Influence Of W And Hf Content On Microstructure And Properties Of High W Nickel Base Superalloy

Nickel base superalloys are widely used in the preparation of hot end parts for aircraft engines because of their good comprehensive mechanical properties,oxidation resistance and structural stability.It's strengthened mainly by solid solution strengthening,?' phase strengthening and carbide strengthening.High alloying is one of the main methods to improve the solid solution strengthening level of nickel base superalloys,where the element W is an efficient alloying element with low price.The control of the W content is extremely important for high W alloyed nickel base superalloys,at the same time,Hf element plays an important role in the structural transformation and performance improvement of nickel base superalloy.However,the influence of changes of W and Hf content on the microstructure and properties of high W alloyed nickel base superalloys has not yet been clarified.In this paper,by means of OM,SEM,XRD,EPMA,DTA,TEM,EBSD and other analysis methods to study the influence of W(14%-18%)and Hf(0.5%-1.5%)on the solidification structure and segregation behavior of high W nickel base superalloys.Based on the microstructure morphology,precipitation phase and stress distribution,the influence of W and Hf content on the room temperature tensile and high temperature lasting properties of the superalloy and its deformation and fracture mechanisms were studied.The main conclusions of the paper are as follows:Both W and Hf can increase the amount and size of eutectic in the alloy,but the mechanisms are different.W promotes the eutectic transformation by enhancing the growth of dendrites with different orientations during the solidification process,and increases the concentration gradients of Al,Ti and other elements in the liquid phase,thereby promoting eutectic transformation.Hf promotes the precipitation of eutectic by increasing the concentration gradient of Hf in the liquid phase.Increasing W content can significantly reduce the grain size of the superalloy.When W content increases to 16%,a high-melting ?-W phase begins to precipitate in the alloy,which acts as a heterogeneous nucleation point during the solidification of the superalloy,thereby promoting the reduction of alloy grain size.The superalloy with 16%W content has the best tensile properties at room temperature.The main deformation mechanism of the superalloy at room temperature is that dislocations slip in the matrix and cross the ?' phase by the Orowan mechanism.Among them,parts of dislocations shearing into the ?' phase can be decomposed to generate antiphase boundaries.And MC carbide is the main Crack source,the main fracture mode is transgranular fracture.The solid solution strengthening and grain refinement of superalloy with 16%W content are stronger than that of superalloy with 14%W content.The ?-W phase precipitated when the W content is further increased will reduce the solid solution strengthening effect,which is the main reason for the reduction of the tensile properties of the superalloy with 18%W content at room temperature.The Hf content has no significant effect on the tensile properties of the superalloy at room temperature.The 0.5%Hf-16%W alloy has the longest high temperature rupture life.The main lasting deformation mechanism of the superalloy under the condition of 1000?/160 MPa is that the dislocations slip in the matrix and climb across the y' phase.The MC phase,eutectic structure and ?-W phase at the grain boundaries are the main crack sources,and the fracture mode is mainly intergranular fracture.The grain size of superalloy with 14%W content is larger,the amount of transverse grain boundaries are more,so the superalloy rupture life is reduced;the amount of ?-W phases in superalloy with 18%W content is larger,and the superalloy rupture life is reduced.As the Hf content increases,the amount of eutectic near the grain boundaries increases,then the rupture life of the superalloy decreases.