Phenomenological study of the densification behavior of Aluminum–Nickel powder mixtures during cold isostatic pressing and differential hydrostatic extrusion M. Andasmas a , P. Langlois a , N. Fagnon a , Th. Chauveau b , A. Hendaoui a ,D.Vrel a, ⁎ a CNRS LIMHP, Université Paris 13, Villetaneuse, France b CNRS LPMTM, Université Paris 13, Villetaneuse, France abstract article info Article history: Received 23 December 2009 Received in revised form 8 October 2010 Accepted 16 October 2010 Available online 13 December 2010 Keywords: Powder consolidation Powder processing Extrusion Cold compaction Densification CompactionandextrusionofAlandNipowdermixtureswereperformedbycoldisostaticpressing(CIP)and differential hydrostatic extrusion (DHE) at room temperature. Relative density and Vickers hardness were determined as a function of the compaction pressure, showing a significant increase in the density with increasing Al content for comparable pressing conditions; at the same time, the addition of a very small quantityofAlinNidecreasesthehardnesssignificantly. Williamson–Hallmethodwasappliedtodetermine microstrains evolution during CIP, and X-ray diffraction (XRD) analyses were performed to investigate textureevolutionduringDHE.FinallyArchimedes'principlewasusedtoestimatethedensificationoccurring during DHE. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Cold isostatic pressing (CIP) is a key stage in many synthesis processes, not only to manufacture products such as ceramics, intermetallics, metals, but also pharmaceutical products [1,2], and mostofthedensificationofthesematerialsoccursduringthisstage,as opposed to sintering [3,4]. Various models have been proposed to depictthepowderbehaviorduringCIP [5,6],includingnanomaterials [7],butfewdataareavailableconcerningpowdermixtureswithvery different bulk moduli and yield strength [8]. Our domain of interest concerns the synthesis of dense nickel aluminidesfromNiandAlpowderswiththe1:1and3:1stoichiometries (i.e. NiAl and Ni 3 Al). As these materials possess a high hardness, CIP compactionisdifficultifperformedafterthesynthesis;wethuschoseto studythedensificationofNi+Alpowdermixtures,i.e.beforereaction, whichwillbecarriedoutinasecondstep,eitherbysolidstatediffusion orbySelf-propagatingHigh-temperatureSynthesis(SHS,seee.g. [11]), withthehopethatareactionfromfullydensereactantsmayyieldafully denseproduct. However, to reach full densification of powders, CIP is not sufficient, even, for the powders we are studying, at pressures close to1Gpa(10,000bar),andwethereforeuseddifferentialhydrostatic extrusion (DHE) to further increase the relative density. Indeed, this method has proved to be successful to produce fully dense materials such as copper nanopowders [9,10]. The reaction between Ni and Al is known to be limited by the presenceofanoxidelayeraroundthealuminumgrains.Wetherefore purposely chose to perform severe plastic deformation during DHE, using a strong reduction ratio, with the aim of increasing Al plastic deformation and thus to break furthermore these oxide layers and facilitate the nickel aluminides formation [12].Indeed,anincreasein the reactivity of SHS powder mixtures has been observed during mechanicalactivationusingmillingtechniques [13,14],butthekinetic improvementobservedmaybedueeithertothegrainrefinementorto thesuppressionoftheoxidelayersduringthisgrainrefinementwhen performed in an inert atmosphere. In this study, no significant grain refinement occurs, but plastic deformation of Aluminum grains performed in the absence of oxygen must induce an at least partial suppressionoftheoxidelayers,asthespecificsurfaceofthealuminum grains increases. Comparing SHS reactions with regular powders and with extruded powders should therefore give us the influence on the sole oxide layers on the kinetic, i.e. independently from the grain refinementobtainedwithmilling,andwillbedetailedinafurtherpaper. Asa firststepinourgeneralinvestigationonExtrusionActivatedSHS reactions(EASHS),thisstudyisaimedatprovidingsomeinsightsonthe densificationofpuremetalsandtheirmixturesandonthebehaviorof these powders (rearrangement, plastic deformation, and preferential crystallographic orientation) during isostatic pressing and anisotropic plasticdeformationbydifferentialhydrostaticextrusion. 2. Experimental procedure The starting powders were provided by Alfa Aesar (Ni powders) and Prolabo (spherical Al powder) whose main characteristics are Powder Technology 207 (2011) 304–310 ⁎ Corresponding author. Tel.: +33 1 49403452; fax: +33 1 49403414. E-mail address: vrel@limhp.univ-paris13.fr (D. Vrel). 0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2010.10.031 Contents lists available at ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec