A pinboard by
Jayshri Dumbre

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.


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

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

Phase equilibria and solidification characteristics of the Al–Sc–Si alloys

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

Effect of Severe Plastic Deformation on Structure and Properties of Al-Sc-Ta and Al-Sc-Ti Alloys.

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