Damage accumulation and mechanical properties of particle-reinforced metal-matrix composites during hydrostatic extrusionIntroductionMaterials and test methodsMechanical properties of the pristine materialsTensile properties following hydrostatic extrusionDamage evolutionDiscussion and concluding remarksAcknowledgementsReferencesDamage accumulation and mechanical propertiesof particle-reinforced metal–matrix compositesduring hydrostatic extrusionD.J. Lahaiea, J.D. Emburya, F.W. Zokb,*aDepartment of Materials Science and Engineering, McMaster University, 1280 Main St. W., Hamilton, Ont., Canada L8S 4L7bDepartment of Materials, College of Engineering III, University of California, Santa Barbara, CA 91306-5050, USAReceived 16 July 2003; received in revised form 31 October 2003; accepted 12 November 2003Available online 17 January 2004AbstractThe effects of hydrostatic extrusion on particle cracking and on the subsequent tensile properties of some prototypical particle-reinforced metal–matrix composites are investigated. In most cases, tensile failure occurs through a plastic instability in accordancewith the Considere criterion for necking. The corresponding failure strain is therefore dictated by the global flow and hardeningcharacteristics of the composites, as influenced by the intrinsic flow properties of the matrix as well as the extent and rate of particlecracking. Such cracking leads to significant reductions in the hardening rate and thus causes a reduction in the failure strain relativeto that of the neat matrix alloy. Extrusion prior to tensile testing has the effect of saturating the flow stress of the matrix and limitingthe tensile ductility to low values, largely because of the very low hardening rate of the matrix. Particle cracking during extrusioncauses a further reduction in ductility. The dominant role of the matrix hardening is demonstrated through re-tempering treatmentsof extruded billets prior to tensile testing. A micromechanical model of particle cracking is developed, taking into account the effectsof both the hydrostatic and the deviatoric stress components in axisymmetric loadings. The model is used to rationalize the observedtrends in damage accumulation with particle content, particle type, and loading configuration (tension vs. extrusion).Ó 2004 Elsevier Ltd. All rights reserved.Keywords: A. Metal–matrix composites; A. Particle-reinforced composites; B. Stress/strain curves; Extrusion; C. Damage mechanics1. IntroductionThe process of hydrostatic extrusion can be used as ameans of imposing large plastic strains on materialswith limited tensile ductility, for the purpose of study-ing damage accumulation and the effects of this damageon the subsequent material properties [1–3]. Thesestudies are relevant to the understanding of the fun-damental fracture processes as well as for identifyingpotential forming operations. When combined with theresults from other types of mechanical tests, they alsooffer the potential for ascertaining the role of stressstate in the damage process, especially in delineatingeffects of the deviatoric and the hydrostatic componentsof stress. In brittle materials, such as most intermetalliccompounds, the hydrost atic stress plays a particularlyprofound role since fracture is dictated largely by themaximum principal tension. Such effects are manifestedin enhanced tensile ductility upon superposition of ahydrostatic pressure during tensile testing [3,4]. Simi-larly, in particle-reinforced metal–matrix composites(PMMCs), fracture of the reinforcing particles is dic-tated largely by the maximum principal tension in theparticle and hence the tensile ductility is enhanced by asuperimposed hydrostatic pressure [5]. However, thetransfer of the macroscopic stresses onto the particles isconsiderably more complicated. That is, a significantcomponent of the particle stress is associated with theplastic flow of the surrounding ductile matrix, a processthat is driven by the macroscopic deviatoric stress.*Corresponding author. Tel.: +1-805-893-8699; fax: +1-805-893-8486.E-mail address: [email protected] (F.W. Zok).0266-3538/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.compscitech.2003.11.006Composites Science and Technology 64 (2004) 1539–1549COMPOSITESSCIENCE ANDTECHNOLOGYwww.elsevier.com/locate/compscitechAdditionally, macroscopic hydrostatic stresses can betransmitted through the matrix elastically, although theelastic mismatch between the particles and the matrixleads to a concentrating effect of this pressure on theparticles. As a consequence, the particle stresses areinfluenced by both the deviatoric and hydrostaticcomponents of the macroscopic stress. One of the ob-jectives of this paper is to delineate these dependenciesof the particle stresses, for the purpose of better un-derstanding the process of particle cracking in PMMCs.This paper deals largely with damage accumulation insome prototypical PMMCs upon hydrostatic extrusionand the effects of such damage on the tensile propertiesof the PMMCs. Particular attention is paid to the con-nections between the flow and hardening characteristicsof the composites before and after extrusion, the con-ditions at the onset of tensile failure, the state of particledamage, and the reinforcement volume fraction andtype. The recovery of mechanical properties followingheat treatment of the extruded billets to their initial stateis also probed, for the purpose of identifying the role ofthe matrix hardening characteristics on tensile fracture.Furthermore, a micromechanical model is developed todescribe the stresses within the particles in tension andunder hydrostatic extrusion, explicitly accounting forthe different dependencies on hydrostatic and deviatoricstress components, and the model is then combined withweakest link fracture statistics to simulate particlecracking in the two loading configurations. Compari-sons with the experimental measurements are used as ameans of both calibrating and validating the model.2. Materials and test methodsThree Al-based particulate composite systems wereinvestigated in this work. Two were DuralcanÔ com-posites consisting of Al alloy A356 reinforced with either10% or 20% by volume angular SiC particles. The otherconsisted of alloy 6061 reinforced with 20% by volumespherical ceramic microspheres (CMSs). The composi-tions of the matrix materials are given elsewhere [6]. TheSiC particles had an