Quantification of the Phase Transformation Kinetics in High Chromium Cast Irons Using Dilatometry and Metallographic Techniques

Metallurgical and Materials Transactions A - Further development of high chromium cast irons (HCCI) is based on tailoring the microstructure, necessitating an accurate control over the phase...     

High Chromium Cast Irons: Destabilized-Subcritical Secondary Carbide Precipitation and Its Effect on Hardness and Wear Properties

This work evaluates the effect of a destabilization treatment combined with a subcritical diffusion (SCD) and a subsequent quenching (Q) steps on precipitation of secondary carbides and their influence on the wear properties of HCCI (16%Cr). The destabilization of the austenite at high temperature leads to a final microstructure composed of eutectic and secondary carbides, with an M₇C₃ nature, embedded in a martensitic matrix. An improved wear resistance was observed in the SCD?+?Q samples in comparison with the Q one, which was attributed to the size of secondary carbides.     

Isothermal Oxidation Behaviour of Nanocrystalline RuAl Intermetallic Thin Films

It has been demonstrated that intermetallic thin films usually show different oxidation rates compared to those of bulk materials. Within the intermetallic phases, RuAl thin films have not been thoroughly investigated. Thereby, new studies of these systems are needed. Single-phase RuAl was found to be a promising candidate for protective coating material in applications that demand oxidation resistance. An important advantage of this system over other B2-aluminides arises on the coefficient of thermal expansion (CTE), which is substantially lower than that of FeAl, CoAl and NiAl, and it is closer to that of the α-Al₂O₃, thus increasing the adherence to the alumina protective layer, by decreasing the CTE mismatch between each other. In the present work, the isothermal oxidation behaviour of single-phase RuAl thin films deposited onto austenitic stainless steel substrates was studied in ambient air at 750 and 900 °C for short times (up to 60 min.). Scanning transmission electron microscopy and X-ray diffraction were performed for the subsequent analysis of the oxide scale. A protective α-Al₂O₃ scale was formed and in comparison to other aluminides, no evidence of the formation of transient alumina was found even at temperatures as low as 750 °C, being this fact an advantage over the most studied aluminides. The presence of particles at the metal/oxide interface is an indication that the oxide growth is dominated by the outward diffusion of Al cations. Moreover, this growth is described by a parabolic law showing an oxidation rate k _x  = 1.3 × 10^?13 cm²/s at 900 °C and the corresponding oxidation activation energy is 116.4 ± 7.5 kJ/mol. The sufficient Al flux to the oxidation front, in combination with the narrow inter-nuclei spacing lead to a faster formation of continuous α-Al₂O₃ compared to bulk RuAl, as a consequence of the high density of grain boundaries.