Advanced small-scale characterization of hydrogen embrittlement in nickel and nickel alloys
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The demand for high strength materials is increasing in the modern industrial development. With more challenging applications and extreme environments, it is essential that materials are able to meet capacity and safety requirements. Due to their high corrosion resistance and higher strength at high and low temperatures, nickel-based alloys (Ni-based alloys) have particularly become valuable materials in demanding applications. However, the material properties of nickel can dramatically change when it is exposed to environments with hydrogen. This phenomenon is known as hydrogen embrittlement (HE) and is a type of deterioration which can be associated with corrosion. The ingress of hydrogen can seriously reduce the ductility and load-bearing capacity of the material, causing cracking and brittle fracture at stresses below the yield stress. HE has long been an area of intensive study but is still not completely understood. In particular, its detection is one of the most complicated aspects of the problem as the phenomenon affects metallic materials differently. Rapid nanotechnology development offers the opportunity to execute small-scale fracture experiments. Furthermore, the development of new nanomechanical testing methods has made it possible to observe the local effect of hydrogen in metals, which potentially can lead to an enhanced understanding of the fracture properties in the presence of hydrogen, required stress levels for crack propagation and ultimately explain the underlying mechanisms that control HE. The sample materials in this project include; NiS, pure Ni, solution annealed Ni725, precipitation hardened Ni725, over aged Ni725 and solution annealed Ni718. Samples were prepared by surface grinding and electropolishing. Electron Backscatter Diffraction (EBSD) analysis of each sample surface was conducted to determine crystal orientations and grain boundary (GB) types. Subsequently, preferred sites on the sample surface were selected and a Focused Ion Beam (FIB) was utilized to fabricate a number of 6, 8 or 10 microcantilevers on each sample, either to include a GB (bi-crystalline cantilevers) or within grains (single-crystalline cantilevers), with dimensions of approximately 3x3x14µm. Microcantilevers were loaded in a controlled manner to obtain load-displacement data by utilizing a Hysitron TI950 TriboIndenter. Hydrogen enhanced cracking along GB was studied in NiS and pure Ni by performing bi-crystalline microcantilever bending tests in air and during cathodic charging. Bending tests were also performed on precipitation hardened Ni725 single-crystalline microcantilevers, but only in air. After the bending tests, high-resolution Scanning Electron Microscope (SEM) imaging was performed to evaluate the crack propagation path. Fracture did not occur for BC NiS cantilevers tested in air, but cracks propagated along the GB plane, suggesting that there is a critical amount of sulfur (S) segregants in the GB. S segregation to the GB causes a reduction in the GB s cohesive strength, which will reduce the stress needed for intergranular failure below the stress needed to operate GB dislocation sources i.e. transgranular failure. For all NiS cantilevers charged by H, it was clearly shown that the presence of H causes HE by promoting crack initiation and crack propagation along the GB, as the bending tests resulted in a complete opening of the GB. With increasing amounts of H present in the GB, the intergranular failure is accelerated. It is therefore proposed that H accommodates the transition from ductile to brittle cracking along GBs. However, since the H-charged pure Ni cantilevers did not exhibit intergranular failure, it is found that the H-induced intergranular brittleness is strongly dependent on the amount of impurities segregated to the GB. There exists a synergetic relationship between S and H leading to GB decohesion, and when there is a critical amount of either S or H present in the GB the intergranular cracking will be accelerated. Thus, when GBs are involved, the H-enhanced decohesion (HEDE) mechanism is presumably the most relevant among the proposed mechanisms for H-assisted cracking.