TiC-(Ti,M)C core-rim structures in solid-state manufactured steel-based MMCs

Core-rim structures in TiC-based metal matrix composites are known to be an important microstructural feature influencing the mechanical properties of the material. While this is a widely investigated topic for metal matrix composites consolidated by techniques utilizing a liquid phase, little research is available for core-rim structures in solid-state manufactured TiC-steel composites. In this work we study core-rim structures of 10 vol% TiC particles in a heat-treatable steel matrix, solid-state consolidated at 1140 °C by hot isostatic pressing. Before consolidation, TiC and steel powders were mixed by either tumble blending or attritor milling, which allows characterizing the influence of TiC particle size on the core-rim structure. SEM and STEM/EDS microstructural investigations show that in both materials a rim phase of the type (Ti,M)C (M = V, Mo, W, Cr) forms during solid-state consolidation and heat treatment. It is thereby shown that TiC in steel-based composites can be thermodynamically instable and form a core-rim structure even in the absence of a liquid phase. While significantly more (Ti,M)C forms in the attritor milled sample, it is less enriched in matrix elements compared to the tumble blended sample. Comparing thermodynamic calculations with the amount and composition of (Ti,M)C formed, it is shown that the attritor milled composite is much closer to thermodynamic equilibrium of matrix steel + 10 vol% TiC than its blended counterpart. Moreover, it is found by TEM and XRD that the rim phase has a NaCl crystal structure and grows epitaxially on the surface of original TiC particles. [Display omitted] •10 vol.% TiC – steel composites are produced by either tumble blending or attritor milling and hot isostatic pressing.•TiC is thermodynamically instable and forms core-rim structures during solid-state consolidation.•Rim structures are NaCl-type mixed carbides containing besides Titanium also Vanadium, Molybdenum, Tungsten and Chromium.•More rim-phase forms in the attritor milled sample, indicating a state closer to the calculated thermodynamic equilibrium.