Experimental and thermodynamic study of the microstructure evolution in cobalt-base superalloys at high temperature

Abstract

Experiments and thermodynamic calculations were performed on five cobalt-base superalloys containing tantalum carbide and chromium carbides in order to evaluate the accuracy of thermodynamic calculation for this alloy family. The studied quantities were the solidus temperature and the phase fractions of the different carbides observed after a long time exposure at different high temperatures. The predicted and observed phases are in good agreement. However the measured phase-fractions and melting temperatures were sometimes higher than the calculated values. The disagreement in the phase fractions can be attributed to the fact that surface fractions and volume-fractions are not necessarily equal.

High temperature properties of several chromium-containing Co-based alloys reinforced by different types of MC carbides (M=Ta, Nb, Hf and/or Zr)

“…In the latter alloy the interdendritic compounds could be considered as matrix, instead the cobalt solid solution, because of its higher volume fraction. The alloys with equal atomic contents for carbon and for the M elements present are the most refractory, with temperatures of fusion's start of at least 1300 • C. When the atomic content of M elements exceeds too much the carbon atomic content, this temperature becomes significantly lower, as it is the case for other alloys in which M elements were less present than carbon and in which chromium carbides appeared in microstructures in addition to MC carbides [4].…”

“…Tantalum is a carbide -former element which is more efficient than chromium. Therefore TaC carbides can be easily obtained in Cr-rich cobalt alloys instead of chromium carbides [4]. It is also true in chromiumrich ferritic or austenitic alloys based on (Fe,Cr) or (Fe,Ni,Cr) [5,6] but TaC are more difficult to obtain in (Ni,Cr)-based alloys [7].…”

Influence of the morphologic evolution of the eutectic carbides at high temperature on the thermal expansion behavior of refractory cast alloys

“…Among the modifications which are usually observed in the microstructure of industrial superalloys, one can give as examples: the establishment of a rafting structure for intermetallic precipitates in ␥ -reinforced nickel-based superalloys [1], coarsening of grains and morphology evolution of carbides [2], the fragmentation of TaC carbide fibers in eutectic Co-based superalloys directionally solidified [3], or the same phenomenon but affecting the interdendritic TaC carbides [4][5][6] or other MC-carbides [7] with which several equiaxed cast superalloys are strengthened. Since the interdendritic carbides play an important role at high temperature for both the mechanical resistance of the alloys (as reinforcing phases [1,2]) and the oxidation/corrosion resistance (grain boundaries = special diffusion paths notably for Cr [8,9]), it appears that such carbides modifications may influence the mechanical/thermomechanical behavior of these alloys, as well as their oxidation rate.…”

High-temperature microstructures of ternary Co-30wt.% Cr-based alloys over the [0–2.0wt.%] carbon range

“…In this field, the absence of the highly oxidizable element Al and the possibility to obtain carbides at solidification with an element which necessarily belongs to the chemical composition to resist high-temperature oxidation and corrosion, let the simple ternary Co-Cr-C alloys family to be very interesting for both high-temperature properties and easiness of fabrication. Indeed, this ternary system is already the base of more complex alloys which also include, for example, less than 10 wt.% of Ni for stabilizing the FCC matrix at intermediate temperatures by preventing the appearance of the HCP low-temperature structure of cobalt [8], and some percents of Ta for obtaining highly refractory carbides [8][9][10], from which the compositions of numerous commercial cobalt-base superalloys. Otherwise, these ternary Co-Cr-C alloys can also be interesting without any addition.…”