| Ni-base wrought superalloys are commonly used to manufacture turbine discs due to excellent thermal stability,heat corrosion resistance,oxidation resistance and creep resistance at high temperature.Mechanical properties of Ni-base wrought superalloys are significantly affected by grain boundary second phases.Among grain boundary phases,topologically close-packed(TCP)phases are usually discovered in wrought superalloys with the addition of refractory metal elements.As a class of complex intermetallic compounds with only tetrahedral interstices,TCP phases are stacked with high packing density of atoms,embodying low plasticity and high brittleness.As a result of the above characteristics,TCP phases tend to promote crack initiation and propagation during creep,thus reducing the creep strength of the alloy.In addition,the formation of TCP phases consumes plenty of refractory elements,accordingly weakening the effect of solid solution strengthening of matrix.As a ubiquitous TCP phase in wrought superalloys,The μ phases can be represented by rectangular and parallelogram structural subunits,which are parallel to the basal plane of μ phases.Basal stacking faults(SFs)are the most common defects in μ phases,SFs with different stacking sequences will form different phases with corresponding structures.In addition,refractory metal elements can combine with boron element to precipitate borides.Borides are strengthening precipitations that can pin grain boundaries to prevent grain boundaries gliding and migration.Orientation relationship(OR)is an important basic way to describe grain boundary precipitations such as borides.The ORs affect the morphologies and distributions of borides in the alloy.The basal SFs of μ phases and ORs with matrixes of M3B2 borides in a Ni-base wrought superalloy were systematically studied by multifarious electron microscopy techniques,such as selected area electron diffraction,transmission electron microscopy,high-resolution transmission electron microscopy,X-ray energy-dispersive spectroscopy and atomic-resolution high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM)of aberration-corrected transmission electron microscope,obtaining the following results.According to the different arrangement of structural subunits,the basal SFs were divided into four types.Type Ⅰ basal SF is equivalent to μ phase with a layer of parallelogram structural subunit reversing to form two layers of micro symmetric structures,the reversed parallelogram structural subunit is symmetrical to the forward one about the rectangular structural subunit;type Ⅱ basal SF is equivalent to type I basal SF with absence of a layer of rectangular structural subunit,forming C14 structure and micro symmetric structure;type Ⅲ basal SF is the result of absence of a layer of parallelogram structural subunit in p phase,forming complete Zr4Al3 phase;type Ⅳbasal SF is the result of absence of a layer of rectangular structural subunit in μ phase,forming C15 structure.Among four types of basal SFs,both type Ⅱ and type Ⅳ basal SFs form Laves phases,but the number of the former is more than that of the latter.This is related to the stability of type Ⅱ basal SF(C14 structure)over type Ⅳ basal SF(C15 structure)revealed by first principles calculations.Atomic-resolution HAADFSTEM was used to study the process and mechanism of nucleation and growth of C14 structures from μ phases.The result shows that C14 structures nucleate in the μ phase through type Ⅱ basal SFs and grow continuously through alternating type Ⅱ basal SFs and absence of rectangular structural subunits.The intersection between pyramidal SF of μ phase and the C14 structure that forms in μ phase only changes the arrangement of structural subunits at the intersection region,but could not affect the thickness of the C14 structure.However,the intersection between the disordered SF termination region of μ phase and C14 structure does not only influnce the arrangement of structural subunits at the intersection,but also changes the thickness of C14 structure outside the intersection region.M3B2 borides with straight interfaces were found to precipitate in the grain boundaries of a Ni-base wrought superalloy.It was counted that a large proportion of M3B2 borides satisfy the OR(a)orientation relationship with the matrixes at straight interfaces.OR(a)is represented as[110]grain 1//[120]M3B2,(002)grain 1//(001)M3B2.However,there are two types of habit interfaces between these borides and the matrixes,namely type ⅰ1 and type ⅰ2 interface.The type ⅰ1 interface parallel to(002)plane of the matrix and(001)plane of M3B2 boride,and the type ⅰ2 interface parallel to(200)plane of the matrix and(310)plane of M3B2 boride.The type ⅰ1 and type ⅰ2 interfaces of the OR(a)correspond to low interfacial energies.The prevalence of straight interfaces and OR(a)are as a result of geometric and energetic advantages of type ⅰ1 and type ⅰ2 interfaces of the OR(a).For type ⅰ2 interface,there are M3B2 borides satisfying two variants of OR(a)respectively on the same grain boundary,that is,while maintaining[001]orientation,the two borides are parallel to the(200)plane of matrix at(310)and(3-10)planes,respectively.The crystallographic symmetry between the two variants were revealed through high-resolution transmission electron microscopy images and atomic model. |
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