Creep properties of different γ′-strengthened Co-base superalloys

Abstract

The influence of various alloying elements on the creep properties of polycrystalline Co-base superalloys hardened by a ternary L1 2 compound, Co 3 (Al,W) (γ'-phase), was investigated. A Ti containing quaternary alloy shows creep strength similar to Ni-base superalloys IN100 and IN713C at 850 °C and strongly superior to conventional Co-base superalloys as Haynes 188.The activation energy for creep between 850 and 950 °C is similar to the polycrystalline Nibase superalloy IN 100 in the same temperature range. Strengthening of the grain boundaries by third phase precipitates was found to be crucial for the mechanical properties. This can be achieved either by precipitation of borides or by additional intermetallic phases which precipitate due to oversaturation. During compressive creep at 850°C only a slight tendency for directional coarsening occurs, while at 950°C distinct γ/γ′-rafts perpendicular to the external compressive stress axis are formed which indicate a positive lattice misfit even at 950°C.

Creep Strength and Microstructure of Polycrystalline γ' - Strengthened Cobalt-base Superalloys

“…Molybdenum ( Figure 8) and vanadium have a similar stabilizing effect on the γ'-phase, however, without a pronounced improvement of the mechanical properties [14]. The increase in the γ'-solvus temperature is moderate [14]. Addition of 2 at.% silicon enhances the oxidation resistance [10] and slightly increases the solvus temperature as well as the volume fraction of the γ'-phase (Table 1).…”

“…For better comparison, creep tests on conventional cast Co-base (Mar-M509) and cast Ni-base (IN100) superalloys were performed and added to the diagrams. In general, at 850 °C the creep strength of the experimental alloys is between that of the conventional alloys Mar-M 509 and IN100 (some data have already been published in [14]). An exception is the 9W-2Ti-0.12B alloy exhibiting a creep strength higher than that of the Ni-base superalloy IN100.…”

“…The creep strength of the B-doped ternary alloy 9W-0.04B at 850 °C is by magnitudes superior to that of the wrought Co-base alloy Haynes 188 [14] and, as shown in Figure 18, also superior to the cast Co-base alloy Mar-M 509. Thus, the effect of the γ' hardening on creep strength is much stronger than that of carbide hardening and solid solution strengthening utilized in conventional Co-base superalloys.…”

“…Additional investigations of the long term stability were conducted on solution heat treated samples encapsulated in argon atmosphere and aged at 750 °C and 900 °C for 1000 hrs, respectively. [9,14]. Compositions marked with "*" could not be determined using EDS.…”

“…The rafting process is caused by a superposition of external applied stress on the coherency stresses due to the lattice misfit between the γ-and γ'-phase [28]. the stress axis in compression [14].…”