Strength and deformation mechanism of tungsten wires exposed to high temperature annealing: Impact of potassium doping

Abstract

Recent efforts dedicated to the assessment of mechanical properties of tungsten wires, as means for fiber-reinforced composites, have shown that potassium (K) doping in the as-drawn state does not modify the mechanical properties of the wire. High temperature annealing (Ta up to 2300°C) leads to the severe embrittlement of the wire associated with the loss of fracture strength. In this work, we assess the transition behavior of pure and K-doped W wires exposed to the annealing in the temperature range of 1000-2300°C to identify and recommend temperatures suitable for operation and fabrication of the fiber-reinforced composites. The results of mechanical tests performed in the temperature range of RT-500°C are reported and substantiated by the electron microscopy analysis. Room temperature tests demonstrate that pure W wires become fully brittle after annealing above 1300°C, whereas K-doped wires loses ductility above 2100°C. With raising the test temperature to 300-500°C, it is found that the strength of pure W wire reduces by a factor of two at Ta=1000°C (as compared to non-annealed wire), and goes down to 100 MPa at Ta=1900°C. The K-doping suppresses the reduction of the fracture strength at least up to Ta=1900°C, thus offering a temperature gap of ~600°C for the use as reinforcement.

Micromechanical and microstructural properties of tungsten fibers in the as-produced and annealed state: Assessment of the potassium doping effect

“…in the necking region) plastic deformation [34]. Indeed, as earlier studies show the determination of the true strain of the whole sample is complicated due to the formation of multiple necking regions [24], while determination of the yield stress is somewhat unambigous because the fabrication of standardized samples from 150 µm wire is impossible. Correspondingly, here we report the ultimate tensilie strength (UTS) which was measured at 22°C (RT), 300°C and 500°C for the two sets of wires annealed in the temperature range 1000-2300°C.…”

“…Correspondingly, here we report the ultimate tensilie strength (UTS) which was measured at 22°C (RT), 300°C and 500°C for the two sets of wires annealed in the temperature range 1000-2300°C. The detailed description of the study of mechanical properties is reported in [24]. The UTS and important features of the fracture surface are summarized in Figs.…”

“…Given that the hardness measured by nanoindentation testing should be related with the flow properties of the material, we made an attempt to establish a correlation of H 0 with the mechanical properties of the wires measured from the tensile tests [22,24]. Since the extraction of the yield/proof stress for the wire is somewhat ambigous due to the lack of the standardized geometry and accuracy of the strain measurement, the most reliable (and important for the reinforcement application) mechanical property is the ultimate tensile strenght.…”

“…The tensile mechanical properties of pure W and K-doped wires were studied at room and elevated temperatures by groups of Riesch [18][19][20][21] and Terentyev [22][23][24][25]. The primary conclusion regarding the potassium doping that can be drawn from those studies is that it helps suppressing massive grain growth at least up to 1600°C for 30 min, remaining ductile at room temperature with ultimate tensile strength exceeding 1 GPa.…”

“…Particular focus was put on the understanding of the role played by potassium doping in the recrystallization process occurring under annealing made in several steps to reach 2100°C. Firstly, we summarize the results of tensile tests of the as-fabricated and annealed wires performed in previous studies [22,24]. Then we proceed with a systematic analysis of the fracture surface of the samples tested at room and elevated temperature, microstructural characterization of grain morphology by electron backscattering diffraction analysis, and nanoindentation testing.…”