Effect Of Various Elements On The Microstructure And Mismatch In Co-Al-W Alloy System

This thesis mainly focuses on the effects of various elements on the microstructure,phase morphology,lattice constant,lattice mismatch of the Co-Al-W superalloys.Ti and Hf elements was introduced in the alloy,with dynamically adjusting the contents of W and Al.A series of characterization methods including XRD?DTA?SEM?TEM?density instrument,etc were employed to investigate the roles of W?Al?Ti?Hf element content on the mismatch,morphology,and properties of cobalt-based superalloy.The following conclusions are drawn:With the sustainable growing of aging time,the morphology of ?? phase has changed a lot to some extent,and the size of ?? phase has coarsened gradually.However,the aging treatment during short period can't make the ?? phase precipitate fully and affect the morphology.In the alloy Co-12Al-9.5W-6Ti,?? phase is continuously embedded in the matrix phase after 96 h aging treatment.The lattice constant of ? 'phase is 0.3592 nm,the lattice constant of ? phase is 0.3573 nm,they are very close.This is very similar to that of dual ?+?? phase structure in Ni-based alloy,which indicates that the alloys have good high temperature performance.The high temperature performance of Ni-based supperalloys is due to the high volume fraction of ?? phase,and the large amount of ?? phase is beneficial to suppressing the dislocation slip in the matrix.When 9% Al and 3% Ti are added into the alloy,it is helpful to promote the precipitation of ?' phase and to increase the size of ?'.However,when the content of Al in the alloy composition is more than 10% or the content of Ti is more than 5%,it shows no benenficial to the precipitation of the ?' phase,and it results in the formation of other heterogeneous phase.Consequently it leads to inhibit the precipitation of the ? 'phase and changing the morphology of the ?' phase particles from a cubic to an irregular state,and finally it decreases in the thermal stability and melting point of the alloy.Hf and Ti are both the elements of the strengthening phase,former,so the effect of Hf on the mismatch degree will improve after Hf is added to the alloy.When there is the same level of Hf and Ti contents,the effect of Hf on the mismatch degree is higher than that of Ti element.However,as for the effect of increasing the size of ?' phase and stabilizing the morphology of ?' phase,the effect of Hf is not as good as Ti.Therefore,Hf and Ti play a irreplaceable role on the strengthening role to the alloy.The high contentof Hf will generate Hf rich phase in the alloy,such as CoHf phase and Co2 Hf phase.The appropriate additionof the Hf element can improve the thermal stability of the material,and the higher the Hf is,the higher the temperature of the ? 'phase will be.Nevertheless,when the Hf content is higher than 3%,the increase of Hf content will lead to formation of a large number of rich Hf phase,and the lack of plenty ?' phase can't play a sufficient precipitation strengthening role on the material.When the content of W in the alloy is less than 5%,the alloy will form a large number of Al-rich interdendritic phase.With the increase of the content of W,it will refine the dendrites and inhibit the formation of interdendritic phases.When the content of Ti of the alloy is too high,the surface of the alloy will appear a large number of Al-rich phase and Ti-rich dendrites,namely,the Co2Al9 phase and Co3(Al,Ti)phase.In the Co-9Al-3W-7Ti,it was observed that the pores formed on the surface of the substrate after the solution treatment at 1280?,this indicates Ti will drop the melting point of the material,Thereby degrade the performance of the material.The higher the atomic percentage of the element W is,the larger the density of the alloy material is.To the opposite,the higher the atomic percentage of the element Ti is,the smaller the density of the alloy is.In summary,the content of W is 9% in general.