Amorphization mechanisms of NiZr 2 by ball-milling
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Amorphization mechanisms of NiZr2 by ball-milling D. Galy, L. Chaffron, and G. Martin Centre d’Etudes de Saclay, CEREM/Section de Recherches de M´etallurgie Physique, 91191 Gif sur Yvette Cedex, France (Received 20 October 1995; accepted 27 March 1996)
The microstructure of NiZr2 in the course of amorphization by ball-milling is studied by transmission electron microscopy (TEM) and x-ray diffraction (XRD). The evolution from the initial fully crystalline alloy to a fully amorphized material is described. It is shown that prior to amorphization, the powder aggregates achieve a 100% nanocrystalline structure; the amorphous phase then appears and develops to the expense of the nanocrystalline phase. No massive chemical disordering is observed, but a small amount cannot be ruled out. It is proposed that amorphization occurs by chemical disordering at interfaces, induced by the scattering of shear waves.
I. INTRODUCTION
Phase transformations induced by ball-milling (BM) in intermetallic compounds have been extensively studied in the past ten years. It is now well established that an intermetallic compound submitted to a high and cyclic deformation can undergo one of the three following transformations. In a first case, the intermetallic compound disorders before amorphizing completely (Ni3 Al1 and Ni3 Si2 ) or partially (Nb3 Sn3 ). In a second case, the compound amorphizes directly without disordering: CuTi2 4 or CoZr.2 Third, the intermetallic compound does not amorphize at all, but disorders partially or fully, depending on the milling conditions (FeAl,5 AlRu,6 CuEr7 ). In the present work, we focus only on the mechanisms involved in this crystal-toamorphous transformation. Indeed, these mechanisms, down to the atomistic scale, have not been yet identified, even if some general scenarios have been proposed.8 There are two different approaches to the amorphization mechanisms: a static approach and a dynamical one. The static approach consists in claiming that the accumulation of distortion centers in a crystal might induce its collapse. Limoge et al.9 were the first to propose this hypothesis and have studied by molecular dynamics simulation the amorphization of a pure Lennard–Jones system under the introduction of point defects, either Frenkel pairs or interstitials. Later Massobrio et al.10 investigated by molecular dynamics the response of a model for the intermetallic alloy NiZr2 to the introduction of antisite defects. They have shown that beyond some concentration threshold of antisite defects, the material loses its crystallinity. Thus, amorphization just results from a simple accumulation of defects. Other simulations have followed on other alloys well known to amorphize under irradiation; in CuTi,11 amorphization is promoted by a simultaneous injection of chemical disorder and Frenkel pairs. On the basis of these simu688
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J. Mater. Res., Vol. 12, No. 3, Mar 1997
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lations, the amorphization mechanisms under i
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