Characterisation of an Al93Fe3Cr2Ti2 alloy obtained by spray forming

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Abstract

Al93Fe3Cr2Ti2 is a highly promising alloy due to its capacity to form quasicrystals. In the present work bulk Al93Fe3Cr2Ti2 obtained by spray forming is microstructurally characterised. A microstructure gradient is observed, in which the external layer consists of an α-Al matrix with Al3Ti precipitates and fine quasicrystals while the internal core presents intermetallics of the equilibrium phases Al13Cr2 + Al13Fe4, with intermediate stages between both areas. The hardness ranges from the 140 Hv of the external layer to the 90 Hv of the core. Spray forming can therefore produce nanoquasicrystalline Al93Fe3Cr2Ti2 which evolves towards equilibrium as the process proceeds, due to the heating produced by the deposition of successive layers.

Introduction

Nano-size quasicrystalline (QC) structures have been a matter of interest in recent years due to their potential for reinforcing advanced engineering alloys [1]. 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 phases for the reinforcement of ductile matrices like Al alloys affords a potential improvement in mechanical properties and enhanced stability at high temperatures.

In previous studies Al93Fe3Cr2Ti2 nanoquasicrystalline (nano-QC) powders have been obtained by gas atomisation [2], [3] and consolidated via warm extrusion into bars, showing high strength with good ductility at intermediate and high temperatures [4]. These good mechanical properties remain up to 450 °C and subsequently decrease with temperature. Melt-spun nano-QC ribbons have also been obtained [5]. The high strength of the alloy is achieved by a combination of solid solution, particle dispersion and grain refinement, while its stability seems to be due to the slow coarsening rate of the icosahedral phase [6].

Spray forming is an alternative casting process for near net shape billets, and although not a conventional rapid solidification (RS) process it may provide an opportunity to produce new types of alloys and bulk structures. Thus, spray-formed Al alloys containing a significant volume fraction of amorphous phases have been reported [7], [8].

Section snippets

Experimental techniques

Al–3Fe–2Cr–2Ti (atomic %) alloys were spray formed using the Sandvik–Osprey plant installed at Oxford University with process conditions described elsewhere [9], obtaining billets of diameter 200 mm, height 155 mm and weight 19.2 kg. Different cross-section areas were studied by means of X-ray diffraction (XRD) using Cu Kα radiation; differential scanning calorimeter (DSC) under an argon flow at a constant rate of 20 °C/min from room temperature to 600 °C; optical microscopy (OM); scanning electron

Results and discussion

Spray forming procedure gives rise to different areas in the billet because of the different cooling rates during solidification and further thermal treatment, due to the contact with progressively deposited powder. These areas would exhibit different microstructures and properties.

A cross-section of the spray-formed billet was cut as shown in Fig. 1 to obtain representative samples of the different areas of the billet. Six areas were studied: top centre (TC), middle centre (MC), bottom centre

Conclusions

Nanoquasicrystalline Al93Fe3Cr2Ti2 has been obtained by spray forming and is stable above 300 °C. The near 20 kg billet comprised a α-Al matrix with a high volume fraction of micron and submicron scale intermetallics, with retention of the nanoquasicrystalline phase, and high hardness at the surfaces.

As the spray forming process proceeds, the material evolves towards equilibrium due to the heating produced by the successive layers deposited, so the Al93Fe3Cr2Ti2 spray-formed billet contains three

Acknowledgements

This work has formed part of a European Union funded Research Training Network Project entitled “Manufacture and Characterisation of Nanostructured Al alloys”. Contract HPRN-CT-2000-00038.

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