Microstructure Evolution And Strain Rate Sensitivity Of Co-Base Superalloys

The new Co-base superalloys have attracted extensive attention because of the similar ?' phase strengthening method with Ni-base superalloys.New Co-base superalloys are used in turbine discs and other components of gas turbines.In order to meet the production needs of aerospace engines,two new Co-base superalloys(0C alloy and 5C alloy)with different carbon content have been developed.The superalloys have brilliant comprehensive properties in the temperature range of 600?-900?.As aeroengine materials,they usually serve at high temperature and high speed.Microstructure evolution behavior during long-term aging and strain rate sensitivity of alloys are very important.In this paper,the microstructure evolution during long-term aging and strain rate sensitivity of two new Co-base superalloys were studied.The evolution of ?' phase and carbide during long-term aging at 900? and the change of mechanical properties at different strain rates were investigated.The results show that the ?' phase in 0C alloy and 5C alloy in as-cast condition is sphere-like.The evolution behavior of ?' phase in as-cast alloy during long-term aging can be divided into four stages:squaring,coarsening,unstable splitting and dynamic equilibrium.The ?' phase in aged alloys is square-like after heat treatment.The aged alloys can be divided into three stages:coarsening,unstable splitting and dynamic equilibrium because the squaring process has been finished during the heat treatment.In the coarsening stage,the evolution of ?' phase follows Ostwald ripening theory.The driving force of coarsening is the decrease of interface energy.The critical size and coarsening rate of aged alloys are larger than those of as-cast alloys.When the critical size is reached,the instability splitting of ?' phase begins.At this time,the evolution behavior is affected by the total amount of interface energy and elastic interaction energy.During the longterm aging,the coherence relationship between ?' phase and ? matrix is maintained.The carbides in 5C alloy are spot-like,strip-like and skeleton-like,rich of Ti and Ta elements.They are MC type carbides.MC type carbides have good stability during longterm aging process.The morphology and composition of 5C alloy did not change significantly during long-term aging at 900?.Tensile tests were carried out on as-cast 0C alloy,as-cast 5C alloy,aged 0C alloy and aged 5C alloy with different strain rates.The results show that the mechanical properties of aged alloys are more stable than those of as-cast alloys at different strain rates due to the more uniform microstructure after heat treatment.When the strain rate is 102 s-1,the mechanical properties of the four alloys are all excellent.The results show that there are secondary cracks along carbide and carbide broken at the fracture surface of 5C alloy.When the strain rate is 102 s-1,the concentration of area of 0C alloys are obvious.