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

Abstract

a b s t r a c tFive cast cobalt alloys based on Co-8Ni-30Cr-0.4/0.45C and containing Ta, Nb, Hf and/or Zr were studied by metallography in the as-cast condition and after treatments at 1300 • C. The obtained MC carbides were all interdendritic with a eutectic script-like morphology. For similar carbon contents, the HfC carbides are the most developed in the as-cast microstructure and the most stable at 1300 • C. As-cast, the TaC carbides are less developed than the former and they tend to become more fractioned and less present in microstructure at 1300 • C. The NbC carbides, which have initially the same morphology and the same fraction as TaC, rapidly dissolve at 1300 • C. The cobalt alloys containing HfC or TaC are chromia -forming at 1300 • C. The NbC-containing alloy catastrophically oxides after only few hours at 1300 • C. The average hardness is the highest for the HfC-containing alloy and the lowest for the NbC-containing alloy.

Oxide Spallation During Post-isothermal High Temperature Oxidation Cooling of Cr-rich Cast Alloys Highly Alloyed with Hf

“…To conclude one can notice a more or less evident beneficial effect of hafnium on the oxide spallation. Initially introduced in the present alloys to obtain script-like eutectic MC carbides which are especially stable at very high temperatures on a longer time [11] to maximize the creep resistance of these alloys [12], hafnium was totally trapped in the HfC carbides, and absent in solid solution in the matrix (or with negligible concentrations). The high stability of the HfC carbides did not allow neither any enrichment of the matrix in hafnium nor Hf diffusion.…”

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.…”

Microstructure evolution in bulk and surface states of chromium rich nickel based cast alloys reinforced by hafnium carbides after exposure to high temperature air

“…7,8 Other MC carbides, such as HfC, which are less common than TaC, can be considered for reinforcing cast chromium rich nickel based alloys. Indeed, these Hf carbides, which were observed earlier with a script-like eutectic shape in the interdendritic spaces 9 in Cr rich Co based cast alloys demonstrated high stability at very high temperatures. They have recently shown the same shapes, locations and high volume fractions in Cr rich Ni based alloys.…”