Cell structure in cold worked and creep deformed phosphorus alloyed copper Rui Wu a , Niklas Pettersson a , Åsa Martinsson a, c , Rolf Sandström a, b, ⁎ a Swerea KIMAB, Box 7074, SE-164 07 Kista, Sweden b Materials Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden c Now at Sandvik Materials Technology, Sandviken, Sweden ARTICLE DATA ABSTRACT Article history: Received 29 August 2013 Received in revised form 7 January 2014 Accepted 9 January 2014 Transmission electron microscopy (TEM) examinations on as-received, cold worked, as well as cold worked and creep tested phosphorus-alloyed oxygen-free copper (Cu-OFP) have been carried out to study the role of the cell structure. The cell size decreased linearly with increasing plastic deformation in tension. The flow stress in the tests could also be correlated to the cell size. The observed relation between the flow stress and the cell size was in excellent agreement with previously published results. The dense dislocation walls that appeared after cold work in tension is likely to be the main reason for the dramatic increase in creep strength. The dense dislocation walls act as barriers against dislocation motion and their presence also reduces the recovery rate due to an unbalanced dislocation content. © 2014 Elsevier Inc. All rights reserved. Keywords: Cell Dislocation Cold work Creep Cu-OFP TEM 1. Introduction In Sweden spent nuclear fuel is planned to be disposed of by encapsulating in cast iron inserts placed inside copper canisters [1]. The cast iron insert is the load bearing part of the waste package and the copper canister acts as corrosion barrier. Due to an external hydrostatic pressure, the copper canister will be exposed to creep deformation [2,3]. The cylindrical copper canister is about 1 m in diameter, 5 m long, and has a wall thickness of 50 mm. It will be made out of phosphorus alloyed oxygen-free copper (Cu-OFP). After the hot working process which is either in the form of extrusion, pierce-and-draw or forging, the canister is in a soft condition. During the subsequent handling, the canister may be subject to local cold working. For example, an incorrect application of a tool might introduce indenting. It is well known that dislocation movement during plastic deformation forms cells and subgrains in many alloys. Cells are built up of nearly dislocation-free regions which are, separated by loosely knit tangles of dislocations. On the other hand, subgrains are separated by boundaries of sets of parallel dislocations [4]. It is well established that the subgrain size d that is developed during creep is frequently inversely proportional to the creep stress σ [5]. Sometimes a stronger stress dependence is observed at higher stresses [6]. In stress dip tests where the creep stress is suddenly reduced while the microstructure is unchanged, it has been found that the creep rate is proportional to the cube of the subgrain size. A survey can be found in [6]. It thus seems that the control of the subgrain size would be a powerful way of improving the creep strength. However, the subgrain size is not stable, and as just pointed out, the subgrain MATERIALS CHARACTERIZATION 90 (2014) 21 – 30 ⁎ Corresponding author. E-mail address: rsand@kth.se (R. Sandström). 1044-5803/$ – see front matter © 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.matchar.2014.01.007 Available online at www.sciencedirect.com ScienceDirect www.elsevier.com/locate/matchar