Yousif A. Al Rabie
Advisor: Prof. Preet Singh

will defend a master’s thesis entitled,

Hydrogen Embrittlement in Precipitation-Hardenable Nickel-Based Alloys

On

Tuesday, November 18 at 3:00 p.m.
MRDC Room 3515

Committee

            Prof. Preet Singh - School of Materials Science and Engineering (advisor)

            Prof. Hamid Garmestani - School of Materials Science and Engineering

     Prof. Chaitanya Deo – School of Mechanical Engineering

Abstract
Hydrogen embrittlement (HE) in precipitation-hardenable (PH) nickel-based alloys poses critical risks for energy infrastructure, including subsea systems, petrochemical processing, and hydrogen production technologies. This thesis quantifies the interplay between hydrogen transport, microstructural trapping, and mechanical degradation across representative PH Ni alloys (UNS N07725, N09925, N09945, N09946, N09955) using coordinated Devanathan–Stachurski electrochemical permeation (ASTM G148) and in-situ slow-strain-rate tensile (SSRT) testing under cathodic charging, supported by scanning electron microscopy fractography and XRF analysis.

A clear HE-susceptibility ranking was established (725 > 945X/945 > 925), consistently supported by ductility-loss ratios (epR​), brittle-area fractions, and hydrogen-transport metrics. HE susceptibility did not correlate with any single transport parameter (e.g., total hydrogen inventory or lattice diffusivity) in isolation. The most robust, mechanistically grounded predictor of ductility was the composite mobility-to-storage ratio, Ddecay​/Cobs​, which showed a strong correlation with epR​ (r=0.96,). Furthermore, in Alloy 945X–140, susceptibility increased from 25–55 °C, attributed to thermal activation of reversible traps that increased the trap bias (TB) ~5.8× and facilitated trap-assisted hydrogen transport.

Overall, HE susceptibility is governed by the mode of hydrogen transport, controlled by the accessible reversible-trap population. Alloys with high reversible-trap accessibility (e.g., 725) exhibit strong kinetic asymmetry (ΔD) and high HE susceptibility. In contrast, HE-resistant alloys (e.g., 945) mitigate embrittlement by minimizing reversible-trap participation, enforcing a slower, lattice-dominated diffusion mode.