Effects of residual elements on surface hot shortness

This study focuses on documenting the effects of Ni, As, Sb and Sn on the interface microstructure, grain boundary cracking, internal oxidation, oxidation kinetics and Cu-rich liquid distribution for surface hot shortness.;A series of Fe-0.3Cu-xNi, Fe-0.3Cu-xAs, Fe-0.3Cu-xSb and Fe-0.3Cu-xSn alloys with x ranging from 0.03 to 0.15 wt% were designed to investigate the isolated effects of Ni, As, Sb and Sn on surface hot shortness. Fe-0.3Cu-xNi-0.03Sn alloys with x ranging from 0.03 to 0.45 wt% were also adopted to study how Ni counteracts with the detrimental effects of Sn. The samples were oxidized in air at 1150°C for 60, 300 or 600 seconds (s) while monitoring the weight change through thermogravimetry (TG) to investigate the oxidation behavior. The post-oxidation microstructures were investigated under scanning electron microscopy (SEM) and the energy dispersive spectroscopy (EDS) technique was used to measure chemical compositions. Selected samples of the Fe-Cu-Ni alloys were prepared by a focused ion beam (FIB) system and liquid/gammaFe interface concentrations were investigated using EDS in a transmission electron microscope (TEM). FIB serial sectioning technique was used to reconstruct 3D-microstructure of grain boundary cracks in the Fe-Cu-Sn alloy. A numerical model was developed and applied to the Fe-Cu-Ni and Fe-Cu-Sn systems to predict liquid/gammaFe interface concentrations and interface morphology and compare with experimental results.;Ni promotes wavy oxide/metal interfaces (∼0.03 wt%) and internal oxidation (∼0.10 wt%), which results from the strong enrichment of Ni near the oxide/metal interface at the gammaFe side. Ni at contents as low as 0.03 wt% also decreases parabolic oxidation rates by a factor of three compared to Fe and Fe-Cu, and consequently decreases the separated liquid thickness at the oxide/metal interface. One possible reason for the lower oxidation rates could be that the presence of Ni lowers the Fe activity in the liquid phase and therefore decreases the outwards transport of Fe cation. Relatively small amounts of occluded Cu-rich phase were found in the external scales in Fe-Cu-Ni alloys of Ni contents up to 0.15 wt%. It is possible that internal oxides of different size and morphology resulting from more reactive elements such as Si are needed for causing significant occlusion. The lower parabolic oxidation rates caused by Ni and consequently less separated copper seem more important for suppressing hot shortness than the role of Ni alone in promoting occlusions.;In the absence of Ni, As, Sb and Sn at contents up to 0.10 wt% stabilize planar oxide/metal interfaces because these elements do not enrich strongly near the liquid/gammaFe interface at the gammaFe side. As, Sb and Sn also decrease the parabolic rates slightly compared to the Fe and Fe-Cu alloys.;Sb and Sn at contents as low as 0.03 wt% cause significant grain boundary cracking during short time exposure to air at 1150°C, while As does not cause any cracking. This is because Sb and Sn have large contents in the liquid phase and they are highly embrittling when present at the gammaFe grain boundaries. Fe oxides were found along the open cracks without liquid coverage at the grain boundaries in Sn and Sb alloys, and the possible oxygen sources are discussed.;The calculated interface concentrations using the numerical model for Fe-Cu-Ni and Fe-Cu-Sn systems matched the experimental results reasonably well. The XXIII prediction of interface morphology using constitutional super-saturation criterion also supports the experimental results.;The additions of Ni at contents greater than 0.3 wt% were found to suppress grain boundary cracking in the Fe-0.3Cu-0.03Sn system due to the elimination of the detrimental Sn containing liquid phase, which is qualitatively supported by the numerical model. (Abstract shortened by UMI.)...