G.M. Song

High temperature oxidation behaviour of Ti2AlC ceramic at 1200°C

Abstract The oxidation process of Ti2AlC ceramics in dry synthetic air at 1200°C was monitored with thermogravimetry. The microstructural evolution of the oxide scale with time was characterised by X-ray diffractometry, scanning electron microscopy, X-ray microanalysis and electron back scattering diffraction. The oxide scale is comprised of a continuous α-Al2O3 inner layer with isolated coarse TiO2 particles on top. At the early oxidation stage (roughly less than 0.5 h), α-Al2O3 and TiO2 are the main reaction products, while at longer reaction times the oxidation only leads to the formation of α-Al2O3. The α-Al2O3 grains in the oxide scale grow in size upon high-temperature oxidation with the grain size being uniform throughout the thickness of the scale. As diffusion of oxygen along the grain boundaries dominates the oxide scale growth, the change in grain size affects the oxide scale growth kinetics. A simple oxide scale growth model, that takes into account this change in fast diffusion paths, describes the experimentally observed oxide scale growth kinetics perfectly.

Interface Microstructure and Adhesion of Zinc Coatings on TRIP Steels

Hot-dip galvanized transformation induced plasticity (TRIP) steel sheets were recently developed for automotive applications. The microstructure and the adhesion of zinc coated CMnSi TRIP steel alloyed with P were studied. The α-Zn coating adjacent to the steel substrate consists of a continuous η-Fe2Al5-xZnx inhibition layer with columnar ζ-FeZn13 intermetallic particles on top. Along the interface between the inhibition layer and the steel substrate Mn/Mn-P oxides were frequently observed. Although these oxides at the steel surface reduce the adhesion between the zinc coating and the TRIP steel, they do not cause any bare spots during galvanizing. Upon tensile deformation of the galvanized steel sheet, cracking along the α-zinc grain boundaries preceded fracture of the interface between the α-Zn layer and the inhibition layer. After 4 % deformation the average interface crack length increased linearly with the applied strain. This interface fracture was strongly influenced by the crystalline orientation of the α-Zn grains.

Evaluation of interface adhesion of hot-dipped zinc coating on TRIP steel with tensile testing and finite element calculation

In this work, a methodology for the determination of the interface adhesion strength of zinc coating on TRIP steel is present. This method consists of a conventional tensile test in combination with finite element calculation. The relation between the average interface crack length and the applied tensile stress is determined on the partially delaminated coating with in-situ tensile test. The delamination process of zinc coating on steel substrate is simulated by using a two-grain finite element model with different interface adhesion strengths. By comparing the experimental observation with the finite element calculations, the interface adhesion strength is estimated. The interface adhesion strength of the zinc coating on transformation induced plasticity (TRIP) steel is found as high as 160 MPa. The influences of microstructural parameters of zinc coating including zinc grain orientation and grain size on the delamination behavior of the zinc coating are also analyzed with the finite element model.