A pinboard by
Jayshri Dumbre

PhD Student, Monash University


Strong and corrosion resistant aluminium alloy

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.

This project aims at development of 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 these alloys already have lower corrosion rate. Sc is added like some magic element which will enhance strength but will not have any effect on corrosion.

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 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, Cu 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 particles. 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 coherent particles in the aluminium.


Mechanical properties and optimization of the aging of a dilute Al-Sc-Er-Zr-Si alloy with a high Zr/Sc ratio

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: 13 Oct '17

Effect of Sc and Zr additions on microstructures and corrosion behavior of Al-Cu-Mg-Sc-Zr alloys

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: 13 Oct '17

Effect of vanadium micro-alloying on the microstructural evolution and creep behavior of Al-Er-Sc-Zr-Si alloys

Abstract: Publication date: 1 February 2017 Source:Acta Materialia, Volume 124 Author(s): Dinc Erdeniz, Wahaz Nasim, Jahanzaib Malik, Aaron R. Yost, Sally Park, Anthony De Luca, Nhon Q. Vo, Ibrahim Karaman, Bilal Mansoor, David N. Seidman, David C. Dunand Al-Er-Sc-Zr-Si alloys, strengthened by L12-ordered, coherent Al3(Er,Sc,Zr) nanoscale precipitates, can be used for automotive and aerospace applications up to 400 °C. Vanadium, due to its small diffusivity in aluminum and its ability to form L12-ordered tri-aluminide precipitates, is a possible micro-alloying addition for further improving the service temperature of these alloys. Moreover, vanadium-containing Al3(Er,Sc,Zr,V) precipitates are anticipated to have a smaller lattice parameter mismatch with the matrix, thereby improving the alloy's coarsening resistance. In this study, the temporal evolution of microstructural and mechanical properties of an Al-0.005Er-0.02Sc-0.07Zr-0.06Si alloy micro-alloyed with V are investigated utilizing isochronal, isothermal and double-aging treatments and compared to the results obtained from an alloy that does not contain V, but otherwise has the same composition. Both isochronal and isothermal aging treatments reveal slower precipitation and coarsening kinetics for the V-containing alloy. A peak microhardness value of ∼600 MPa is obtained after a double-aging treatment at 350 °C/16 h, followed by aging at 400 °C for 12 h. Transmission electron microscopy reveals a duplex-size precipitate microstructure, with the smaller precipitates having a mean radius <3 nm. Despite the expectation of a reduced creep resistance due to a lower precipitate/matrix lattice mismatch, both alloys have similar creep behavior at 400 °C, characterized by a threshold stress of 7.5 and 8 MPa under peak-aged and over-aged conditions, respectively. Thus, micro-additions of V to an Al-Er-Sc-Zr-Si alloy lead to enrichment of V in the Al3(Er,Sc,Zr,V) nano-precipitates, improving their coarsening resistance without deteriorating their ability to block dislocations under creep at 400 °C. Graphical abstract

Pub.: 25 Nov '16, Pinned: 13 Oct '17

Effects of Sb micro-alloying on precipitate evolution and mechanical properties of a dilute Al-Sc-Zr alloy

Abstract: An Al-Sc-Zr aluminum alloy with Sb micro-addition (Al-0.066Sc-0.050Zr-0.021Sb at.%) is cast and heat-treated to study the effects of Sb on the nucleation, growth, coarsening kinetics and precipitate morphology, and resulting mechanical properties at ambient and elevated temperatures. When isochronally aged, the Sb-containing alloy exhibits a peak microhardness (607 ± 12 MPa) at 475 °C, which is greater than that of a comparable Sb-free alloy at the same temperature (549 ± 17 MPa), and a smaller rate of decrease of microhardness values due to precipitate coarsening for aging temperatures >475 °C. When isothermally aged, the Sb-containing alloy achieves larger peak microhardness values at 300 °C for more than a month (~80 MPa difference) and 400 °C for ~8 hours (~200 MPa difference) than the Sb-free alloy. Atom-probe tomography of the peak-aged Sb-containing alloy demonstrates that Sb partitions to the precipitates, and is enriched in the Zr-rich shell (up to 0.35 at.% Sb). For creep testing at 300 °C, the Sb-containing alloy exhibits smaller steady-state strain-rates than the Sb-free control alloy at applied stresses >15 MPa. It is hypothesized that the effects of Sb micro-alloying (partitioning to precipitates, enhanced precipitate coarsening and higher creep resistance) are linked with the following mechanisms: (i) enhanced Zr diffusion in the matrix due to attractive Sb-Zr interactions; (ii) reduction in matrix/precipitate interfacial free energy, when Sb is present; and (iii) an increase in precipitate/matrix lattice parameter mismatch resulting in stronger elastic interactions with dislocations.

Pub.: 24 Oct '16, Pinned: 13 Oct '17

Nano-structure evolution of secondary Al3(Sc1−xZrx) particles during superplastic deformation and their effects on deformation mechanism in Al-Zn-Mg alloys

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: 13 Oct '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: 13 Oct '17