Computational Polarized-Light Microscopy for Microtextured Regions Characterization in Titanium Alloys

Microtextured regions (MTRs) or macrozones in titanium alloys are spatially and crystallographically clustered alpha grains that cause significant debits in fatigue life under cold dwell loading conditions such as those experienced during operation of jet engines. Large-area electron backscatter diffraction (EBSD) has evolved over the past two decades as the primary tool to characterize MTRs (size, shape, and crystallographic orientation), typically covering an area of 10mm x 10mm at 5-10 micrometer resolution. While EBSD is the ground truth for MTRs characterization, limitations such as stringent sample preparation requirements, sample size limitation (to fit within the evacuated SEM chamber), limited data collection rates, and high cost restrict production implementation of this method for quality control. In this talk, we present an alternative method for MTRs quantification based on orientation reconstruction from computational polarized-light microscopy (CPM) response data for the optically anisotropic hexagonal close-packed crystal structure. Using a polarized-light instrument developed by MRL (TiPolar^TM), data were recorded at an effective 200,000 pixel per second at 0.4 micrometer resolution (1000x faster than EBSD) without the attendant size and atmosphere limitations typical of EBSD. Data analytics were applied to determine c-axis orientation with respect to the sample coordinate system to generate c-Maps and (0001) pole figures. This data can be utilized to affordably characterize MTRs in large areas of interest. Examples of application of CPM to different titanium alloys will be presented to demonstrate the capabilities and robustness of the method with comparisons to EBSD measurements.