Thermodynamic calculations for studying high temperature oxidation of superalloys

Abstract

Microstructure phenomena resulting from high temperature oxidation of a simple nickel alloy and two cobalt-base superalloys were studied by microstructure examinations and thermodynamic calculations. The latter allowed to better quantify what happened to carbon during oxidation, which was not possible with microstructure examination and microanalysis alone, and to explain the microstructure changes due to oxidation. At 1000°C, due to the Cr oxidation, the grain boundary carbides dissolve and the carbon diffuses inwards, where new carbides form. However, at 1100 and 1200°C, the carbon diffuses outwards to the surface. Thermodynamic calculations were used to explain the seemingly different mechanisms of carbon diffusion.

Experimental and thermodynamic study of tantalum-containing iron-base alloys reinforced by carbides: Part II — Case of (Fe, Ni, Cr)-base austenitic steels

“…They also contain tantalum, with two targeted contents: 3wt.% for the FENITA1 and FENITA2 alloys, and 6wt.% for the FENITA3 alloy. Thus, these alloys are similar to the FETA1, FETA2 and FETA3 ferritic iron-base alloys previously studied [5]. Carbon is present in the three alloys with a low content (0.2wt.%) in FENITA1 and a higher content (0.4wt.%) in FENITA2 and FENITA3.…”

“…Each surface fraction was the average of five measures, and was supposed to be close to the corresponding volume fractions. Fractions of carbides were also determined from the Cr and Ta contents in the whole alloy and in the matrix, following a procedure already described in the first article [5] which presents the equations that were used.…”

“…Indeed, they are very refractory and are stable in significant amounts in cobalt-base superalloys for example [3], while they are not stable in other systems such as nickel-base alloys as seen earlier in some nickel alloys [4]. Tantalum carbides can also be present in great quantities in ferritic iron-base alloys, even if their microstructure may evaluate when exposed to very high temperatures and long times [5].…”

Study of Carbide Transformations during High-Temperature Oxidation of Nickel-Base Superalloys

“…Such phenomena were seen and studied earlier for Ni-Cr-C alloys and cobalt-base superalloys. 9 All the metallographic measures realized on both zones affected by oxidation and bulk for comparison, are listed in Table III: the mean depth of the carbide-free zone, the mean thickness of the zone where carbides are still here but modified (nature, surface fractions, new precipitation), the nature (determined by WDS micro-analysis) and the mean surface fraction of carbides in this latter zone. For comparison, the mean surface fraction of carbides in the bulk are also listed.…”

Oxidation Behavior and Mechanism of Inconel 740H Alloy for Advanced Ultra-supercritical Power Plants Between 1050 and 1170 °C

“…The light grey oxide was mainly rich in Ti and the dark grey one was rich in Al, indicating that a mixture of Al 2 O 3 and TiO 2 internal oxidation zone [2, 12, 14] quickly formed at the initial stage of oxidation. It is easy to be accepted since Al 2 O 3 and TiO 2 are the thermodynamically most stable oxide and are able to form at the lowest oxygen partial pressure [14,16]. The formation of these compounds suggests that O permeated to these locations either by travelling across oxide scale and metal or due to the adsorption during the early stage or prior to oxidation [14].…”