Microstructures and Surface Stabilities of {Ni-0.4C-6Ta-xCr, 0 ≤ x ≤ 50 Wt Pct} Cast Alloys at High Temperature

Abstract

Nickel-based cast alloys rich in chromium and reinforced by TaC carbides are potentially very interesting alloys for applications at elevated temperatures. Unfortunately, unlike cobalt-chromium and ironchromium alloys, it is difficult to obtain exclusively TaC as primary carbides in Ni-Cr alloys. In alloys containing 30 wt.% Cr tantalum, carbides coexist with chromium carbides. The latter tend to weaken the alloy at elevated temperatures because they become rapidly spherical and then quickly lose their reinforcing effect. In this work, we attempted to stabilize TaC as a single carbide phase by testing different chromium contents in the [0; 50 wt.%] range. Six alloys containing 0.4C and 6Ta, weight contents corresponding to equivalent molar contents, were elaborated by foundry, and their as-cast microstructures were characterized. Samples of all alloys were exposed to 1127°C and 1237°C for 24 hours to characterize their stabilized microstructures. The surface fractions of chromium carbides and tantalum carbides were measured by image analysis, and their evolutions versus the chromium content were studied. For the chosen C and Ta contents, it appears that obtaining TaC only is possible by decreasing the chromium content to 10 wt.%. At the same time, TaC fractions are unfortunately too low because a large portion of tantalum integrates into the solid solution in the matrix. A second consequence is a critical decrease in oxidation resistance. Other possible methods to stabilize TaC as a single carbide are evocated, such as the simultaneous increase in Ta and decrease in chromium from 30 wt.% Cr.

Oxidation of a 2wt.%Hf–doped (Ta,C)-rich nickel–based alloy between 1100 and 1250°C

“…)-25Cr-0.4C-6Ta, all contents in weight percents. A previous study was devoted to the behavior in oxidation at high temperature of a series of alloys including this one [16]. It was noticed that this alloy was composed, in its ascast state, of an austenitic matrix and of carbides of two types: TaC in majority, but also chromium carbides for a minor part.…”