Computational simulation of H embritlement in the Fe-Cr-Ni alloy: analysis of the chemical bonding and the electronic structure

C. LANZ; A. JUAN; G. BRIZUELA; S. SIMONETTI

Congreso; 21st International Congress of Chemical and Process Engineering; 2014

It has long been recognized that the introduction of atomic hydrogen into metallic alloys during service causes the metal to become embrittled. The embrittlement is often manifested by a significant increase in the ultimate tensile strength and work-hardening rate and a reduction in the strain to fracture. This work contributes to analyze the changes in the Fe55Cr25Ni20 electronic structure and the chemical bonding after the H adsorption, to better understanding the H embrittlent by decohesion mechanism of iron-chromium-nickel alloys. In order to study the phenomenon in detail, it is necessary to use computer-simulation techniques. Calculation shows that the H atoms are trapped in the zone of a γ-Fe55Cr25Ni20 vacancy. The impurities are located aligned both along 1-10 direction and with a vacancy, in the (111) plane. This behavior can be related with the easiness of hydrogen trapping and forming a lineal hydrogen-vacancy clusterization, as a precursor to crack initiation. An electron transfer to the H atoms from the Fe, Cr and Ni nearest neighbor atoms is observed. The electron transfer process helps to form the new H-metal bonds. The atomic orbital occupations of the metallic bonds close to the H atoms are affected. The mainly changes are presented in Cr 4py and Fe 4px orbitals. The 3dx2-y2, 3dz2 and 3dxz metallic orbitals have also participation in the hydrogens-metal interactions. The strengths of the metallic bonds nearest neighbors to the H atoms decrease. The Cr atoms have an important role in the embrittlent process, the H-Cr OP is the highest and the Cr-metals bonds strengths are the most affected after H adsorption. We found same H-H interaction that could be associated with the precursor of hydrogen bubble but it is far away to a typical H2 chemical bond. The embrittlement mechanism helps to form the bonds between metals and H atoms, and finally break the metal?metal bonds. These processes play a key role in the stress corrosion cracking initiation.