Combustion synthesis of W–Cr alloys with hierarchical microstructure and improved hardness

Significance Statement

Tungsten, a promising material for high temperature applications because of its high melting point is limited by its susceptibility to oxidation at moderate temperatures. Inter-diffusion of W-Cr is very slow and it is difficult to prepare dense W-Cr alloys by powder sintering which normally requires the addition of appropriate sintering aids.

Liu et al. (2016) reported an alternative method of preparing Tungsten-Chromium (W-Cr) alloys by combustion synthesis. The work published in journal, Materials Letters, used a method where dense W-Cr alloys are produced by melt solidification instead of powder sintering.

Two major approach of sintering is addition of palladium and high energy mechanical milling which reduces particle size to nanoscale. The two sintering approach has drawbacks in terms of low melting phase and high cost in case of palladium usage and impression energy consumption which contaminates the milling media.

Reactants were prepared by mixing powders of aluminum (Al), chromium (III) oxide (Cr2O3) and Tungsten trioxide (WO3) according to chemical equation formula of (2+4x)Al + Cr2O3 + 2xWO3 = 2CrWx + (1+2x)Al2O3 where four compositions of x= 0.1, 0.25, 0.5 and 1.0 were investigated. Each batch of 200g reactant powder was cold-pressed into a cylindrical compact with diameter and porosity 40mm and 40% respectively. The reactant compact was ignited by passing an electric current of 10A for 2s in tungsten coil and burning time depended on the composition of reactant powders usually 3-5s. After combination reaction, product consisted of two layers; the upper layer was Al2O3 ceramic and lower alloy was W-Cr alloy.

Bulk density of Tungsten-Chromium alloy was measured according to Archimedes Principle. Vickers hardness was measured by indentation method with a load of 98N and dwelling time of 15s. Microstructure was examined by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS).

From the result obtained, reaction of (2+4x) Al + Cr2O3 + 2xWO3 = 2CrWx + (1+2x) Al2O3 is highly exothermic and adiabatic combustion temperature (Tad) was much higher than the Cr-xW system as well as melting point Al2O3 (2050oC) leading to synthesized CrWx and Al2O3 in molten state. The upper layer of Al2O3 and lower layer of CrWX alloy was formed due to their difference in density.

X-ray diffraction analysis revealed that as synthesized CrWx alloys were solid solutions between tungsten and chromium showing that their lattice parameter increases with increasing x value.

Scanning Electron Microscopy images and Energy Dispersive Spectroscopy results of CrWx showed all samples has a heterogeneous microstructure composed of W-rich and Cr-rich areas. CrW1.0 showed a hierarchical microstructure consisting of both micro- and nano-sized grains which is thought to be produced by spinodal decomposition.

Density and hardness of CrWx also increases with increasing x-values. Hardness of CrW1.o sample reached 9.6±0.2 GPa which is about 50% higher than convectional tungsten-chromium alloys with same composition but prepared by conventional powder sintering. The improved hardness of CrW1.0 sample is attributed to its hierarchical structure with nano-sized grains. The density and hardness of the CrWx alloys can be further improved by introducing a high-gravity field to assist the combustion synthesis process. By combustion synthesis under a high-gravity of 800*g (g=9.8 m/s2), the prepared CrW alloys exhibited a hardness of 10.7±0.1 GPa.

Combustion synthesis offers a new, fast, and furnace-free way for the fabrication of W-Cr alloys, and the W-Cr alloys have an interesting micro/nano hierarchical microstructure and improved hardness of 9.6GPa which is 50% higher than that of W-Cr alloys prepared by conventional powder sintering.

Combustion synthesis of W–Cr alloys with hierarchical microstructure and improved hardness. Advances in Engineering

Journal Reference

Guanghua Liu1, Fan Dingdong2, Jiangtao Li1, Kexin Chen3, Gang He1, Zengchao Yang1, Shibin Guo1. Combustion Synthesis of W-Cr Alloys with Hierarchical Microstructure and Improved Hardness. .  Materials Letters, Volume 166, 2016, Pages 43–45.

[expand title=”Show Affiliations”]
  1. Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
  2. School of Metallurgy Engineering, Anhui University of Technology, Hefei 243002, PR China
  3. State Key Laboratory of New Ceramics & Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
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