Al Sc Zr Alloys Research Articles

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.

Microstructure, mechanical properties and corrosion behaviour of dilute Al–Sc–Zr alloy prepared by powder metallurgy

Dilute Al–0.5 wt.% Sc–0.4 wt.% Zr alloy was prepared by powder metallurgy. Fine (sub)grain structure developed and fine coherent Al 3(Sc, Zr) particles precipitated during extrusion at 350 °C. The particles coarsened and the structure recovered only slightly during annealing at 550 °C for 20 min without a sign of recrystallization. Mechanical properties of the PM AlScZr alloy are comparable to those of Al alloy AA6061-T6 at room temperature and better at temperatures over 250 °C also than those of PM Al–6 wt.% Mg–Sc alloy with 1.1 and 1.9 wt.% Sc. Potentiodynamic polarisation experiments and intergranular corrosion test ascertained a greatly higher corrosion resistance of the PM AlScZr alloy compared with the commercial AA6061-T6 Al alloy.

Optimization of hardening of Al–Zr–Sc cast alloys

The effects of composition, cooling rate after the end of solidification, and annealing regime on the structure and hardening of binary and ternary alloys of the Al–Sc–Zr system are studied. The liquidus in Al–Sc–Zr alloys is experimentally assessed in order to facilitate the correct choice of casting temperatures. The precipitation during slow cooling after the end of solidification causes hardening in the as-cast state and decreases the hardening effect during annealing. It is shown that the full hardening ability of precipitates can be achieved only upon their homogeneous distribution in the matrix. The optimum total concentration of Sc and Zr in aluminium alloys should be about 0.3 wt% at the ratio Zr:Sc ≥ 2. That allows conventional casting temperatures and considerable hardening during annealing.

Effects of Ti additions on the nanostructure and creep properties of precipitation-strengthened Al–Sc alloys

Three-dimensional atom-probe tomography, transmission electron microscopy and microhardness were used to investigate the temporal evolution of nanosize, coherent Al 3(Sc 1− x Ti x ) precipitates (L1 2 structure) in a coarse-grained Al–0.06at.%Sc–0.06at.%Ti alloy aged between 300 and 450 °C. At 300 °C, most scandium atoms partition within one day to the precipitates, whereas titanium atoms, even after 64 days of aging, remain predominantly in solid solution in the matrix, resulting in precipitates with average composition Al 3(Sc 0.94Ti 0.06). While titanium is very effective at retarding the coarsening kinetics of the precipitates, the low levels of titanium substitution result in only modest hardness increases over the binary Al–0.06at.%Sc alloy. When crept at 300 °C, the peak-aged alloy exhibits a threshold stress, which when normalized by the Orowan stress, increases with increasing radius, as predicted by a recent model considering elastic interactions between dislocations and coherent precipitates, and as previously observed in Al–Sc–Zr alloys.

Segregation in Al3(Sc,Zr) precipitates in Al–Sc–Zr alloys

Abstract An unusual precipitate structure in ternary Al(Sc,Zr) alloys has been revealed by analytical and high resolution transmission electron microscopy. Coherent precipitates with L1 2 structure and highly anisotropic shapes were found to consist of a core containing Al and Sc surrounded by a Zr-rich shell. The shell thickness ranged from about 4 to 7 nm in different particles. An abrupt change in composition between core and shell was indicated by both, energy-dispersive X-ray microanalysis and high resolution phase contrast imaging. It is likely that this phenomenon is responsible for the increased resistance to coarsening of Al 3 (Sc,Zr) precipitates in these ternary alloys.

The effects of trace Sc and Zr on microstructure and internal friction of Zn–Al eutectoid alloy

The damping properties of Zn–22 wt.% Al alloys without and with Sc (0.55 wt.%) and Zr (0.26 wt.%) were investigated. The internal friction of the determined by the microstructure has been measured in terms of logarithmic decrement (δ) using a low frequency inverted torsion pendulum over the temperature region of 10–230 °C. An internal friction peak was separately observed at about 218 °C in the Zn–Al alloy and at about 195 °C in Zn–Al–Sc–Zr alloy. The shift of the δ peak was found to be directly attributed to the precipitation of Al3(Sc, Zr) phases from the alloy matrix. We consider that the both internal friction peak in the alloy originates from grain boundary (GB) relaxation, but the grain boundary relaxation can also be affected by Al–Sc–Zr intermetallics at the grain boundaries, which will impede grain boundary sliding. In addition, Al–Sc–Zr intermetallics at the grain boundaries can pin grain boundaries, and inhibit the growth of grains in aging, which increases the damping stability of Zn–22 wt.% Al alloy.