Grain boundaries

1. Advancement of Fatigue Damage across Grain BoundariesTongguang Zhai, University of Kentucky, DMR 0645246 This research project explores the mechanisms for the early advancement of fatigue damage across grain boundaries (GBs) in engineering alloys. In this project, it was revealed that the orientation of a GB could affect the path of a short fatigue crack in high strength Al alloys. In Fig. 1, the fatigue crack was initiated in grain 3, and propagated onto the {111} plane in grain 2, despite the {111} plane had the largest twist angle (47.5) with crack plane in grain 3. This was due to the GB being tilted by 38, thereby the twist angle of the {111} plane became the minimum (close to 0) among the 4 possible slip planes (Fig. 2). This result could contribute to the development of a technique for more accurate life prediction of components in engineering systems such as airplanes and steel bridges, etc. Fig. 1 Electron micrograph of the path of a short fatigue crack across GBs in an AA8090 Al-Li alloy. Detailed crystallographic relations between crack planes across GBs were analyzed. Sub-grain 3 Fracture surface 2 Grain boundaries surface  111  1-11  -111  11-1 GB tilted by 38 Vertical GB Fig. 2 Calculated twist angles of 4 possible crack planes across the boundary between grains 3/2 as a function of GB tilt.

2. Advancement of Fatigue Damage across Grain Boundaries Tongguang Zhai, University of Kentucky, DMR 0645246 • A undergraduate student from mechanical engineering, Denis Livchak, participated in this research project by developing 3D animation models for short crack growth across a grain boundary perpendicularly and vertically, stereotype pole figure projection and Euler space construction, etc. These models help students understand those often hard-to-understand 3D concepts and processes in materials science. The video shows the stereotype projection of crystal orientations to form a pole figure. • The PI developed an introductory course on crystallographic texture, focusing on the representation of crystal orientation, formation of texture, texture control during thermomechanical processing and property-texture relations in alloys. Video file here