Abnormal WC crystal growth from liquid Co flux occurs via eta phase decomposition. (September 2021)

• Record Type: Journal Article
• Authors: Vreeswijk, Maxim; Kot, Alexandre; Giuliani, Finn; Humphry-Baker, Samuel
• Is Part Of: International journal of refractory metals & hard materials. Volume 99(2021)
• Publication Date: 2021-09
• Subjects: Crystal growth -- Abnormal grain growth -- Eta phase -- Tungsten carbide; Heat resistant alloys -- Periodicals; Refractory materials -- Periodicals; Metallography -- Periodicals; Alliages réfractaires -- Périodiques; Matériaux réfractaires -- Périodiques; Métallographie -- Périodiques; Heat resistant alloys; Metallography; Refractory materials; Periodicals; Electronic journals; 669.73
• DOI: 10.1016/j.ijrmhm.2021.105589
• Languages: English
• ISSNs: 0263-4368

Abstract

Abstract: The growth mechanism of large WC crystals from a liquid Co-based flux is identified. This is achieved by systematically varying the growth temperature and Co content from 1200 to 1400 °C and 70–83 at.% respectively. Crystal growth was characterised using metallography and X-ray diffraction. The WC grains were bimodally distributed, consisting of a smaller (10–20 μm) population of grains, which nucleated homogenously from the liquid, and a secondary population of abnormally large grains, several millimetres in size. The abnormal grains nucleated on the (W, Co)6 C eta phase, and subsequently consumed it through a carburisation reaction. The size of abnormal grains was enhanced by adopting the eutectic composition, such that the first solid phase to form was the eta phase, whilst at the same time cooling slowly through the eta➔WC transformation temperature, at ~1300 °C. This growth mechanism could be exploited for a variety of metal carbides with similarly sluggish diffusion rates. Graphical abstract: Unlabelled Image Highlights: Growth mechanism of abnormal WC crystals from a Co-based flux is identified. Abnormal grains nucleate on eta phase and subsequently consume it by a carburisation reaction. Grain size enhanced at 83 at.% Co (eutectic composition), where eta is the first solid phase to form. Slow cooling through the eta➔WC transformation temperature, at ~1300 °C, is also required.