Mechanochemically synthesized NbC cermets: Part I. Synthesis and structural development
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Mechanochemically synthesized NbC cermets: Part I. Synthesis and structural development B.R. Murphy and T.H. Courtney Department of Metallurgical and Materials Engineering, Michigan Technological University, Houghton, Michigan 49931 (Received 22 July 1998; accepted 20 September 1999)
The mechanochemical synthesis of NbC-based cermets is described. Nanocrystalline NbC is synthesized by room-temperature milling of Nb and graphite (or hexane) mixtures. While some structural coarsening occurs during powder consolidation to full density, a nanoscale structure is maintained. Grinding media wear occurs during milling, and milled powders contain Fe from this abrasion. This phase, homogeneously distributed in milled powders, segregates during consolidation and heat treatment, and a cermetlike microstructure results. Copper added to the powder charge yields NbC–Cu or NbC–Cu–Fe cermets. Copper-containing materials have different phase morphologies. In particular, relatively large NbC particles are dispersed in a matrix containing finer NbC and metal particles. Higher Cu-content materials also develop a pure Cu constituent on heat treatment. A companion paper, “Mechanochemically synthesized NbC cermets: Part II. Mechanical properties,” addresses aspects of the mechanical behavior of these materials.
I. INTRODUCTION
Mechanical treatment of powders can generate materials that display intriguing characteristics and properties. Amorphous metals can be produced by mechanical milling some elemental mixtures.1–4 Extended solubility (the extent being system- and process-specific) is almost invariably found when immiscible metals are mechanically treated.5,6 Nanocrystalline structures or substructures are ubiquitously developed when particulates are subjected to the high-strain deformation characteristic of highenergy mechanical treatment.7–9 Mechanical treatment can also be employed for material synthesis. Intermetallics often form from their elements when they are mechanically milled.10,11 Inorganic compounds, such as oxides, sulfides, silicides, nitrides and carbides, can be similarly synthesized.12–16 Such reactions are often termed mechanochemical reactions (MCR); alternatively, the process is called mechanochemical synthesis (MCS). Many of the inorganic compounds noted have high melting temperatures. Thus their formation at what is nominally room temperature has commercial promise, although the quantities produced in a typical batch reaction (on the order of 10 g or so) are small at present. Carbides and nitrides are currently being considered for high-temperature structural use. Their MCS offers a further advantage; namely, the fine crystallite size of mechanochemically synthesized powders. Because ma4274
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J. Mater. Res., Vol. 14, No. 11, Nov 1999 Downloaded: 16 Mar 2015
terial hardness generally increases with refinements in microstructural scale, this is another positive aspect of MCS. A number of factors impede the utilization of MCS. There is the
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