Effect of hydrogen charging on dislocation behavior in Ni-Cr and Ni 2 Cr alloys

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Effect of Hydrogen Charging on Dislocation Behavior in Ni-Cr and Ni2Cr Alloys KAORI MIYATA The effects of hydrogen charging on the dislocation behaviour in Ni-Cr binary alloys have been investigated by means of transmission electron microscope (TEM) observations using single-crystalline specimens. The deformation mode of Ni-Cr alloys in the absence of hydrogen is characterized by planar dislocations. However, hydrogen charging changed the dislocation configurations to promote curved dislocations, such as dislocation loops and dipoles. The hydrogen-affected dislocation configurations are enhanced with increasing Ni content and reducing Cr content. Weak-beam images show that the Shockley partials of the hydrogen-affected dislocations frequently constrict to make kinks and cross-slip, as if the dislocations were generated by a thermally activated process. The effect of hydrogen charging on superdislocations of a Ni2Cr superstructure has been also investigated using an aged 70Ni-30Cr alloy. While the deformation mode in the Ni2Cr superlattice is classified as five variants of superdislocation triplets and one variant of ordinary dislocations, the hydrogen charging has preferred the ordinary dislocations to the superdislocation triplets. The results suggest that the charged hydrogen changes the local plasticity to affect the deformation dynamics in Ni-Cr alloys, where the influence of hydrogen on the plasticity is sensitive to the Ni/Cr concentration and the symmetry of atomic arrangement.

I. INTRODUCTION

NI-BASED superalloys have been used extensively as structural materials in severely corrosive environments such as deep sour-gas and oil fields, chemical process industries, and commercial nuclear reactors, because of their excellent corrosion resistance and high strength.[1,2,3] In an environment with hydrogen, the highly strengthened Ni-based superalloys often experience intergranular fracture, resulting in a marked reduction in ductility.[4,5] Since the susceptibility to hydrogen embrittlement increases with increasing Ni content, many commercial Ni-based alloys are optimized with Ni contents not exceeding 60 mass pct.[6] Other metallurgical factors controlling the hydrogen embrittlement generally include (1) segregation of metalloid elements such as sulfur and phosphorus to grain boundaries,[7,8] (2) a deformation mode controlling the stress concentration to grain boundaries,[9,10] and (3) the grain size and orientation. In previous studies on C276 alloys with minimized sulfur and phosphorus contents, the author has shown that a hydrogen crack initiates and propagates preferentially along highangle grain boundaries having specific crystallographic orientation relationships with slip lines.[11,12] The inhomogeneous stress concentration at grain boundaries is, therefore, considered to play an important role in the hydrogen embrittlement in Ni-based alloys. The author has also found a change in the fracture mode due to hydrogen embrittlement from grain boundaries to twin boundaries when the disordered fcc structure