Influences of the Co content and of the level of high temperature on the microstructure and oxidation of cast {Ni, Co}-based Cr-rich TaC-containing cast alloys

Abstract

A series of six alloys derived from a Ni-25Cr-0.4C-6Ta (wt.%) base one was developed by substituting nickel by cobalt. They were synthesized by casting and exposed to oxidative environment at two high temperatures. Their bulk microstructures were studied in as-cast condition and in two high temperature aged states. Their surfaces after oxidation during aging were characterized. The cobalt enrichment succeeded in avoiding chromium carbides formation and in stabilizing the TaC carbides at high temperature. As the high temperature morphologic stability of TaC was not perfect, it was much better than the one of the chromium carbides, but can be improved by the total removal of nickel. Unfortunately, at the same time, the oxidation behavior, initially good, shows increased rate of the oxides formation. The room temperature hardness was also significantly increased by the substitution of Ni by Co, and decreased after aging when carbides became rounder or fragmented.

Oxide scale spallation behaviour of cast chromia-forming TaC-strengthened superalloys

“…The isothermal oxidation behaviours in air of alloys based on cobalt and nickel simultaneously and containing 25 wt-%Cr in addition to these 6 wt-%Ta and 0.4 wt-%C were recently studied at high temperatures. These previous tests were performed for short durations at temperatures close to 1150°C and 1250°C [22,23], to look for the best {Co,Ni} combinations allowing both good resistance of carbides against morphologic evolution and good oxidation resistance. Despite these high test temperatures the alloys appeared to be well oxidation resistant, but problems of spallation of the protective external oxide scales during cooling were pointed out, especially for the alloys richer in nickel than in cobalt.…”

Effect of Re Addition on Microstructure and Oxidation Resistance of Co-Cr-Ta-Ti-C Alloys

“…Cobalt was chosen as the primary component due to its high-temperature performance, chromium was added for improved oxidation resistance, and titanium and tantalum were added to strengthen the alloys by forming (Ta and Ti) carbides, respectively. The composition of Re-0 was determined based on the design of Berthod et.al., as described in Section 1 in the literature [7,8]. Rhenium was included in the range of 0-15 at% to examine its effect on the microstructure and oxidation kinetics.…”

“…Employing Co-Cr-based alloys might be the sole feasible resolution for the glass industry. Berthod et al proposed Co-Cr-Ta-Ti-C alloys with reasonable mechanical properties at high temperatures [7,8]. The alloy system is strengthened by the formation of Ti and Ta carbides [9,10], having a reasonable combination of mechanical properties and oxidation resistance at high temperatures.…”

“…Increasing the melting point of alloys is one potential approach, as metallic materials typically experience strength loss when heated near melting temperatures. Since the Co-Cr-Ta-Ti-C alloy system developed by Berthod et al is predicted to have a melting point of 1200 to 1400 • C [7], further alloy design is needed to increase the melting point, considering its applications at temperatures above 1000 • C. Oxidation properties are another issue. In the Co-Cr-Ta-Ti-C alloy system, Cr 2 O 3 is formed on the alloy surface.…”

Room temperature tensile behaviour of K640S Co-based superalloy

“…Under the application of stress, the carbide region can continuously generate and release dislocations into the matrix [19][20][21], enabling the dislocations to continuously multiply and interact to strengthen the alloy matrix [22][23][24][25]. In addition, in high-temperature aging or service process, a large amount of secondary carbides of tiny M 23 C 6 and MC can be precipitated in the matrix [26][27][28], causing an interaction with defects such as dislocations and stacking faults, etc., thereby increasing the deformation resistance of alloy matrix [29,30]. However, the room-temperature tensile behaviour of K640S cobalt-based alloys has rarely been reported.…”