Towards a Fundamental Basis for Controlling Shear Flow Instabilities in HCP Metals

Purdue University, West Lafayette IN

Investigators

Abstract

Non-Technical Abstract When metals are deformed to large strains they often undergo a transition from smooth, steady flow to unstable flow modes, which are usually undesirable. Shear banding, in which the strain becomes concentrated in highly localized bands is one mechanism for the flow instability. Another mechanism, recently discovered, is sinuous flow, which is characterized by a folding process near surfaces, analogous to vortex-like fluid flow. These instabilities can cause failure during the deformation itself, as well as leave behind defects that initiate failure in service. They are thus of critical importance in product quality for a wide range of applications (e.g., biomedical, automotive and aerospace). The instability phenomena will be characterized in model metal systems (magnesium, titanium, zinc) using a controlled shear deformation apparatus, in concert with direct, high-speed imaging of the microscopic flow fields and image analysis. Complementary ex situ characterization of the flow will be done using advanced microscopy methods, and low-load indentation. The experiments will be coupled with analytical and numerical modeling to develop flow mechanism maps that depict the various instabilities and the conditions of their occurrence. The research will provide a fundamental basis for developing methods of broad applicability for controlling flow instabilities in advanced metals, advance experimental techniques for flow analysis, and facilitate understanding of other types of instability phenomena in nature. The research results will broadly impact synthesis of metal structures for energy absorption, friction and wear, metals processing and discrete products manufacturing. Complementing the research is an education program involving undergraduate and graduate students in creating a video gallery of flows in materials; and a modest focus on fostering entrepreneurship in graduate study. Technical Abstract The proposed research seeks to advance our understanding of meso-scale plastic flow instabilities in large-strain deformation of metals, and how these instabilities mediate transitions from laminar to unsteady fluid-like flows, e.g., shear band, sinuous and serrated flows. Prior work has established a suite of experimental techniques to characterize flow fields at high resolution that will be built upon in this work. Three coordinated thrusts will study flow instabilities at the meso-scale to establish a fundamental basis for controlling unsteady flows. First, key flow attributes will be mapped, combining direct in situ analysis at high spatial and temporal resolution, with complementary ex situ characterization by microscopy and profilometry. Second, modeling approaches will be developed to describe the flow instabilities and development of unsteady flow dynamics. Third, by integrating the experimental results with model analyses, a phase diagram will be constructed for flows, demarcating unstable regimes and flow transitions in terms of quantitative deformation parameters. The study will be conducted specifically on model HCP alloys (Mg, Ti and Zn) selected for their experimental suitability and range of deformation responses. The resulting flow phase diagram will provide a basis for tailoring and controlling simple-shear flows, from suppressing flow instabilities to enhancing unsteady flows for energy dissipation. The research will foster an education program involving graduate and undergraduate students in developing video galleries to illustrate diverse flow and instability phenomena; student internships in research labs; and graduate student entrepreneurship.