The high precision of HEDM measurements at FAST offer new insight into the microscopic processes that cause dwell fatigue, pointing toward new alloying strategies for mitigation.
What is the discovery?
Titanium alloys exhibit a phenomenon known as dwell fatigue: when the alloys are held under persistent loads as low as 60% of yield stress, their fatigue lifetime is gradually reduced. The culprit for this degradation in performance is believed to be dislocation slip, which is an intermittent, scale bridging phenomenon, not unlike a nanoscale earthquake occurring in the alloy. Sudden dislocation slips can induce large stress bursts and initiate crack formation. In a new publication appearing in Nature Communications, a team lead by Felicity Worsnop (MIT) and David Dye (Imperial College London) has used high-energy diffraction microscopy at the FAST beamline at CHEXS to observe and quantify thousands of sudden “stress drop” events in thousands of different crystalline grains inside titanium alloys held under dwell fatigue conditions. The team was able to collect unprecedentedly precise statistics for the probability for different types of stress drop events to occur in different alloys. The figure below shows the probability for stress drops with magnitude equal to or greater than Δτ̅ and associated with the possible 3 slip modes (illustrated at right), in 4 different alloys (a – disordered Ti-7AL, low oxygen content; b – higher oxygen content; c – low oxygen, after aging; d – high oxygen, after aging). They discover that interstitial oxygen promotes slip homogeneity, with a higher frequency of smaller stress drops being observed, whereas precipitation of regions with aluminum ordering results in fewer, larger events. Basal slip is observed to be the most common of the slip modes and gives rise to the largest slip events.
Read more on the CHESS website