Abstract:In-situ TiB/TiC particle-reinforced titanium matrix composites (TMCs) based on a near-β Ti-5Al-5Mo-5V-3Cr alloy (Ti-5553) were synthesized by solid-state reaction with B 4 C and graphite particles during spark plasma sintering (SPS). In this study, investigations were focused on the influence of the molar TiB:TiC ratio on the mechanical properties of the composites. With respect to the adjustment of the molar TiB:TiC ratio, the formation of stoichiometric TiC or nonstoichiometric TiC y was considered as the literature provides conflicting information in this respect. Furthermore, the solid-state reaction behavior influenced by the matrix alloying elements is discussed in comparison to a pure titanium matrix. The hardness, compressive strength and bending strength of the TMCs were improved successfully due to the TiB and TiC particles maintaining acceptable levels of ductility. However, X-ray diffraction experiments revealed that for the adjustment of the molar TiB:TiC ratio, the stoichiometry of the TiC y particles formed must be considered as nonstoichiometric TiC 0.5 resulted from the solid-state reaction of carbon and titanium. Compared to TMCs with pure titanium matrices, more sluggish solid-state reaction kinetics were observed. This was attributed to the matrix alloying elements molybdenum, vanadium and chromium, which formed solid solutions within the reinforcing particles.
A combination of the high damage tolerance of TRIP-steel and the extremely low thermal conductivity of partially stabilized zirconia (PSZ) can provide controlled thermal-mechanical properties to sandwich-shaped composite specimens comprising these materials. Sintering the (TRIP-steel-PSZ)/PSZ sandwich in a single step is very difficult due to differences in the sintering temperature and densification kinetics of the composite and the ceramic powders. In the present study, we successfully applied a two-step approach involving separate SPS consolidation of pure (3Y)-TZP and composites containing 20 vol % TRIP-steel, 40 vol % Al2O3 and 40 vol % (3Y)-TZP ceramic phase, and subsequent diffusion joining of both sintered components in an SPS apparatus. The microstructure and properties of the sintered and bonded specimens were characterized. No defects at the interface between the TZP and the composite after joining in the 1050–1150 °C temperature range were observed. Only limited grain growth occurred during joining, while crystallite size, hardness, shear strength and the fraction of the monoclinic phase in the TZP ceramic virtually did not change. The slight increase of the TZP layer’s fracture toughness with the joining temperature was attributed to the effect of grain size on transformation toughening.
Composite materials, which consist of a metastable austenitic TRIP-steel matrix (CrMnNi TRIPsteel; TRansformation Induced Plasticity) reinforced by alumina particles (25 vol.% ceramic, designated as AT 25/75) and reinforced by alumina and MgO partially stabilized zirconia particles (Mg-PSZ) (35 vol.% ceramic, designated as AT 25/75 + MgPSZ) were synthesized through spark plasma sintering (SPS). In the AT 25/75 + MgPSZ, the steel particles were mainly surrounded by alumina. Hence, mostly steel/alumina and alumina/MgPSZ interfaces existed. The mechanical behavior of the as-sintered samples was characterized by compression tests at room temperature and 40 °C and in a range of strain rates between 103s-1and 103s1. The influence of the ceramic content, strain rate and temperature on TRIP-effect of the steel matrix was investigated. Due to the increasing ceramic volume fraction, AT 25/75 + MgPSZ exhibits the highest compressive yield strength under all loading conditions and no strain rate sensitivity. This composite showed no measurable TRIP-effect, due to the low fracture strain. The deformation-induced α’martensite within the steel particles in pure steel and AT 25/75 primary depends on the testing temperature and the strain rate. This is attributed to an increase of stacking fault energy with rising temperature. High strain rates cause adiabatic heating, counteracting the martensitic transformation.
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