An Investigation of Ti-6Al-4V Micromechanical Evolution Under Dwell Fatigue Conditions Using High Energy X-rays

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Restricted (Penn State Only)
Author:
Peterson, Kenneth
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 22, 2026
Committee Members:
Venkatraman Gopalan, Program Head/Chair
Darren Pagan, Chair & Dissertation Advisor
Jingjing Li, Outside Unit & Field Member
Allison Beese, Major Field Member
Long-Qing Chen, Major Field Member
Keywords:
Titianium
dwell fatigue
X-ray
Mechanical testing
stress redistribution
Abstract:
Precisely predicting a time and location of failure initiation continues to be an area of ongoing study in engineering alloy design and performance modeling. A challenge of determining conditions for failure lies in the complex interplay of deformation mechanisms across length scales, local loading conditions, and microstructural heterogeneity that all evolve with time and have varying influence on failure nucleation. Cold dwell fatigue failure of titanium alloys is one such failure mechanism that has gained notoriety for markedly reducing alloy performance and is also challenging to characterize in situ, as failure nucleation events are rare and occur within the material bulk. This work pushes the state-of-the-art in high energy X-ray diffraction microscopy, a suite of techniques capable of nondestructively interrogating microstructure and mechanical response of bulk material volumes in situ, to study popular engineering alloy Ti-6Al-4V in various loading conditions exploring the early stages of dwell fatigue. Monotonic and cyclic loading experiments reveal neighborhood scale load redistribution results in stress concentrations in grain orientations associated dwell fatigue failure and transient slip system softening that is often not represented in crystal plasticity models. Stress relaxation studies present a novel method for determining slip system specific rate sensitivity that quantifies the rate anisotropy of basal, prismatic, and pyramidal slip and demonstrates the effects of this anisotropy using finite element method crystal plasticity simulation. In the final set of experiments, the fastest far-field high energy X-ray diffraction microscopy measurements to date, reveal long hypothesized transient stress redistribution during dwell periods, defect reconfiguration, and stress concentration in grain orientations associated with dwell fatigue failure in agreement with popular theorized failure mechanisms. Lastly, a multiscale diffraction simulation framework is presented that incorporates stress contributions from dislocations, grain-to-grain interaction, and macroscale boundary conditions. Comparisons of simulation and monotonic tension experiments are made to describe changes in defect population as the material deforms. These first-of-their-kind in situ observations help paint a clearer picture of the complex micromechanical conditions leading up to failure of engineering alloy Ti-6Al-4V in support of improved material modeling and alloy design.