Primary Al3 Sc Research Articles

Determination of homogenizing conditions and aging behaviors of cast 62Be-37.6Al-0.4Sc alloy

In this work, the effects of the homogenizing and aging conditions on the microstructural evolution of Al3Sc precipitates and the mechanical properties of cast 62Be-37.6Al-0.4Sc alloy were investigated. Stereoscopic observations by selective phase leaching indicated that the dendritic Be matrix remained continuous with weakened secondary dendrite arms compared to those of other cast Be-Al alloys. The activation energy of diffusion Q was calculated to be 160.3 kJ/mol for Sc in the cast alloy though resistivity measurements, which indicated that the diffusion of Sc in the Be matrix could be neglected below 640 °C. The homogenization kinetics curves of the cast alloy were acquired, and the homogenizing conditions were optimized to be 620 °C for 2 h followed by water-quenching. The temporal evolution of the hardness and thermal analysis indicated that Al3Sc started precipitating at approximately 250 °C and underwent a coarsening stage when heated at temperatures greater than 300 °C for times longer than 1 h during isochronal aging. The Vickers hardness during isothermal aging revealed four distinct stages in the aging response of the cast alloys, and the aging treatment at 400 °C for 2 h followed by water-quenching was sufficient for considerable precipitation of Al3Sc in the as-homogenized alloys. The precipitating behaviors of Al3Sc during the aging processes were qualitatively obtained. The coarsening process of the Al3Sc precipitates due to the overall increase in the lattice mismatch between Al3Sc and Al was illustrated. Notably, it was observed for the first time that secondary Al3Sc phases with morphologies controlled by the elastic free energy grew preferentially along certain directions into elongated particles from the larger spherical primary Al3Sc particles in the Al matrix. Thermal analysis also revealed that the heating processes influenced the enthalpy needed by the Al matrix during melting and cooling processes owing to the effects of Al3Sc particle density and supersaturated solid solution of Sc.

Effects of Sc and Zr on microstructure and properties of 1420 aluminum alloy

Effects of Sc and Zr on microstructure and properties of 1420 aluminum alloy were investigated. With the additions of Sc and Zr, the coarse primary Al3(Sc,Zr) particles formed at the solidification process were difficult to be dissolved into the matrix during homogenization. The precipitations of Al3Li and Al3Li/Al3(Sc,Zr) occurred during aging process. The growth rates of Al3Li and δ′-PFZs were retarded with the additions of Sc and Zr. The mechanical properties were significantly improved with the additions of Sc and Zr. The mechanical properties (UTS = 300 MPa, YS = 211 MPa and EL = 6.8%) were obtained in 1420-Sc-Zr alloy aged at 190 °C for 16 h. Moreover, the thermal diffusivity and corrosion resistance were significantly affected with the additions of Sc and Zr. With the additions of Sc and Zr, the thermal diffusivity was efficiently increased by 28% during aging process, however, the corrosion resistance decreased.

Effect of Sc and Zr on Al<sub>6</sub>(Mn,Fe) Phase in Al–Mg–Mn Alloys

The effects of Sc and Zr on the shape, size, and number density of Al6(Mn,Fe) phase in Al–5Mg–0.7Mn alloys were investigated by this article. The results showed that Sc and Zr can form strengthening particles of Al3Sc and Al3(Sc,Zr), which have the effect of refining grains and modifying the morphology and distribution of Al6(Mn,Fe) phase. The OM, SEM, and TEM images indicate that with the formation of primary Al3Sc and Al3(Sc,Zr), the shape of Al6(Mn,Fe) became more regular, size of Al6(Mn,Fe) became smaller, and the number density of Al6(Mn,Fe) increased when compared with base alloy. Zr could motivate the precipitation of primary Al3(Sc,Zr) and Al–5Mg–0.7Mn–0.2Sc–0.2Zr had better refining effect than Al–5Mg–0.7Mn–0.4Sc. The grain refinement and finely dispersed Al6(Mn,Fe) phase can improve the mechanical properties such as yield strength and ultimate tensile strength and the corrosion resistance by the results of intergranular corrosion (IGC) also improved.

Segregation behaviors of Sc and unique primary Al3Sc in Al-Sc alloys prepared by molten salt electrolysis

This work mainly deals with the segregating behaviors of Sc and the growth of unique primary Al3Sc in Al-Sc alloys prepared by molten salt electrolysis. The alloys contain 0.23–1.38 wt%Sc where Sc segregation is observed. It is found that a high current density and long electrolysis time are in favor of high Sc content, and so do the high temperature and the addition level of Sc2O3. Sc content at the edge of Al based alloy (average Sc content: 0.75 wt%) can be as high as 1.09 wt%, while it is merely 0.24 wt% at the central area. The cooling rates have a strong impact on the morphology and particle size of primary Al3Sc, but a weak influence on Sc segregation. The cusped cubic and dendritic primary Al3Sc can precipitate in the prepared Al-Sc alloys. In a slightly hypereutectic Al-0.67 wt%Sc alloy, a large and cusped dendrite grows from the edge into the center. The primary and secondary dendritic arms can be as long as 600 and 250 μm, respectively. The Sc segregating behaviors in Al-Sc alloys is due to the mechanism controlled by the limited diffusion rate of Sc in liquid Al. This can involve the establishment of a near spherical discharge interface between liquid Al and the electrolyte. The Sc rich layer near Al-molten salt interface may provide the potential primary nuclei and sufficient Sc atoms for the growth of large dendritic primary Al3Sc.

Ambient-temperature mechanical properties of isochronally aged 1420-Sc-Zr aluminum alloy

Ambient-temperature mechanical properties of isochronally aged 1420-Sc-Zr aluminum alloy were investigated. A significant grain refinement is caused with the additions of Sc and Zr. No primary Al3Sc or Al3Zr is found in as-cast 1420-Sc-Zr alloy. However, primary Al3(Sc,Zr) particles exist in as-cast 1420-Sc-Zr alloy. During the isochronal aging process, these particles of Al3Li, Al3Sc and Al3Zr begin to precipitate at 100 °C, 250 °C and 450 °C, respectively. These precipitation particles make 1420-Sc-Zr alloy obtain three peak microhardness: 845 MPa at 150 °C, 1000 MPa at 350 °C, and 985 MPa at 500 °C. The predicted strengthening increments are contrasted to the measured strengthening increment to confirm the operative precipitation mechanism at different isochronal aging. The shearing mechanism is operating in 1420 and 1420-Sc-Zr alloys aged isochronally to 150 °C. A mixed mechanism (the shearing and bypass mechanisms) is predicted to be operative in 1420-Sc-Zr alloy aged isochronally to 350 °C. The Orowan strenthening mechanism is operative in 1420-Sc-Zr alloy aged isochronally to 500 °C. During isothermal aging at 475 °C after aged isochronally to 450 °C, 1420-Sc-Zr alloy exhibits excellent thermal stability.

Mechanical properties and thermal stability of 7055 Al alloy by minor Sc addition

This study investigated the effect of 0.25 wt% Sc addition on the microstructure and mechanical properties of AA 7055 alloy. The addition of Sc obviously refined the grains of AA 7055 alloy during casting, homogenizing, rolling, solution, and aging treatments due to the formation of primary and precipitate Al3(Sc,Zr) phase. The recrystallization and precipitation of AA 7055 alloy were inhibited during heat treatments by Sc addition. The Sc-containing AA 7055 alloy exhibited higher thermal stability than AA 7055 alloy during homogenizing treatment, because of the grain boundary pinning effect of nano-sized Al3(Sc,Zr) particles. Given its structure characteristics such as fine grains, fine η′ phase, and less η phase, AA 7055 alloy with added Sc showed good mechanical properties after aging at 120 °C for 24 h, with an ultimate tensile strength (UTS) of 679 MPa and elongation (EL) of 14%. This work provides an effective strategy to fabricate Al–Zn–Mg(–Cu) series (7xxx) alloys with excellent mechanical properties.

Effects of synergetic ultrasound on the Sc yield and primary Al3Sc in the Al-Sc alloy prepared by the molten salts electrolysis

In this work, ultrasound has been employed to help prepare Al-Sc alloys using the molten salt electrolysis. The effects of synergetic ultrasound on the Sc content and primary Al3Sc particles of Al-Sc alloys are specifically investigated. Ultrasound can strongly promote the Sc content of the electrolytic Al-Sc alloy and transform the large Al3Sc phase clusters into fine cubic particles. Meanwhile, it also homogenizes the distribution of Al3Sc phase. The Sc content increases by 74.4% up to 1.43 wt% when applying ultrasound during both electrolysis and solidification process. It also transforms the molten salts/liquid Al interface by enhancing the wettability. The huge primary Al3Sc phase clusters transform into fine cubic particles by ultrasound. The average particle size reduces from 77 ± 36 μm down to 21 ± 8 μm. The concerned mechanism have been discussed in detail.

Microstructure and Mechanical Properties of Al-Mg-Sc-Zr Alloy Variable Polarity Plasma Arc Welding Joint

Al-Mg-Sc-Zr alloy plates were welded by variable polarity plasma arc welding (VPPAW) with Sc-containing filler wire. The microstructure and mechanical properties were investigated. Results show that the weld zone and heat-affected zone (HAZ) are narrow due to the high concentrated heat input of VPPAW. Primary Al3(Sc,Zr) particles are nice nucleating sites for α-Al, which can significantly refine the grains of α-Al, helping the weld zone maintain an outstanding strength. The unmelted Al3(Sc,Zr) precipitates of the base metal aggregated near the fusion boundary result in the formation of a fine equiaxed grains zone, which improves the bonding degree between the weld zone and the base metal. The Al3(Sc,Zr) precipitates in the HAZ have excellent inhibition effect on recrystallization. According to the observed microhardness distribution and microstructure characteristics, the microstructure model of welded joint is obtained. The fractured position of welded joint locates in the weld zone. The tensile strength of the welded joint without reinforcing rib is 330 MPa. The welding coefficient reaches 74%.

Influence of Sc content on the microstructure and mechanical properties of cast Al-2Li-2Cu-0.5Mg-0.2Zr alloy

In this work, the influence of Sc content on the microstructure and mechanical properties of cast Al-2Li-2Cu-0.5Mg-0.2Zr alloy was investigated. It was found that the grain size of As-cast alloy was significantly reduced in steps with the increase of Sc addition, accompanied by the arising and increasing of cubic primary Al3(Sc, Zr) phases. No evidence for the presence of the ternary W (Al8−xCu4+xSc) phase was obtained. The volume fraction of nanoscale Al3(Sc, Zr) dispersoids was improved with the increase of Sc content, and the presence of Sc was observed to refine the S′ (Al2CuMg) morphology during ageing. The addition of Sc lowered the growth rates of δ′ (Al3Li) particles and δ′-Precipitate-free zones (δ′-PFZs) of the base alloy. The tensile property results showed that, with the increasing Sc content and ageing times, the yield strength (YS) and ultimate tensile strength (UTS) were continuously increased, and the highest elongation of 6.0% was obtained in the 0.2Sc-containing alloy aged for 32 h at 175 °C. The results indicated that the optimal Sc addition in cast Al-2Li-2Cu-0.5Mg-0.2Zr alloy was 0.2 wt%.

Microstructures and properties evolution of spray-deposited Al-Zn-Mg-Cu-Zr alloys with scandium addition

In this work, the effect of Sc content on the microstructures and mechanical properties of Al-Zn-Mg-Cu-Zr alloys prepared using spray deposition was studied. Microstructure observations reveal that not only the grain size, but also the coarse secondary phase can be refined by Sc addition through the formation of a hybrid structure coupled by η-MgZn2 and Al3(Sc, Zr) phases near and at the grain boundaries. It is inferred that the primary Al3(Sc, Zr) particles can act as nucleation sites for the η-MgZn2 phase during the deposition process. With the increase of Sc content, the continuous secondary phase at the grain boundaries converts gradually into small discontinuous particles, which contributes to the considerable improvement in tensile strength and elongation. The alloy with 0.2 wt% Sc exhibits excellent combinations of ultimate tensile strength of 831±3 MPa and elongation of 10.2 ± 0.4% after T6 heat treatment, and the elongation increases by almost 50% compared to the Sc-free base alloy.

Synergetic effects of Sc and Zr microalloying and heat treatment on mechanical properties and exfoliation corrosion behavior of Al-Mg-Mn alloys

Mechanical properties, exfoliation corrosion behavior and microstructure of Al-5.98Mg-0.47Mn and Al-6.01Mg-0.45Mn-0.25Sc-0.10Zr (wt%) alloy sheets under various homogenizing and annealing processes were investigated comparatively by tensile tests, electrochemical measurements, X-ray diffraction technique and microscopy methods. The as-cast alloys mainly consist of Fe and Mn enriched impurity phases, Mg and Mn enriched non-equilibrium aluminides and Mg3Al2 phases. During homogenization treatment, solvable intermetallics firstly precipitate and then dissolve into matrix. The optimized homogenization processes for removing micro-segregation and obtaining maximum precipitation strengthening of secondary Al3(Sc, Zr) particles are 440°C×8h and 300°C×8h, respectively. Sc and Zr additions can make the yield strength of Al-Mg-Mn alloy increase by 21MPa (6.9%), 120MPa (61.2%) and 127MPa (68.3%), when annealed at 270°C, 300°C and 330°C, respectively, indicating that Orowan precipitation strengthening caused by secondary Al3(Sc, Zr) nano-particles is much greater than grain boundary strengthening from primary Al3(Sc, Zr) micro-particles. Increasing homogenization and annealing degrees and adding Sc and Zr all can decrease corrosion current density and improve exfoliation corrosion resistance. The exfoliation corrosion behavior is dominant by anodic dissolution occurring at the interface between intermetallics and ɑ(Al) matrix. After homogenizing at 440°C for 8h and annealing at 300°C for 1h, yield strength, ultimate strength, elongation to failure and exfoliation corrosion rank are 196MPa, 360MPa, 20.2% and PA (slight pitting corrosion) in Al-Mg-Mn alloy, and reach to 316MPa, 440MPa, 17.0% and PA in Al-Mg-Mn-Sc-Zr alloy, respectively, revealing that high strength, high ductility and admirable corrosion resistance of Al-Mg-Mn alloys can be achieved by the synergetic effects of Sc and Zr microalloying and heat treatment.

Impurity effects on the solidification of primary Al3(Sc,Zr) phase in Al alloys

The nucleation and growth of primary Al3Sc phase has been investigated in high purity Al alloys and commercial purity Al alloys, with a special focus on the impurity effects. In the case of high purity Al alloys, most primary Al3Sc phases were pushed to grain boundaries during moving solidification front. Such type of primary Al3Sc phase does not contribute to the heterogeneous nucleation and thereby no significant grain refinement of Al alloys was observed, although some Al3Sc particles remain the same orientation with the Al matrix. In the case of commercial purity Al alloy, the presence of impurities, e.g. Ti, Fe and Si, enhances the heterogeneous nucleation of primary Al3Sc phase. Most primary Al3Sc phases are located within the Al matrix, and keep the same orientation with the Al matrix. Furthermore, the presence of impurities also changes the growth mode of primary Al3Sc phase. In the case of commercial purity Al alloy, a layer by layer growth was observed. This investigation demonstrates that impurities have important effects on the nucleation and growth of primary Al3Sc phases in Al based alloys.

Evolution of microstructure and properties of Al–Zn–Mg–Cu–Sc–Zr alloy during aging treatment

As one of the most efficient trace element to improve the mechanical properties of alloys, scandium (Sc) has attracted much attention for aluminum alloys in recent years. In this study, a new kind of Al–Zn–Mg–Cu–Sc–Zr alloy has been designed, and the evolution of microstructure and properties during the processes including homogenization, hot extrusion, solid solution and aging has been investigated. The alloy aged at 120 °C reveals a dual-peak aging phenomenon which is attributed to the overlap of the formation of Guinier–Preston (GP) zones and η′ phases. Consequently, excellent mechanical properties can be obtained (UTS = 747 MPa, YS = 721 MPa and EI = 10.9%) when the alloy is aged at 120 °C for 15 h. The improvement of the mechanical performance is mainly attributed to the grain refinement by the primary Al3(Sc, Zr) and the dispersion strengthening of the secondary Al3(Sc, Zr). Thus, the combined additions of Sc and Zr play an important role for the strengthening of the alloy.

Microstructural evolution in Al-Zn-Mg-Cu-Sc-Zr alloys during short-time homogenization

Microstructural evolution in a new kind of aluminum (Al) alloy with the chemical composition of Al-8.82Zn-2.08Mg-0.80Cu-0.31Sc-0.3Zr was investigated. It is found that the secondary phase MgZn2 is completely dissolved into the matrix during a short homogenization treatment (470°C, 1 h), while the primary phase Al3(Sc,Zr) remains stable. This is due to Sc and Zr additions into the Al alloy, high Zn/Mg mass ratio, and low Cu content. The experimental findings fit well with the results calculated by the homogenization diffusion kinetics equation. The alloy shows an excellent mechanical performance after the short homogenization process followed by hot-extrusion and T6 treatment. Consequently, a good combination of low energy consumption and favorable mechanical properties is obtained.

Correlative characterization of primary Al3(Sc,Zr) phase in an Al–Zn–Mg based alloy

Three-dimensional electron backscatter diffraction, focused ion beam, transmission electron microscopy and energy filtered transmission electron microscopy were employed to investigate the structural information of primary Al3(Sc,Zr) phase, i.e. size, shape, element distribution and orientation relationship with the α-Al matrix. It was found that (i) most primary Al3(Sc,Zr) phases have a cubic three-dimensional morphology, with a size of about 6–10μm, (ii) most primary Al3(Sc,Zr) phases are located within the α-Al matrix, and exhibit a cube to cube orientation relationship with the α-Al matrix, and (iii) a layer by layer growth was observed within primary Al3(Sc,Zr) phases. Al, Cu, Si and Fe are enriched in the α-Al matrix between the layers of cellular eutectic Al3(Sc,Zr) phase, while Sc, Ti and Zr are enriched in small Al3(Sc,Zr) phases. A peritectic reaction and subsequent eutectic reaction between Al3Sc and Al was proposed to interpret the observed layer by layer growth. This paper demonstrates that the presence of impurities (Fe, Si, Cu, Ti) in the diffusion field surrounding the growing Al3(Sc,Zr) particle enhances the heterogeneous nucleation of Al3(Sc,Zr) phases.

Effect of minor Sc and Zr addition on microstructure and properties of ultra-high strength aluminum alloy

The Al–9Zn–2.8Mg–2.5Cu–xZr–ySc alloys (x=0, 0.15%, 0.15%; y=0, 0.05%, 0.15%), produced by low-frequent electromagnetic casting technology, were subjected to homogenization treatment, hot extrusion, solution and aging treatment. The effects of minor Sc and Zr addition on microstructure, recrystallization and properties of alloys were studied by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that Sc and Zr addition can refine grains of the as-cast alloy by precipitation of primary Al3(Sc,Zr) particles formed during solidification as heterogeneous nuclei. Secondary Al3(Sc,Zr) precipitates formed during homogenization treatment strongly pin the movement of dislocation and subgrain boundaries, which can effectively inhibit the alloys recrystallization. Compared with the alloy without Sc and Zr addition, the Al–Zn–Mg–Cu–Zr alloy with 0.05% Sc and 0.15% Zr shows the increase in tensile strength and yield strength by 172 MPa and 218 MPa, respectively. Strengthening comes from the contributions of precipitation, substructure and grain refining.

Impurity effects on the nucleation and growth of primary Al3(Sc,Zr) phase in Al alloys

Impurity effects on the nucleation and growth of primary Al3(Sc,Zr) phase have been investigated in high purity Al alloys and commercial purity Al alloys, respectively. In the case of high purity Al alloys, primary Al3(Sc,Zr) phases were found to be pushed to grain boundaries ahead of the solidification front. Such type of primary Al3(Sc,Zr) phase did not contribute to the heterogeneous nucleation, and thereby the grain refinement of Al alloys. In the case of commercial purity Al alloys, the presence of Fe, Si, Cu, Mg, Ti, and other impurities significantly enhanced the heterogeneous nucleation of primary Al3(Sc,Zr) phase. Most primary Al3(Sc,Zr) phases were found to be located within the α-Al matrix, and kept an identical orientation relationship with the α-Al matrix. Furthermore, the presence of the impurities also changed the growth mode on the primary Al3(Sc,Zr) phase. In the case of commercial purity Al alloys, a peritectic to eutectic reaction was induced due to the presence of the impurities. A layered growth was observed leading to a narrow particle size distribution. In contrast, in the case of high purity Al alloys, a featureless structure was observed. This investigation demonstrates that impurities and their concentrations are important factors affecting the nucleation and growth of primary Al3(Sc,Zr) phases, and thereby for the successful grain refinement in Al-based alloys.

Investigation of primary Al3(Sc,Zr) particles in Al–Sc–Zr alloys

This paper studied the primary Al3(Sc,Zr) particles formed during solidification in three Al–Sc–Zr alloys with various Zr contents. It has been shown that the primary Al3(Sc,Zr) particles formed during solidification since the Sc and Zr concentrations are above the solid solubility limit in Al matrix. Scanning electron microscopy line scanning results indicated that the distributions of Sc, Zr and Al atoms in the primary Al3(Sc,Zr) particles differ a lot from those in the secondary Al3(Sc,Zr) particles formed during annealing. In the primary Al3(Sc,Zr) particles, both Sc and Zr contents are found from rim to the centre of the particles, with an increasing trend from the rim to the centre, while the Al content drops sharply on the rim of the particle and slightly from the rim to the centre.

High resolution cone beam X-ray computed tomography of 3D-microstructures of cast Al-alloys

X-ray computed tomography (XCT) has become a very important method for non-destructive 3D-characterisation of materials. XCT systems with cone beam geometry, micro- or nano-focus tubes and matrix detectors are increasingly used in research and non-destructive testing. Spatial resolutions down to 1μm can be reached with such XCT-systems for heterogeneities in metals with high absorption contrast. High resolution cone beam XCT is applied to five different Al-alloys: AlMg5Si7, AlCu4Mg1, AlZn6Mg2Cu2, AlZn8Mg2Cu2 and AlSi12Ni1. Up to four different types of inhomogeneities are segmented in one alloy using voxel sizes between (0.4μm)3 and (2.3μm)3. Target metallography and elemental analysis by energy dispersive X-ray analysis are used to identify the inhomogeneities. The possibilities and restrictions of XCT applied to Al-alloys are discussed.AlMg5Si7 XCT-data with a voxel size of (0.4μm)3 show inhomogeneities with brighter grey-values than the Al-matrix identified as elongated Fe-aluminides, and those with lower grey-values identified as pores and Mg2Si-particles with a “Chinese script-like” structure. Higher-absorbing interdendritic Al–Al2Cu-eutectic regions appear brighter than the Al-dendrites in the CT-data of AlCu4Mg1 with (1.1μm)³/voxel, whereas pores>4μm appear darker than the Al-matrix. The size and the 3D-structure of the α-Al dendrite arms with a diameter of 50–100μm are determined in samples from chill cast billets of AlCu4Mg1 and AlZn6Mg2Cu2 alloys. The irregular interdendritic regions containing eutectic segregations with Cu- and Zn-rich phases are >5μm wide. Equally absorbing primary equi-axed Al3(Sc, Zr) particles >5μm are distinguished in the centres of the dendrites by the level of sphericity values. The distribution of Ni- and Fe-aluminides in a squeeze cast AlSi12Ni1-alloy is imaged with (0.4μm)³/voxel, but the Si-phase cannot be segmented.

Microstructural Characteristics of Electron Beam Processed Al-2Sc

Plate samples of Al-2Sc were processed with an electron beam (EB) gun in a vacuum of 10-3 Pa at 50kV and 30 µA. EB processing suppressed the formation of primary Al3Sc, altered the morphology of the eutectic (Al)-Al3Sc, and increased the solubility of Sc in the eutectic (Al). In addition, its cyclic heating effect induced the precipitation of Al3Sc. EB processing resulted in more profound hardening than conventional high temperature solid solution and ageing.