Atomic-Scale Analysis of Oxide Inclusion in Weld Metal Using Atom Probe Tomography

Atomic-scale analysis of a spherical oxide inclusion of several micrometers in weld metal containing Ti was performed using a pulsed-laser atom probe. Many types of field-evaporated metallic oxide molecular ions were generated, some of which had overlapping peaks in the mass spectrum. Reliable ion assignment through peak deconvolution allowed us to obtain three-dimensional atom maps and concentration profiles of constituent elements from the oxide core to the ferrite matrix through the interface. It was found that multiple thin oxide layers with different compositions were non-uniformly stacked on the oxide core; the outermost oxide layers contained a high concentration of Ti. It was also found that a Mn-depleted zone approximately 30 to 40 nm in thickness was generated around the oxide in the steel. The formation mechanism of oxide inclusions with multiple thin oxide layers was proposed based on the results of atom probe tomography analysis and thermodynamic calculation.