Surface analysis of the selective oxidation during austenitic annealing of CMnSi high strength steel

The selective oxidation of three CMnSi steels with different C levels was studied by XPS. The formation of a gradually covering SiO2 film is observed until a transition temperature above which the film disappears, resulting in a renewed appearance of metallic iron at the surface. This change in oxidation behavior is thought to be due to the SiO2 reduction by solute C which is thermodynamically favorable at high temperature. At the same time the precipitation of a BN film on the metallic surface was observed which prevented any further oxidation

Experimental investigation of microstructure and ageing behaviour of bulk Zn-(1-18) wt% Al-(0-0.06) wt% Mg alloys

International audienceFine scale microstructure and hardness properties of bulk Zn-Al-Mg alloys have been investigated after solution-annealing, during ageing at room temperature and after ageing at 95 degrees C. Small-angle X-ray scattering measurements evidence nucleation, growth and coarsening of tiny elongated precipitates (probably of the (Al) phase) in Zn-Al-Mg alloys during ageing, with a characteristic size of 10 nm, and a characteristic time scale of 1 h at 95 degrees C to complete the precipitation reaction. Small additions of magnesium strongly enhance this precipitation. A strong effect on both hardness and two-stage precipitation enthalpy was evidenced. The nodules of both alpha-(Al) and eta-(Zn) phases formed by decomposition of the former alpha' phase showed a bimodal size distribution. An ageing treatment at 15 and 95 degrees C mainly affected the bulk of former alpha' particles i.e. the finest areas of this microstructure