Characterization of microtexture in Ti-6Al-4 V using ultrasonic polar forward scattering

The manufacturing of billets of Ti6Al4V leads to a distribution of microtextured regions (MTRs) within the billet. The presence of MTRs is hypothesized by some to increase the risk of cold dwell fatigue in components forged from such billets. The position and orientation of a critical MTR within a component can increase the local stress state which may lead to fatigue crack initiation. An MTR orientation is characterized by its C-axis which defines the direction of the MTR which is mechanically the stiffest. Thus, it is imperative that critical MTRs within billets and forgings are detected and that their C-axis orientation is quantified. Ultrasonic inspections are currently used for Ti6Al4V billets during production, primarily to identify hard-alpha inclusions which also act as stress risers. These inspections have increased airline safety by ensuring that such inclusions are not present within billets prior to forging. Unfortunately, MTRs can result in false positives during these inspections, but little is known about the specific material state that leads to such indications. Here, we examine the ultrasonic scattering from suspected MTRs using a unique experiment to quantify the ultrasonic through-transmission polar scattering from an MTR. First, a Ti6Al4V billet was scanned using a cylindrically-focused transducer to identify potential MTRs. Possible MTRs were identified and 2″ cubes were cut from the billet with the goal to position specific MTRs near the center of each cube. The cubes were then scanned from all six faces using a spherically focused transducer to locate the primary MTR and any other interesting features. Scans of the six faces of the cubes provide partial information about the backscatter response for an MTR from different directions. These samples were used for additional through-transmission ultrasonic measurements that used a custom rotating fixture to reveal the strong dependency of the angular scattering from suspected MTR locations. Finally, the angular scattering profiles are compared with predictions of MTR scattering that are based on DREAM.3D models of textured microstructures. The results are expected to provide additional insight into the detection and characterization of MTRs, especially with respect to their C-axis orientation.