Melting of secondary-phase particles in Al-Mg-Si alloys

The melting of secondary-phase particles—or, more precisely, the melting of such particles together with the surrounding matrix—in two ternary Al-Mg-Si alloys has been studied. In the quasi-binary Al-Mg₂Si alloy, one melting reaction is found. In the alloy with an Si content in excess of that necessary to form Mg₂Si, three different melting reactions are observed. At upquenching temperatures above the eutectic temperature, the reaction rates are very high, and it is assumed that they are controlled by diffusion of the alloying elements in the liquid. Melting is also observed after prolonged annealing at temperatures below the eutectic temperature in these alloys, which is explained by the different diffusion rates of Mg and Si. The rate of the melting reaction is in this case assumed to be controlled by diffusion of the alloying elements in the solid α-Al phase. It is shown that calculation of the particle/matrix interface composition, which determines when melting is possible, cannot be made solely on the basis of the phase diagram, but must also include the rate of diffusion of Mg and Si. The melting temperatures observed differ somewhat from the accepted eutectic temperatures for these alloys. On prolonged annealing, the liquid droplets formed dissolve into the surrounding matrix and their chemical composition is found to change during dissolution. The resulting eutectic structure after quenching of a droplet is explained by the phase diagram and the different diffusion rates of Mg and Si as well as by the nucleation conditions of the constituents involved.     

Time dependence of internal friction and shape change in Cu-Zn-Al shape memory alloys

The time evolution of internal friction and shape change in different microstructural states of two Cu-Zn-Al shape memory alloys has been investigated. In the single-martensite phase state, the monotonic decrease of internal friction with time is attributed to the pinning of martensite/ martensite interfaces. When the parent phase is present, the change in the internal friction is associated with the formation of stress-induced martensite and the subsequent pinning of the martensite/parent and martensite/martensite interfaces.     

Microstructural characterization of secondary-phase particles in a hot-deformed Al-Cu-Mg-Zr alloy

Torsion tests, on a 2014 + 0.13Zr alloy, were performed at temperatures in the range 573 to 773 K under strain rates ranging from 10⁻³ to 10 s⁻¹. Transmission electron microscopy (TEM) inspection was performed in order to establish the role of the hot deformation on the hardening second-phase particles. The pinning effect of Al₃Zr particles was also investigated. At the testing temperatures, the Al₃Zr particles were stable, and no significant statistic changes, in terms of density and mean size, occurred during the tests. Small Al₃Zr dispersoid particles inhibit recrystallization by pinning the grain and subgrain boundaries during hot deformation. Yet, they are particularly resistant to dislocation shear microstructure mechanism. Grains were elongated and contained a large number of sub-grains a few microns in width.     

Hydrogen embrittlement of steels: III. Influence of secondary-phase particles

The influence of secondary-phase particles (carbides) on the hydrogen embrittlement resistance of 35KhGM and 33KhM1F steels with various contents of carbide-forming elements has been studied. A combined analysis of the mechanical behavior of the material during uniaxial extension, the acoustic emission signal intensity, and metallographic and fractography patterns demonstrate the influence of carbide particles on the mechanisms and kinetics of fracture of the steels under hydrogen embrittlement.     

Preaging effects in Al-Mg-Si alloys

The aging characteristics of aluminum alloy extrusions containing 0.60 to 0.90 wt pct Mg₂Si were determined. At low Mg₂Si levels, preaging treatments at room temperature and at elevated temperatures refined the G.P. zone dispersion and increased the alloy’s hardness after final aging. Preaging had little effect on hardness at the high end of the Mg₂Si range. These results are explained on the basis of current aging theories which invoke the concept of a critical temperature, above which homogeneous nucleation does not take place. This temperature varies from ≃150°C at 0.6 pct Mg2Si to ≃220°C at 1.5 pct Mg₂Si. The apparent activation energy for final aging was estimated to be 21 kcal/mole, a value which is intermediate between the activation energies for vacancy motion and solute (silicon and magnesium) diffusion in aluminum.     

Influence of Si on Interfacial Combination of SiCp/Al-Mg-Si Composite

The scanning electron microscopy (SEM) analysis results of Si distribution in the interface between SiC reinforcements and aluminum matrix of a stir casting SiCp/Al-Mg-Si composite were presented. Results show that there is Si precipitation deposit on the interface of the composite and Si connects with SiC reinforcements in one side and connects with aluminum matrix in the other side. Si phase plays as a connecting bridge, which contributes to the interfacial combination of SiCp/Al composite.     

Creep regimes for directionally solidified Al-Al 3 Ni eutectic composite

Creep characteristics of Al-Al₃Ni eutectic composites directionally solidified at 2.2 × 10^-2 mm/s were determined over a wide range of stress and temperature. Four distinct regions of creep were observed. The rate controlling mechanisms for the four regions appear to be high-temperature dislocation climb in the Al matrix, low-temperature climb in the Al matrix, boundary sliding, and a mechanism involving deformation of the Al₃Ni fibers. Creep rates of the Al-Al₃Ni composite are several orders of magnitude smaller than for pure Al, and apparently, in the regions where deformation of the Al matrix is rate controlling, only a very small fraction of the matrix is deforming during creep of the composite. Formerly Graduate Student, Department of Mechanical and Industrial Engineering, University of Manitoba     

Heterogeneous Nucleation of Eutectic Structure in Al-Mg-Si Alloys

Metallurgical and Materials Transactions A - The microstructure of Al-5Mg-2Si-0.4Mn-0.7Fe alloy solidified at different conditions were examined. Different kinds of eutectic structures such as...     

Superplastic behavior and cavitation in high-strain-rate superplastic Si3N4p /Al-Mg-Si composites

High-strain-rate superplastic behavior has been investigated for Si₃N_4p /Al-Mg-Si (6061) composites with a V _f =20 and 30 pct, respectively, where V _f is the volume fraction of reinforcements. A maximum elongation was attained at a temperature close to the onset temperature for melting for both composites. The maximum elongation for the 30 vol pct composite was larger than that for the 20 vol pct composite. Development of cavities transverse to the tensile direction is responsible for the lower maximum elongation of the 20 vol pct composite. However, development of the transverse cavities was limited to the optimum superplastic temperature for the 30 vol pct composite. The differential scanning calorimetry (DSC) investigation showed that a sharp endothermic peak appeared for the 30 vol pct composite, indicating that sufficient partial melting occurs. It is, therefore, likely that the stress concentrations are sufficiently relaxed by a liquid phase and that the development of transverse cavities is limited for the 30 vol pct composite.     

Metastable phases in an Al-Mg-Si alloy containing copper

The Q′ phase in an Al-1.0 mass pct Mg₂Si-0.5 mass pct Cu alloy at a peak-aged condition of 523 K was observed by a high-resolution transmission electron microscope (HRTEM), in order to identify its structure and chemical composition, and was compared with the type-C precipitate in an Al-1.0 mass pct Mg₂Si-0.4 mass pct Si alloy (the excess-Si alloy). The Q′ phase has similar features to the type-C precipitate, according to results of electron diffraction patterns and images taken by the HRTEM, that is, they have similar hexagonal crystal lattices (a=1.04 nm and c=0.405 nm) to each other. The type-C precipitate in the excess-Si alloy was the ternary Al-Mg-Si phase, and the Q′ phase was the quaternary Al-Mg-Si-Cu phase in the Al-Mg-Si-Cu alloy, as determined by energy-dispersive X-ray spectroscopic (EDS) analysis.     

Low-Temperature Differential Scanning Calorimetry of an Al-Mg-Si Alloy

The clustering behavior at room temperature of a pure ternary Al-0.59 wt pct Mg-0.82 wt pct Si alloy was investigated by low-temperature differential scanning calorimetry (DSC). We find three clustering reactions that take place in two stages. The first two reactions are linked to each other and are completed after 1 hour. The third reaction starts around 1 hour after quenching and is completed after 2 weeks. Only the latter reaction exhibits a strong shift of the peak position of the thermal signal, indicating a change in the activation energy during aging at room temperature caused by changing solute supersaturation or increasing trapping of vacancies. The first two stages are closely linked to the known adverse effect of room-temperature preaging on the ensuing age-hardening step, since 60 to 80 pct of cluster formation is sufficient to establish the full negative effect.     

Deformation and fracture behavior of two Al-Mg-Si alloys

Deformation and fracture behavior of two Al-Mg-Si alloys in different aging conditions has been studied by tensile testing, transmission electron microscope (TEM), and scanning electron microscope (SEM) observation. Tensile test results show that the strain hardening exponents (n values) of the two alloys decrease sharply at the early stage of artificial aging and are only 0.045 and 0.06, respectively, in the overaged condition. The sharp decrease of work hardening rate is believed to be one major reason that results in the rapid decrease of elongation to failure at the early stage of artificial aging. In fully aged conditions, dislocations are concentrated in narrow bands during plastic deformation of these alloys, which is responsible for the very low n values of the Al-Mg-Si alloys in peak aged and overaged conditions. The Si particles formed in the interior of grains of the higher Si containing alloy reduce the inhomogeneous deformation behavior. The TEM results show that large precipitates and precipitate-free zones (PFZs) along grain boundaries are formed in peak aged and overaged conditions, and SEM observations demonstrate that the tensile fracture modes of the two alloys in these aging conditions are completely intergranular with many small cusps decorated on facets of the fractured grain boundaries. Thus, the fracture process of both alloys is suggested to be that in which the high local stresses, built up where the slip band impinges on the grain boundaries, nucleate voids at the grain boundary precipitates by decohesion of the particle/PFZ interface, and then coalescence of these voids within the PFZ leads to the final fracture of these alloys.     

Al-Mg-Si系合金的热加工工艺与可挤压性

Al Mg Si(6××× )系合金是最重要的挤压合金 ,其中又以 6 0 6 3、 6 0 82、 6 0 6 0和 6 0 0 5四种合金及其变种应　　用最广泛。本文全面系统而深入地分析了Al Mg Si系合金锭坯的均匀化及冷却方式、锭坯挤压前加热温度与加热方式、挤压成形工艺及在线淬火方式、人工时效及停放时间等对上述四种合金的可挤压性、挤压力、最大生产能力以及力学性能等的影响     

Clustering and Vacancy Behavior in High- and Low-Solute Al-Mg-Si Alloys

The precipitate microstructure and vacancy distribution in Al-Mg-Si alloys with different amounts of solute and different heat treatments were investigated by transmission electron microscopy and muon spin relaxation measurements. A high amount of vacancies is normally present in Al-Mg-Si alloys as these bind to atomic clusters. We observe these vacancies to leave the material not before over-aging at very high temperatures such as 623 K (350 °C), meaning that vacancies do not bind to incoherent over-aged precipitates. For samples only stored at room temperature after solution heat treatment, a reduction of muon trapping was found at a temperature of 140 K (?133 °C) when reducing the amount of solute in the alloy. This might be connected to a lower number density of Cluster (1), which contrary to Cluster (2) do not nucleate precipitates upon further aging of the material.     

Improving Thermal Stability in Cu-Containing Al-Mg-Si Alloys by Precipitate Optimization

It is demonstrated that good thermal stability in Al-Mg-Si-Cu aluminum alloys correlates with a high density of fine lath-shaped, Cu-containing, disordered L-precipitates. Alloys optimized for L retained hardness above 90 HV after 3 weeks over-aging at 473 K (200 °C). Further improvement was achieved by substituting Si by Ge in one alloy. High-angle annular dark-field scanning transmission electron microscopy showed that at peak-hardness conditions, L coexists with more common needle-shaped precipitates, often with Cu-enriched interfaces.     

Continuous cooling deformation testing of steels

In the processing of steel, the design of any kind of heat treatment and/or thermomechanical processing schedule, to obtain a given microstructure, is greatly facilitated by the knowledge of the austenite-to-ferrite transformation characteristics. In the past, isothermal and continuous cooling tests were used in the laboratory to create time-temperature-transformation and continuous cooling transformation diagrams, respectively, which then served as the source of transformation data. The problem with such information is that it is only truly applicable to one particular microstructure, usually one resulting from a simple reheating cycle in the austenite region. Most industrial steel processing operations additionally involve several stages of high-temperature deformation leading to changes in the microstructure emerging from the final pass. To account for this situation, a novel laboratory method for the determination of the transformation characteristics, based on continuous cooling deformation testing, was developed. A major attraction of this test technique is that the specific microstructure, for which the transformation characteristics are required, can be generated by hot deformation and then immediately evaluated by continuous cooling deformation. In this article, the basic continuous cooling deformation test technique and general methods of data analysis are illustrated, using results from several different grades of steel. Formerly with the Department of Mining and Metallurgical Engineering, McGill University,     

Microstructural Evolution and Solidification Behavior of Al-Mg-Si Alloy in High-Pressure Die Casting

Microstructural evolution and solidification behavior of Al-5 wt pct Mg-1.5 wt pct Si-0.6 wt pct Mn-0.2 wt pct Ti alloy have been investigated using high-pressure die casting. Solidification commences with the formation of primary α-Al phase in the shot sleeve and is completed in the die cavity. The average size of dendrites and fragmented dendrites of the primary α-Al phase formed in the shot sleeve is 43 μm, and the globular primary α-Al grains formed inside the die cavity is at a size of 7.5 μm. Solidification inside the die cavity also forms the lamellar Al-Mg₂Si eutectic phase and the Fe-rich intermetallics. The size of the eutectic cells is about 10 μm, in which the lamellar α-Al phase is 0.41 μm thick. The Fe-rich intermetallic compound exhibits a compact morphology and is less than 2 μm with a composition of 1.62 at. pct Si, 3.94 at. pct Fe, and 2.31 at. pct Mn. A solute-enriched circular band is always observed parallel to the surface of the casting. The band zone separates the outer skin region from the central region of the casting. The solute concentration is consistent in the skin region and shows a general drop toward the center inside the band for Mg and Si. The peak of the solute enrichment in the band zone is much higher than the nominal composition of the alloy. The die casting exhibits a combination of brittle and ductile fracture. There is no significant difference on the fracture morphology in the three regions. The band zone is not significantly detrimental in terms of the fracture mechanism in the die casting. Calculations using the Mullins and Sekerka stability criterion reveal that the solidification of the primary α-Al phase inside the die cavity has been completed before the spherical α-Al globules begin to lose their stability, but the α-Al grains formed in the shot sleeve exceed the limit of spherical growth and therefore exhibit a dendritic morphology.     

The influence of alloy composition on precipitates of the Al-Mg-Si system

To study how changes in solute elements affect precipitation, six Al-Mg-Si alloys aged at 175 °C were investigated by transmission electron microscopy (TEM). In alloys with 1.3 at. pct solute, when the Si/Mg ratio exceeds 5/6, a sharp hardness peak appears after 3 hours that correlates with a high density of fine Guinier-Preston (GP) zones. A second, broader peak correlates with β″ precipitates and U phases. With high Si/Mg ratios, GP zones survive for long aging times. The β″-Mg₅Si₆ phase becomes very stable in the alloy with its Si/Mg ratio closest to 6/5. Deviation from this ratio increases fractions of β′, U-phases and disordered precipitates. In Mg-rich alloys less GP zones form and the first peak is suppressed. A coarse precipitate microstructure of β″ and β′ develops, the volume fraction being much higher than in Si-rich alloys. The Mg-rich alloys overage faster. Reducing the content of solutes causes alloys with high Si/Mg ratios to have a more Mg-rich behavior.