pair interactions between same atoms species, between different atom species and between an atom and a vacancy, were derived from known thermodynamic or kinetic quantities. A small addition of Mg to Al–Cu alloys exhibits a strong tendency to form Mg/Cu/Vacancy complexes in the atom configurations, which is more remarkably revealed in Al–Li–Cu alloys. The combined addition of Ag or Si with Mg significantly increases the number of Mg/Cu/Vacancy complexes in Al–Cu–Mg alloys. From the comparison with experimentally reported results, these complexes are reasonably regarded as an effective heterogeneous nucleation site for GP zones, GPB zones and/or the Ω phase. The utilized simulation model, furthermore, permits the role of microalloying elements to be well classified in terms of the characteristic features of each element.

Determining the composition of small features in atom probe: bcc Cu-rich precipitates in an Fe-rich matrix.

Aberrations in the ion trajectories near the specimen surface are an important factor in the spatial resolution of the atom probe technique. Near the boundary between two phases with dissimilar evaporation fields, ion trajectory overlaps may occur, leading to a biased measurement of composition in the vicinity of this interface. In the case of very small second-phase precipitates, the region affected by trajectory overlaps may extend to the centre of the precipitate prohibiting a direct measurement of composition. A method of quantifying the aberrant matrix contribution and thus estimating the underlying composition is presented. This method is applied to the Fe-Cu-alloy system, where the precipitation of low-nanometre size Cu-rich precipitates is of considerable technical importance in a number of materials applications. It is shown definitively that there is a non-zero underlying level of Fe within precipitates formed upon thermal ageing, which is augmented and masked by trajectory overlaps. The concentration of Fe in the precipitate phase is shown to be a function of ageing temperature. An estimate of the underlying Fe level is made, which is at lower levels than commonly reported by atom probe investigations.

Quantitative characterization of precipitate free zones in Al -Zn -Mg( -Ag) alloys by microchemical analysis and nanoindentation measurement

Abstract To correlate quantitatively the mechanical properties of precipitate free zones (PFZ) with the corresponding microstructural and compositional characteristics, TEM observation, EDX analysis and nanoindentation measurement have been performed in the vicinity of grain boundaries in Al–4.9 mass%Zn–1.8 mass%Mg (–0.28 mass%Ag) alloys. The remarkable decreases in hardness and solute concentrations were observed towards grain boundaries even in the regions just outside PFZ. With increasing aging time, it is firstly revealed that the hardness inside PFZ monotonously decreases although the hardness inside grains increases in the earlier stage of aging. Three distinct regions of “PFZ”, “Transition-area” and “Grain-region” were therefore proposed to explain the origins of such age-hardening behavior observed in this work. In the Ag-added alloy, on the other hand, the hardness could be maintained up to closer regions to grain boundaries at the same level as that inside grains.

Combined atomic–scale modelling and experimental studies of nucleation in the solid state

The process of solid–state nucleation in highly supersaturated solid solutions has been investigated on the atomic scale by a combination of three–dimensional atom probe analysis and atomistic modelling using dynamical Ising models. In binary Cu–Co alloys, a simple atom–exchange model with a single thermodynamic parameter derived from phase–diagram data was able to reproduce the atomic–scale microstructures observed in the atom probe, and also match the measured peak precipitate density. Modelling solute effects in complex copper–bearing steels required a more sophisticated model based on a vacancy–hopping mechanism and a larger number of thermodynamic and kinetic parameters derived from independent experimental data and theoretical calculations. The model gave an excellent match to the experimentally observed microstructures, and it reproduced features such as the clustering of Ni and Mn before the precipitation of Cu. The model also allowed time–dependent behaviour to be investigated, and it showed that solute clustering of Ni and Mn occurs during the cooling of the alloy. These clusters then act as heterogeneous nucleation sites for the formation of copper precipitates. Understanding such complex solute interaction effects through combined experiment and modelling is an essential step to controlling nucleation and hence the fine–scale microstructures in advanced engineering alloys.

Effects of Mg addition on the kinetics of low-temperature precipitation in Al–Li–Cu–Ag–Zr alloys

Abstract The thermal stability at around 343 K is one important problem of Al–Li alloys related to the microstructural changes during the low temperature exposure. The present work aims to investigate the effects of Mg addition on the age-hardening behavior and precipitation kinetics of an Al–Li–Cu–Mg–Ag–Zr alloy in the low temperature range from 278 to 373 K. The microstructures observed with transmission electron microscopy (TEM) and the hardness changes indicate that a small amount of Mg markedly accelerates the formation of GP(1) zones, not the δ ′ (Al 3 Li) phase, resulting in an enhanced age- hardening. Quantitative analysis of the precipitation kinetics determined by the electrical resistivity changes also elucidates the more detailed effects of Mg addition on the characteristic precipitation phenomena in the Al–Li–Cu alloys, i.e. a decreased activation energy for the GP(1) zone nucleation and a suppressed growth of both GP(1) zones and the δ ′ phase (and/or its precursory structures). The complicated effects of Mg addition are well explained in terms of both the enhanced nucleation rate of GP(1) zones with the aid of Mg/Cu/Vacancy complexes and the pronounced decrease in free-vacancies available for Cu and Li diffusion due to the preferential vacancy trapping by Mg atoms.

First-Principles Calculation of Interaction Energies between Solutes and/or Vacancies for Predicting Atomistic Behaviors of Microalloying Elements in Aluminum Alloys

The atomistic behaviors of microalloying elements during phase decomposition of Al- Cu-Mg, Al-Zn-Mg and Al-Mg-Si alloys have been systematically predicted in terms of two-body interaction energies between solutes and/or vacancies. The utilized first-principles calculation based on generalized gradient approximation (GGA) and full-potential Korringa-Kohn-Rostoker (FPKKR) Green’s function method accurately estimated such fundamental energies in good agreement with experimentally reported behaviors: e.g. vacancy-trapping model, vacancy-sink model and nanocluster assist processing. The proposed interaction energy maps (IE maps), in which the estimated interaction energies are plotted along the rows of the periodic table, are quite useful for designing new aluminum alloys with microalloying elements.

Improvement of Bake-Hardening Response of Al-Mg-Cu Alloys by Means of Nanocluster Assist Processing (NCAP) Technique

In this work, the bake-hardening (BH) response of an Al-3.0Mg-1.0Cu (in mass%) alloy has been improved by the small addition of Ag as a good example of our proposed Nanocluster Assist Processing (NCAP) technique. From the detailed observation through high resolution transmission electron microscopy (HRTEM), it is found that the origin of the increased hardness in the Ag-added alloy is attributed to the densely and uniformly formed Z phase at the expense of Guinier-Preston- Bagaryatsky (GPB) zones and the S’ phase. It is new findings that the Z phase is formed even in the ternary alloy although the chemical composition lies in the (α+S+T) phase field. Based on the threedimensional atom probe (3DAP) results, furthermore, it is suggested that nanoclusters of Mg, Ag and/or Cu provide effective nucleation sites for the Z phase, whereas nanoclusters of Mg and Cu do less. Such unique characteristics of Ag are clearly seen in the newly constructed interaction energy map (IE map).

Quantitative Correlation between Strength, Ductility and Precipitate Microstructures with PFZ in Al-Zn-Mg(-Ag, Cu) Alloys

The quantitative correlation between strength, ductility and precipitate microstructures in the vicinity of grain boundaries with precipitate free zones (PFZ) was evaluated for Al-Zn-Mg(-Ag, Cu) alloys using transmission electron microscopy (TEM), three-dimensional atom probe (3DAP) and tensile test. In the Al-Zn-Mg ternary and Cu-added alloys aged at 433K, larger widths of PFZ were observed by TEM and resulted in lower elongations to fracture, independent of the size of grain boundary precipitates. On the other hand, the elongation of the Ag-added alloy was higher, if compared at the same levels of proof stress, due to the much smaller width of PFZ. This strongly suggests that PFZ is harmful to fracture of the investigated alloys. From a 3DAP analysis, furthermore, it was revealed that Ag and Cu atoms are incorporated in the nanoclusters from the initial stage of aging. In this work, the elongation was well correlated to the width of PFZ, size of grain boundary precipitates and the level of proof stress, enabling to predict ductility of the alloys from known microstructural factors.

Monte Carlo computer simulation of the atomistic behaviour of microalloying elements in Al-Li alloys

The atomistic behaviour of various microalloying elements during low-temperature precipitation in Al-Li alloys has been investigated using a Monte Carlo computer simulation. The utilized simulation parameters derived from known thermodynamic quantities can well reproduce the phase decomposition of Al-Li alloys involving simultaneous reactions of clustering and ordering processes. The spatial and temporal evolution of the simulated microstructures provides useful information on microscopic events such as the nucleation and growth of precipitates, the change in the degree of order and the preferential partitioning of microalloying elements. The role of microalloying elements is well classified in terms of the characteristic features of individual elements.

Nano-Scale Clusters Formed in the Early Stage of Phase Decomposition of Al-Mg-Si Alloys

The formation of nano-scale clusters (nanoclusters) prior to the precipitation of the strengthening b” phase significantly influences two-step aging behavior of Al-Mg-Si alloys. In this work, the existence of two kinds of nanoclusters has been verified in the early stage of phase decomposition by differential scanning calorimetry (DSC) and three-dimensional atom probe (3DAP). Pre-aging treatment at 373K before natural aging was also found to form preferentially one of the two nanoclusters, resulting in the remarkable restoration of age-hardenability at paint-bake temperatures. Such microstructural control by means of optimized heat-treatments; i.e. nanocluster assist processing (NCAP), possesses great potential for enabling Al-Mg-Si alloys to be used more widely as a body-sheet material of automobiles.