Amorphisation and Devitrification of Al-Transition Metal- Rare Earth Alloys
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Amorphisation and Devitrification of Al-Transition Metal- Rare Earth Alloys Livio Battezzati, Marcello Baricco, Martin Kusy’, Mauro Palumbo, Paola Rizzi, Viktoria Ronto’ Dipartimento di Chimica IFM, Centro di Eccellenza “Superfici ed Interfasi Nanostrutturate”, Università di Torino Via Pietro Giuria 7 10125 Torino, Italy ABSTRACT Amorphisation studies by rapid solidification of Al-based alloys containing a transition metal (TM) and a rare earth element (RE) are reported. Results on primary formation of Al nanocrystals are given and discussed in relation to possible nucleation mechanisms considering the effect of various RE elements (RE = La, Ce, Nd and Sm) in Al87Ni7RE6 alloys. Ti or Zr, immiscible with RE’s, are added to the ternary alloys with the aim of revealing possible phase separation in the melt. Calculations of ternary Al-Ni-Ce metastable phase equilibria are helpful in understanding the transformation sequence. Composition profiles ahead of nanocrystals are computed using the DICTRA software which correctly predicts the occurrence of composition gradients. The surface fracture of mechanically tested samples are observed in TEM to check whether crystallisation is induced by deformation. INTRODUCTION Al-based amorphous alloys can be obtained by rapid solidification of systems containing a transition metal (TM) and a rare earth (RE) element. Once they are properly annealed, copious Al nanocrystals form, well dispersed in the amorphous matrix. The nanocrystalline alloys with such microstructure have attracted interest because of improved mechanical properties with respect to both conventional light alloys and amorphous Al alloys [1-2]. In the recent literature the devitrification behaviour of Al amorphous alloys has been thoroughly studied in view of microstructure optimisation. The rare earth elements and Y are often considered as vicariant although differences in the amorphising range and devitrification mechanism were demonstrated for binary Al-RE and ternary Al-TM-RE systems [1, 3-6]. Fe and Ni are the most commonly used transition metals. However, several metastable intermetallic compounds compete with glass formation during rapid solidification when Fe is alloyed to Al and a rare earth. So, amorphisation is difficult [7]. Systematic studies on Al-Ni-Gd alloys have indicated the Al87Ni7Gd6 composition as a relatively good glass former [3]. Employing different rare earth elements in the same amount as Gd, induces a variation in transformation mechanism: there are two stages when RE = La or Ce, whereas they become three when RE = Nd, Sm or Gd [5, 6]. Furthermore, in the case of Al-Ni-La alloys of close composition, the primary formation of a metastable intermetallic phase was reported instead of Al [4, 8, 9]. In all cases the nucleation rate is high, leading to formation of a high number of fine crystals, and growth becomes soon sluggish, possibly because of the occurrence of composition gradients in the matrix around the nanocrystals [10 - 14]. Recently it has been suggested that separation within
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