Processing of Composites by Blending of Very Fine Powders

Preparation of iron-base materials by blending of elemental powders offers wide possibilities in the design of thermally stable microstructures by adding carbides or nitrides that are resistant to coarsening. This applies to the carbides and nitrides of elements which have high affinity for carbon and nitrogen and are strong formers and stabilizers of carbides and nitrides (e.g., V, Nb, and Ta). This approach has been utilized for the design of wear-resistant iron-base materials containing unusually large volume fractions of thermally stable carbides, nitrides, and carbonitrides (Ref 3, 38). For example, the abrasive wear resistance of cold sintered Fe-25VC-5Cr3C2 and Fe-50VC compositions containing large volume fractions of a hard VC carbide is two times higher than that of the wear-resistant T15 high-speed tool steel. Representative microstructures of the cold sintered and heat treated Fe-25VC-5Cr3C2 material with the homogeneous distribution of carbides are shown in Fig. 15. The initial chromium carbide, Cr3C2, has been dissolved at a relatively high austenizing temperature (1200 °C) and has reprecipitated in the form of fine mixed carbides. Vanadium carbide particles — 1 /m size have been partially dissolved, but they are stable to coarsening at austenizing temperatures <1300 °C. For Fe-50VC, carbides form a skeleton after austenizing treatment at T > 1200 °C. Still finer microstructures can be obtained if finer starting powders of elemental iron and milled carbides, nitrides, etc. are used. It should be mentioned that compositions containing higher volume fractions (up to 80 vol%) of micron-size carbides in micron-size iron- and nickelbase matrices have been successfully consolidated to full density employing cold sintering.

Fig. 15 A representative microstructure (SEM) of cold sintered and heat treated Fe-25VC-5Cr3C2 alloy prepared from the fine elemental powder blend

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