S5E14

Abstract

Hydrogen embrittlement (HE) of metals causes unexpected failures and remains imperfectly understood. Prevailing theories emphasize either hydrogen (H) enhanced decohesion (HEDE) or H-enhanced plasticity (HELP) as the key factors responsible for HE. Identifying the true mechanism is crucial for the lifetime prediction of metals operating in H environment, and can help develop new HE-resistant alloys.

In this work, we use digital image correlation (DIC) during in situ tensile tests in a scanning electron microscope (SEM) to analyze localized plastic strains, and clarify the role of slip in the initiation cracks in hydrogen embrittled alloy 725. Our work reveals no tendency for H to enhance localized slip. Intense, localized slip in alloy 725 occurs predominantly along grain boundaries, rather than in grain interiors. At low H concentration, cracks initiate primarily in the vicinity of slipping boundaries, and with the increase of H concentration, a greater fraction of cracks initiate without localized slip. Our results suggest that slip is not essential for crack initiation in H embrittled alloy 725, and are inconsistent with HELP.

Introduction of speaker

Dr. Mengying Liu is an assistant professor at Washington and Lee University, Physics and Engineering department. She received her Ph.D. degree in Materials Science and Engineering from Texas A&M University, and her Bachelor’s degree in Materials Science and Engineering from Tianjin University, China.

Abstract

High-strain rate mechanical testing is demanded to improve the design of future structural materials across broad application conditions. Advances in several standard high-strain rate mechanical testing platforms allow a deeper fundamental understanding of material response under extreme conditions to refine the designing process. However, the throughput on a given sample in a given time is limited due to complicated testing procedures. In this work, we utilize a simple, high-throughput, cost-efficient nanoindentation instrument to measure hardness evolution and unveil the deformation mechanism of metal crystals from low (10-2 s-1) to ultra-high strain rate (up to 105 s-1) using FCC-aluminum and BCC-molybdenum as the model systems. Precession electron beam diffraction microscopy and recently developed quasistatic reloading technique demonstrate that, at ultra-high strain rate, the deformation mechanism of aluminum is governed by the generation of new dislocations, whereas that of molybdenum is dominated by thermal activation. Our results agree well with the studies from standard high strain-rate testing platforms, even though the boundary conditions are different, and also indicate a promising pathway of a high-throughput small-scale testing technique that could remarkably accelerate testing times and substantially reduce costs.

Introduction of speaker

Yuwei Zhang is a postdoctoral associate in the Department of Material Science and Engineering at Texas A&M University. He received his Ph.D. degree in Mechanical Engineering at Texas A&M University in 2021. During Ph.D., his research focused on small-scale mechanical testing in the field of energy storage and conversion. During postdoc appointment, his research focuses on the development of the high strain rate nanoindentation technique.