PhD candidate, Monash University
Use of Scandium in aluminium to produce strong and corrosion resistant alloy
On 4th October 1992, Boeing 747 aircraft crashed on 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 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, Al-Mg-Si base is selected since this alloy already has a lower corrosion rate. 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. The first part of my research is to focus on alloy design. The chemical composition of the new alloy has to be optimised in order to get the best balance of end properties. This will be done by varying the amount of Sc, Zr and adjusting the ratio of Mg to Si in the melt. The alloy will be cast and rolled into the sheets.
Well, just adding Sc does not help. Sc reacts with aluminium and forms spherical particle. Then, Zr reacts with these particle 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 however is to get a very stable, nano-sized and well distributed particles in the aluminium.
Towards the end of my research, I will build the relationships between this structure and the properties of the new alloy.
Abstract: 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℃× 8 h and 300℃× 8 h, respectively. Sc and Zr additions can make the yield strength of Al-Mg-Mn alloy increase by 21 MPa (6.9%), 120 MPa (61.2%) and 127 MPa (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℃ for 8 h and annealing at 300℃ for 1 h, yield strength, ultimate strength, elongation to failure and exfoliation corrosion rank are 196 MPa, 360 MPa, 20.2% and PA (slight pitting corrosion) in Al-Mg-Mn alloy, and reach 316 MPa, 440 MPa, 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.
Pub.: 12 Apr '16, Pinned: 24 Aug '17
Abstract: The phase composition and recrystallization of Al-based alloys with small amounts (up to 4 wt %) of transition metals, such as Sc, Mn, and Zr, are studied by metallography and electrical resistivity and hardness measurements. No new phases, besides phases in equilibrium with an Al-based solid solution in the associated ternary systems, are found for the portion of the quaternary Al-Sc-Mn-Zr system under study. It is also found that the manganese and zirconium solubilities in the Al-based solid solution decrease in the presence of scandium and that zirconium additions in Al-Sc-Mn alloys increase their recrystallization temperature.
Pub.: 20 Jul '10, Pinned: 24 Aug '17
Abstract: The possibility of formation of a nanocrystalline structure (with a grain size smaller than 100 nm) in four Al-Mg-Sc alloys with 3.1–5.9% Mg during severe plastic deformation by torsion at a hydrostatic pressure of 6 GPa (high-pressure torsion (HPT)) has been studied. Room-temperature HPT of the alloys is shown to produce a nanocrystalline structure if the magnesium content is more than 4% (in the range 0.16–0.31% Sc). As the magnesium content increases, the grain size decreases and is minimal (40–50 nm) in a 01570 alloy with 5.9% Mg and 0.3% Sc. The structure of the HPT-processed 01570 alloy remains nanocrystalline upon heating to 200°C or at a deformation temperature as high as 200°C. Postdeformation heating is found to cause aging processes. The hardening of all the Al-Mg-Sc alloys is maximal after HPT at 20°C followed by aging at 300°C.
Pub.: 01 Dec '06, Pinned: 24 Aug '17
Abstract: By means of scanning electron microscopy, electron probe microanalyses, and differential scanning calorimetry, 21 alloys in both as-cast and annealed states were investigated to study the phase equilibria and phase transformations of the Al–Sc–Si system in the Al-rich corner. Based on the observed microstructure, the solidification paths of the as-cast alloys were analyzed. In addition, the phase equilibria of the Al–Sc–Si system at 500 °C were determined and phase transition temperatures for representative alloys are measured by means of DSC. For the sake of providing missing thermodynamic data, the enthalpies of formation at 0 K for the compounds ScSi, Sc5Si3, and τ (AlSc2Si2) were obtained by first-principles calculations. Based on the data from the present work and literature, the thermodynamic descriptions of the Sc–Si and Al–Sc–Si systems were developed. The Al–Sc–Si phase diagram in the entire composition range and the solidification characteristic in the Al-rich corner were then calculated. It is shown that the predicted solidification paths could describe the experimental observations reasonably. Based on the thermodynamic description obtained in this work, solution heat-treated and aged conditions of some Al–Sc–Si alloys are discussed, which suggests that the validated thermodynamic descriptions of this work could be helpful for the microstructure design of the Al–Sc–Si alloys.
Pub.: 15 Oct '15, Pinned: 24 Aug '17
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: 24 Aug '17
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: 24 Aug '17
Abstract: Experiments were conducted to evaluate the flow properties of an Al-3Mg-0.2Sc alloy both without and with processing using equal-channel angular pressing (ECAP). The initial grain size was ~300 µm and this was reduced to ~250 nm by ECAP and then increased to ~600 nm by annealing at 673 K for 10 min. Tests were conducted to determine the mechanical properties over seven orders of magnitude of strain rate from ~10−4 to ~103 s−1. The results confirm the validity of the Hall-Petch relationship in the material processed by ECAP. Shear banding occurred in the coarse-grained material during dynamic testing but in the ECAP-processed alloy there was only minor grain coarsening. There was evidence for dynamic strain ageing in both the coarse and the ultrafine-grained (UFG) alloy with a transition in flow mechanism at high temperatures from dislocation climb in the coarse-grained material to superplasticity in the UFG alloy.
Pub.: 07 Jan '17, Pinned: 24 Aug '17
Abstract: As a typical system in the family of heat-resistant aluminum alloys, the Al-Sc-Zr ternary system was thermodynamically studied by experimentation and theoretical calculation. Employing the scanning electron microscopy/energy dispersive X-ray spectroscopy and X-ray diffraction, the isothermal section at 873 K was determined with equilibrated alloys. Two stable ternary compounds Al75Sc16Zr9 and Al75Sc10Zr15 were newly discovered. By means of the differential scanning calorimetry, phase transformation temperatures of selected samples were obtained. In addition, the as-cast microstructure of individual alloys was observed to identify the primary crystallization phase during solidification. Based on the experimental information mentioned above, the Al-Sc-Zr system was thermodynamically optimized combining the first-principles calculation and the CALPHAD method, with the constituent binary system Sc-Zr optimized for the first time. Using the thermodynamic parameters obtained in this work, the isothermal section, vertical section and liquidus projection were all calculated and then compared with available experimental data, most of which could be accounted for satisfactorily.
Pub.: 17 Mar '17, Pinned: 24 Aug '17
Abstract: Correlations between densification, hardness and electrical conductivity were investigated over a wide range of applied volumetric energy densities (E) for an Al−Mg−Sc−Zr alloy fabricated by selective laser melting. It is shown that porosity dominates electrical conductivity in the low E region up to 77 J/mm3, while the contribution of solute in solution is more significant in the medium−high E region. Reasons for these differences are discussed and a linear relationship between electrical conductivity and densification in the absence of solute effects is presented.
Pub.: 18 Mar '17, Pinned: 24 Aug '17
Abstract: Axial loading fatigue tests were carried out in a friction stir welded Al-Mg-Sc alloy. Local strain evolution under fatigue loading was monitored using small strain gages. The weld joint had very complex heterogeneous microstructure and hardness distributions. The cyclic stress-strain responses were strongly dependent on the local hardness distribution. The cyclic plastic strain was observed intensively on the advancing side in the soft stir zone. Transgranular fatigue cracks initiated in the stir zone on the top surface, where high plastic strain was observed. To the contrary, intergranular crack initiations were observed on the bottom surface. The crack initiation was caused mainly by the cyclic plastic straining on the top side. However on the bottom side, the residual initial oxide layer, which was not sufficiently stirred, as well as the local cyclic plastic straining could be attributed to the intergranular crack initiations.
Pub.: 28 Feb '17, Pinned: 24 Aug '17
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: 24 Aug '17
Abstract: The comparative analysis of the effect of monotonous and non-monotonous severe plastic deformations (SPD) on the structure and properties of aluminum alloys has been carried out. Conventional hydrostatic extrusion (HE) with a constant deformation direction and equal-channel angular hydroextrusion (ECAH) with an abrupt change in the deformation direction were chosen for the cases of monotonous and non-monotonous SPD, respectively. Model cast hypoeutectic Al-0.3%Sc alloys and hypereutectic Al-0.6%Sc alloys with Ta and Ti additives were chosen for studying. It was demonstrated that SPD of the alloys resulted in the segregation of the material into active and inactive zones which formed a banded structure. The active zones were shown to be bands of localized plastic deformation. The distance between zones was found to be independent of the accumulated strain degree and was in the range of 0.6-1 μm. Dynamic recrystallization in the active zones was observed using TEM. The dynamic recrystallization was accompanied by the formation of disclinations, deformation bands, low-angle, and high-angle boundaries, i.e., rotational deformation modes developed. The dynamic recrystallization was more intense during the non-monotonous deformation as compared with the monotonous one, which was confirmed by the reduction of texture degree in the materials after ECAH.
Pub.: 28 Mar '17, Pinned: 24 Aug '17
Abstract: Formation of submicrocrystalline and nanostructured states in scandium- and zirconium-alloyed Al – Mg – Mn alloys obtained by rapid quenching from melt and torsion under high quasi-hydrostatic pressure is studied. The evolution of the structure and the change in the hardness of the alloys are considered depending on the composition, casting conditions, and mode of heat treatment.
Pub.: 12 Mar '09, Pinned: 24 Aug '17
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: 24 Aug '17