Composition Design And Optimization Of Coni-Based Wrought Superalloys Based On Materials Genome Engineering Approach

γ’-strengthened CoNi-based superalloys have a higher γ’ volume fraction and a lower γ’ solvus temperature compared with Ni-based wrought superalloys,which make they have a higher high-temperature strength and a wider hot processing window.Therefore,γ’-strengthened CoNi-based superalloys may be more suitable to be as next generation of wrought superalloys for application at 750℃-850℃.However,the composition design and optimization method as well as design criteria of this type of alloy have not been established,and the reported alloys were mainly designed by "trial and error" method,which makes that their comprehensive performance cannot fully meet the requirements.Microstructural stability,oxidation resistance and high-temperature mechanical properties are the key issues to the composition design and optimization of wrought superalloys,but the related researches are very limited.Moreover,due to the complexity of alloying elements in multicomponent alloy system,the traditional "trial and error"method is quite inefficient and costly,especially for investigating synergistic alloying effects.Hence,it is critical to develop an effective method to accelerate the exploration and design of multicomponent CoNi-based superalloys.Based on the previous work of our group and the concept of "Materials Genome Engineering",the following researches were carried out for CoNi-based wrought superalloys:1)The physical and chemical features such as phase transformation temperature,density and phase compositions of CoNi-based wrought superalloy were studied by CALculation of PHase Diagrams(CALPHAD)and experiments.The alloying effects on the coarsening behavior ofγ’ precipitates at 850℃ and the oxidation behavior of the alloy at 800℃-1000℃were identified.The rate-limiting element for the γ’ coarsening and the composition criteria of good oxidation resistance were determined;2)The alloying effects on the microstructural stability of CoNi-based wrought superalloys were studied efficiently and systematically by multicomponent diffusion-multiple at 850℃.More than 4000 experimental data on the relationship between compositions and microstructural parameters were obtained;3)Based on the large number of microstructural data,machine learning was employed to establish the prediction models between compositions and microstructures.According to the microstructural inverse design concept and multi-objective optimization strategy,the composition design and optimization method as well as the design criteria of CoNi-based wrought superalloys were establishd.With the help of experimental verification of microstructural stability,oxidation resistance and mechanical properties,some prototype of CoNi-based wrought superalloys for engineering application were developed.CALPHAD can effectively guide the design of CoNi-based wrought superalloy,and the designed alloy had a low γ’ solvus temperature and density,large hot processing window,good oxidation resistance and high temperature strength.During the long term aging at 850℃,the coarsening behavior of γ’precipitates followed the Lifshitz-Slyozov-Wagner(LSW)mechanism and Cr was determined to be the rate-limiting element.Lower γ/γ’ lattice misfit,higher Cr content in the y matrix and larger Cr concentration difference across the γ/γ’interface are good for decreasing the γ’ coarsening rate.At 800℃,the oxidation resistance of the alloy was attributed to the formation of a dense Cr2O3 layer.A higher content of Cr or Al was beneficial to the oxidation resistance of the alloy,while the effect of Ti was not obvious.At 900-1000℃,Al played a dominant role in the oxidation resistance of the alloys in comparison with Cr.However,Ti was detrimental to the oxidation resistance.The compositional criteria of CoNi-based wrought superalloy with good oxidation resistance are:Al≥8at.%,Al+Cr>20at.%,Ti<3at.%.The alloying effects on the microstructural stability of CoNi-based wrought superalloys at 850℃ studied by multicomponent diffusion-multiple show that Mo and W are the major elements that affect the phase equilibrium in the studied composition space.On the basis of the base alloy,Mo addition will strongly promote the precipitation of a deleterious secondary phase(μ phase),which is detrimental to the microstructutal stability.Adding Mo while reducing W can effectively prevent the precipitation of this phase,and maintain the γ+γ’ two-phase microstructure.The addition of Al,Ti,Nb and W effectively increase the γ’volume fraction,while the addition of Cr can significantly decrease the γ’ volume fraction based on the base alloy.On the basis of the base alloy,the addition of Al,Ti and Mo significantly accelerate the coarsening of γ’ precipitates and increase the γ’ size,while the addition of Cr or using Nb to replace Ti will decrease the γ’size to a certain extent and slow down the coarsening of γ’ precipitates.The addition of Mo,Cr and Ni to the base alloy can significantly increase the γ’roundness,that is,result in the more spherical γ’ morphology.Meanwhile a decrease in Ni(addition of Co)or the addition of Nb,Ta,Al and Ti to the base alloy will decrease the γ’ roundness to different extents,i.e.promote a cuboidal or elongated γ’ morphology.Based on the compositional and microstructural data obtained from multicomponent diffusion-multiple and the γ’ solvus temperature data collected by our group and literatures,several machine learning models were estabilished to predict the relationship between alloy composition and phase equilibrium,γ’volume fraction,γ’ size,γ’ morphology and γ’solvus temperature of CoNi-based wrought superalloys.Based on the design criteria of phase equilibrium,γ’volume fraction,γ’ size,γ’morphology,γ’solvus temperature,oxidation resistance and alloy density,a prototype of CoNi base superalloy with good comprehensive properties was designed.The results show that the established machine learning models provide a reliable and accurate prediction on the microstructures and γ’solvus temperature of CoNi-based wrought superalloys.Compared to the Ni-based superalloy U720Li,the designed alloy has better oxidation resistance and mechanical properties in the service temperature range from 750℃ to 850℃.Based on the above results,this article systematically studied the alloying effects on the microstructural stability and oxidation behavior of CoNi-based wrought superalloys.The insight obtained in this study will provide guidance to the subsequent alloy design.The composition design and optimization method by integrating CALPHAD,multicomponent diffusion-multiple and machine learning was established,which not only accelerate the development and application of CoNi-based wrought superalloys for engineering applications,but also can be used in other metallic materials to accelerate their development.