Quasicrystalline Al93Fe3Cr2Ti2 alloys

Aluminium alloy powder having a nominal composition of Al93Fe3Cr2Ti2 (at%) has been prepared using gas atomisation. The atomised powder present a microstructure of an aluminium matrix reinforced with a spherical quasicrystalline icosahedral phase, in the range of nanometre in size. The powder was consolidated into bars using warm extrusion. The microstructure of the extruded bars retains the quasicrystalline microstructure and the bars present outstanding mechanical properties, i.e. proof stress of 280 MPa at 300 °C. Upon heating the microstructure evolves towards the equilibrium. The thermal evolution was investigated by means of x-ray diffraction, differential scanning calorimeter, scanning electron microscopy and transmission electron microscopy. According to these observations a transformation in two steps is proposed. A first step consists in the decomposition of the supersaturated solid solution of the matrix and the quasicrystals, and a second step in the transformation of the quasicrystals into the equilibrium phases.


INTRODUCTION
Nano-size quasicrystalline structures have been a subject of interest over recent years due to their potential use as reinforcement in advanced engineering alloys (Kimura et al., 2000).The icosahedral phases are hard and brittle due to the difficulty of dislocation movement in the quasiperiodic lattice without long range periodicity.However, the use of such quasicrystalline phase as reinforcement of ductile matrix, such as aluminium alloys, offers the potential improvement of mechanical properties together with enhanced stability at elevated temperatures.Aluminium alloys for high temperature applications are of considerable interest, especially in the automotive and aerospace industries.
The Al 93 Fe 3 Cr 2 Ti 2 extruded bars (Chlup et al., 2003) show high strength at elevated temperatures, with a proof stress of 280 MPa at 300 °C and an elongation of 1.8%, Table 1.These good mechanical properties decrease with temperature.A combination of solid solution strengthening, precipitation strengthening and grain size refinement would be responsible for the high strength of this alloy (Galano et al., 2009a).
Nanoquasicrystalline Al 93 Fe 3 Cr 2 Ti 2 powder was obtained by gas atomisation, the powder was sieved into well defined size fractions and consolidated into bars by warm extrusion.The stability and the evolution of bulk Al 93 Fe 3 Cr 2 Ti 2 extruded bars upon thermal treatments is studied as a function of initial gas atomised powder particle size range, and therefore, of the solidification rate.Thermal treatments at 400 °C, which was the extrusion temperature, were performed up to 1000 hours.

EXPERIMENTAL TECHNIQUES
Atomised powder particles of composition Al 93 Fe 3 Cr 2 Ti 2 (at %) were produced by helium atomisation of pure Al and Ti, and master Al-Fe and Al-Cr alloys.The Al 93 Fe 3 Cr 2 Ti 2 melt was superheated in a crucible up to above the liquidus, and then teemed via a refractary tundish through a confined nozzle.Atomisation of the melt was achieved at the nozzle exit by an annular jet of argon.Figure 1 presents a squeme of the gas atomiser employed.
Consolidation of the different powder particle size fractions, PPSF, −25, 25-50 and 50-100 μm, was performed without prior degassing by means of a horizontal warm extrusion press with a container diameter of 42 mm and maximum available pressure of 1600 MPa.After several trials at different temperatures and extrusion ratios (Todd et al., 2004), the extrusions were carried out at 400 °C, extrusion ratio 10 to 1 and extrusion ram velocity 0.3 mm s −1 .Prior to the extrusion, the encapsulated powder batches were submitted to heating at the extrusion temperature for 15 minutes.
The extruded bars, that have a diameter of 14 mm, were heat treated for 1, 10, 100 and 1000 hours at 400 °C.The powder, the as-extruded and the heat treated bars were characterised by means of X-Ray Diffraction (XRD), using Cu Kα radiation, Differential Scanning Calorimeter (DSC), under argon flow at a constant rate of 40 °C min −1 from room temperature up to 600 °C, Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM), equipped with Energy Dispersive X-ray analysis units (EDX).

RESULTS AND DISCUSSION
Direct observations of powder particle surfaces in the SEM showed they are spherical in shape (Fig. 2).The surface of powder particles is smoother the smaller the particle size (Fig. 3).Cross section observations of the powder particles by scanning electron microscopy (Fig. 4) show the copious presence of near spherical precipitates into the Al matrix.As it was shown in previous work (Todd et al., 2004), x-ray diffraction patterns of the powder showed mainly signal of the α-Al phase together with peaks indexed as an icosahedral phase, i-phase, by using the indexation scheme for icosahedral quasiperiodic crystals (Bancel et al., 1985).In the 25-50 and 50-100 μm powder particle size ranges, some smaller peaks corresponding to the intermetallic Al 23 Ti 9 were also identified.The DSC traces revealed a large exothermic peak around 500 °C (Todd et al., 2004), which was attributed to the transformation of the i-phase.
Figure 5 collects the XRD patterns of the bars processed with the −25 μm powder particle size fraction, in the as-extruded and the heat treated states, in the most representative range of 2θ between 35° and 50°.In the as-extruded state, signals of α-Al and the i-phase are seen.In the bars extruded with larger powder size fraction, some peaks of Al 3 Ti are also detected.The XRD pattern of the bar treated 1 hour at 400 °C is similar to that of the as-extruded bar.In the 10 and 100 hours treated bars the Al 3 Ti phase grows, and appears the metastable Al 6 Fe phase, which gets its maximum with the 100 hours treatment at 400 °C, and disappears afterwards.Meanwhile, the bar treated for 1000 hours presents the equilibrium phases: α-Al, Al 3 Ti, Al 13 Cr 2 and Al 13 Fe 4 .
The thermal evolution of the as-extruded bars was followed by differential scanning calorimeter (García Escorial et al., 2015).The DSC traces showed a large exothermic reaction at around 500 °C, which was larger the smaller the particle size and occurred at a slightly lower temperature, in a similar way to the DSC traces of the powder.
The presence of the icosahedral i-phase in the Al 93 Fe 3 Cr 2 Ti 2 as-extruded bar was confirmed by TEM (García Escorial et al., 2007).Selected area diffraction patterns revealed the two, three and five fold reflection spots as can be seen in Fig. 6.Quasicrystals in the atomised powder as well as in the extruded bars of Al 93 Fe 3 Cr 2 Ti 2 have the clear and characteristic spherical shape shown in Fig. 4 and Fig. 6.TEM observations reveal the presence of Al 3 Ti, also in the −25 μm size powder bar.This phase was not observed in this bar through XRD probably due to its small volume fraction.
Figure 7a shows a SEM micrograph of an asextruded bar, which present bands of different precipitation density formed by the elongation that suffered each powder particle during the extrusion process, with a copious presence of nearly spherical precipitates, identified by TEM as icosahedral phase.Figure 7b shows a TEM micrograph of an icosahedral quasicrystal and the selected area diffraction pattern.The quasicrystals are quite enriched in alloying elements, as determined by EDX analysis which pointed out to a composition around the Al 84.5 Fe 7 Cr 6 Ti 2.5 in agreement with other studies (Kimura et al., 2000;Galano et al., 2009b).With the 1 hour treatment, the microstructure hardly changes with respect to that of the as-extruded bars.After the  10 hours and 100 hours thermal treatments the spherical QC are mostly substituted by less spherical and larger precipitates (Fig. 8), which resemble the previous QC with darker regions of several shapes as tiebow, helix, triangle and cross (Fig. 8a), Ti enriched according with EDX analysis.Figure 8b shows a corresponding TEM micrograph.After heating for 1000 hours at 400 °C, (Fig. 9), numerous Fe enriched needle-like precipitates are seen, together with large agglomerates of Cr and Ti rich precipitates.
The atomised powder particles contain α-Al and QC and the extruded bars retain this structure, with an increasing amount of Al 3 Ti as particle size increases.Upon heating the Al 3 Ti phase grows, and the metastable Al 6 Fe phase appears.With the 1000 hours annealing, the metastable QC transforms into the equilibrium phases: Al 13 Fe 4 , Al 13 Cr 2 and Al 3 Ti.
Several authors have studied the transformation of quasicrystalline Al-based alloys upon thermal treatments mainly on melt-spun ribbons and by TEM (Lui and Köster, 1991;Galano et al., 2010) and report different ways of transformations: continuous and discontinuous transformations, as peritectoid, polymorphic, eutectoid and precipitation.In this work the evolution of bars, which is a bulk material, has been studied by SEM and TEM at 400 °C as a function of holding time, up to 1000 hours.Similarly to the microstructural evolution observed by other authors in other QC-Al base alloy ribbons, the i-phase seems to be supersaturated, since thermal treatment leads to a precipitation reaction of Al 3 Ti inside the QC, relaxing the QC prior to decomposition, meanwhile the supersaturated matrix leads to the precipitation of Al 6 Fe and Al 3 Ti.So, the quaternary QC-AlFeCrTi would evolve towards ternary QC-AlFeCr, because Fe and Cr are QC formers and they would reject Ti to form Al 3 Ti precipitates, although the presence of an intermediate phase of the type of Al 13 (Fe,Cr) 2-4 should be also considered, because Fe and Cr replace each other.With further thermal treatments the remaining QC particles may transform eutectoidcally to Al 13 Fe 4 and Al 13 Cr 2 and the Al 6 Fe phase evolves to the equilibrium Al 13 Fe 4 phase.
As has been seen above, the QC phase present in the alloy studied in this work is quite stable, and thus, it would contribute to the high mechanical properties of this alloy at high temperatures (Chlup et al., 2003;Galano et al., 2009a;Todd et al., 2004).Effectively, Fe and Cr are quasicrystal formers in Al base alloys, and Ti produces a significant  refinement of the icosahedral particle size as well as an increase of the stability (Galano et al., 2004).With enough time of permanence at high temperature, i.e. 1000 hours at 400 °C in our case, the metastable QC phase transforms into the equilibrium ones, with the result of the decay of mechanical properties.
At the present, we are studying composites of QC Al 93 Fe 3 Cr 2 Ti 2 matrix with Al fibres to improve ductility, which is one of the properties that should be improve in this type of materials.In the second step the QC are fully transformed into equlibrium phases, Al 13 Fe 4 , Al 13 Cr 2 and Al 3 Ti, as observed after 1000 hours at 400 °C.

FIGURE 4 .
FIGURE 4. SEM micrograph of a cross section of atomised Al 93 Fe 3 Cr 2 Ti 2 powder.

FIGURE 5 .
FIGURE 5. XRD patterns of as-extruded and heat treated bars obtained with −25 μm powder particle size fraction.

FIGURE 7 .
FIGURE 7. Micrographs of an Al 93 Fe 3 Cr 2 Ti 2 as-extruded bar: a) SEM and b) TEM, in the inlet the selected diffraction pattern.

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As-atomised Al 93 Fe 3 Cr 2 Ti 2 powder particles exhibit a microstructure of an aluminium matrix reinforced with a spherical nanometric icosahedric phase.-Bars obtained by warm extrusion at 400 °C of as-atomised −25, 25-50 and 50-100 mm powder particle size fractions retain the nanoquasicrystalline microstructure.-The QC are enriched in alloying elements, with a composition around Al 84.5 Fe 7 Cr 6 Ti 2.5 .-The as-extruded bars evolve upon heating through two steps.During the first one the supersaturated solid solution of the matrix and the QC decomposes, forming Al 6 Fe and Al 3 Ti in the matrix and Al 3 Ti precipitates in the QC, as observed after 10 and 100 hours at 400 °C.