Quasicrystals, crystals and multiple twins in rapidly solidified Al-Cr-Si, Al-Mn-Si and Al-Mn-Cr-Si alloys

The Al-Mn icosahedral phase is found to precipitate and grow at room temperature from rapidly quenched Al-4 at % Mn f.c.c. solid solutions, with the composition 13•5 ± 1•5at.%Mn, the surrounding matrix having a composition of 3•1 ± 0•3 at.% Mn. Dodecahedral-shaped precipitates with orientation relationships with the f.c.c. matrix are produced. The above information is rationalized in terms of symmetry arguments and growth processes. Comparison of the compositions obtained here with that of other work extends our knowledge of the Al-Mn metastable equilibrium between the icosahedral phase and the f.c.c. Al-Mn solid solution in the solid state.

Dodecahedral-shaped quasicrystalline precipitates in dilute Al-Mn solid solutions

The microstructural characteristics of AI-Cu-Li-Mg-Zr alloys have been studied after rapid solidification by melt spinning and after subsequent annealing at temperatures in the range 160 to 500°C, by using a combination of optical microscopy, scanning and transmission electron microscopy, X-ray diffraction, differential scanning calorimetry and microhardness measurements. The as-melt-spun alloys consist of a cellular microstructure with fine scaleδ′ precipitates and icosahedral particles distributed within the cells and at cell boundaries. The icosahedral structure is equivalent to the T2 phase reported by Hardy and Silcock. Annealing the melt-spun alloys leads to a complex precipitation sequence: α + I +δ → α + I + S′ +θ′ → α + I → α +δ′ + T1 + T2 (bcc) + two other phases. The icosahedral particles coarsen progressively during annealing, especially at higher annealing temperatures. Fine-scaleδ′ precipitates grow during annealing at low temperature, dissolve at higher annealing temperatures below 500°C, and then reprecipitate during cooling after annealing at 500°C. During annealing at low temperature, plates ofθ′ and S′ precipitate and then dissolve, providing solute atoms for icosahedral particle growth. Stable T1, T2 (bcc) and two other phases precipitate after decomposition of the icosahedral particles during annealing at 500°C.

Structure and decomposition behaviour of rapidly solidified AI-Cu-Li-Mg-Zr alloys

Two FeCSi alloys containing 2.20 and 4.16wt.% C are incrementally deposited on a steel substrate by flame spraying. The substrate temperature gradually increases during spraying. The maximum temperature is in the range of about 330 to 670 K depending on the cooling conditions. The cooling rate of the flattened metal droplets during solidification is almost independent of the substrate temperature. As the maximum substrate temperature rises, metastable austenite and a metastable h.c.p. ϵ phase formed on rapid solidification start to decompose during spraying. The cooling rate, the cause of the self-annealing, the decomposition of the metastable phases and the hardness of the deposit layer are examined.

Self-annealing of rapidly solidified deposit layers of FeCSi alloys by flame spraying

The AlCo decagonal phase

A study has been made of the relationship between the microstructures formed in rapidly solidified Al-15 wt pct Mn ribbons and the precipitation processes following annealing in the regime 300° to 600 °C from 15 minutes to 100 hours. The as-melt spun ribbons exhibit the icosahedral structure (in an Al-matrix), which is found to be stable up to ∼320 °C as deduced from electrical resistivity measurements. X-ray and TEM analyses reveal that decomposition of this phase is detectable after annealing at 400 °C/1 h, giving rise to Al6Mn. At 450 °C, four phases, viz., the G, Al6Mn, G’, and G″ intermetallic phases, are observed (in addition to Al), which result from the direct decomposition of the icosahedral phase. At higher temperatures, only the Al6Mn phase + Al is observed. The effect of increase in annealing time is simply to coarsen the Al6Mn particles. Our X-ray and electron diffraction data favor the model proposed by Shechtman and Blech for the icosahedral structure. The investigations were carried out using density and electrical resistivity measurements, differential thermal analysis, as well as X-ray diffraction and optical and transmission electron microscopy.

A. M. Samuel

Papers

On the microstructure of rapidly solidified Al-15 wt pct mn ribbons: effect of annealing in the temperature range 300° to 600 °C