Materials Science Forum Vols. 519-521

Abstract: The role of alloying elements in the improvement of the high temperature strength of Al-12Si(CuNiMg) cast alloys used for automotive piston applications was investigated. The addition of alloying elements such as Mn, Cr, Ti and Ge was studied and the detailed characterization of the composition and morphology of the constituent phases after over aging at 350
 for 1000 hrs was performed. The compositions and volume fractions of the equilibrium phases determined by thermodynamic calculation were compared with the experimental results. The addition of transition elements, including Mn, Cr and Ti, increased the volume fraction of the intermetallic phases, which effectively enhanced the high temperature strength of the alloys. Among these transition elements, Mn turned out to be the most effective alloying element. After adding up to 0.5wt% of Mn, a large number of intermetallic phases, α-Al(Mn,Fe)Si as well as fine Al6(Mn,Fe) particles were precipitated and a significant improvement in the elevated temperature properties was achieved. The addition of Ge promoted the precipitation of the θphase (metastable phase, θ-Al2Cu), due to the formation of GeSi precipitates, thereby improved the mechanical properties of the alloy after T6 heat treatment. However, the presence of these GeSi precipitates did not affect the coarsening of the θ phase to form Qphase( Al5Cu2Mg8Si6) during aging and, thus, the elevated temperature properties were not improved by the addition of Ge.

Abstract: Scandium additions are known to offer a number of benefits to aluminium alloy performance. Many of these benefits can be attributed to the precipitation of fine Al3Sc particles. These particles are fully coherent with the aluminium matrix when they are small, but can lose coherency as they grow or coarsen. In this work, the change in coherency has been studied in an Al- 0.12 wt%Sc alloy over the temperature range 300-425oC. Three coherency regimes were identified, consistent with previous observations. The time and temperature range over which coherency changes occur have been measured for a range of conditions and correlated with the precipitation kinetics and the predictions of a model for Al3Sc precipitation. The effect of the coherency change on the particle morphology has also been investigated.

Abstract: The influence of the cadmium and the indium on the kinetics of the δ’ metastable phase precipitation has been studied in Al-8.5at.%Li and Al-8.5%at.%Li- 0.2at.%In(0.3%at.%Cd) solid solutions. From the hardness isotherms the volumic fraction of the formed δ’ metastable phase is determined for different times of ageing at 150, 180 and 200°C. The incubation times of δ’ deduced from the precipitation kinetics show that In and Cd atoms retard the δ’ precipitation.

Abstract: Main properties and structure of 2 mm thick sheets and 20×100 mm extruded strips made under industrial conditions from Al-Cu-Mg heat-resistant B-1213 alloy with Ag, Cr, Mn and Zr additions were investigated. It was stated that wrought B-1213 alloy semiproducts are superior to analogous semis made of widely used commercial heat resistant AK4-1ch-type alloys (analogues - AU2GN, 2618 alloys) in strength properties and heat resistance characteristics (by 10-20 %) as well as in crack resistance and fatigue life (by 20-40 %). B-1213 alloy is intended for replacement of traditional heat-resistant alloys of system Al-Cu-Mg-Fe-Ni in advanced aircraft primary structure components subjected heating at elevated temperatures (up to 200-250 oC) and allows one to increase weight efficiency, service life and reliability.

Abstract: A study on the microstructures developed during artificial aging of the 2024 alloy in nondeformed and 8% pre-deformed specimens is presented. The microstructural characterization was carried out using transmission electron microscopy (TEM), specifically, diffraction contrast and highresolution imaging. The S phase was found to precipitate heterogeneously on dislocations in both nondeformed and pre-deformed alloys. After the same artificial aging time, a larger density of S phase precipitates was obtained in the pre-deformed specimens. In non-deformed specimens, a high density of small nanometer-sized aggregates was found, that was absent in the pre-deformed specimens, that were identified as GP zones. To further analyze the small sized aggregates, artificially aged specimens with no previous deformation were also studied with Coincidence Doppler Broadening (CDB) of positron annihilation radiation. With this technique, information on the chemical environment around vacancy sites contained in the GP zones was obtained. Specifically, a composition close to (AlCu)2Mg was derived.

Abstract: The interaction between vacancies and Sn and Cu solute atoms in an Al-1.7at.%Cu- 0.01at.%Sn alloy was investigated by exploring the effect of incorporating natural ageing into conventional age hardening treatment. It was found that provided the artificial ageing temperature does not exceed a critical value between 160°C and 200°C, a narrow window of natural ageing (3-100 h) will result in a significant acceleration of the age hardening response and no decrease in peak hardness. Transmission electron microscopy showed that this effect reflects a large and rapid increase in number density of Cu GP(I) zones, and, to a lesser extent, of θ". The distribution and number density of θ' are essentially unaffected. Three-dimensional atom probe provided strong evidence that refinement of GP(I) zone distribution is not due to clustering of Cu atoms onto pre-existing Sn clusters. Instead it appears to be caused by a subtle interaction between vacancies, Sn and Cu atoms.

Abstract: In commercial aluminum alloys, Zr is recognized as an important additional element improving their properties such as strength, corrosion resistivily, and so on. It forms very fine particles of the metastable Al3Zr phase with L12 ordered configuration, in the case of alloys without Si. On the other hand, the DO22-type (Al, Si)3Zr phase besides the L12-type Al3Zr phase is precipitated in alloys including large amounts of Si, such as 6000 series. In this study the main stress falls on precipitation behavior of these Zr compounds in a Zr bearing Al-Mg-Si alloy. The Al-0.67Mg-0.97Si-0.37Zr alloy was cast into a metal mold and hot-rolled to a 5 mm thick plate. A lot of fine L12-type Al3Zr particles were coherently precipitated in the matrix by aging the plate at 673K. The aged plate was cold-rolled to a 1 mm thick sheet and annealed at 553-813K, and then microstructure was investigated by using a transmission electron microscope. It was realized that fine coherent L12-type Al3Zr particles are transformed to incoherent and coarse DO22-type (Al, Si)3Zr particles through recrystallization. The DO22-type (Al, Si)3Zr particles reduced recrystallized grain size owing to their pinning effect.

Abstract: In this work, the age-hardening of Al- 1.0 mass% Mg2Si- 0.4 mass% Mg – 0.5 mass% Ag (ex.Mg-Ag alloy) alloy has been investigated. It showed increase of hardness and age-hardening response. Precipitates in this alloy aged at 523 K have been observed by high resolution transmission electron microscopy (HRTEM) and classified into five types based on characteristics in their HRTEM images.

Abstract: The purpose of this study is identity the crystal structure of metastable phase in Ag added Al-Mg-Si alloy by comparing the β’-phases in Al-Mg-Si alloy without Ag, using images of high resolution transmission electron microscope (HRTEM), selected area electron diffraction patterns (SADPs) and an energy dispersive X-ray spectroscopy (EDS). The result of SADPs and HRTEM images have been simulated and compared with images and SADPs obtained from actual precipitates. SADPs and HRTEM images obtained from metastable phase in the Ag added Al-Mg-Si alloy showed similar to those of β’-phase in Al-Mg-Si alloy without Ag and had different lattice spacings because of the effect of Ag.