PhD Student, Monash University
Development of a high strength, corrosion resistant Al-Si-Mg-Sc alloy
On 4th October 1992, Boeing 747 aircraft crashed on the ground in Amsterdam killing all the 4 people on board and over 50 people in the building. This event is being considered as one of the engineering disasters in the aircraft industry. The investigation report indicated that corrosion was the root cause for this crash. Even today, corrosion is the second largest failure mode for aircrafts; fatigue being the first! Aluminium alloys are the primary material in aircraft. These alloys are alloyed with either Cu or Zn which results in very high strength. However, corrosion is the major problem for these alloys. My aim is to develop a new alloy which will have strength comparable to these conventional aerospace alloys and a significantly lower corrosion rate. To achieve this, an aluminium alloy which is already rust-free is selected. Now, I need some magic element which will enhance strength but will not have any effect on corrosion. Scandium is one such element which can be added to aluminium like a pinch of salt. The only difference is that the cost of Scandium is nowhere similar to salt, in fact, it is super-expensive and should be compared with saffron! Having said that, a very small addition of Sc of the order of just 0.3 wt. % has proven to make a dramatic effect on strength. However, the story doesn't end here and there is a need of another element to make Sc addition successful. Zirconium can help here. Scandium reacts with aluminium and forms the spherical particles. Then, Zr reacts with these particles and form a stable shell around the Sc core. The final core-shell structure looks like a macadamia nut. This structure is responsible for increased strength. The challenge is to get very stable, nano-sized and well-distributed particles in the aluminium. I am confident that I will produce a novel aluminium alloy which will avoid any further disastrous aircraft crashes.
Abstract: The nano-structure evolution of secondary Al3(Sc1−xZrx) particles during high-strain-rate (0.01 s−1) superplastic deformation at 500 °C was investigated by high resolution transmission electron microscopy. The results show that by increasing the true stains from 0.69 to 2.40, the mean radii of spheroidal secondary Al3(Sc1−xZrx) particles increase from 9.8 ± 3.4 nm to 16.4 ± 6.8 nm, and the lattice misfit value increases from 1.04% to 1.30%. Before deformation, Al3(Sc1−xZrx) nano-particles are completely coherent with Al(α) matrix. As the deformation proceeds, the accumulated sever plastic deformation introduces misfit dislocations at the interface between particles and matrix. Superplastic flow deformation data indicate that with the increase of true strains, the strain rate sensitivity of Al-Zn-Mg alloy decreases from 0.29 to 0.19, and the deformation activation energy increases from 112 to 121 kJ/mol. However, for Al-Zn-Mg alloy with secondary Al3(Sc1−xZrx) nano-particles, the strain rate sensitivity increase from 0.33 to 0.45, and the deformation activation energy decreases from 107 to 84 kJ/mol. Based on the microstructural results and the established constitutive equations at different strains, it can be concluded that at the working hardening stage, two kinds of alloys are both controlled by dislocation viscous glide creep mechanism. At the dynamic softening stage, dislocation creep mechanism operates in Al-Zn-Mg alloy, whereas grain boundary sliding mechanism is dominant in Al-Zn-Mg-Sc-Zr alloy. Secondary Al3(Sc1−xZrx) nano-particles play an important role in accelerating the cooperative grain boundary deformation and only affect dynamic softening deformation mechanism of Al-Zn-Mg alloys.
Pub.: 12 Oct '16, Pinned: 11 Feb '18
Abstract: The precipitation behavior, mechanical properties and corrosion resistance of a novel Al—Zn—Mg—Sc—Zr alloy aged at different time were studied by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), tensile tests, potentiodynamic polarization and electrochemical impedance spectroscopy. The results revealed that with increasing aging time at 120 °C, the hardness and tensile strength of the alloy increased rapidly at first and then slightly decreased. The resistance of exfoliation corrosion (EXCO) and intergranular corrosion (IGC) increased gradually with increasing aging time. The same trend of corrosion properties was demonstrated by electrochemical polarization curves and EIS test. The characteristics of grain boundary precipitates and precipitate free zone (PFZ) had a significant influence on the mechanical and corrosion behaviors of the studied alloy. On the basis of TEM observations, the size of grain boundary precipitates and the width of PFZ became larger, and the distributed spacing of grain boundary precipitates was enhanced with increasing aging time.
Pub.: 07 Oct '16, Pinned: 11 Feb '18
Abstract: Precipitation strengthening behavior during aging of an Al-0.014Sc-0.008Er-0.08Zr-0.10Si (at.%) alloy was investigated utilizing microhardness, electrical conductivity and scanning electron microscopy. This new composition, with a Sc/Zr ratio (in at.%) smaller than 1/5 represents a significant reduction of the alloy's cost, when compared to more usual Al-0.06Sc (at.%) based alloys with typical Sc/Zr ratios of 3. The research presented herein focuses on identifying the optimal homogenization duration at 640 °C and additionally the temperature range at which a single-step aging treatment will achieve the highest possible microhardness in the shortest time. Due to a compromise between dissolution of Er-Si rich primary precipitates, homogenization of the Zr distribution and precipitation of large Al3Zr precipitates, 8 h at 640 °C appears to be the optimal homogenization duration for this alloy, leading to an hardness of 571 ± 22 MPa after aging for 24 h at 400 °C. To study the precipitation behavior of this low-Sc concentration alloy, isochronal aging to 575 °C with two different heating rates was performed. The small Sc concentration, compensated by a high Zr concentration, permits the alloy to achieve a similar peak microhardness during isochronal aging (587 ± 20 MPa) as the corresponding Sc-richer and Zr-leaner alloys. The isochronal aging experiments permits us to identify the best aging temperature as between 350 and 425 °C.
Pub.: 11 Aug '16, Pinned: 11 Feb '18
Abstract: Publication date: Available online 8 January 2017 Source:Journal of Materials Science & Technology Author(s): Fangfang Sun, Guiru Liu Nash, Qunying Li, Enzuo Liu, Chunnain He, Chunsheng Shi, Naiqin Zhao The effects of adding the alloy element Sc to Al alloys on strengthening, recrystallization and modification of the grain microstructure have been investigated. The combination of Sc and Zr alloying not only produces a remarkable synergistic effect of inhibition of recrystallization and refinement of grain size but also substantially reduce the amount of high-cost additional Sc. In this work, the microstructures and corrosion behavior of a new type of Al-Cu-Mg-Sc-Zr alloy with Sc/Zr ratio of 1/2 were investigated. The experimental results showed that the Sc and Zr additions to Al-Cu-Mg alloy could strongly inhibit recrystallization, refine grain size, impede the segregation of Cu element along the grain boundary and increase the spacing of grain boundary precipitates. In addition, adding Sc and Zr to Al-Cu-Mg alloy effectively restricts the corrosion mechanism conversion associated with Al2CuMg particles, which resulted in the change of the cross-section morphology of inter-granular corrosion from an undercutting to an elliptical shape. The susceptibility to inter-granular corrosion was significantly decreased with increasing Sc and Zr additions to the Al-Cu-Mg alloy. The relationships between microstructures evolution and inter-granular corrosion mechanism of Al-Cu-Mg-Sc-Zr alloys were also discussed. Graphical abstract
Pub.: 15 Jan '17, Pinned: 11 Feb '18
Abstract: The effects of homogenization treatments on the microstructure evolution, microhardness and electrical conductivity of two dilute Al-Sc-Zr-Er alloys with different Zr contents were investigated. In both as-cast alloy, Zr and Sc are respectively concentrated at the dendrite interiors and the interdendritic regions, and Er is segregated at the primary Al3Er phases within grain and discontinuous intergranular phases with traces of Sc and Fe. The severe microsegregation of Zr induces the formation of the coarse Zr-rich precipitates in the high Zr alloy during homogenization. The composition of the coarse phase is analyzed by EPMA and STEM-HAADF and a corresponding model of its formation mechanism is proposed. The optimal homogenization duration of the both studied alloys depends on the segregation degree of Zr in the dendritic cells. For the high Zr alloy, the homogenization for 2 h at 640 °C eliminates the primary Al3Er phases and the intergranular Er/Sc-rich phases without the coarse Al3(Er,Sc,Zr) phases. For the low high Zr alloy, 24 h of homogenization at 640 °C eliminates successfully the microsegregation of Zr, Sc and Er, and induces the maximum aging strengthening increment.
Pub.: 07 Feb '17, Pinned: 11 Feb '18
Abstract: Using published experimental phase equilibria, a self-consistent thermodynamic database of the Al-Si-Mg-Sc quaternary system in Al-rich corner was established by application of the CALPHAD (CALculation of PHAse Diagram) technique. The reliability of the thermodynamic database was tested in two quaternary model cast alloys by comprehensive comparison of their experimentally measured solidified microstructure characteristics and phase transition temperatures with the calculated ones. Scheil-Gulliver simulations were performed, allowing construction of a solidification diagram for Sc-added A356 alloys to examine the influence of Sc levels on the solidification behavior of cast A356 alloys. With the aid of theoretical solidified microstructure analysis and its qualitative relationship with mechanical properties, the optimal amount of added Sc in A356 alloys was determined to be 0.54 wt.%. The experimental mechanical property measurements and microstructural characterizations confirmed that the A356-0.54 wt.% Sc alloy exhibits the best comprehensive mechanical performance. Additionally, the grain refining mechanisms in Sc-supplemented A356 alloys were described, suggesting a path for further improvement of the overall mechanical performance.
Pub.: 14 Dec '16, Pinned: 11 Feb '18
Abstract: Ductility and electrical conductivity of metallic materials are inversely correlated with their strength, resulting in a difficulty of optimizing all three simultaneously. We design an Al-Sc-Zr-based alloy using semisolid extrusion to yield a good trade-off between strength and ductility along with excellent electrical conductivity. The Al-0.35Sc-0.2Zr wire with a diameter of 3 mm exhibited the best combined properties: a tensile strength of 210 ± 2 MPa, elongation of 7.6% ± 0.5%, and an electrical conductivity of 34.9 ± 0.05 MS/m. The average particle size of nanosized Al3(Sc, Zr) precipitates increased from 6.5 ± 0.5 nm to 25.0 ± 0.5 nm as the aging time increased from 1 h to 96 h at 380 °C, accompanied by the corresponding volume fraction variation from (6.2 ± 0.1) × 10−4 to (3.7 ± 0.1) × 10−3. As proved by transmission electron microscopy observation, the high strength originates from the effective blockage of dislocation motion by numerous nanosized Al3(Sc, Zr) precipitates whilst both electrical conductivity and ductility remain at a high level due to the coherent precipitates possessing an extremely low electrical resistivity.
Pub.: 16 Jan '17, Pinned: 11 Feb '18
Abstract: Microstructures and formability of scandium and zirconium added Al-Mg-Mn alloy sheets with various heating conditions were examined to improve their mechanical properties. Formability of these samples were judged by the Lankford value, r-value. It was possible to fabricate mechanically balanced Al-Mg-Mn-Sc-Zr alloy with high hardness 76.2 Hv and with high formability with r=1.2, by not only adding scandium and zirconium but also optimizing the heat treatment conditions.
Pub.: 22 Jul '17, Pinned: 11 Feb '18
Abstract: An Al–3% Mg–0.2% Sc alloy was solution treated and processed through 10 turns of high-pressure torsion (HPT) at 450 K. Afterwards, the HPT-processed alloy was annealed for 1 h at temperatures ranging from 423 to 773 K and its mechanical properties and microstructural evolution were examined using microhardness measurements and electron backscattered diffraction (EBSD) analysis. The results demonstrate that HPT processing at an elevated temperature leads to a more uniform microhardness distribution and to an early saturation in the hardness values in the Al alloy compared with high-pressure torsion at room temperature. In addition, detailed EBSD analysis conducted on the HPT-processed samples immediately after annealing revealed that the Al–Mg–Sc alloy subjected to HPT processing at 450 K exhibits superior thermal stability by comparison with the same material subjected to HPT at 300 K.
Pub.: 01 Jun '17, Pinned: 11 Feb '18
Abstract: •Sc refines both the α-Al grain and the eutectic Si in hypoeutectic Al-Si alloy.•Sc results in the formation of AlSi2Sc2 in two eutectic reactions.•The AlSi2Sc2 were significantly larger when they grew in the L→Al+AlSi2Sc2 reaction.•The as-cast yield strength was improved by the addition of Sc.
Pub.: 01 Jan '18, Pinned: 11 Feb '18
Abstract: The characteristics of strength, static crack resistance, rate of growth of fatigue cracks and cyclic endurance of annealed sheets from weldable alloys of the Al – Mg – Sc system are studied as compared to annealed sheets from alloy AMg6. The compositions of aluminum alloys with scandium (01545 and 1570S) with characteristics substantially superior to those of AMg6 and level of properties comparable to that of the base aircraft not weldable alloy 1163 in state T6 are determined.
Pub.: 17 Nov '17, Pinned: 11 Feb '18
Abstract: Publication date: 1 February 2018 Source:Acta Materialia, Volume 144 Author(s): Anthony De Luca, David C. Dunand, David N. Seidman The precipitation hardening behavior of an Al-0.08Zr-0.014Sc-0.008Er-0.10Si (at.%) alloy was investigated utilizing microhardness, electrical conductivity, atom-probe tomography (APT), and compressive creep-measurements. This new composition, with a Sc:Zr atomic ratio of less than 1:5 represents a significant reduction of the alloy's cost when compared to the more usual Al-0.06Sc-0.02Zr based alloys with typical Sc:Zr atomic ratios of 3:1. To study the precipitation behavior of this low-Sc alloy, isothermal aging experiments between 350 and 425 °C for a duration of up to 6 months were performed. The low concentration of Sc, compensated by the high Zr concentration, permits the alloy to achieve a higher peak microhardness than the corresponding Sc-richer, Zr-leaner alloys. The low-Sc alloy also shows better over aging resistance, as anticipated from the smaller diffusivity of Zr when compared to Sc, leading to slower coarsening kinetics. Atom-probe tomography demonstrates that the high microhardness is due to the formation of a high number density of nano-precipitates, ∼1023 m−3 for peak aging conditions, with a mean radius of 1.9 nm, thus yielding a high volume fraction (0.35%) of nano-precipitates. Like alloys with much higher Sc and Er concentrations, the (Al,Si)3(Sc,Zr,Er) nano-precipitates still exhibit a core-shell structure with a concentration of Zr in the shell of up to 25 at.%, and a Sc- and Er-enriched core. Compressive creep experiments at 300 °C demonstrate that the new alloy, with only 0.014 at% Sc, is as creep resistant as a binary Al-0.08Sc at.% alloy, displaying a threshold stress of 17.5 ± 0.6 MPa at peak aged condition. Graphical abstract
Pub.: 11 Nov '17, Pinned: 11 Feb '18
Abstract: The second-generation aluminum-magnesium-scandium (Al-Mg-Sc) alloy, which is often referred to as Scalmalloy®, has been developed as a high-strength aluminum alloy for selective laser melting (SLM). The high-cooling rates of melt pools during SLM establishes the thermodynamic conditions for a fine-grained crack-free aluminum structure saturated with fine precipitates of the ceramic phase Al₃-Sc. The precipitation allows tensile and fatigue strength of Scalmalloy® to exceed those of AlSi10Mg by ~70%. Knowledge about properties of other additive manufacturing processes with slower cooling rates is currently not available. In this study, two batches of Scalmalloy® processed by SLM and laser metal deposition (LMD) are compared regarding microstructure-induced properties. Microstructural strengthening mechanisms behind enhanced strength and ductility are investigated by scanning electron microscopy (SEM). Fatigue damage mechanisms in low-cycle (LCF) to high-cycle fatigue (HCF) are a subject of study in a combined strategy of experimental and statistical modeling for calculation of Woehler curves in the respective regimes. Modeling efforts are supported by non-destructive defect characterization in an X-ray computed tomography (µ-CT) platform. The investigations show that Scalmalloy® specimens produced by LMD are prone to extensive porosity, contrary to SLM specimens, which is translated to ~30% lower fatigue strength.
Pub.: 04 Jan '18, Pinned: 11 Feb '18
Join Sparrho today to stay on top of science
Discover, organise and share research that matters to you