Pressure effect on crystallization of mechanically alloyed amorphous Al85Fe15 alloy
Introduction
In recent years, Al-based Al-TM-RE (TM, transition metals; Fe, Co, Ni; RE, rare earth; Y, La, Ce, Gd) metallic glasses have attracted increasing attention due to their novel mechanical properties, especially the high strength [1], [2], [3], [4]. When the Al-based amorphous alloy is partially crystallized with precipitation of nm-sized fcc A1 crystallises in the amorphous matrix, the strength could be even enhanced [5], [6], [7]. The strength/weight ratio of the nm-sized Al/amorphous composite materials is extremely high that can be comparable to that for some ceramics [8]. The excellent mechanical properties make this new family of Al-based metallic glasses and nanocrystal/amorphous composite materials to be a promising candidate as advanced engineering materials.
The Al-based amorphous alloys are normally prepared by means of melt-quenching techniques (splate-quenching, melt-spinning or gas atomization etc.), forming either thin ribbon samples or powders, which are not suitable for most engineering applications. Therefore, consolidation of the amorphous powders or ribbons into bulk samples is necessary. In fact, many attempts have been made to fabricate bulk amorphous alloy samples by using various consolidation techniques from amorphous powders or ribbons [9], [10], [11], [12]. In the consolidation process, one of the fundamental issues is the pressure effect on thermal stability of the amorphous phase that is critical for optimizing the consolidation parameters in order to obtain bulk samples with desired structure and properties.
Pressure effect on crystallization kinetics of amorphous alloys has been investigated in several systems. It was found that pressure significantly affected the transformation behaviour of amorphous alloys and often led to the formation of dense, metastable crystalline phases that were not encountered during transformation at ambient pressure [13], [14]. The crystallization kinetics were found to be evidently altered by application of high pressures: the crystallization process can be enhanced [15] or suppressed [16], [17], depending upon systems. The pressure effect on crystallization kinetics is associated with the atomic diffusion process and the volume change effect during the initial stage of nucleation of crystals in the amorphous phase.
In the present paper, we prepared an amorphous Al85Fe15 alloy by means of mechanical alloying from elemental powders. In order to consolidate the as-milled amorphous powders into bulk samples, we investigated the pressure effect on the crystallization process under an applied pressure ranging from 0 to 1.4 GPa. The applied pressure was found to enhance precipitation of nm-sized fcc Al crystallises from amorphous matrix, that does not occur at ambient pressure. The precipitation temperature of nanocrystallites decreases with an increase of the applied pressure. It suggests that, if properly applied, annealing amorphous alloys under high pressures provides an effective approach for synthesizing bulk nanocrystalline/amorphous composite materials from amorphous alloy powders or ribbons.
Section snippets
Experiment
Elemental powders of Al and Fe (purity, 99.9%; the mean particle size is about 50 μm) were mixed with composition of Al85Fe15 (at%). Ball milling was performed in a planetary ball mill in an argon atmosphere using hardened steel container and balls with a ball-to-powder ratio of 20:1. The milled powders were transformed completely into an amorphous phase when the milling time exceeds 300 h, as indicated by X-ray diffraction analysis (Fig. 1). Chemical analysis of the as-milled amorphous powders
Results and discussion
Fig. 1 shows the XRD spectra for the milled powders with different milling times. It is seen that amorphization of the Al85Fe15 blended powders was completed after ball milling for 300 h. Upon heating the as-milled amorphous powders at a constant rate of 8°C/min, two exothermal peaks are observed in the DSC curve, as shown in Fig. 2. The onset temperatures of the two peaks were determined to be 366 and 461°C, respectively. Fig. 3 shows the XRD patterns of the as-milled amorphous powders after
Conclusions
Under high pressures, the mechanically milled amorphous Al85Fe15 alloy crystallizes primarily into a fcc Al nanophase dispersions, while no precipitation of fcc Al phase from amorphous matrix was observed at ambient pressure. The precipitation temperature of fcc Al nanophase decreases from about 380 to 200°C when the applied pressure increases from 0.4 to 1.0 GPa. The high-pressure annealing of mechanically alloyed Al-based amorphous powders provides an effective approach to optimize the
Acknowledgements
The financial support from the Chinese Academy of Sciences and the National Science Foundation of China (Grants No. 59431021, 59625101 and 59701002) is acknowledged.
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