Influence of Elevated Temperatures on Mechanical Properties and Microstructure of C106 Copper Investigated by In Situ He
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C106 copper (Cu-DHP: phosphorus deoxidized copper) is extensively used in industrial applications such as heat exchangers, water piping, and pressure vessels[1] due to its good formability, corrosion resistance, and weldabiltiy. Studies have shown that the residual phosphorous in the copper improves its weldability as well as its creep resistance compared to Cu-OF (oxygen free) copper particularly at elevated temperatures over 150 C.[2,3] Extant industrial processes operate at elevated temperatures (~ 500 C) according to process requirements. At these temperatures, materials used previously (C106) may present an instability within the system due to a propensity to recover and recrystallize. This study investigates the use of C106 copper in elevated temperature processes by means of experimental analogs representative of industrial processing. C106 has a recrystallization temperature of around 300 C, but the
S. TAYLOR, I. MASTERS, Z. LI, and H.R. KOTADIA are with the WMG, the University of Warwick, Coventry CV4 7AL, UK. Contact e-mail: [email protected] Manuscript submitted May 22, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
influence of strain on the initiation of recrystallization at extant operating temperatures has not been investigated.[4,5] C106 strain hardens and thus achieves increases in strength following forming due to the formation of point defects and dislocation tangles.[6] Within this study, forming limit curve (FLC) testing was conducted using a Nakajima hemispherical tool in direct contact with test pieces to establish material formability.[7] Nakajima testing is well established for the determination of forming limit curves for various materials including copper and steel[8, 9]; samples from this testing were then used to establish hardness values to look at the strain hardening achieved and subsequent strength reductions following exposure to elevated temperatures. To glean better understanding of recrystallization characteristics of metallic materials in situ techniques have been widely employed. These allow for direct observation of events rather than post facto sample preparation where previous events must be inferred.[10–13] In situ observations are extremely useful as recrystallization is a highly complex process effected heavily by stored energy which is itself impacted by many variables [strain extent, grain size, grain boundary characters, chemical elements, crystal orientation, and stacking fault energy (SFE)].[14–16] Understanding better the recrystallization region of the material and related microstructural evolution will help to better inform industrial processes and avoid any potential problems. Pre-rolled commercially available C106-DHP (99.85 pct Cu, 0.15 pct P) was used throughout this study. The material was nominally supplied in the annealed condition as 0.5 mm sheet; no further thermomechanical processing was applied prior to testing. To offer analogs of industrial processes materials were subjected to testing using a Nakajima punch operating with a constant pu
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