Looking for New Polycrystalline MC-Reinforced Cobalt-Based Superalloys Candidate to Applications at 1200°C

Abstract

For applications for which temperatures higher than 1150°C can be encountered the currently best superalloys, the γ/γ′ single crystals, cannot be used under stress because of the disappearance of their reinforcing γ′ precipitates at such temperatures which are higher than their solvus. Cobalt-based alloys strengthened by refractory and highly stable carbides may represent an alternative solution. In this work the interest was focused on MC carbides of several types. Alloys were elaborated with atomically equivalent quantities in M element (among Ti, Ta, Nb, Hf, or Zr) and in C. Script-like eutectic TiC, TaC, NbC, HfC, and ZrC carbides were successfully obtained in the interdendritic spaces. Unfortunately, only one type, HfC, demonstrated high morphological stability during about 50 hours at 1200°C. The concerned alloy, of the Co-25Cr-0.5C-7.4Hf type (in wt.%), was further characterized in flexural creep resistance and air-oxidation resistance at the same temperature. The creep behaviour was very good, notably by comparison with a more classical Co-25Cr-0.5C-7.5Ta alloy, proving that the interest of HfC is higher than the TaC one. In contrast the oxidation by air was faster and its behaviour not really chromia-forming. Significant improvements of this chemical resistance are expected before taking benefit from the mechanical superiority of this alloy.

Effect of NbC Addition on the High-Temperature Oxidation Resistance of Co- and Ni-Based Chromium-Rich Alloys

“…Beside TaC and HfC, other MC carbides may be considered, involving metals not so expensive as tantalum and hafnium in particular. Many of them may crystallize with a mechanically favorable script-like morphology, as demonstrated with recently investigated polycrystalline cobalt-based [11] and nickel-based [12] alloys, with consequently potential creep resistance at very high temperature (e.g. 1200°C).…”

The Effect of Heat Treatment on the Oxidation Resistance of Cobalt-Based Superalloys

“…Similarly, Mn has been observed to increase oxidation resistance by stabilising Cr 2 O 3 and Al 2 O 3 scales [21]. It has also been shown that benefits to oxidation resistance may be derived through incorporation of Hf to form more resilient refractory metal carbides [22,23]. Whilst initial results appear encouraging, particularly for the high temperature morphological stability and creep resistance of HfC-containing alloys, research is still required to identify the elemental additions that provide optimal environmental resistance without compromising mechanical performance.…”

Influence of Co Content on the High-Temperature Oxidation Performance of (Ni,Co)–25Cr–0.4C–6Ta Cast Alloys

“…Beside chromium carbides, tantalum carbides can play a special role, for example in the MarM-509 alloy [3][4][5]. Even if many MC carbides may be obtained in conventionally cast Ni-based [6] or Co-based [7] alloys, TaC carbides keep an interest high enough to be still employed in numerous superalloys [8]. Primary carbides are principally used in equi-axed polycrystalline alloys and these ones often contain also high contents in chromium to resist oxidative chemical aggression at high temperature [9].…”