Early stages of the mechanical alloying of TiCeTiN powder mixtures Giovanna Mura a , Elodia Musu b , Francesco Delogu c, * a Dipartimento di Ingegneria Elettrica ed Elettronica, Università degli Studi di Cagliari, via Marengo 2, 09123 Cagliari, Italy b Industrial Telemicroscopy Laboratory, Sardegna Ricerche, Polaris, Technology Park of Sardinia, Edificio 3, Loc. Piscinamanna, 09010 Pula (CA), Italy c Dipartimento di Ingegneria Meccanica, Chimica, e dei Materiali, Università degli Studi di Cagliari, via Marengo 2, I-09123 Cagliari, Italy highlights < Mechanically processed TiCeTiN powder mixtures form two solid solutions. < An analytical model was developed to describe the mechanical alloying kinetics. < The amount of powder alloyed at collision was indirectly estimated. < A few nanomoles of material participate in the alloying process at each collision. < The chemical composition of the solid solutions was shown to change discontinuously. article info Article history: Received 9 July 2012 Received in revised form 1 November 2012 Accepted 5 November 2012 Keywords: Ceramics Cold working X-ray scattering Microstructure abstract The present work focuses on the alloying behavior of TiCeTiN powder mixtures submitted to mechanical processing by ball milling. Accurate X-ray diffraction analyses indicate a progressive modification of the unit cell parameters of the TiC and TiN phases, suggesting the formation of TiC- and TiN-rich solid solutions with an increasingly larger content of solutes. Once the discrete character of the mechanical treatment is taken into due account, the smooth change of the unit cell parameters can be explained by a sequence of mutual dissolution stages related to individual collisions. At each collision, the average chemical composition of small amounts of TiC- and TiN-rich phases changes discontinuously. The discontinuous changes can be tentatively ascribed to local mass transport processes activated by the mechanical deformation of powders at collisions. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Ceramic metals, generally referred to as cermets, are structural materials composed by a hard ceramic dispersed into a softer binding metal [1,2]. This combination of phases keeps the desirable properties of metals and ceramics, and suppresses the undesirable ones [1e3]. For example, cermets are lightweight, and character- ized by remarkably high strength, toughness, and heat conductivity [3,4]. The temperature gradients responsible for thermal stress and cracks are also lowered, which allows cermets to exhibit very good high-speed cutting performances [3,4]. The commercially available cermets presently employed for cutting tool applications are substantially based on Ti carbo- nitrides, Ti(C,N) [4e6], which permit to achieve significantly higher hot hardness, transverse rupture strength, oxidation resis- tance, and heat conductivity, as well as to improve the high temperature creep resistance [4e6]. In the attempt of optimizing their properties by tuning composition and microstructure, Ti(C,N)- based cermets have been fabricated by different methods including hot pressing, hot isostatic pressing, and sintering [4e6]. In most cases, TiC, TiN, and Ti(C,N) phases, have been used as starting materials [4e6], synthesized in turn by a variety of methods. In this work, attention is focused on the synthesis of a Ti(C,N) solid solution by mechanical alloying of TiC and TiN powders. The process attracts remarkable interest due to the brittleness of the TiC and TiN phases. Based on the first mechanistic hypotheses invoking the accumulation of lattice defects [7,8], the formation of homo- geneous alloys of brittle elements and compounds by mechanical alloying was initially ruled out. However, homogeneous alloys in brittleebrittle systems were soon obtained [9e13]. The formation of solid solutions during the mechanical alloying of TiC and TiN powder mixtures was also clearly demonstrated by X-ray diffrac- tion (XRD) [14]. In particular, it was shown that the mechanical processing induces a progressive change of the unit cell parameters of the TiC and TiN phases, which can be ascribed to a progressive * Corresponding author. Tel.: þ39 070 6755073; fax: þ39 070 6755067. E-mail address: francesco.delogu@dimcm.unica.it (F. Delogu). Contents lists available at SciVerse ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.11.023 Materials Chemistry and Physics 137 (2013) 1039e1045