Materials Science and Engineering A 463 (2007) 122–127 Exploring the dislocation/twin interactions in zirconium G.C. Kaschner a,∗ , C.N. Tom´ e a , R.J. McCabe b , A. Misra c , S.C. Vogel d , D.W. Brown a a MST-8, Los Alamos National Laboratory, Los Alamos, NM 87545, USA b MST-6, Los Alamos National Laboratory, Los Alamos, NM 87545, USA c MPA-CINT, Los Alamos National Laboratory, Los Alamos, NM 87545, USA d LANSCE-LC, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Received 30 March 2006; received in revised form 22 September 2006; accepted 22 September 2006 Abstract This paper explores the ‘barrier’ effect that tensile and compressive twins exert upon propagation of dislocations and other twins in zirconium (Zr). We do so by pre-deforming textured Zr at liquid nitrogen to selectively induce either tensile or compressive twins; next we anneal the dislocations while preserving the twinning structure; finally we reload the material at room temperature, where only prism slip, pyramidal slip and tensile twins are active. An analysis of the yield stress upon reload, and of the subsequent hardening response allows us to conclude that the twins play a dominant role in determining the hardening, while dislocations only have a second order effect. Published by Elsevier B.V. Keywords: Zirconium; Deformation twinning; Dislocation recovery 1. Introduction We have been applying a multi-faceted approach over the last several years to elucidate the dominant deformation mechanisms in hexagonal close-packed (HCP) metals [1–3]. The material of interest has been clock-rolled zirconium (Zr), processed to a moderately strong fiber texture, that exhibits strong asymmetry in its mechanical response. The mechanical experiments have included quasi-static uniaxial tension and compression, as well as high rate compression, along different directions with respect to the material’s c-axis and at various temperatures (76–450 K). These samples have, in turn, been characterized using optical microscopy, neutron diffraction, orientation image microscopy (OIM, also known as automated electron backscatter diffrac- tion or EBSD), and transmission electron microscopy (TEM) [4–9]. Our predictive modeling is based on the visco-plastic self-consistent code (VPSC) and has been modified to accom- modate the growing fraction of reoriented matrix commensurate with deformation twinning [10–12]. Building on successes of first capturing the characteristic response of monotonic loading at room temperature and equi- librium liquid nitrogen temperature and then applying those ∗ Corresponding author. Tel.: +1 505 665 1179; fax: +1 505 667 8021. E-mail address: kaschner@lanl.gov (G.C. Kaschner). parameters to predict the deformation of beams bent in four- point loading [11], we challenged the models with ever more interesting loading conditions. The predominant twin reorienta- tion (PTR) model keeps track, for each grain of the aggregate, of the increasing fraction of reoriented matrix as twins accommo- date shear [11]. This volume fraction increases until a threshold value is met and the entire grain is considered saturated and is assumed to adopt the orientation of the predominant twin sys- tem. The limitation of the model lies in the fact that it does not recognize such physical factors as twin morphology and the strength of twin boundaries as obstacles to dislocation motion. We determined that, although the PTR model was sufficiently adept at capturing the nuances of temperature change tests, it would not be capable of predicting the response of samples suf- ficiently loaded in one orientation to produce a population of deformation twins and then reloaded in another orientation. This set of loading orientation change tests compelled the development and inclusion in the VPSC code of a new twin model that we term “composite grain” [13]. In this application, twins are assigned a representative morphology and spacing within each grain. Such “composite grain” is idealized as an ellipsoidal inclusion embedded in and interacting with a homo- geneous medium. Parameters and criteria [14–16] exist for evaluating the strength of boundaries. Specifically, we are inter- ested in the penetration of twin boundaries by dislocations, and an associated Hall–Petch type of strengthening with accumulat- 0921-5093/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.msea.2006.09.115