Binary Al Sc Alloys Research Articles

Highly interdependent dual precipitation and its effect on mechanical properties of Al–Cu-Sc alloys

Dual precipitation in a microalloyed Al-2.4 wt% Cu-0.1 wt% Sc alloy was investigated in comparison with single precipitation in binary Al-2.4 wt% Cu and Al-0.1 wt% Sc alloys, respectively. Although Al3Sc nanoparticles are more readily produced at higher aging temperatures in the binary Al–Sc alloys, the Al3Sc precipitation in the Cu-added ternary alloy was completely suppressed at 723 K while unexpectedly promoted at 573 K-aging. Meanwhile, a significant Sc-dependence of θ′ precipitation was also evidenced in the 573 K-aged Al–Cu-Sc alloy that the thickness of θ′ precipitates kept almost unchanged even to prolonged time, resulting in a high aspect ratio of >200 for the θ′ precipitates. The complex precipitation behaviors are rationalized in terms of strong interactions between the Sc and Cu solutes, which could be tailored to optimize the Al3Sc + θ′ dual precipitation and hence to improve mechanical properties. Moreover, the Sc atoms fully stabilized in Al solid solution by Cu solute at 723 K was discussed, and the high strength in Al–Cu-Sc alloy with dual precipitation was evaluated by quantifying the strengthen contribution from different strengthening mechanism.

Isothermal aging of Al-Ni-Sc alloy containing Al 3 Ni microfibers and Al 3 Sc nanoprecipitates

Binary Al-Sc alloys have excellent strength at room and elevated temperature up to 300°C due to coherent of Al 3 Sc nanoprecipitates. Binary Al-Ni alloys are an alternative alloy for high-temperature applications owing to high-thermal stability of Al 3 Ni microfibers. In this study, the hardness evolution at 300°C for up to 672 h for eutectic ternary Al-6Ni-0.4Sc (wt%) was studied and compared with binary Al-0.4Sc and Al-6Ni alloys. The Al-6Ni alloy maintains a constant hardness after aging at 300°C up to 672 h. Both Al-0.4Sc and Al-6Ni-0.4Sc alloys show strong precipitation strengthening response, with peak hardness reached after about 3 h aging, due to Al 3 Sc precipitates. These precipitates were studied in Al-6Ni-0.4Sc via local electrode atom probe tomography and contain small amounts of Ni (0.04 at%) which do not affect the kinetics of precipitation. The microhardness evolution for Al-6Ni-0.4Sc can be explained through superposition of binary Al-0.4Sc and Al-6Ni alloys, indicating that both Al 3 Sc nanoprecipitates and Al 3 Ni microfibers contribute to strength.

Assessing the corrosion resistance of binary Al–Sc alloys in chloride‐containing environment

The corrosion behaviour of binary Al–Sc alloys (Al‐0.3Sc and Al‐1Sc, wt%) is investigated in de‐aerated 0.6 M NaCl solution, performing potentiodynamic polarisation, potentiostatic transients and electrochemical impedance spectroscopy (EIS) measurements complemented with scanning electron microscopy (SEM). In the presence of chloride ions, both Al–Sc alloys experience localised corrosion (pitting). However, increasing Sc alloying addition leads to an increase of the breakdown potential and the extent of passivation domain, indicating improved resistance to initiation of pitting corrosion. Furthermore, EIS measurements performed at the breakdown state evidenced a higher Rct and a lower depression angle value for the Al‐1Sc alloy compared to the Al‐0.3Sc one, confirming its better localised corrosion behaviour. Spatial distribution and volume fraction of the Al3Sc secondary phase are microstructural features which play a very important role in determining the corrosion resistance of the Al‐1Sc alloy.

A mathematical model coupled to CALPHAD to predict precipitation kinetics for multicomponent aluminum alloys

A mathematical model was developed to simulate the precipitation kinetics during heat treatment of multicomponent aluminum alloys. The model is based on the general numerical framework proposed by Kampmann and Wagner, and features a full coupling with CALPHAD software for the evaluation of the Gibbs–Thomson effect. It also does not rely on the assumption that precipitate phase composition is stoichiometric or uniform, and is therefore applicable for predicting complex precipitation kinetics encountered in industrial practices. Applications of the model to various aging treatments of binary Al–Sc alloys and a ternary Al–Sc–Zr alloy were conducted. It was found that the model predictions for extended time coarsening kinetics are in good agreement with the analytical Lifshitz–Slyozov–Wagner coarsening theory. Its ability to reproduce the complex precipitation pathways in multicomponent alloys was demonstrated by simulation of the precipitation kinetics for an Al–0.09at.% Sc–0.03at.% Zr alloy. Comparison of the simulation results with experimental measurement has also highlighted research directions that require further effort.

Effects of Yb and Zr microalloying additions on the microstructure and mechanical properties of dilute Al–Sc alloys

It is known that Zr and Yb partition to the Al3Sc precipitates created during aging when microalloyed separately in dilute binary Al–Sc alloys. Addition of Zr delays precipitate coarsening, thereby improving the coarsening resistance of the ternary Al-Sc-Zr alloys. Addition of Yb increases the resistance against dislocation climb, thereby improving the creep resistances of the ternary Al-Sc-Yb alloys. A combination of microhardness, creep, and atom probe tomography measurements provide evidence that these effects of Zr and Yb additions are cumulative in quaternary dilute Al–Sc–Yb–Zr alloys: Yb increases their creep resistance at 300°C compared with ternary Al–Sc–Zr alloys and Zr improves their coarsening resistance at 300°C compared with ternary Al–Sc–Yb alloys. Additionally, excellent coarsening resistance is observed at 350 and 375°C.

Study of decomposition of oversaturated solid solution in Al-Sc-Zr alloys at different ratio of scandium and zirconium

Using electrical resistivity and hardness measurement methods and also electron microscopy analysis, the aging processes were studied in the Al-rich alloys of the Al-Sc-Zr system for contents of 0.1–0.3% Sc and 0.04–0.28% Zr. A significant increase in the alloy hardness as compared with the quenched state was already established during aging at a temperature of 350°C after 1h. The character of the change in electrical resistivity and hardness during aging turned out to be similar for all studied alloys. However, the strengthening effect in the ternary Al-Sc-Zr alloys was higher than that in the binary Al-Sc alloys with the same Sc contents. Decomposition of the supersaturated Al solid solution was accompanied by precipitation of nanosize particles of the strengthening phases.

Microstructural evolution and creep properties of precipitation-strengthened Al–0.06Sc–0.02Gd and Al–0.06Sc–0.02Yb (at.%) alloys

The aging behavior at 300 °C of Al–0.06Sc–0.02Gd and Al–0.06Sc–0.02Yb (at.%) alloys is studied by local-electrode atom-probe tomography, transmission electron microscopy and microhardness measurements. The ternary alloys exhibit high number densities of coherent L1 2 precipitates ( N v ≅ 10 22 m - 3 ) at aging times up to 1536 h (64 days). In the Al–0.06Sc–0.02Gd alloy, the Al 3(Sc 1− x Gd x ) precipitates are always Sc-rich, displaying a small Gd concentration ( x < 0.12) in the precipitates. In the Al–0.06Sc–0.02Yb alloy, the precipitates are initially Yb-rich, Al 3(Yb 1− x Sc x ), with Sc diffusing subsequently to the precipitates, resulting in a core/shell structure and an overall Sc-rich composition, Al 3(Sc 1− x Yb x ). Gd and Yb, like other lanthanides but unlike the transition metals Zr and Ti, do not retard the coarsening kinetics compared with binary Al–Sc alloys. Additionally, the creep resistance of these alloys is greater than that of Al–Sc alloys. The coarsening kinetics and creep properties of Al–0.06Sc–0.02Gd and Al–0.06Sc–0.02Yb alloys are compared with other Al–Sc-based alloys and with coarsening models for ternary alloys.

Microstructural investigations on as-cast and annealed Al–Sc and Al–Sc–Zr alloys

Al–Sc and Al–Sc–Zr alloys containing 0.05, 0.1 and 0.5 wt.% Sc and 0.15 wt.% Zr were investigated using optical microscopy, electron microscopy and X-ray diffraction. The phase composition of the alloys and the morphology of precipitates that developed during solidification in the sand casting process and subsequent thermal treatment of the samples were studied. XRD analysis shows that the weight percentage of the Al 3Sc/Al 3(Sc, Zr) precipitates was significantly below 1% in all alloys except for the virgin Al0.5Sc0.15Zr alloy. In this alloy the precipitates were observed as primary dendritic particles. In the binary Al–Sc alloys, ageing at 470 °C for 24 h produced precipitates associated with dislocation networks, whereas the precipitates in the annealed Al–Sc–Zr alloys were free of interfacial dislocations except at the lowest content of Sc. Development of large incoherent precipitates during precipitation heat treatment reduced hardness of all the alloys studied. Growth of the Al 3Sc/Al 3(Sc, Zr) precipitates after heat treatment was less at low Sc content and in the presence of Zr. Increase in hardness was observed after heat treatment at 300 °C in all alloys. There is a small difference in hardness between binary and ternary alloys slow cooled after sand casting.

Effects of Zr on the Microstructure and Mechanical Properties of Al-Sc-Yb Alloys

Cast, coarse-grained Al-Sc alloys are interesting candidates for high temperature, creepresistant applications due to the formation of a high number density of nanometer sized Al3Sc precipitates [1]. Upon addition of Yb to Al-Sc alloys, Yb replaces the Sc forming Al3(Sc1-xYbx) precipitates (L12 structure) [2]. The resulting alloys have higher creep resistance due to the larger lattice parameter mismatch between the matrix and precipitate phases as a result of the substitution of Yb for Sc. The precipitates in Al-Sc-Yb alloys form rapidly at 300 C and the coarsening kinetics are not slowed when compared with binary Al-Sc alloys [3]. Additions of Zr to Al-Sc alloys do, however, retard the coarsening kinetics of the Al3(Sc1-xZrx) precipitates [4]. In the present study, Zr is added to an Al-Sc-Yb alloy. The compositions of the alloys studied are Al-0.06Sc-0.02Yb0.02Zr (at.%) and Al-0.06Sc-0.02Yb-0.005Zr (at.%).

Complex precipitation pathways in multicomponent alloys

One usual way to strengthen a metal is to add alloying elements and to control the size and the density of the precipitates obtained. However, precipitation in multicomponent alloys can take complex pathways depending on the relative diffusivity of solute atoms and on the relative driving forces involved. In Al-Zr-Sc alloys, atomic simulations based on first-principle calculations combined with various complementary experimental approaches working at different scales reveal a strongly inhomogeneous structure of the precipitates: owing to the much faster diffusivity of Sc compared with Zr in the solid solution, and to the absence of Zr and Sc diffusion inside the precipitates, the precipitate core is mostly Sc-rich, whereas the external shell is Zr-rich. This explains previous observations of an enhanced nucleation rate in Al-Zr-Sc alloys compared with binary Al-Sc alloys, along with much higher resistance to Ostwald ripening, two features of the utmost importance in the field of light high-strength materials.

Effect of Mg addition on the creep and yield behavior of an Al–Sc alloy

The relationships between microstructure and strength were studied at room temperature and 300 °C in an Al–2 wt% Mg–0.2 wt% Sc alloy, containing Mg in solid-solution and Al 3Sc (L1 2 structure) as nanosize precipitates. At room temperature, the yield strength is controlled by the superposition of solid-solution and precipitation strengthening. At 300 °C and at large applied stresses, the creep strength, which is characterized by a stress exponent of ~5, is significantly improved compared to binary Al–Sc alloys, and is independent of the size of the Al 3Sc precipitates. At small applied stress, a threshold stress exists, increasing from 9% to 70% of the Orowan stress with increasing Al 3Sc precipitate radius from 2 to 25 nm. An existing model based on a climb-controlled bypass mechanism is in semi-quantitative agreement with the precipitate radius dependence of the threshold stress. The model is, however, only valid for coherent precipitates, and the Al 3Sc precipitates lose coherency for radii larger than 11 nm. For semi-coherent precipitates with radii greater than 15 nm, the threshold stress remains high, most likely because of the presence of interfacial misfit dislocations.

Interaction of recrystallization and precipitation: The effect of Al 3Sc on the recrystallization behaviour of deformed aluminium

The kinetics and mechanisms of recrystallization in high purity binary Al-Sc alloys have been investigated. In an Al-0.25wt.%Sc alloy, precipitation always precedes recrystallization, which is severely inhibited. However, in an Al-0.12wt.%Sc alloy, precipitation is found to either precede, follow or occur concurrently with recrystallization, depending on the processing conditions, and the resultant complex microstructures are interpreted in terms of the interactions between precipitation and recrystallization. Coherent precipitates are found to coarsen and become semi-coherent during the passage of low-angle boundaries during recovery. The passage of high-angle boundaries through semi-coherent precipitates during recrystallization occurs by migration of the boundary through the particle, resulting in the precipitates maintaining semi-coherency with the new grain.

Solidification structure of aluminium with trace additions of Sc and Zr

The effect of hypoeutectic Sc addition, by itself or in combination with 0.14% Zr, on the solidification structure of pure Al was investigated. A strong grain refining effect was observed in the alloy containing 0.45% Sc and 0.14% Zr. No grain refinement occurred in binary Al-Sc alloys or in the ternary alloy containing 0.24% Sc. Detailed microstructural investigations showed the presence of coarse cuboidal particles in the grain refined alloy. These particles were found to be enriched in Sc and Zr, had an L12 crystal structure, and shared a cube-on-cube orientation relationship with the α-Al matrix. Consequently, the Al-Sc-Zr particles were considered to be responsible for the observed grain refinement.

Precipitation of Al 3Sc in binary Al–Sc alloys

The precipitation of coherent Al 3Sc particles in Al–Sc alloys containing 0.06, 0.12 and 0.18 at.% Sc was investigated. The alloys were aged at 350°C for times up to 4663 h and the kinetics of particle growth, the particle size distributions and the evolution of particle morphology were measured and evaluated using transmission electron microscopy. Al 3Sc precipitates did not nucleate homogeneously in the most dilute alloy; this result was unexpected because 0.06 at.% Sc exceeds the solubility limit at 350°C. Persistent dislocation networks were observed in the alloy containing 0.12 at.% Sc under normal solution treatment conditions (e.g. 1 h at 600°C) and the dislocations acted as heterogeneous nucleation sites. The dislocations were ultimately eliminated using a very long solution treatment time of ∼70 h near the melting temperature. Aging of both of the more concentrated alloys produced coherent precipitates. At short aging times the particles in the alloy containing 0.12% Sc were cauliflower-shaped and became spherical at longer times. At 4663 h some of the precipitates in this alloy were cuboidal, while others appear to have become semicoherent. The precipitates in this alloy were highly resistant to coarsening, and their size distributions were for the most part narrower than that predicted by the classical theory of Lifshitz, Slezov and Wagner (the LSW theory). The shapes of the precipitates in the alloy containing 0.18% Sc evolved from spherical to cuboidal with increasing aging time. The kinetics of growth of the precipitates in this alloy were consistent with the predictions of the LSW theory, the average size, 〈 r〉, increasing with aging time, t, according to an equation of the type 〈 r〉 3≃ kt. The experimentally measured rate constant, k, was in very good agreement with that calculated theoretically for this alloy.

The solidification behaviour of dilute aluminium–scandium alloys

The solidification behaviour of dilute Sc containing Al alloys has been investigated. In binary Al–Sc alloys, Sc additions greater than the eutectic composition (0.55wt%) were found to produce a remarkable refinement in the grain size of aluminium castings, of two orders of magnitude, due to the formation of the primary Al3Sc intermetallic phase during solidification. The refinement in grain size only occurred in hypereutectic compositions and was shown to be far greater than can be achieved by conventional Al grain refiners. Grain refinement by the addition of Sc is accompanied by a change in growth morphology from dendritic, in the large unrefined grains, to fine spherical grains with a divorced eutectic appearing on the grain boundaries in the refined castings. Similar levels of refinement were observed in Al–Sc–Zr and Al–Cu–Sc alloys. In the latter, a change in the segregation behaviour of Cu was observed, from a strongly interdendritic segregation pattern to a more homogeneous distribution. The supersaturated Al–Sc solid solution can decompose via a discontinuous precipitation reaction to form coherent rod-like precipitates of the L12 Al3Sc phase.