Precipitation Behavior And Deformation Mechanisms Of Topologically Close-packed Phases In Ni-based Single Crystal Superalloys

Ni-based single crystal(SX)superalloys have been widely used for gas turbine blades in most advanced aero-engines.In order to impart a remarkable resistance of creep deformation at increasingly higher temperatures,significant amounts of refractory metals are added into the Ni-based SX superalloys for forming ?'-precipitates or strengthening ? solid solution.An excessive content of refractory metals,however,promotes the precipitation of topologically close-packed(TCP)phases.Nucleation and growth of TCP phases can greatly deteriorate mechanical performances,especially creep resistance,of these alloys by depleting the ?-matrix of solid solution strengtheners and formation of internal cracks induced by local stress concentrations at brittle TCP phase/matrix interfaces.Studying the precipitation behavior and deformation mechanism of TCP phase in nickel-based single crystal superalloys is of great significance to inhibiting the precipitation of TCP phase,adjusting the microstructure of superalloys and optimizing alloy properties.The present work investigates the precipitation behavior,structure evolution and intrinsic deformation mechanisms of TCP phases in a nickel-based single crystal superalloy containing Re and Ir during the creep process of high temperature and low stress conditions(1140?,137Mpa)by using aberration-corrected HAADF-STEM technique.After 6 hours crept deformation,?,P and ? phases coprecipitation occurs in the 3Ir Ni-base single crystal superalloys.The crystallographic orientation relationship of the intergrowth TCP phases can be described as:(112)?//(010)P//(001)?,[111]?//[001]P//[100]?.TCP precipitates grow along the intergrow planes((112)?[111])with the short rod-like shape at the initial creep stage to lath-like,while the thickness of the intergrowth lamellae doesn't change significantly.By combining of atomic resolution HAADF-STEM and energy dispersive X-ray spectroscopy(EDS),the atomic occupancy information of the pentagonal anti-prism and hexagonal anti-prism structural units in the intergrowth phase was analyzed respectively,and the preferential occupacion on different Wycokff sites in the crystal structure of the intergrowth phase was determined.The anti-prism structural units in each cryatal are connected to each other and topologicallly close-packed structure maintained across the intergrowth interphase.The atomic coni figurations of(112)? twin was firstly established by the atomic resolution HAADF-STEM observation.The(112)?twin boudary actually is a mirror-glide plane and it is asymmetric and coherent.The twin boundary consists of ditorted pentagonal anti-prism.? topologically close-packed phase commonly forms in superalloys and its intrinsic deformation is critical for their performance.Atomic configurations of planar defects in ? phase have been investigated by aberration-corrected scanning transmission electron microscopy and geometrical structure analysis.The formation of type I basal defect is due to the basal-slip in the ? phase which probably caused by synchroshear inside Laves triple-layers.While for the formation of pyramidal planar faults on(1101)planes and(1102)pyramidal planar defects involves atomic transportations and local atomic rearrangements.Displacement vectors of pyramidal Ia and pyramidal Ib planar faults deviating from the(1101)slip plane lead to long-range outward or inward diffusion and result in the?0.048 nm contraction and?0.049 nm dilation perpendicular to the slip plane.This study provides a fundamental understanding of deformation mechanisms of ? phase in Ni-based alloys which could shed light on improving the ductility of complex structured intermetallic compounds.Through volume fractions variation of intergrowth phases,the phase transition from ? to P and then to ? phase was determined.In the creep process,the first flaky ?phase is precipitated and then transformed into the P phase.The P phase will continue to transform into the ? phase in the subsequent creep process.The initially precipitated? phase is rich in refractory elements.As the precipitated phase grows up,the surrounding ?' phase restricts the diffusion of refractory elements to the precipitated phase,thereby restricting the growth of the TCP phase.At this time,the ? phase went through self-diffusion of the refractory element and itself undergoes a phase change,so that the precipitated phase continues to grow along the intergrowth planes.The P phase formed by the phase transformation can further diffuse the refractory element along the intergrowth planes and transform into a refractory element containing less ? phase.