Compressive and tensile deformation behaviour of TRIP steel-matrix composite materials with reinforcing additions of zirconia and/or aluminium titanate

Abstract

The mechanical properties and the related microstructure of metal-matrix composites (MMC) based on a high-alloyed CrMnNi steel with varying particle reinforcements (5% or 10 vol%) of magnesia partially stabilized zirconia and/or aluminium titanate were investigated. The powder metallurgical processing comprised the cold extrusion and conventional solid-state sintering at 1350 °C. The mechanical properties were examined by quasi-static compressive and tensile loading tests at ambient temperature. The microstructure characteristics contributing to significant changes in strength and ductility and affecting the failure mechanisms during deformation were characterised by scanning electron microscopy including energy-dispersive X-ray spectroscopy and electron backscatter diffraction, and by X-ray diffraction. For all compositions the stress-strain and the deformation behaviour were mainly controlled by dislocation hardening and α’-martensite formation in the steel matrix, but it was further improved by the particle strengthening of the ceramic reinforcement. Thus, the composite materials showed higher strength and work hardening than unreinforced steel specimens over a wide strain range. The pressureless sintering triggers several reactions at the metal/ceramic interface, which leads to pronounced destabilisation of the initial metastable zirconia particles with a lack of transformation toughening capability, and the formation of invalid olivine. These MMC suffer from early particle/matrix displacement and particle fracture under loading. A more positive effect of interfacial reactions was observed in the composites with addition of aluminium titanate. Here, the formation of a dense spinel structure and the reliable matrix/particle interface bonding during firing provides a significant particle strengthening effect. The combination of zirconia and aluminium titanate provides mechanical and microstructural benefits as well. Thus, aluminium titanate facilitates the consolidation of powder metallurgical processed steel-matrix MMCs via the conventional sintering for advanced load applications. The results of the study help to understand essential hardening mechanisms in composite materials and provide potential for future cellular MMC structures.