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 and#730;C) creep strength as compared to Ni-base superalloys which are strengthened by and#947;and#8242; precipitates having L12 crystal structure coherent with the and#947; 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 (and#947; + and#947;and#8242;) two-phase microstructure similar to Ni-base superalloys. Co Ti system has thermodynamically stable and#947;and#8242; phase but the lattice misfit of and#947; and#947;and#8242; 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 and#947;and#8242; solvus but increases the solidus and liquidus temperatures. Thus, it improves the homogenisation temperature window. The and#947;and#8242; precipitate morphology changes from cuboidal to cuboidal with round corners with an increase in V concentration. The composition of and#947;and#8242; phase suggests Ti as better and#947;and#8242; 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 and#947; and#947;and#8242; phases decreases with V addition. The and#947;and#8242; phase is off-stoichiomet...