In situ measurement of deformation mechanics and its spatio-temporal scaling behavior in equal channel angular pressing
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Deformation mechanics in equal channel angular pressing (ECAP) was studied in situ using digital image correlation (DIC) and infra-red (IR) thermography. In a prototypical experiment in an optical and IR transparent die, the deformation of commercially pure lead (Pb) is observed using high-speed optical and IR cameras. From the resulting time-sequence images of metal-flow in the deformation zone, DIC is used to characterize the zone of severe plastic deformation (SPD) as a function of the scale of deformation (sample dimensions), deformation speed, and die geometry. The temperature rise in the deformation zone was characterized using IR thermography and the results were compared against theoretical estimates. These observations provide direct insights into the mechanics of SPD in ECAP, which can offer strategies for microstructure control, process optimization, and miniaturization of ECAP.
I. INTRODUCTION
Equal channel angular pressing (ECAP) has been established as a scalable route for manufacturing ultrafine grained (UFG)/nanostructured metals in bulk form (comprising large cross-sections) by imposing severe plastic deformation (SPD).1,2 The process involves pressing a ram against a workpiece (billet), which sits in a channel. Due to a bend in the channel, the workpiece undergoes deformation involving high levels of strain as it advances through the bend [Fig. 1(a)]. Here, grain refinement takes place during imposition of high levels of shear strain without any significant change in the cross section geometry of the workpiece, even after several deformation passes.2 Imposition of SPD during ECAP to process fully-dense bulk forms is also facilitated by substantial hydrostatic pressure that prevails in its deformation zone.3,4 Additionally, much like other plastic deformation processes, microstructure evolution here is governed by the spatio-temporal distribution of the strain, strain-rate, and temperatures in the deformation zone.5 In ECAP, a steady state flow of material under plane strain conditions is associated with simple shear in a narrow zone at the intersection plane of the entry and exit channels of die.2 To characterize this, plastic deformation zone (PDZ) models involving slip-line field theory and finite element modeling have been utilized.6–8 From these analyses, it was found that the intrinsic material properties, processing parameters, and die geometry are important factors that determine the final microstructural Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.38 798
J. Mater. Res., Vol. 30, No. 6, Mar 28, 2015
http://journals.cambridge.org
Downloaded: 14 Sep 2015
features and define the mechanics of plasticity for a given material system. The fidelity of such models is ultimately determined by the validity of the underlying assumptions of the constitutive models and the boundary conditions. A direct validation of these models is limited by the lack of an in situ delineation of the mechanics of material flow in the deformati
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