Microscopic mechanism of elastic softening in hydrogen-induced amorphization

Hydrogen-Induced Amorphization (RIA) has been focussed as a hopeful method to control the microstructure. We have investigated the microscopic mechani sm of HIA. on AB(2) C15 Laves phase compounds using a molecular dynamics (M D) method. In our previous paper, we reported the softening phenomena of bu lk modulus by hydrogenation. We found two softening mechanisms. The first i s due to the volume expansion, originating from the non-linearity of the in teratomic potentials; this also appears in non-HIA materials. The second is due to the atomic relaxations of A and B atoms, and is characteristic of H IA materials. In HIA materials of C15 Laves phase compounds, A atoms (A=rar e earth) contract and B atoms (B=Fe, Co, Ni) are expanded compared to the p ure crystals. It realizes the second softening mechanism. In this study, we calculate the elastic constants at finite temperature directly from the MD results by fluctuation formula. We observed the third mechanism due to the pressure-tenser fluctuation in addition to two mechanisms described above. The third is characteristic of HLA materials as well as the second. It can be considered that the softening of the bulk modulus due to these mechanis ms is an indication of the increase of the elastic instabilities by hydroge nation.