Chemical Physics Letters 639 (2015) 23–28 Contents lists available at ScienceDirect Chemical Physics Letters jou rn al hom epage: www.elsevier.com/locate/cplett Influence of temperature on the mechanical alloying of Cu–Nb powder mixtures Antonio Mario Locci a , Giorgio Ligios a , Michele Mascia a , Stefano Enzo b , Francesco Delogu a,∗ a Dipartimento di Ingegneria Meccanica, Chimica e dei Materiali, Università degli Studi di Cagliari, via Marengo 2, 09123 Cagliari, Italy b Dipartimento di Chimica e Farmacia, Università degli Studi di Sassari, via Vienna 2, 07100 Sassari, Italy a r t i c l e i n f o Article history: Received 23 July 2015 In final form 31 August 2015 Available online 5 September 2015 a b s t r a c t This study focuses on the mechanical alloying behaviour of equiatomic Cu–Nb powder mixtures sub- jected to ball milling at temperatures between 300 and 900 K. A homogeneous amorphous phase forms at room temperature. The increase of processing temperature promotes the formation of mixtures of amorphous and nanostructured Cu and Nb phases below 650 K, and of nanostructured Cu–Nb compos- ites above 650 K. The grain size of Cu and Nb phases increases with temperature according to a power-law kinetics. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Ball milling is a powder metallurgy processing method with considerable technological potential [1–5]. Experiments are per- formed on powders that partially fill the reactor along with milling balls in the desired mass or volume ratio. As the reactor, or part of it, move along the periodic path determined by the mill working principles, balls collide with each other and with the reactor inter- nal walls. During each collision, a fraction of powder is trapped between milling tools and loaded at high strain rate. Intense non- hydrostatic stresses arise at the points of contact between powder particles, which causes severe mechanical deformation processes that further induce cold-welding and fracturing. Driven by mechanical deformation, physical and chemical trans- formations can also take place. In this regard, mechanical alloying (MA) is the typical example [1–5]. MA consists of the gradual dis- solution of two or more metals into each other to form crystalline and amorphous alloys [1–5]. Initially, the process was tentatively explained hypothesizing that the high content of lattice defects attained in severely deformed metals could enhance the rate of thermal diffusion and activate local melting phenomena [1,6]. However, numerical findings progressively pointed out the crucial role of dislocation activity in promoting the local atomic rearrange- ments that mediate chemical mixing at sheared interfaces [7–17]. ∗ Corresponding author. E-mail address: francesco.delogu@unica.it (F. Delogu). At present, two fundamental processes are thought to govern MA. On the one hand, ballistic atomic displacements activated by non-hydrostatic stresses, which tend to homogenize chemical composition via athermal mixing [7–14]. On the other, thermal diffusion, which tends, instead, to homogenize chemical poten- tial according to equilibrium thermodynamics [6]. Depending on the system, the two processes can strengthen each other, or com- pete against each other [15]. Competition occurs, in particular, in immiscible systems, where the positive enthalpy of mixing contrasts the mutual dissolution of chemical species, and yet dis- ordered solid solutions can be obtained [16–25]. In this case, the final MA product depends on the relative importance of athermal ballistic mixing and thermal demixing. Since the rate of demixing increases with temperature, per- forming experiments at different temperatures represents the most suitable approach to throw light on the relationship between prod- uct selection and experimental conditions [1,15]. Unfortunately, commercial ball mills are not equipped with temperature control systems, and this significantly hindered the investigation about the role of temperature in MA. Only a few studies on this subject have been carried out [1,21–23]. In all cases, a discontinuous range of temperature was investigated. Cryo-milling at liquid nitrogen tem- perature was performed by closing the reactor inside a container filled with liquid nitrogen [21,22]. Temperatures approximately between 300 and 500 K were reached, instead, by wrapping a heat- ing tape around the reactor [21,23]. Similar studies can be found occasionally in literature [1], always concerning a short tempera- ture interval. A systematic approach to temperature variation, and to its effects on MA processes, is lacking, which results in relevant http://dx.doi.org/10.1016/j.cplett.2015.08.072 0009-2614/© 2015 Elsevier B.V. All rights reserved.