Elsevier

Materials Letters

Volume 304, 1 December 2021, 130728
Materials Letters

Ultrafine-grained W-Cr composite prepared by controlled W-Cr solid solution decomposition

https://doi.org/10.1016/j.matlet.2021.130728Get rights and content

Highlights

  • Thermal decomposition of SPS samples results in UFG microstructure (<200 nm).

  • Decomposed samples have up to 65 % higher flexural strength, while 10 % lower hardness.

  • Elastic properties of present phases can be accurately calculated by computer modelling.

Abstract

The application of non-equilibrium methods such as Mechanical Alloying and Field Assisted Sintering enables the formation of a single solid solution in immiscible systems, or the ones characterized by a miscibility gap. This work shows that W-Cr solid solution can be used as an intermediate step to produce ultrafine-grained composite alloys with rod-like microstructural features. It is shown that a significant increase in strength can be achieved without an increase in hardness using this top-down material design.

Introduction

Tungsten is a refractory metal with the highest melting point, exceeding 3400 °C. However, low oxidation resistance is a serious drawback of tungsten, setting some restrictions regarding the high temperature operation conditions [1]. Chromium is a refractory metal commonly used as an alloying element to enhance oxidation resistance. Tungsten and chromium are, however, very difficult to process into an alloy or composite. Conventional technologies such as casting or thermal spraying are complicated as the boiling point of chromium is lower than the melting point of tungsten. Therefore, tungsten-based composites are frequently produced by various sintering processes of elemental powder blends. Due to the mutual interdiffusion and grain coarsening, sintered composites are limited to a micron-scale level.

W-Cr single solid solution based alloys have been recently produced by Mechanical Alloying (MA) and Field Assisted Sintering (FAST) [2], [3]. The equilibrium solubility of tungsten and chromium is limited, and the solid solution tends to decompose at temperatures below ca. 1650 °C, depending on the composition [4], [5]. However, the operation window of the solid solution is still quite large, i.e., between RT and 700 °C, as the decomposition kinetics below 700 °C was observed to be very slow [3].

In this work, we report on production and properties of an ultra-fine grained W-Cr composite material. The composite consisting of 3 phases, namely a Cr-rich and two W-rich phases is produced by the decomposition of W-10Cr single solid solution.

Section snippets

Materials and methods

W-10wt.%Cr-1wt.%Hf alloy was prepared by MA and FAST from micron-size powders using the procedure reported in [3]. Microstructure was examined using Zeiss EVO MA 15 scanning electron microscope. TEM analysis was performed at 200 kV acceleration voltage using JEOL JEM 2200FS microscope equipped with JED-2300 T Energy Dispersive X-ray Spectrometer. Phase compositions and lattice parameters were determined from X-ray diffraction (XRD) patterns obtained using Bruker D8 Discover and evaluated by

Results and discussion

After MA and FAST, the sample consists of a W-Cr solid solution and HfO2 particles (Tab. 1, 0 h). The solid solution is unstable, thus, a suitable thermal treatment within the miscibility gap of the W-Cr system triggers the decomposition. At 1000 °C, the decomposition has a convenient kinetics allowing material design with respect to the volume of emerging phases. Fig. 1 (top row) shows the decomposition progress on SEM micrographs of 0, 15, and 40 h heat-treated samples. The decomposition

Conclusions

W-Cr solid solution decomposition has shown to be a feasible top-down method for producing ultra-fine grained W-Cr composites. The W-Cr solid solution alloy with HfO2 particle dispersion decomposes during heat treatment at 1000 °C into a Cr-rich and W-rich tungsten phase, creating a composite with a fine rod-like morphology. Stability of the composite at 700 °C is comparable to single solid solution alloy. Flexural strength of the composite is significantly higher than that of single solid

CRediT authorship contribution statement

Jakub Veverka: Investigation, Formal analysis, Writing – original draft, Visualization. František Lukáč: Investigation. Andrzej P. Kądzielawa: Methodology, Software, Writing – review & editing. Martin Koller: Investigation. Zdeněk Chlup: Investigation, Formal analysis. Hynek Hadraba: Resources, Investigation. Miroslav Karlík: Investigation. Dominik Legut: Conceptualization. Jiřina Vontorová: Investigation. Tomáš Chráska: Supervision. Monika Vilémová: Conceptualization, Investigation, Writing –

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Financial support by the Czech Science Foundation through grant No. 20-18392S is acknowledged as well as the ERDF in the IT4Innovations national supercomputing center - path to exascale project CZ.02.1.01/0.0/0.0/16_013/0001791 within the OPRDE and the project e-INFRA CZ (ID:90140) by Czech MŠMT. M. Karlík acknowledges financial support from the ERDF project CZ.02.1.01/0.0/0.0/15_003/0000485.

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