Properties of cryo-drawn copper with severely twinned microstructure A. Kauffmann a,b , J. Freudenberger a,c,n , H. Klauß a , V. Klemm c , W. Schillinger d , V. Subramanya Sarma e , L. Schultz a,b a IFW Dresden, P.O. Box 270116, 01171 Dresden, Germany b TU Dresden, Institute of Materials Science, 01062 Dresden, Germany c TU Bergakademie Freiberg, Institute of Materials Science, Gustav-Zeuner-Str. 5, 09599 Freiberg, Germany d Wieland-Werke AG, Graf-Arco-Straße 36, 89079 Ulm, Germany e Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India article info Article history: Received 10 July 2013 Received in revised form 28 August 2013 Accepted 3 September 2013 Available online 14 September 2013 Keywords: Deformation twinning Copper Low temperature deformation Twin orientation relationship abstract We present the work hardening behaviour, mechanical and electrical properties of pure copper subjected to wire drawing at 77 K and 295 K, respectively. The deformation per pass is increased up to true strain of 0.4 by adopting pressure die/drawing die combinations in order to optimize lubricant residuals of MoS 2 on the wire surface at 77 K. The onset of deformation twinning for wire drawing at 77 K was found to be 0.3 and 0.4 fora true strain of 0.1 and 0.4 per pass, respectively. Twinning activity, texture strength and homogeneity are enhanced by increasing deformation per pass while the number of processing steps required for a certain deformation are reduced significantly. A considerably altered electrical conductiv- ity, medium strength increase accompanied with a loss of ductility and a limited thermal stability suggest the formation of non-coherent twin boundaries or destructed twin orientation relationship in cryo- drawn wires. Evidence was found for the latter possibility by local investigation of deformation twins in the final stage of deformation. & 2013 Elsevier B.V. All rights reserved. 1. Introduction In metallic materials twinning is a commonly observed phe- nomena which can occur during (i) crystal growth in bulk as well as thin films [1–3], (ii) recovery and recrystallisation [4], (iii) deformation [5,6] and (iv) transformation. Usually twin boundaries are inherent parts of the microstructure and in specific cases they allow for combinations of contradictory properties. The most prominent examples are probably severely twinned micro- structures observed in copper thin films prepared by special deposition methods [1–3]. These thin films usually combine high ultimate tensile strength with a slightly reduced electrical con- ductivity when compared to bulk well-annealed copper. Both properties are usually opposing since defects which impede the movement of dislocations and lead to strength increase act as scattering centres for the propagating electron wave. Hence, the goal of defect engineering is the realisation of defects with maxi- mum hardening effect but reduced electron scattering potential. For example, the Σ3 twin boundary in a face-centred cubic (fcc) metal shows properties which make it favourable for increasing strength without loss in electrical conductivity. The origin of electrical resistivity lies in the perturbation of the periodic potential. In this respect, grain boundaries provide a disorder of long-range order by the orientation misfit of the grains and of short-range order by the structure of the boundary itself [7]. Since the Σ3 twin boundary is a coincidence site lattice [8] the contribution by the change of crystal orientation across the boundary is reduced when compared to arbitrary high angle grain boundaries due to the appearance of a long-range order across the boundary described by the coincidence site lattice [9]. If the boundary is coherent the disruption of short-range order is reduced additionally since the interface between matrix and twin is a part of the lattices of both grains. The experimental determination of the excess resistivity of several boundaries in aluminium [9] revealed that the latter contribution seems to be the dominant one. Thus, the coherent Σ3 twin boundary provides the lowest contribution to electrical resistivity of all investigated grain boundaries in Ref. [9]. For interpreting the impact of coherent Σ3 twin boundaries on the mechanical properties of materials, the dislocation–boundary interaction has to be considered. Jin et al. investigated the interaction of pure screw and mixed dislocations with these boundaries in Refs. [10,11]. Since copper shows a rather low stacking fault energy in comparison to the energy necessary to enlarge a twin about one additional f111g plane, screw disloca- tions with Burgers vector within the twinning plane are first Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A 0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.09.022 n Corresponding author at: IFW Dresden, P.O. Box 270116, 01171 Dresden, Germany. E-mail address: j.freudenberger@ifw-dresden.de (J. Freudenberger). Materials Science & Engineering A 588 (2013) 132–141