Study On High Temperature Properties And Creep Mechanism Of Co-Al-W Novel Cobalt-base Superalloys

The discovery of L12 hardened cobalt-base superalloys has provided a new opportunity for improving the high-temperature properties of aerospace turbine blade.The Co-Al-W ternary superalloys possess a similar microstructure with γ/γ’ nickel-base superalloys.However,the higher melting point of cobalt seems to be promising a better strength at higher temperature.At present,the thermal stability of γ’ phase,oxidation resistance and creep properties of theses alloys have not form unified conclusion,and the plastic deformation mechanism at high temperature is not mature.These kinds of problems are crucial for further perfecting this alloy system.The thermal stability of γ’ phase as well as the oxidation resistance of superalloys are critical important as they always work in air at elevated temperature for very long time.Three alloys with different W contents Co-9Al-xW-0.1B(x=8,9,11)were aged at 900℃ for 200,500,1000,2000,5000,1000Oh to study the thermal stability of γ’ phase.The morphological evolution,volume fraction varing and sizes of γ’ precipitates were investigated under Scanning and Transmission Electron Microscopy.The images showed that the volume fraction of γ’ phase decreased continuously,and the sizes of γ’ precipitates coarsened according to the LSW theory.The Electron Back Scattered Diffraction and X-Ray Diffraction were conducted to indentify the phases balance among γ-Co phase,γ’-Co3(Al,W)phase,DO19-Co3W phase and B2-CoAl phase.The results showed that the y’phase was metastable at 900℃,which would decompose into DO19 phase.Combined the images taken by Transmission Electron Microscopy the phase transformation mechanism of Co3(Al,W)→Co3W was explained in the scale of atoms.Three alloys with different W contents were isothermally exposed in air at 900℃ for 1000,5000 and 1000Oh to study the oxidation resistance.Scanning Electron Microscopy combined with Energy Disperse Spectrum and X-Ray Diffraction were carried out to to identify the structure of the oxides layers that included Al2O3,mixture oxides and CoO from the substrate to the surface.Additionally,a zone of Co3W/γ was adherent in the edge of the substrate.The morphological,structural evolution and thickness varing were investigated by Scanning Electron Microscopy,showing that the thinnest oxides layer and slowest oxidation rate were presented in Co-9Al-11W-0.1B alloy.The increasing content of W is benefit for the thermal stability of γ’ phase and oxidation resistance.Creep properties are main indicator to evaluate a superalloy since the turbine blades normally withstand low stresses at elevated temperature.Single crystallines possess improved high temperature strength and oxidation resistance without grain boundaries that are weak and harmful at high temperature.Single crystals of Co-9Al-xW(x=8,9,11)ternary alloys were produced by directional solidification combied with a spiral selector Compressive creep at 850℃ and 460MPa were carried on single crystal alloys.Subsequently,the creep curves and strain rate were analysised to figure out that reasonable higher content of W would improved the strength to some extent at high temperature.Transmission Electron Microscopy was used to reveal the development of dislocations and stacking faults in three crept alloys.The highest dentisy of dislocations and lowest dentisy of stacking faults existed in alloy containing the lowest W content,as higher content of W increased the opportunity of cutting γ’ precipitates by 1/3<112>dislocation pairs and decreased the motion of 1/2<110>dislocations in γ matrix channels.A series compressive creep tests at 850℃ were conducted on Co-Al-W single crystal alloys respectively.The morphological evolution of γ’ precipitates and γ matrix channels during different periods of creep were investigated using Scanning Electron Microscopy.The γ’ precipitates rafted strongly along the direction perpendicular to the external stress axis.The quantitative of rafting demonstrated that the γ’ precipitates started to raft when the transient strain rate reached minimum,while the γ matrix channels broadened continuously.The development of dislocations and stacking faults in each period was also investigated by Transmission Electron Microscopy.In the γ matrix channels perpendicular to the stress axis existed strong movement of 1/2<110>{111} dislocations,while the γ matrix channels parallel to the stress axis were almost free of dislocations.The preferential orientation of dislocation arose from the different local stress situations due to the external compressive stress.The stacking faults were induced by the cutting ofγ precipitates by 1/3<112>dislocation pairs.Moreover,the details of rafting was revealed by Transmission Electron Microscopy that the γ particles always merged from the corners where stacking faults concentrated.These stacking faults were believed to provide channels for the necessary directional diffusion.The rielief of coherency stresses at horizontal γ/γ’ interface by the 1/2<110>{111} dislocations drived the rafting.The γ/γ’ lattice misfit plays key role in the creep of superalloys,which may vary during the creep.Therefore the in-situ measurement of γ/γ’ lattice misfit is benefit to discover the plastic deformation mechanism of these novel Co-Al-W superalloys.The lattice parameters and misfit of γ/γ’ pahses of three Co-Al-W-0.1B alloys with different W contents during heating from 20℃ to 1000℃ were measured by Synchrotron High Energy X-Ray Diffraction.In lower temperature range,the lattice parameters of γ and γ’phases increased quite similarly,resulting in the stable misfit,which was refered to the difference of thermal expansion coefficient between γ and γ’ phases.In higher temperatue range,the dramatical decrease of misfit was caused by the decomposition of γ’ phase and diffusion of W atoms.Synchrotron High Energy X-Ray Diffraction was used to in-situ measuring the lattice parameters and misfit of γ/γ’ pahses of Co-9Al-8W single alloy during compressive creep.The diffraction patterns and peaks were analysised and fitted to obtain the evolution of lattice parameters and misfit.The lattice parameters of γ’ phase in the direction perpendicular to the stress axis increased obviously,due to the rielief of coherency stresses at horizontal γ/γ’ interface dislocations.As a result the misfit in the same direction increased.Differently,the lattice parameters of γ/γ’ pahses and the misfit in direction parallel to the stress axis presented no obviouse change.