Thermal and mechanical properties of infiltrated W/CuCrZr composite materials for functionally graded heat sink application

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Abstract

Functionally graded tungsten/copper composite materials are considered as interlayer material for the water-cooled divertor target of fusion reactors consisting of a tungsten base armor and a copper alloy heat sink. The W/Cu composite interlayer is supposed to reduce the thermal expansion mismatch and to strengthen the heat sink. A critical drawback of this composite is loss of strength at elevated temperatures owing to the softening of the copper matrix. To solve this problem, we developed a novel tungsten/copper composite using precipitation-hardened Cu1CrZr alloy instead of pure copper. To this end, a fabrication route based on melt infiltration into a tungsten skeleton was established. Comprehensive characterizations and tests were performed on the specimens of three compositions (30, 50 and 70 vol.% of tungsten) at temperatures of 20, 300 and 550 °C. In this paper, extensive data of thermal and mechanical properties are presented. It turned out that the composites possess a strongly enhanced strength compared to the W/Cu composites and unreinforced alloy. The tensile behavior exhibits a significant hardening effect even for small W content while the rupture strain is decreased as well. Nevertheless, the composites show a still acceptable ductility for W content up to 50 vol.%. The composite of higher W content becomes fully brittle. Graded composites were also produced. Metallographic analysis confirms a good bonding between the layers. The thermal conductivity and thermal expansion data exhibit a typical rule-of-mixture behavior indicating a high quality of the materials.

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:cW/CuCrZr=1ρw/CuCrZr(fWρwcW+fCuCrZrρCuCrZrcCuCrZr)withfW=1-fCuCrZrwhere 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.

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