Materials Science and Engineering A 527 (2010) 6956–6962 Contents lists available at ScienceDirect Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea The influence of post-processing on creep and microstructure of rolled Cu–8Cr–4Nb J.L. Walley 1 , J.L. Heelan, L.G. Vettraino, J.R. Groza, J.C. Gibeling ∗ Department of Chemical Engineering and Materials Science, University of California, One Shields Ave., Davis, CA 95616, USA article info Article history: Received 10 March 2010 Received in revised form 28 June 2010 Accepted 16 July 2010 Keywords: Copper Cu–Cr–Nb alloy Particle-strengthened Processing Creep Microstructure abstract Previous work has shown that rolling of an extruded Cu–8Cr–4Nb (GRCop-84) alloy results in higher steady-state creep rates over a range of stresses, leading to concern that other post-processing methods could have substantial deleterious effects on creep properties. To explore that possibility, constant-stress creep tests were conducted at 773 K on rolled GRCop-84 after it was subjected to either friction stir weld- ing (FSW) or a 24 h 1073 K simulated-life heat treatment. The FSW had no measurable effect on the creep rate of the rolled material, but did have detrimental effects on the creep ductility caused by softening, and thus strain localization, in the heat-affected zone of the weld. The simulated-life heat treatment decreased the steady-state creep rate by approximately 45% as compared to the rolled material, leading to longer creep life at a particular applied stress. Electron backscatter diffraction methods were used to compare the microstructural features of extruded, rolled and annealed material forms. It was determined that the rolling procedure decreased the grain size, and decreased the intensity of the texture as com- pared to the extruded form. The simulated-life heat treatment allowed for the substantial development of twins along with minor grain growth, and a substantial decrease in texturing. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Creep resistant copper alloys are of interest for moderate to high-temperature applications where the design limiting proper- ties are the strength and thermal conductivity of the material at the service temperature. In particular, actively cooled structural com- ponents in rocket nozzles and combustion chamber walls require a material with high strength to weight ratio, good creep resistance, and high thermal conductivity. Dispersion strengthened copper alloys are a natural choice for these applications because of their inherently excellent thermal conductivity and high creep strengths in oxygen and hydrogen rich environments. Copper alloys are also relatively easy to machine, form, and weld into desired compo- nents [1]. A dispersed, high melting temperature, second phase increases room temperature strength and creep resistance by pin- ning grain boundaries to limit recrystallization and grain growth and by also interfering with dislocation motion [2]. One such dis- persion strengthened copper alloy is the NASA Glenn Research Center developed GRCop-84, in which copper is alloyed with 8 at% Cr and 4 at% Nb, resulting in the formation of 14 vol% Cr 2 Nb precip- itates dispersed in the copper matrix [3]. One proposed application ∗ Corresponding author. Tel.: +1 530 752 2050; fax: +1 530 752 6222. E-mail address: jcgibeling@ucdavis.edu (J.C. Gibeling). 1 Current address: Department of Materials Science and Engineering, The Ohio State University, 477 Watts Hall, 2041 College Dr, Columbus, OH 43210, USA. of this alloy, regeneratively cooled rocket engine combustion cham- ber liners, requires good creep resistance at temperatures ranging from 773 to 1073 K for 100 launch cycles. For this application, depending upon the specifics of engine design, GRCop-84 could be used in extruded or rolled conditions, with or without friction stir welding needed for joining. Recent work has shown that there is an order of magnitude difference in creep rates between extruded and rolled material conditions [4]. Due to the substantial differences in creep rate with respect to material processing conditions, it is important to further understand this material’s mechanical prop- erties as a function of processing methods. The goal of the present study was to compare creep properties of extruded and rolled GRCop-84 with rolled GRCop-84 subjected to subsequent process- ing: a heat treatment simulating the service life cycle and a friction stir welding (FSW) step [5,6]. An additional goal was to explain changes in the creep properties in relation to the microstructural changes caused by the different post-processing methods. 2. Materials and methods All forms were produced from conventionally argon gas atom- ized -140 mesh (<106 m) Cu–8Cr–4Nb powders that were consolidated via extrusion at 1133 K (extruded form) as described in earlier work [4]. Some of the extruded material was subsequently warm rolled to 1 mm thick sheets (rolled form). Each rolling pass consisted of a 15% reduction at temperatures between 473 and 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.07.051