Effect of micro-structural changes on mechanical properties of La 66 Al 14 (Cu, Ni) 20 amorphous and crystalline alloys Y. Zhang a, * , M.L. Lee a , H. Tan b , Q. Jing b , Y. Li a,b a AMM and NS, Singapore–MIT Alliance, National University of Singapore, Singapore, Singapore 117576 b Department of Materials Science, National University of Singapore, Singapore, Singapore 119260 Abstract The micro-structures and phase selections of La 66 Al 14 (Cu, Ni) 20 alloy were studied by Bridgman solidifications with withdraw velocity V between 0.008 and 4.8 mm/s. Composites of a-La dendrites in amorphous matrix or in micro- and nano-sized eutectic matrix were formed with different cooling rates. The volume percent of the dendrite phase reaches a maximum at the cooling rate of about 15 K/s, while the dendritic secondary arm spacing l 2 decreases from 4.3 to 0.6 mm with the increase in the cooling rate, and obeys the equation of l 2 V 0:57 ¼ 19 mm 1.57 s 20.57 . The compression strength, as well as the elastic strain limit of the dendrite/amorphous composite are 600 MPa and 2.3%, respectively. Improved ductility was observed for the amorphous matrix composites with high volume percent of dendrite phase which are obtained by slow cooling rate. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Composites; B. Mechanical properties at ambient temperature; C. Casting (including segregation); D. Microstructure 1. Introduction There are increasing attentions on the research of bulk amorphous alloys (also known as bulk metallic glass), because of their considerable significance in industry applications [1–10]. Composites with micro-sized den- drite primary phase embedded in an amorphous or a nanocrystalline phase matrix exhibits improved mechani- cal properties, e.g. apparent plastic deformation before fracture [8–10]. The volume percent of the dendrite phase ðF d Þ was usually controlled by changing the compositions [8,11]. Cooling rate during solidification has a significant effect on the phase formation and the resulting micro-structure [12–14]. Bridgman solidifica- tion is a widely used unidirectional solidification method by which the cooling rates can be precisely controlled [15,16]. In this paper, Bridgman solidification method was used to study the micro-structures of La 66 Al 14 (Cu, Ni) 20 alloy, to obtain dendrite/metallic amorphous or dendrite/micro- and nano-sized crystalline eutectic com- posites. Their mechanical properties were characterized. 2. Experimental procedures The ingots were prepared by arc-melting a mixture of pure La (99.9%), Al (99.9%), Ni (99.98%) and Cu (99.999%) in an argon atmosphere. The composition, La 66 Al 14 (Cu, Ni) 20 , is nominally expressed in atomic percent, and the alloy was designated L66 for simplicity. Each master ingot was first melted five times, and then crushed, and remelted four times. Bridgman solidifications were carried out by induction melting of the alloys in vacuum sealed quartz tubes with 3 mm internal diameter and a wall thickness of 1 mm. The alloys were then remelted at 923 K and kept for 20 min, and subsequently withdrawn at constant velocities ðV Þ in the range of 0.008 – 4.82 mm/s through a temperature gradient ðGÞ of 15 K/mm into a water bath. The cooling rate ðRÞ can be calculated as: R ¼ GV [15]. The rod samples with 3 mm diameter were mounted and polished for characterization under a scanning electron microscope (SEM) and optical microscopy (OM). The volume percent F d ; and the secondary dendrite arm spacing l 2 were measured using an image analyzer. The degree of amorphicity of all samples was examined by the X-ray diffraction (XRD) with Cu K a radiation and differential scanning calorimetry (DSC) at a heating rate of 40 K/min. 0966-9795/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2004.04.008 Intermetallics 12 (2004) 1279–1283 www.elsevier.com/locate/intermet * Corresponding author. E-mail address: smazy@nus.edu.sg (Y. Zhang).