Novel L12 precipitate hardened Co-base alloys

Abstract

Conventional cobalt base superalloys relied on solid-solution and carbide precipitate based strengthening. They lacked high temperature (more than 800 ˚C) creep strength as compared to Ni-base superalloys which are strengthened by γ′ precipitates having L12 crystal structure coherent with the γ matrix. The blades of land-based gas turbines for electricity generation, need to possess hot corrosion resistance from Sulphur in the gasified coal used as fuel. This motivates the present work, which is to investigate Co–Ti–V alloys to develop Co-base superalloys possessing (γ + γ′) two-phase microstructure similar to Ni-base superalloys. Co–Ti system has thermodynamically stable γ′ phase but the lattice misfit of γ–γ′ phases is unacceptably high. Vanadium being smaller in atomic size as compared to titanium can be added to minimize the misfit. Therefore, the objective of this thesis was to investigate the effect of vanadium addition to cobalt-titanium system on the physical and mechanical properties. The Co–Ti–V alloys were vacuum arc melted and cast into rod form, followed by heat treatments and microstructural characterization. Mechanical testing was carried out to evaluate the strength from room temperature to high temperature. First principles density functional theory calculations, finite element modelling, and discrete dislocation dynamics were performed to analyze the effects of various parameters on physical and mechanical properties. The addition of V to Co–Ti system decreases the γ′ solvus but increases the solidus and liquidus temperatures. Thus, it improves the homogenisation temperature window. The γ′ precipitate morphology changes from cuboidal to cuboidal with round corners with an increase in V concentration. The composition of γ′ phase suggests Ti as better γ′ phase former than V. The extent of discontinuous precipitation at the grain boundaries in Co–Ti system decreases with V addition. The constrained lattice parameter misfit of γ–γ′ phases decreases with V addition. The γ′ phase is off-stoichiometric with Co antisite defects in 20% of Ti sublattice in Co3(Ti, V). Co–Ti–V ternary alloys possess improved strength over Co–Ti binary alloy. The strength at room temperature initially decreases with V addition followed by an increase at higher concentration of V. The alloys show yield stress anomaly; increase of strength with increase in temperature with the maximum strength observed at 750 ˚C. The peak high temperature strength at 750 ˚C decreases with V addition to Co–Ti system. Dislocations are predicted to shear the γ′ precipitates due to very narrow γ matrix channels. The γ′ precipitates are predicted to be semi-coherent in the Co–Ti alloy due to high lattice misfit. The anti-phase boundary (APB) energy and complex stacking fault (CSF) energy on {111} plane are predicted to increase with V addition to Co3Ti. Whereas the stacking fault energy in both phases is predicted to decrease with V addition. The yield stress anomaly is possibly due to anisotropy in anti-phase boundary (APB) energy on {111} and {100} planes as well as elastic constants anisotropy. The creep strength of Co–Ti–V alloys at 700 ˚C is found to be better than γʹ phase strengthened Co–Ti alloy and conventional solid-solution strengthened Co-base superalloys. Grain boundary cracking is observed as the creep damage mechanism. The creep strength decreases for high V addition probably due to deviation of γ′ precipitate morphology from perfect cuboidal and decrease of extent of grain boundary precipitates.