Microstructure Evolution And Finite Element Analysis Of Single Crystal Nickel-Based Superalloys With Different Orientations During Creep

By means of tensile/compressive creep properties measurement and microstructure observation of single crystal superalloys with [001], [Oil], [111] orientations, an investigation has been made into the microstructure evolution of single crystal superalloys with different orientations during tensile/compressive creep at elevated temperature, and the existing modes of the raftedγ' phase in three-dimensional space of the single crystal nickel based superalloys with different orientations have been constructed. Moreover, the von Mises stress distributions and strain energy changes of the cubicalγ/γ'phases in the single crystal nickel based superalloy with different orientations are calculated by means of the finite element methods for investigating the influence of the applied tensile/ compressive stress on the microstructure evolution of the different oriented single crystal superalloys. In addition, the influences of the defects (such as the cavity, cracks and impurity) on microstructure evolution and creep properties of a single crystal nickel-base superalloy are investigated, and the main results of the investigations are given as follows:After the as-cast alloy is solution treated at high temperature and cooled in air, the butterflyγ' phase is coherently precipitated from the y matrix. After the first aged, the sharp regions of the butterflyγ' phase are dissolved for filling the concavity regions, and the fine matrix channel in between the butterflyγ' phases disappears to make theγ' phase transformed into the cubical configuration. Moreover, the cubical extent of theγ' phase increases during the second aging. After fully heat treated, the microstructure of the single crystal nickel-based superalloys with different orientations consists of the cubicalγ' phase coherent embedded in theγmatrix, and the cubicalγ' phase is regularly arranged along the <100> directions. During the primary stage of the tensile/compressive creep, the cubicalγ' phases in the [001] orientation single crystal nickel-based superalloy are transformed into the N-type or P-type rafted structure along the direction vertical or parallel to the applied stress axis, respectively. During the rafted transformation of theγ' phase, no dislocations are detected within the raftedγ' phase, and the dislocations movement occurs only in the y matrix, which suggests the plastic deformation occuring in the y matrix, and the elastic strain occurs only within the raftedγ' phase. Therefore, the inflence of the applied stress on the von Mises stress distribution and configuration evolution ofγ/γ' phases in the single crystal nickel-based superalloy can be analysed by means of the elastic-plasticity finite element methods.During tensile creep of single crystal superalloys with [001] and [111] orientations, and during compressive creep of [011] orientation superalloy, the cubicalγ' phase in the alloys is transformed into the mesh-like rafted structure. But during compressive creep of [001] oriented superalloy, and during tensile creep of [011] oriented superalloy, the cubicalγ' phase in the alloy is transformed into the stripe-like rafted structure along the direction parallel to the stress axis, and the y matrix phase is continuously filled in between the.raftedγ' phase.The finite element analysis shows that the lattice extruding and expanding strain occur, to a different extent, on the different crystal planes of the cubicalγ' phase under the action of the principal stress component during tensile/compressive creep of the superalloys with different orientations. Thereinto, the extruding lattice strain may exclude the Al, Ti atoms with bigger radius, and the expanding lattice strain may trap the Al, Ti atoms for promoting theγ' phase grown up directional along the normal of the expanding lattice, which is the main reason of resulting in theγ' phase grown directional into the mesh-like or stripe-like rafted structure along the specifical orientations.Under the action of the tensile/compressive stress at high temperature, the different changes of the stain energy density occur on the different crystal planes of the cubicalγ' phase, and theγ' phase is directionally grown along the crystal planes with bigger strain energy. Moreover, the different changes of the interface area in the alloys withγ/γ' phases occur during the rafting transformations of the mesh-like or stripe-like structures. The changes of the potential energy among atoms, interface energy and lattice misfit stress are thought to be the driving force for promoting the elements diffusing andγ' phase growing up directional. And the different driving forces are displayed during the different rafted transformation.During the elements diffusing andγ' phase directional growing up, the ordering transformation, due to the atomic concentration, in the y matrix near theγ' phase occurs to form theγ' phase with Ll2 structure, which is a spontaneity process of the free enery decrease. Thereinto, the Al, Ti atoms with bigger radius transfer to{100} plane to form the stable stacking model of Ll2 structure which is in the form of the combination bond of the different atoms, which is thought to be the main cause of theγ' phase transformed into the mesh-like and stripe-like rafted structure along<100> orientations.The defects in the alloys, such as the cavity and cracks, may decrease to a great extent the creep lifetime of single crystal nickel-basded superalloy. As the creep goes on, the circinal cavity in the alloy is elongated into the ellipse-like configuration along the direction vertical to the stress axis. The single cavity in the alloy occurs only the smaller strain, and the strain value of the alloy with defects during creep consists of the strains of the multi-cavities and cracks propagation. During creep at high temperature, the symmetrical distribution of the butterfly-like von Mises stress occurs in the region near the cavity along the stress axis, and the bigger von Mises stress appears in the 45°angle directions relative to the stress axis, which is the main reason ofγ' phase transformed into the rafted structure along the 45°direction relative to the stress axis.During creep, the different stress distribution occurs in the regions near the cavities with/free cracks. Compared to the cavity with free-cracks, the bigger von Mises stress appears in the apices regions of two sides in the cavity with cracks to promote the cracks propagated along the direction vertical to the stress axis, which is thought to be the main reason of the alloy with cavity possessing the shorter creep lifetime. The maximum stress value occurs in the two sides of the cavity, and the stress value increases as the creep goes on, which may promote the initiation and propagation of the cracks in the apices regions along the direction vertical to the stress axis. This is thought to be the fracture mechanism of the alloy with cavity during creep.