טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentLarisa Popilevsky
SubjectHyrogenation-Induced Microstructure Evolution in as-Cast
and Severly Plastically Deformed Mg-10wt%Ni Alloy
DepartmentDepartment of Materials Science and Engineering
Supervisor Full Professor Rabkin Eugen


Abstract

Increasing air pollution and anticipated exhaustion of fossil resources in the near future encourage intensive research in the field of renewable energies. Hydrogen is considered as an environmentally friendly carrier of energy, however the main obstacle is the difficulty of storing hydrogen. The safest way of storing hydrogen is in the form of metal hydride, since it is reversibly bound to the metal by chemical bonds.

Magnesium is one of leading candidates for hydrogen storage, due to its capability to store reversibly up to 7.6 wt.% of H2. Yet, slow hydrogenation kinetics is the main drawback that prevents its use in hydrogen storage. Increase of surface-to-volume ratio, alloying with transition elements and increase of defects density were found to accelerate the hydrogenation kinetics of Mg and its alloys.

The aim of the study was to establish the correlation between hydrogenation properties of as-cast and equal channel angular pressing (ECAP) processed Mg-10 wt.% Ni alloys and their microstructure, which evolved with increasing number of hydrogenation cycles.

Formation of large faceted Mg crystals during the first hydrogenation cycle was found to have a crucial effect on the observed kinetics during further cycling. ECAP has a dual effect on the kinetics of hydrogen absorption: on the one hand, increased atomic mobility in the ECAP-modified alloy results in faster hydrogen absorption during the first hydrogenation cycle. On the other hand, the same increased mobility results in the growth of very large Ni-free Mg crystals, which inhibit hydrogenation kinetics during the subsequent cycles. The hydrogen absorption rates of the alloy in the two conditions became identical only after full disintegration of the large Mg crystals due to accumulation of hydrogenation-induced defects after nine hydrogenation cycles.

The size of large Mg crystals is contingent upon the presence of large primary Mg grains or large compact areas of polycrystalline Mg where volumetric expansion is restricted due to the geometrical constrains.  

The re-distribution of Mg2Ni particles in the partly hydrogenated alloy was observed in both types of samples. The volumetric density of Mg2Ni particles was higher in the untransformed metal core of a chip than in the outer hydride shell. This phenomenon was interpreted in terms of dissolution of small Mg2Ni particles in the vicinity of transformation front under the action of compressive stresses induced by the metal-hydride transformation. Mg2Ni particles were found to accelerate the movement of metal-hydride front only when they are located close to the surface.