Learning Module: Atomic Picture of Plastic Deformation in Metals

by Joseph M. Cychosz, Alejandro Strachan

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The main goal of this learning module is to introduce students to the atomic-level processes responsible for plastic deformation in crystalline metals and help them develop a more intuitive understanding of how materials work at molecular scales. The module consists of:
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• Two introductory lectures (50 minutes each) available online as audiovisual presentations
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• Hands-on lab involving online molecular dynamics (MD) simulations via nanoHUB.org
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Jump directly to the learning module or continue reading for the module’s rationale, learning objectives, and target audience.
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Why MD simulations?
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MD provides a very detailed description of materials and its processes by describing the dynamics of each individual atom in a material. Such a realistic descript of materials has an enormous educational potential and, unlike simple toy models or cartoons, can help students understand how materials look and work at atomic scales.
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If you are interested in learning more about MD, click here [link to MD topics page].
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Learning objectives
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Upon completion of this learning module most students will be able to:
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• Compute stress strain curves of metallic nanowires using online MD simulations with the nanoMATERIALS simulation tool and explore the role of size, temperature, and strain rate;
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• Compute the strength of perfect nanowires and compare it with that of polycrystalline samples;
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• Understand the role of pre-existing dislocations on the yield stress of metals;
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• Understand the orientation of the active slip plane with respect to the tensile axis for uniaxial deformation;
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• Understand the atomic displacements that lead to plastic deformation in single crystal nanoscale wires
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Some students are expected to:
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• Understand the difference in activation associated with dislocation nucleation and their propagation;
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• Identify the active slip system (slip plane and slip direction) from the MD simulations
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• Explore strain hardening focusing on the difference between annealed and cold worked macroscopic samples and nanoscale specimens;
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• Explore and understand compressive vs. tensile asymmetry in uniaxial deformation of nanowires;
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Instructors can build on this learning module to teach Schmid law and the calculation of Schmid factors for fcc crystals as well as the specimen rotation during single glide.
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Audience
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This learning module was designed for and used in an introductory course for second-year students of Materials Engineering at Purdue University. Students will find this learning module most useful if they are familiar with the following topics:
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• Basic physics of classical mechanics;
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• Mechanical response of metals (elastic and plastic deformation, yield strength, and work hardening)
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• Basic knowledge of crystal structures (common crystal structures of metals, crystalline planes)
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We expect this module to be useful in introductory and advanced courses of the mechanical response of materials and nanoscience at the undergraduate and graduate levels.