Oxidation and creep properties at 1200 °C of cast quaternary Ni-Cr-C-Ti alloys

Abstract

Three nickel-based alloys rich in chromium (25wt%) and containing carbon and titanium in quantities chosen to promote the appearance of titanium carbides (0.25C-1Ti, 0.50C-1Ti and 0.50C-2Ti, in wt%) were elaborated by casting in inert atmosphere. Their microstructures and their thermodynamic, chemical and mechanical behaviours at high temperature were studied. The as-cast microstructures of the obtained alloys are composed of a dendritic nickel matrix and of interdendritic carbides. These carbides are not TiC as expected, but Cr7C3, the whole titanium remaining in solid solution in the matrix. The applied {46h, 1200°C}-exposure in air led to the development of an external duplex oxide scale constituted of a thick inner chromia scale and a thin outer rutile scale. These oxide layers resisted spallation more than the ones of Ti-free ternary reference alloys in similar conditions. The bulk's chromium carbides became rounder and coarsened, with as result a decrease in room temperature hardness. The {5 or 20MPa, 1200°C}−3 points flexural creep tests showed a rather good behaviour, regarding the high test temperature. It is concluded that titanium seems hardening these bases of alloys at low and high temperatures. However the effects on the oxidation behaviour may be deleterious for the oxidation rate and subsurface Cr-impoverishment. In contrast Ti appears as beneficial for the resistances against spallation and possibly chromia volatilization. This needs to be further investigated.