Study of structure-corrosion property correlations in three Al-Ni-Y based metallic glasses

Microstructural control through heat treatments is one of the most important means of designing alloys to gain desired mechanical properties. As microstructural modifications in alloys are associated with creation of heterogeneities both in terms of chemistry (precipitates) and energy (defects such as dislocations, stacking fault and grain boundaries), they affect corrosion, especially the localized corrosion tendency of alloys. Contrary to this general wisdom, certain precipitation treatment in aluminium alloys has been found to impart high strength without sacrificing on uniform and localized corrosion resistance. Thus, the role of precipitates in influencing corrosion seems to be complex and needs a thorough understanding which may help to develop the guidelines for developing corrosion resistant structural alloys having high strength. In this study, three Al-based (Al-Ni-Y) metallic glasses with varying Ni content, namely Al87Ni9Y4, Al86Ni10Y4 and Al83Ni13Y4 were chosen as model alloy systems to investigate the mechanistic role of thermally-induced transformations or nanoscale precipitates on the corrosion properties. Annealing conditions (temperature and time) corresponding to different thermally-induced transformations such as structural relaxation, partial and full crystallization were selected based on the DSC thermograms. Synchrotron X-ray diffraction (XRD), nuclear magnetic resonance (NMR) spectroscopy and transmission electron microscope (TEM) were employed to characterize the structure of these metallic glasses before and after annealing. Surface characterization was done using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Potentiodynamic polarization and electrochemical impedance techniques were employed to study the corrosion behaviour of these glasses in a freely exposed 3.5 wt.% NaCl solution of pH 6.5. Annealing in all the three glasses below their respective first crystallization temperatures caused reduction in passive current density, although they were not found to undergo any structural relaxation which is in contrast with the published literature. The present study found that the Y segregation to the surface due to annealing causes reduction in passive current density. Y segregation has also been found to affect electrochemical corrosion behaviour of the glasses even in the crystallized state. Formation of FCC Al precipitates caused an increase in passive current density in all the conditions, but improves the pitting resistance. It is found that Y is the main element responsible for better corrosion resistance of Al-Ni-Y metallic glasses and since FCC Al precipitates is free of Y; these precipitates tend to dissolve faster than the matrix causing an increase in passive current density. On the contrary, stable pits are formed only when the pit size exceeds a critical level. Below the critical size pitting potential might raise as the precipitates may be covered with Y and/or Ni oxides. Enrichment of Ni and Y in the matrix in the immediate vicinity of the precipitate might help to cover the precipitates with more Ni and Y, and also suppress the pit growth beyond the pit. In contrasts to FCC Al precipitates, the intermetallic phases (Al3Ni and Al19Ni5Y3) rich in Y and Ni resist selective dissolution, but their formation results in Y and Ni lean matrix surrounding them and so they can induce galvanic corrosion. Therefore, as high temperature crystallization caused coarsening of FCC Al and increase in the volume fraction of Y and Ni containing intermetallic phases, and Y and Ni lean glassy matrix, the glasses lost their passivity. Thus, this study emphasizes the role of the size of the precipitates, chemistry of the precipitates and the chemistry of the surrounding matrix in electrochemical polarization tendency of the glasses. Based on these observations, a phenomenological model has been proposed to satisfactorily explain the change in the observed electrochemical behaviour (both the passive current density and pitting potential, which was not done in the published literature) of the alloys due to crystallization. Thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy of the Indian Institute of Technology Bombay, India and Monash University, Australia