The relation between microstructure and mechanical properties of cobalt- and nickel-rich Co-Ti-C and Ni-Ti-C alloys

Sammanfattning: Age-hardening processes up to l000°c which might be important in the binder phase of the Co-Ti-C and Ni-Ti-Chard metal systems have been studied mainly by high voltage electron microscopy, (HVEM), optical microscopy (OM) and scanning electron microscopy (SEM) as well as by micro-and macrohardness measurements and tensile testing at room and elevated temperatures (500°-800°). In addition,the allotropic transformation of cobalt has been followed by X-ray diffraction in order to permit a more complete interpretation of the changes in microstructure reflected in the mechanical testing. Various degrees of FCC stabilization to room temperature are obtained from carbon and titanium in solid solution, from predeformation, (related to fine grain size), and from the very early stages of decomposition.In the Co-Ti-C system a wide range of precipitation morphologies is possible on ageing; the only precipitated phase is TiC usually with a cube coincidence orientation relationship to the FCC-Co matrix. The nucleation sites depend sensitively on ageing temperature with homogeneous precipitation of coherent particles dominating at low temperatures (600°c) together with grain boundary precipitates associated with precipitate free zones along the grain boundaries. Stacking fault precipitation is common at medium temperatures (700°-850°c) and dislocations are the main nucleation sites at high temperatures (850°-l000°C). The grain boundary precipitates are fairly fine (above 600°C) and often associated with twin boundaries; a dispersion of rods in unusual orientations are observed over most of the temperature range investigated.The homogeneous precipitation produces the best strengthening in Co-Ti-C alloys with a peak in hardness and yield strength after 600°C/64h. This material is, however, rather brittle at all temperatures; alloys with stacking fault precipitation give cleavage at room temperature but are ductile at elevated temperatures. The solution-treated metastable FCC-structure is characterized by high work hardening rates, high ultimate tensile strengths and large elongations to fracture due to stress- and strain-induced martensite transformation and dynamic strain ageing.In comparison to the Co-system, the microstructure in the Ni-Ti-C system is affected by the higher stacking fault energy of the matrix and solubilities of Ti and C. After a satisfactory solution treatment, followed by rapid quenching, carbide precipitation occurs as a sparse distribution of homogeneously nucleated precipitates at soo0c and for higher temperatures as lath-shaped dislocation nucleated precipitates as well as grain boundary particles. Under certain conditions, depending sensitively on solution treatment, repeated nucleation on climbing dislocations is obtained. The principal differences from the Co-system, are the very rapid precipitation at all temperatures and the more complex relationships as regards interlattice orientations and growth directions. At room temperature, the solid-solution hardening effects are of the same order as peak hardness after 600 C/500h. At elevated temperatures (500° -800°C) the coarse TiC distribution exerts some influence on the mechanical properties (particularly UTS).