Thermal and mechanical properties of infiltrated W/CuCrZr composite materials for functionally graded heat sink application
Introduction
One of the widely accepted design models of the water-cooled divertor for next generation fusion reactors is based on a dual-material joint structure consisting of a tungsten (W) armor and a copper alloy (Cu1CrZr) heat sink [1], [2]. Such joint component is subject to a structural integrity issue due to thermal stresses resulting from thermal expansion mismatch between the bonding partners. For example, the mismatch in the coefficient of thermal expansion (CTE) between W and the Cu1CrZr alloy amounts to 13.8 × 10−6 strain per degree Celsius at 300 °C [3]. Thermal stresses are likely to cause a failure of a joint component at the free surface edge of the bond interface [4], [5], [6], [7]. Indeed, this problem turned out to be a critical reliability issue for flat tile type divertor designs [8], [9].
Grading a bond interface by gradual material transition is a well established approach for mitigating thermal stresses in a joint structure. In practice, an interlayer of a functionally graded material (FGM) defined by intermediate material properties is inserted between the bonding partners. In the case of a tungsten/copper (W/Cu) FGM, one can additionally benefit from a strengthening effect, since the hard W particles embedded in the soft Cu matrix act as a reinforcing element. Thus, a W/Cu FGM can locally strengthen the heat sink while reducing the thermal stresses at the same time.
One of the critical drawbacks of a W/Cu FGM is the rapid loss of plastic strength at elevated temperatures owing to the pronounced thermal softening of pure Cu. A possible solution to overcome this limitation is to use a precipitation-hardened copper alloy (e.g. Cu1CrZr alloy) as matrix instead of pure copper. To the author’s knowledge, all the hitherto reported works on W/Cu FGMs are based on a soft matrix of pure copper. Thus, for developing a novel W/Cu type composite with a hardened copper alloy matrix, a dedicated research on processing and characterization is demanded.
Various metallurgical methods have been employed for producing W/Cu FGMs, for instance, powder metallurgy (resistance sintering, hot pressing) [10], [11], plasma spraying [12], [13] and infiltration [10], [13], [14], among others. In the infiltration technique, one utilizes the capillary force of molten copper infiltrating into the open pores of a porous tungsten green compact. A well-defined composition profile of a FGM can be obtained by tailoring the porosity fraction. Porous W skeletons can be prepared by several different methods, for example, mixing of powders with different particle sizes [10], electrochemical gradation [14], space holders [15] and powder segregation [16], etc. The architecture of a W skeleton needs to have a continuous network of open pores for complete infiltration and interconnection of Cu matrix [17]. The quality of a FGM is determined by the homogeneity of microstructure and residual porosity.
In this paper, we present the recent results of our metallurgical efforts for developing a novel W/Cu1CrZr FGM. Comprehensive testing data as well as fabrication method based on melt infiltration are reported. In addition, microstructural features of tensile rupture are discussed. Focus is placed on the temperature effect and composition dependence.
Section snippets
Fabrication process
The W/Cu1CrZr composites were fabricated in two steps. Firstly, a set of porous tungsten skeletons was prepared for a wide range of porosity volume fraction. The starting material was a W powder with a mass-median-diameter (i.e. d50-value) of 4 μm provided by company Plansee. The d50-value is considered to be the average particle diameter by mass. In addition, an amid wax (ethylene bis stearamide) delivered from company Hoechst was used as a space holder. The amid wax had a slightly higher d50
Characterization
The thermal conductivity was measured using a laser flash device (Netzsch LFA427) and the CTE values according to DIN 51045-1. The hardness and elastic modulus were measured using a Vickers indenter (force: 294 N) and resonance frequency analysis (RFA), respectively. 4-point bending test was made at room temperature (RT). Finally, a series of tensile tests were conducted at RT, 300 °C and 550 °C using inductive heating at the heating rate of 20 K/s and strain rate of 0.005/s. In this study, we
Thermal conductivity
The thermal conductivity was estimated from the thermal diffusivity data measured by the laser flash device by multiplying the diffusivity values with the specific heat and the density of each composite specimen. The density was determined by the Archimedes principle and the specific heat by the rule of mixture as follows:where f, ρ and c denote the volume fraction, density and specific heat, respectively. Materials data for the
Conclusions
In this study, the overall thermal and mechanical performance of infiltrated W/Cu1CrZr alloy composites were investigated for three compositions (30, 50 and 70 vol.% of tungsten) and temperatures (20, 300 and 550 °C), respectively. Firstly, the metallurgical processing route was established based on the melt infiltration technique and using tungsten skeleton preforms of controlled porosity. The measured thermal, elastic and hardness properties roughly show a rule-of-mixture type behavior and
Acknowledgement
This work was funded by Deutsche Forschungsgemeinschaft (DFG).
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Current address: MTU Aero Engines, München, Germany.