Accounting for Climb and Cross-slip in the Crystal Plasticity of Non-Cubic Metals

University Of Virginia Main Campus, Charlottesville VA

Investigators

Abstract

NON-TECHNICAL DESCRIPTION: Metal alloys with atypical crystal structures (atomic arrangements) exhibit key properties that enable them to be employed in a wide range of advanced technologies that contribute to environmental protection (lead-free tin-based solders); energy conservation through transportation weight reduction (magnesium (Mg) and titanium alloys); and energy production via nuclear technology (zirconium alloys). The current research focuses on lightweight Mg metal deformation, which is of interest to the automotive, aerospace, and defense industries. The research (including both experimental activities and computational modeling) provides insights into improved design and manufacture for these applications. Students involved in this research find employment within the materials, automotive or aerospace manufacturing sectors, and some go on to research careers which serve the nation's interests in energy, defense, or education. An outreach program seeks to develop enthusiasm for materials science, and science and engineering more generally in local elementary school students in Charlottesville and the surrounding rural area. Within this program, the role of strong, yet light-weight materials is demonstrated in efficient transportation - from the Wright brothers' first flight to modern vehicles, like Tesla electric cars and drones. TECHNICAL DETAILS: The objective of the proposed research is to determine the degree to which dislocation climb and cross-slip accommodate strain during the polycrystal plasticity of non-cubic metals. Lightweight Mg and its alloys are selected for this study because of the world-wide interest in increasing the efficiency of transportation as well as the scientific convenience that the near-thermo-elastic isotropy of Mg crystals simplifies the analysis of plastic anisotropy. Textured samples of pure Mg and commercial alloys will be tested under creep and stress-relaxation conditions along different directions with respect to the initial texture. While there is already published data for these materials, a controversy regarding the roles of cross-slip and climb remains. Quantitative accounting for the effect of initial texture, various dislocation types (with a, c, and c+a Burgers vectors), as well as the evolution of the dislocation substructure (largely unstudied in non-cubic materials) will settle the issue. The approach is multi-scale, addressing the macroscale, mesoscale and microscale. MACROSCALE creep and stress relaxation experiments are performed on textured polycrystalline samples, and texture evolution and strain anisotropy are measured on samples crept to significant strain levels. The results of macroscale experiments guide selection of specific stress and temperature conditions to explore with meso- and micro-scale characterization. A new technique known as in situ high energy X-ray diffraction (HEXD) reveals the MESOSCALE distribution of full stress tensors and dislocation contents (including the densities of different types, Burgers vector and line direction) within individual grains of a polycrystal. Information that was previously relegated to single crystal experiments or very small numbers of grains in a polycrystal has only recently become available for statistically significant numbers (hundreds) of grains within polycrystals, and a new polycrystal plasticity model, which accounts for dislocation climb, is used to interpret the HEXD data. Finally, X-ray diffraction line profile analysis and transmission electron microscopy (including in situ straining) experiments elucidate the MICROSCALE dislocation densities, reveal critical dislocation configurations and their kinetic descriptions in terms of specific thermally activated unit processes. The interpretation of the results is aided by discrete dislocation dynamics (DDD) simulation, used to explore the effects of these dislocation ensembles and how they recover. Graduate students are trained in advanced experimental characterization and modeling techniques. With guidance, undergraduate students also perform some of the experiments and analyses, especially at the macroscale. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.