UC Santa Barbara

Titanium alloys have been extensively used throughout the cold section of turbine engines for many decades due to their high strength to weight ratio, corrosion resistance, and excellent fatigue performance. However, many titanium alloys develop millimeter-scale microtextured regions (MTRs) during forging which have been shown to significantly reduce dwell fatigue life. While steps have been taken to develop new alloys and forging pathways that inhibit MTR formation, the mechanisms behind their detrimental effects are poorly understood, and the remaining lifetime for existing MTR-containing components continues to be difficult to predict.

The overarching objective of this research is to understand the local stress states that form within MTRs and link them to dwell fatigue cracking. The extreme grain neighborhoods present within MTRs in combination with the severe elastic and plastic anisotropy of α titanium can induce local stress states that are highly rotated and transformed compared to the nominal macroscopic stress. Understanding these local deviations is critical to predicting the life-limiting microstructural features of titanium alloys.

To accomplish this, experimentally collected, 3-dimensional datasets of deformed titanium microstructures that contain MTRs are analyzed in detail. In-situ experiments as well as elastic and crystal plasticity modeling enable the tracking of local stresses and strains within these datasets on a sub-grain length scale. Metrics derived from these local stresses have been developed to statistically and quantitatively characterize how MTRs alter the local stress state and identify grain boundaries and grain neighborhoods that may be more prone to cracking under dwell fatigue loading. These measurements and models are used to evaluate several dwell fatigue fracture mechanisms presented in the literature that are related to local stresses.