Investigation On Microstructure Control And Mechanical Properties Of Novel ?'-strengthened Co-Ti-(V,Ta)-based Superalloys

Since the discovery of the ?'-Co₃?Al,W?phase with L1₂ structure,novel ?'-strengthened Co-based superalloys have become one of the hot research point in the field of superalloys.The melting points of Co-Al-W-based alloys are higher than that of nickel-based superalloy Waspaloy,and Co-Al-W-2Ta alloy shows excellent high-temperature mechanical properties.Co-Al-W-based alloys possess high mass densities due to the requirement of high tungsten content?7?10 at.%?for stabilizing ?'?L1₂?-precipitates.Because of high densities,Co-Al-W-based alloys are inappropriate for applications such as turbine blades for aircraft engines where the strength to weight ratio is important.Moreover,It has been proven that ?'-Co₃?Al,W?phase is metastable at 900 ?.These problems severely limit the the development of Co-Al-W-based superalloys.A stable ?'?L1₂?-precipitates phase Co₃Ti exists in the binary Co-Ti system.Because of low melting point and high ?/?' lattice parameter misfit of Co-Ti-based alloys,the development of ?'-Co₃Ti is severely limited.Our previous studies have confirmed the existence of ?'?L1₂?-precipitates Co₃?Ti,V?and Co₃?Ti,Ta?phase in ternary Co-Ti-V and Co-Ti-Ta system.Addition of V has significantly decreased the?/?' lattice parameter misfit of ?'?L1₂?-precipitates phase Co₃Ti and V is also proven to increase the ?' solvus temperatures of the alloys containing ?'-Co₃Ti.Moreover,Co₃Ta is considered to be a metastable ?' phase,and Ta addition can significantly improve high-temperature mechanical properties of Co-Al-W-based and Co-Al-Mo-based superalloys.However,the current research on Co-Ti-Ta-based alloys has not been reported yet.Therefore,this paper optimized and designed compositions of master alloys based on the phase equilibrium of the Co-Ti-V and Co-Ti-Ta ternary systems which were measured by our group.The influence of some alloying elements to Co-Ti-?V,Ta?-based alloys on ?/?' microstructures,?' phase stability,and high-temperature mechanical properties were investigated.The main contents of this study are as follows:?1?Co-?84?Ti₄V₁₂?at.%?was selected as the composition of master alloy based on the phase diagram of the Co-Ti-V ternary system.The effects of 2 at.%alloying element X?X:Nb,Mo,W,Al,Si,Cr?on the ?/?' microstructures,?' ph ase stability and room-temperature mechan ical properties of Co-Ti-V alloy have been investigated.The results showed that Nb or W addition significantly improved the stability of ?' phase and room-temperature mechanical properties of the Co-Ti-V master alloy.The crystal structure of?' phase,partitioning behavior of different elements across the ? matrix and ?'precipitate,and high-temperature mechanical properties in the Co-Ti-V-2X?X:Nb,W?quaternary alloys have been investigated.Novel ?/?two-phase Co-Ti-V-2X?X:Nb,W?quaternary alloys with low densities and great high-temperature mechanical properties were prepared,and 0.2%flow stresses of Co-Ti-V-2Nb alloy were better than those of IN939 at 750?-1000?.?2?The influence of 2 at.%Ta addition to Co-Ti-V-2X?X:Nb,W?alloys on ?/?'microstructures,?' solvus temperature,and high-temperature mechanical properties were investigated.It showed that the ?/?' two-phase Co-Ti-V-2W-2Ta alloy we designed had excellent high-temperature mechanical properties,and its solvus temperature of ?'phase was higher than that of IN939.Moreover,0.2%flow stresses of Co-Ti-V-2W-2Ta alloy were higher than those of IN939 at 25 ??1000 ?.The influence of 1 at.%Ir or Ru addition to Co-Ti-V-2Nb alloy on ?/?' microstructures,?' solvus temperature,and high-temperature mechanical properties were investigated.Research showed that both Ir and Ru improved the stability of the ?'phase,but both of them deteriorated mechanical properties of Co-Ti-V-2Nb alloy at 800 ??1000?.?3?The composition of Co-Ti-Ta ternary alloy was designed and the reasonable heat treatment process was formulated based on the Co-Ti-Ta ternary phase diagram.The effects of 2 at.%Ni or W to Co-Ti-Ta alloy on the ?/?'microstructures,?' solvus temperature and mechanical properties were investigated.The ?/?' two-phase Co-Ti-Ta-2W alloy with excellent high-temperature strengths was prepared.The curve of 0.2%flow stresses verse temperature was similar to that of IN939 and 0.2%flow stresses of Co-Ti-Ta-2W alloy were higher than those of IN939 at 600 ??850 ?.