Fabricating FE-base alloys containing rapidly solidified eutectic cementite and ultra-fine graphite by heat treating laser-melted ductile iron

A novel modification layer on the surface of pearlite-ferrite matrix ductile iron was fabricated under irradiation of Nd:YAG laser beam equipped with self-designed diffractive optical element (DOE) which produces a 5 × 5 two-dimensional array distribution at the focal plane. The microstructure of the layer along the surface and the direction of the layer depth had obvious gradient distribution, and therefore the two-dimensional microhardness map of the layer alternated higher hardness with lower hardness. The results showed that the novel modification layer is expected to have excellent combination of strength and toughness.

This paper deals with rapidly solidified binary Fe–C alloys containing 3.8 wt% and 4.3 wt% C. Their microstructure reveals that, as an effect of undercooling, the conventional eutectics have been suppressed and there is occurrence of the ferrite–cementite eutectic. Both phases are metastable: ferrite is supersaturated in C and cementite has a C deficiency. This is demonstrated by means of determination of phase fractions in metallographic sections and of lattice parameters via Rietveld refinement of X-ray diffraction patterns. A major consequence of non-stoichiometricity of cementite is the reduced value of the Curie temperature (up to 17 °C) with respect to that of the equilibrium compound. The maximum C deficiency in Fe₃C_1−x is estimated as x = 0.02. The free energy of defective cementite has been computed by means of the two sublattice model and compared with that of ferrite and austenite obtained from an assessed phase diagram. It is shown that the non-stoichiometric phase is close in energy to the equilibrium one for a composition range of several atomic per cent. It is suggested that this helps in explaining the mechanism of cementite dissolution by heavy deformation of ferrous alloys, and the ease of cementite nucleation in castings.

The effect of laser surface remelting on corrosion resistance and pitting potential of pure titanium (ASTM grade 2) sheets was investigated. A Nd-YAG laser was used to remelt the surface of the titanium sheets. Corrosion performance of laser surface remelted and titanium base sheets were then evaluated qualitatively and quantitatively by both potential dynamic measurements (PDM) and electrochemical impedance spectroscopy (EIS) in a 3% NaCl solution. It was found that there were significant differences in corrosion properties for the two materials. Analyses of the polarization curves obtained from the PDM illustrated that no pitting corrosion could be perceived in laser surface remelted titanium sheet, while clear pitting corrosion could be seen in the base titanium sheet. EIS diagrams also showed that laser surface remelted titanium has higher impedance than the base titanium. Both results are evidence that laser surface remelting can improve the corrosion resistance of titanium. Further observation using optical microscopy and scanning electron microscopy showed clear pitting corrosion on the surface of the base titanium sheet, while no pitting was in evidence on laser surface remelted sheets. The improved pitting corrosion resistance is obtained due to the microstructural changes caused by rapid solidification after laser remelting. The study clearly demonstrates that laser surface remelting can be a potential way to improve pitting corrosion resistance of titanium and to enlarge its application range in harsh environments.