Stress-induced Atomic-scale Structural Relaxation and Damages in Bulk Metallic Glasses (BMGs)

Description

Bulk metallic glasses (BMGs), a new class of metallic materials, have many attractive mechanical properties, such as extremely high strength and hardness, large elastic strain limit and high wear resistance, due to their complex disordered atomic structures. Such non-crystalline structures are thermodynamically metastable in nature, and they have a strong tendency for structural relaxation upon suffering from stress perturbations or temperature changes. Although encouraging progresses have been made in the study of temperature-induced structural relaxations in BMGs, the stress-induced structural relaxation process is rarely addressed. Also, an in-depth understanding of the damage mechanism induced by external stresses in BMGs remains a long-standing fundamental problem. As a result, this proposed research is aiming at elucidating the stress-induced relaxation and damage process in the atomic scale and then correlating them with the unique mechanical properties of BMGs. We intend to develop a scientific scheme to quantitatively correlate stress-induced atomic migrations/rearrangements with mechanical behaviors, especially fatigue properties, through a combination of theoretical calculations and the state-of-the-art experimental techniques. Based on the simulation and experimental outcome, the ultimate goal is to understand the stress-induced deformation and relaxation mechanisms of BMGs in the atomic scale.An BMG with a composition of Zr50Cu40Al10 (at.%) will be selected as the model material, cyclic deformations (tension and compression) of the BMG will be modeled utilizing classical molecular dynamics (MD) simulations with embedded atom method (EAM) potentials to describe the interatomic interactions. During the cyclic deformation, the corresponding atomic structure will be recorded simultaneously. Through a systematic variation of applied stresses and cyclic frequencies, the structural change in atomic clusters and free volumes will be quantitatively identified as a function of the deformation parameters. Besides the systematic theoretical calculations, comprehensive cyclic loading experiments will be performed on the Zr50Cu40Al10 BMG to induce different levels of atomic-scaled relaxation and damage. Then, multi-scale mechanical tests and structural characterizations will be carried out to progressively reveal atomic migration/rearrangement information caused by the relaxation and damage. These systematic investigations coupled with our previous structural model will allow us to identify the microscopic evolution of stress-induced relaxation and damage and thereby elucidate their atomistic mechanisms.The implementation of this proposal will expect to lead to an in-depth understanding the correlation between the atomic structure and mechanical properties. Furthermore, it will offer a scientific knowledge for the design of novel BMGs with superior fatigue and other properties.