Low and high temperature hardness of WC-6 wt%Co alloys

doi:10.1016/S0263-4368(97)81231-0

International Journal of Refractory Metals and Hard Materials

Volume 15, Issues 1–3, 1997, Pages 97-101

$International Journal of Refractory Metals and Hard Materials$

https://doi.org/10.1016/S0263-4368(97)81231-0 Get rights and content

Abstract

This paper reports hardness measurements on WC-6 wt%Co of three different grain sizes in the temperature range from −196 to 900 °C. Coarser grades have been found to soften with increasing temperature at a higher rate than finer grades. It has been confirmed that hardness decreases with increasing grain size over the whole range of temperatures and it has been shown that the decrease in hardness with increasing grain size follows a Hall-Petch-type relationship at all the temperatures tested.

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Extraordinary high-temperature mechanical properties in binder-free nanopolycrystalline WC ceramic
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Citation Excerpt :
In comparison, the Hv of WCCo alloys underwent a drastic decrease at around 600 °C. The Hv decreased by more than 50% with increasing testing temperature from room temperature to 900 °C [7,8]. YG10, for instance, its Vickers hardness at 1000 °C drops severely from 16.5 GPa at room temperature to 6.8 GPa (Fig. 5(a and d)).
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Hardmetals or cemented carbides are used in a wide range of applications due to their excellent mechanical properties. WC-Co hardmetals with the same room temperature hardness can be obtained by different combinations of the WC grain size and cobalt content. However, the thermal conductivity of such hardmetal grades is not equal. Applications such as cutting may require a certain combination of hardness and thermal conductivity, which means that a targeted adjustment is desirable. In this study a wide range of hardmetal grades was studied in respect of microstructure, hardness and thermal conductivity in the temperature range between 20 °C and 1000 °C. Results show that thermal conductivity is considerably influenced by Co content, WC grain size and Cr₃C₂ content. Furthermore, hardmetal grades with the same hardness at room temperature retain hardness very differently at elevated temperatures. For the selection of hardmetal grades for high temperature applications these findings help to choose the right composition in regard to Co content and WC grain size.
Preparation of tungsten carbides by reducing and carbonizing WO<inf>2</inf> with CO
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Tungsten carbides, which can be widely used as cutting and drilling tools, chemical catalysts and aerospace coatings, have attracted widespread attentions. The present work reported a simple method to prepare tungsten carbides by using WO₂ as the raw materials and CO as the reduction and carbonization agent. It was found that at lower and moderate temperatures, all the final product was WC, while at higher temperatures, the final products were W₂C or W. The reduction and carbonization mechanism was also studied theoretically and experimentally. WO₂ was reduced and carburized to WC in one step at lower temperatures; WO₂ was first reduced and carburized to W₂C and then to WC at moderate temperatures; while at higher temperatures, WO₂ was first reduced to metallic W and then carbonized to WC or W₂C, but with the temperature increasing, the carbonization became difficult and required much higher CO content.
Effects of milling on the microstructure and hardness of Al<inf>2</inf>NbTi<inf>3</inf>V<inf>2</inf>Zr high-entropy alloy
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Lightweight Al₂NbTi₃V₂Zr alloy was fabricated through mechanical milling and vacuum hot pressing. The effects of milling on the microstructure, phase evolution, and hardness of the fabricated alloy were investigated. High-energy (HE) milling decreased the size of crystalline grains and activated the powders for subsequent sintering. HE-annealed powders at above 900 °C and all sintered bulks consisted of body-centered cubic (BCC) and intermetallics α and β. Low-energy (LE) milling only provided homogeneity to the powders. LE bulks exhibited a microstructure similar to that of HE bulks at 1450 °C–1550 °C. The hardness of the HE bulk was higher than that of the LE bulk and exhibited a peak value of 781 HV at 1250 °C. All Al₂NbTi₃V₂Zr bulks possessed low density ranging from 5.05 g/cm³ to 5.25 g/cm³. Two empirical equations for hardness of the bulks were proposed for the BCC matrix bulks and simple cubic matrix bulks respectively and the estimated results are consistent with the experimental measurements.
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Citation Excerpt :
However, it is well known that high temperature properties are more difficult to obtain than room temperature properties because of the requirement of more complicated experiment instrumentation and multiple competing deformation mechanisms at high temperature [21,22]. In spite of these challenges, many studies have revealed that high temperature hardness is proportional to room temperature hardness and both of them follow the Hall-Petch relationship shown in Eqs. (1), (2) and (3) [23,26,27]. Furthermore, deformation of WC-Co changes from a brittle manner to a ductile flow at around 700 °C [20,22–27].
The correlations of the microstructure, room temperature hardness and toughness, and high temperature compressive stress and strain of a series of WC–6 wt% Co materials with grain sizes at sub-micrometer ranges (< 0.30 μm) have been investigated. It is found that the average size and size distribution of WC grains play a critical role in both room temperature and high temperature (600 and 800 °C) properties. Furthermore, the maximum compressive stresses at 600 and 800 °C increase with increasing the room temperature hardness and decreasing grain sizes. However, the fracture strain in compression at 600 and 800 °C cannot be predicted from the room temperature toughness. This absence of a relationship is attributed to the facts that the fracture strain of compressive tests at high temperatures is dictated by significant plastic flow of the Co phase, whereas the toughness at room temperature is controlled by crack propagation with little plasticity. In general, small grain sizes (at ~ 0.20 μm) with or without a bi-modal grain size distribution are beneficial to the maximum compressive stress and fracture strain at 600 and 800 °C. In contrast, grain sizes at ~ 0.27 μm with a bi-modal grain size distribution are good for room temperature toughness, but offer inferior high temperature compressive stresses and fracture stains. Thus, the design of the microstructure at a fixed Co composition depends on the intended applications.

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Low and high temperature hardness of WC-6 wt%Co alloys

Abstract

Mater. Sci. Engng

Mater. Sci. Engng

Mater. Sci. Engng

Metall. Trans. A

Powder Metall. Int.