Hot Deformation Behavior and Processing Maps of Diamond/Cu Composites
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INTRODUCTION
HIGH thermal conductivity materials are widely used in extreme environments with high heat flux and consistent thermocycling.[1–3] Recently, diamond/Cu composites, which combine diamond (which has the highest thermal conductivity) and low-cost Cu, have attracted attention because of their high thermal conductivity and thermal stability. However, Cu is non-wetting and has no chemical affinity for diamond, leading to interfacial debonding. The interfacial voids lead to high thermal resistance and significantly decrease the thermal conductivity of the composites.[4,5] In order to ameliorate the diamond/Cu interface, metal matrix alloying[6,7] or diamond surface metallization (achieved via the molten salt method,[8–10] magnetron sputtering,[11] and vapor deposition[12]) can be used. Because of its operability and efficiency, the molten salt method is widely used for diamond surface metallization. In addition, the
HONGDI ZHANG, YUE LIU, FAN ZHANG, DI ZHANG, and TONGXIANG FAN are with the State Key Laboratory for Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. Contact e-mail: [email protected] HANXING ZHU is with School of Engineering, Cardiff University, Cardiff, CF24 3AA, UK. Manuscript submitted August 14, 2017. Article published online March 16, 2018 2202—VOLUME 49A, JUNE 2018
transition element Cr can generally be used to enhance the interface bond at a low reaction temperature, leading to a composite with high thermal conductivity.[9] However, voids still remain during powder metallurgy sintering (such as spark plasma sintering (SPS)) of diamond/Cu composites even with diamond surface metallization.[13] Processing by plastic deformation, such as extrusion, rolling, or forging, can be used to improve the quality and density of these composites as these processes are generally used in the powder metallurgy processing of composites (e.g., TiCp/AZ91D, SiCp/2009Al and SiCw/ 2124A1).[14–16] However, relatively few studies on the hot deformation of diamond/Cu composites have been reported. The introduction of reinforcement severely reduces the workability of metal matrix composites (MMCs) because of the resulting increase in the dislocation density in the matrix.[17–19] In addition, most studies have focused on particle-reinforced MMCs with low particle content (< 30 pct).[18] A high level of residual stress is generated in composites containing a high volume fraction diamond (> 50 pct) following processing, which limits dislocation slip and leads to deformation instability.[20] It is thus necessary to investigate the effect of the reinforcement on the workability of diamond/Cu composites. In general, constitutive equations and hot processing maps are used to investigate the workability of alloys and composites. The hot
METALLURGICAL AND MATERIALS TRANSACTIONS A
deformation behavior can be fully understood through constitutive equations proposed by Sellars and Mctegart by correlating the strain and stress.[21] The hot processing maps base
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