Effect Of Long-Term Thermal Exposure On Microstructures And Mechanical Properties Of Nickel-Based Superalloy With High W Content

Nickel-based alloy with high W content has high temperature capacity and excellent mechanical properties,which is thought as one of the important materials for turbine blades of aeroengines.The alloy must suffer from tough working environment during high-temperature service,and few related research about the effect of long-term thermal exposure on the microstructure and properties of high W nickel-based alloys were reported.Therefore,in this thesis,the effect of long-term thermal exposure on microstructural evolution,room temperature tensile properties and stress rupture properties was investigated in high W nickel-based alloys by SEM,TEM,XRD,DTA and EPMA.Simultaneously,the effect of W and Hf element content on the microstructural stability of high W nickel-based alloy was investigated.The results showed that:The as-cast K416B alloy was mainly composed of ?' phase,? matrix,MC carbide and M6C carbide.After long-term thermal exposures at 1000? for 100 h,500 h,1000 h and 1500 h,the volume fraction of ?' phase and the width of grain boundary increased;the size of ?' phase was decreased with the increase of thermal exposure time.The primary MC carbides were gradually transformed into M6C carbides.The secondary M6C carbides of the alloy were distributed in different forms.Granular M6C carbides were precipitated along the grain boundary discontinuously and needle-like M6C carbides were precipitated in dendrite regions.Rhombohedral and rod-like carbides were precipitated near the eutectic structure in the interdendritic region.As the increase of thermal exposure time,the yield strength of tensile at room temperature decreased in the alloy.The main deformation mechanism of as-cast alloy during tensile at room temperature was the dislocation slipping in the matrix and shearing the ?' phase.After long-term thermal exposure,fine M6C carbides precipitated at the interface of ?/?' phase and dislocation networks had formed.The slipping dislocation crossed over the M6C by Orowan bowing mechanism.M6C carbides were the main source of cracks during tensile at room temperature.The fracture characteristics of the alloy gradually changed from transgranular to intergranular,caused by increase of the number of M6C carbides at grain boundary.The stress rupture life of the alloy at 1000?/160 MPa gradually decreased with the extension of thermal exposure time.The main deformation mechanism of the as-cast alloy during stress rupture test was the dislocation slipping in the matrix and climbing over the ?' phase.With the increase of thermal exposure time,the width of the dislocation network decreased at the interface of ?/?' phase.The deformation mechanism gradually had changed,which was dislocation slipping in the matrix and shearing the ?' phase.The cracks of the alloy during the stress rupture test mainly initiated between the interface of the secondary granular M6C carbide and ?' film,and propagated along grain boundaries.The fracture characteristics of the alloy were intergranular.The 1Hf16W alloy had the best stability of the structure.With the increase of W content,the number of needle-like and rhombohedral M6C carbides increased significantly,and the precipitation tendency of rod-like M6C carbides had also enhanced.With the increase of W content,the number of needle-like and rhombohedral M6C carbides decreased.The change of W and Hf content had no obvious influence on the ?' phase of the alloy.