Analyzing the possibility of significant strain accommodation by dislocation climb in Mg alloys via dislocation density measurements
Guest Speaker： Prof. Sean R. Agnew, University of Virginia, U.S.A
Inviter: Assoc. Prof. Jingya Wang
Date&Time: Wednesday, 6.Nov., 9:30-11:00
Venue: Yiucheng Lecture Hall (500), Xu Zuyao Building
Biography:
Sean R. Agnew (William G. Reynolds Professor of Materials Science & Engineering, MSE) earned a B.S. (double major) in Mechanical Engineering and MSE from Cornell University (1993) and Ph.D. in MSE from Northwestern University (1998). He was a Eugene P. Wigner Postdoctoral Fellow at Oak Ridge National Laboratory (1999-2001) and has served on the faculty at the University of Virginia since 2001. He was named the inaugural recipient of the Helmholtz Zentrum Geesthacht Magnesium Research Award (2008), the Heinz & Doris Wilsdorf Chaired Associate Professor of MSE (2010), a Fellow of ASM international (2015), and served as Associate Editor of the International Journal of Plasticity (2018-2023). He represents UVA on the board of the Virginia Nuclear Energy Consortium and the Virginia Innovation Nuclear Hub. He specializes in electron, X-ray, and neutron scattering-based characterization of materials structure (especially dislocations, internal/residual strains, and crystallographic texture), mechanical property characterization, and computational modeling (especially crystal plasticity) of the mechanical behaviors of a diverse range of materials: from lightweight metals, alloys, and intermetallic compounds (to improve transportation system efficiency), all the way to cermets, shape memory alloys, compositionally complex alloys, and heavier metals and alloys.
Abstract:
Recent crystal plasticity modeling studies have suggested that dislocation climb may be an important contributor to strain accomodation at moderately elevated temperatures (e.g., 200 °C). If true, this would provide a straightforward explanation for the radical transition in formability which many wrought Mg alloys undergo at such temperatures. In this work, novel electron backscattered diffraction (EBSD) and X-ray line profile analysis (XLPA) techniques are employed to examine the densities of dislocations with <a>, <c> and <c+a> Burgers vectors within traditional Mg alloys, AZ31B and ZK10, tested in uniaxial tension at various temperatures. The current and previously published ex-situ XLPA results suggest that the relative activity of <a> dislocations drops with increasing temperature, being replaced by the activity of <c> and <c+a> dislocations in AZ31B, whereas the relative densities of these dislocations are relatively constant in the ZK10 samples examined in this study. EBSD results paint a somewhat different picture, which is consistent with the notion that dislocation climb activation is the relevant transition which explains the radically improved formability of Mg alloys at moderately elevated temperatures.
Novel, EBSD noise reduction strategies have revealed conventional EBSD methods (vs. high-resolution HR-EBSD based on diffraction pattern cross-correlation) can have a much higher sensitivity to detect relatively low densities of geometrically necessary dislocations (GNDs) than previously thought. Such filtered EBSD data reveals GND accumulation near grain boundaries during low temperature plasticity of Mg, whereas GNDs are primarily associated with grain subdivision after deformation under conditions where the constitutive response is often denoted as power law creep, and which is well understood to be due to dislocation climb and glide. Furthermore, results of the EBSD-based approach reveal the relative populations of GNDs with <a>, <c> and <c+a> Burgers vectors is relatively insensitive to the deformation temperature for both alloys examined. It is noted that GNDs are much less prone to static recovery, as compared to statistically stored dislocations (SSDs). As such, it is hypothesized that non-basal slip modes are active and important at all temperatures and the main reason ex-situ XLPA measurements have suggested a transition in the relative densities of <a>, <c> and <c+a> with increasing temperature is because SSDs with <a> Burgers vectors are more inclined to undergo recovery during unloading.