Mechanical behavior of highly reactive nanostructured MgH2

• Particle size measurements show a bi-modal distribution of the as-milled powders.
• A dynamic recrystallization of the nanoparticles occurred during hydrogen cycles.
• Recrystallization strongly depends on the nature and amount of additive.
• Recrystallization is responsible for the progressive swelling of MgH2 composites.
• The Young modulus increases sharply with the number of cycles.

This study analyzes the mechanical behavior of compacted disks made of MgH2 powders co-milled with vanadium or Ti–V–Cr alloy as an additive, through the evolution of the microstructure and mechanical properties upon hydrogen cycling. The recrystallization of MgH2 particles results from a dynamic recrystallization phenomenon associated with the reaction of hydrogenation itself. The coalescence of the nanometric particles tends to create large agglomerates, which induces an increase in porosity, and explains the progressive swelling of the composites. A relaxation of the maximum strain is observed after 10 cycles for vanadium whereas for Ti–V–Cr the expansion increases until 200 cycles. This difference of behavior is correlated to the ability of vanadium particles to prevent the recrystallization mechanisms, then to limit the agglomeration of the MgH2 particles. From the Vickers hardness measured on compacted powders, a hardness of 0.58 GPa was estimated for highly densified magnesium hydride. Nano-indentation tests performed on compacted pellets show an enhancement of about 20% of the Young modulus as the amount of additives raises from 4 to 8 wt. %. The Young modulus markedly improves as the number of hydrogen cycles increases up to 10. The H/E ratio calculated from these data is characteristic of an intermediate state in between elastic and plastic behavior.