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Boron segregation and boride formation at grain boundaries are generally accepted in Ni-based superalloys. However, the addition of boron was found to result in the formation of a high density of nanosized M 5 B 3 (where M is a mixture of Cr, Mo) particles in γ/γ′ eutectics of a Ni-based superalloy during high-temperature creep in this work. M 5 B 3 phase precipitated at γ/γ′ eutectics keeps a well-defined orientation relationship with the eutectics, which is [001] γ/γ′ //[001] M5B3 , (020) γ/γ′ //(130) M5B3. And these nanosized borides could block dislocation slip, result in stress concentration surrounding γ/γ′ eutectics and facilitate crack initiation at γ/γ′ eutectic/matrix interface during the creep at high temperatures. • The structrural characteristic of γ/γ′ eutectics in a crept polycrystalline superalloy was analyzed. • High density of nano-sized M 5 B 3 -type boride precipitated at γ/γ′ eutectic regions was observed. • Nanosized borides can aggravate the deformation incompatibility of γ/γ′ eutectics during high temperature creep. [ABSTRACT FROM AUTHOR]

This study investigates the effects of nitrogen (N) content (0.0005–0.0045 wt%) on the microsegregation, microstructures and high temperature mechanical properties of cast superalloy K417G. The results show that the segregation of Ti to interdendritic region becomes very serious with increased N content. In all three samples with different N contents, the alloys are typically dendritic morphology and γ′ phases show good cubic structure. With the N content increasing from 0.0005 wt% to 0.0045 wt%, the volume fraction of γ′ phase in the dendrite decreases, the sizes of γ/γ′ eutectic and MC carbide increase, and the volume fractions of γ/γ′ eutectic, MC carbide and micro-porosity firstly increase but then decrease. N addition changes the morphology of MC carbide from script-like shape to blocky shape, and Ti(C, N) is identified in the alloy with 0.0045 wt% N. With the N content increasing, the ultimate and yield strength decrease at elevated temperature, and the ductile and stress rupture life firstly have a sharp decrease and then have a slight increase. The specimen with the minimal content of N shows the best comprehensive mechanical properties. [ABSTRACT FROM AUTHOR]

Metal carbides (MC) are typical strengthening phases in steels and superalloys. However, the decomposition of carbides can occur during high-temperature creep, which is generally considered to be detrimental to the high-temperature performance of superalloys. Here, through transmission electron microscopy and atomistic simulation, we report the atomistic to micro-scale mechanisms of MC decomposition. We identify that MC can deform plastically during high-temperature creep, which is supported by the observation of high-density dislocations within MC and steps at matrix/MC interfaces. This could release the stress concentration at the matrix/MC interfaces, thus improving the creep resistance. With Cr segregation along the dislocation line and continuous dislocation climb, decomposition of MC into M 23 C 6 occurs in grain interiors, which could lead to crack inside the MC carbides, i.e., fracture at the partially coherent M 23 C 6 /MC interfaces. This is new to the existing knowledge that MC decomposition and fracture occur at matrix/MC interfaces. [Display omitted] [ABSTRACT FROM AUTHOR]