Influences of Hydrogen on the Nanomechanical Behavior of Advanced Structural Materials

Abstract

Since the deleterious effect of hydrogen (H) on mechanical properties of materials, or H embrittlement (HE), was first noted more than a century ago, there have been intensive efforts to characterize and understand the HE of materials. HE is a severe environmental type of failure that affects almost all metals and alloys, and has therefore been the primary factor of materials failure in petrochemical engineering, nature gas, nuclear power, chemical processing, aircraft and spacecraft industries. In this context, with the developments of novel structural materials, for example metallic glasses (MGs), high-entropy alloys (HEAs), and nanocrystalline (nc) metals, concerns related to the H in potential working environment loom large. Therefore, despite the significant advances in the processing, characterizing, and structural applications of these materials, it is necessary to explore the effects of H on the mechanical behavior since it plays a very important role in structural applications and has been critical to the exploitation of the materials. Although a number of experimental findings on mechanical behavior of advanced structural materials are available, there are still issues remaining unsolved. In this thesis, I attempt to shed a light on the issues and thus to extend our current knowledge over the influences of H on the mechanical behavior of advanced structural materials. First of all, the key differences in H charging methods (electrochemical versus gaseous) and their consequences on the mechanical response were elucidated through thermal desorption spectroscopy and nanoindentation techniques on a low carbon steel. While electrochemical charging enhances the hardness, gaseous charging reduces it. This contrasting behavior is rationalized in terms of the dependency of the strength on the absorbed amount of H during charging and the H concentration gradient in the specimen. In the MG studies, a systematic investigation was made by performing nanoindentation tests with cube-corner and spherical indenter tips on a series of Ni–Nb–Zr MG ribbons having various Zr contents. Weight gain measurements after H charging and thermal desorption spectroscopy studies were utilized to identify how the total H is partitioned into mobile and immobile parts. These, in turn, indicate the significant role of H mobility in the amorphous structure on the mechanical properties. In high-Zr alloys containing immobile H, both hardness and shear yielding stress (τmax) increase significantly; while in low-Zr alloys having only mobile H, decrease in hardness and τmax were noted. The changes in shear transformation zone (STZ) volume, estimated through cumulative analysis of τmax, caused by H absorption were also found to depend on H mobility such that immobile H reduces the STZ volume while mobile H increases it. In the HEA studies, as one of the early steps to elucidate the H effects in the new alloy, I explored the mechanical behavior of hydrogenated CoCrFeMnNi HEA prepared by either gaseous or electrochemical charging. Micro-tensile and nanoindentation tests revealed that the HEA has an excellent resistance to gaseous HE (which is much better than 304 and 316L austenitic stainless steels), despite the impressive H solubility of the HEA examined through thermal desorption analysis. This behavior suggested that the deleterious effects of H can occur, only if local H concentration reaches a critical level (as in the case of electrochemical H charging). By electrochemical H charging, significant hardness increase up to ~1.6 times was induced. In addition, upon subsequent aging of electrochemical charging, the hardness decreased very rapidly to a value which is even lower than that of the uncharged specimen. The results are discussed in terms of H-induced strengthening/softening and irreversible microstructural change. In the nc-metal study, the influence of H on the plastic deformation of nanocrystalline nickel was analyzed by recourse to nanoindentation on the uncharged and H-charged samples. It was revealed that, in nanocrystalline Ni, H significantly decreases hardness but does not alter the strain rate sensitivity. Through thermal desorption spectroscopy measurement, charged H was expected to reside in face-centered cubic lattice, grain boundaries (GBs) and vacancies rather than dislocations. The H-induced softening behavior is discussed in terms of the possible roles of H in GB-assisted dislocation flow mechanism.