Journal of Alloys and Compounds 434–435 (2007) 481–484 Study of early stages of Cu–NbC nanocomposite synthesis M.T. Marques a,∗ , V. Livramento a , J.B. Correia a , A. Almeida b , R. Vilar b a INETI-DMTP, Estrada do Pa¸ co do Lumiar 22, 1649-038 Lisboa, Portugal b IST, Department of Materials Engineering, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Available online 11 October 2006 Abstract Cu–NbC nanocomposites with nominal compositions of 5, 10 and 20 vol.% of NbC were produced in situ via MA from elemental powders. The powders were milled for 1, 2, 4, 8, 16 and 32 h. The as-milled powders were characterised by X-ray diffraction (XRD) and scanning electron microscopy (SEM). X-ray diffraction results showed that the niobium carbide phase is formed in the early stages of milling and that the time necessary to complete the reaction ranges from 1 to about 8 h of milling. © 2006 Elsevier B.V. All rights reserved. Keywords: High-energy ball milling; Metals; Nanostructures 1. Introduction Copper nanocomposites reinforced with ceramic particles can be produced by several methods, such as internal oxida- tion and mechanical alloying (MA). In almost all of the reported experiments MA alone is not able to produce directly the rein- forcing phase, without additional and appropriate heat treatment at relatively high temperatures [1–4]. Results reported by Biseli et al. [2] showed that the in situ production of TiB 2 particle rein- forced copper nanocomposites via MA is only achieved after a suitable heat treatment. They reported that the reaction between Ti and B does not occur readily during milling, but instead takes place during annealing, for short periods at temperatures of 873–1073 K, although the mechanism of in situ formation of TiB 2 was not very clear. Takahashi [1] has synthesised copper alloys reinforced with NbC and TaC particles by MA. The in situ formation of the carbide phases was not observed in the as-milled powder but only after appropriate heat treatment in the temperature range 873–1323 K. Recently, Marques et al. [5] have produced in-situ copper-niobium carbide (Cu–NbC) nanocomposites via mechanical alloying without additional heat treatment. The authors observed that relatively short milling time (less than 20 h) is sufficient to form NbC nanoparticles in a cop- per matrix. The aim of this research is to study the early stages of synthesis of Cu–NbC nanocomposites via mechanical alloying. ∗ Corresponding author. Tel.: +351 217 165 141; fax: +351 217 166 568. E-mail address: tmarques@ineti.pt (M.T. Marques). 2. Experimental procedure The starting materials used in the milling experiments were Cu (99.9% purity; particle size 44 m< d < 149 m), Nb (99% purity; average particle size 65 m) and synthetic graphite (99.9995% purity; average particle size 74 m), used as carbon source. Details of the milling operation are reported in Ref. [5]. Different powder batches were produced with nominal compositions of 5, 10 and 20 vol.% NbC. For nominal composition of Cu–10, 20 vol.% NbC the milling was carried out for 1, 2, 4 8, 16 and 32 h and up to 8 h for Cu–5 vol.% NbC. In all experiments two steps were used; initially the Cu and carbon powders were milled for 4 h, then niobium was added and the milling proceeded for the required time. Milling time is considered after Nb addition. For each milling time and nominal composition, the phase identification of the as-milled powders was per- formed by XRD using Cu K radiation. From XRD data the average crystallite size 〈D〉 of the copper matrix and, when possible, that of the NbC phase were assessed using the Scherrer’s equation with the most intense Cu and NbC reflec- tions, (1 1 1) in both cases. The matrix lattice parameter was determined from the peak shift of the copper reflections; (2 2 2), (3 3 1) and (4 2 0) Cu reflections were used. To follow the niobium carbide synthesis reaction the ratio between the NbC and Cu integral peak intensities, I NbC /I Cu , was estimated. This proce- dure was only possible for Cu powders with 10 and 20 vol.% NbC, in which the NbC reflections are well defined. After milling, samples were mounted in conductive resin, metallographically prepared and characterised by SEM/EDS. 3. Results Fig. 1 shows the XRD pattern of the as-milled Cu–20 vol.% NbC powder for different milling times and also a spectrum of the Cu powder used as starting material. As can be seen, the niobium carbide phase starts to form after 1 h of milling. From 1 up to 8 h of milling time the peak intensity of niobium car- bide phase becomes more intense and well defined, showing 0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.08.307