Lecture12 - Strengthening Mechanisms The Million Dollar...

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Chapter 6 - 1 Strengthening Mechanisms The Million Dollar Question: Knowing how structures affect mechanical properties how can we use this knowledge to “tailor” the structure to achieve desired properties? To strengthen metals: restrict the dislocation movement Imagine yourself walking across the classroom as a dislocation traveling through a crystal. What types of obstacles might you encounter that would slow you down?
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Chapter 6 - 2 Strengthening Mechanisms point defects - solid solution strengthening, alloying lattice strains of impurities interact with dislocation movement ex: Sterling Silver, C in Fe Steel (desks in room) other dislocations - strain hardening, cold working material becomes harder and stronger with plastic deformation due to accumulation of dislocations ex: Blacksmithing, rolling (other people in the room) grain boundaries - grain refining smaller grains -> greater total grain boundary area (processing) ex: nanocrystalline materials, rapid solidification (walls) volume obstacles - precipitation hardening, multiphase alloys dislocations cut through or go around large obstacles
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Chapter 6 - 3 Strengthening The ability of a metal to deform depends on the ability of dislocations to move Restricting dislocation motion makes the material stronger • Mechanisms of strengthening in single-phase metals: grain-size reduction solid-solution alloying strain hardening and precipitation hardening Thus, strengthening reduces ductility
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Chapter 6 - 4 Stronger - grain boundaries pin deformations ( , ) change from one crystal to another. R will vary from one crystal to another. The crystal with the largest R yields first. Other (less favorably oriented) crystals yield later. Adapted from Fig. 7.10, Callister 7e. (Fig. 7.10 is courtesy of C. Brady, National Bureau of Standards [now the National Institute of Standards and Technology, Gaithersburg, MD].) Dislocation Motion in Polycrystals 300 m
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Chapter 6 - 5 Strategies for Strengthening: 1: Reduce Grain Size Grain boundaries are barriers to slip. Barrier "strength" increases with Increasing angle of misorientation. Smaller grain size: more barriers to slip. Hall-Petch Equation: 2 1 / y o yield d k Adapted from Fig. 7.14, Callister 7e. (Fig. 7.14 is from A Textbook of Materials Technology , by Van Vlack, Pearson Education, Inc., Upper Saddle River, NJ.)
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Chapter 6 - 6 Ex: Grain Size Strengthening
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Chapter 6 - 7 Plastic deformation of polycrystalline materials Slip directions vary from crystal to crystal Some grains are unfavorably oriented with respect to the applied stress (i.e. cosφ cosλ low) Even those grains for which cosφ cosλ is high may be limited in deformation by adjacent grains which cannot deform so easily Dislocations cannot easily cross grain boundaries because of changes in direction of slip plane and at grain boundary As a result, polycrystalline metals are stronger than single crystals (the exception is the perfect single crystal without any defects)
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This note was uploaded on 11/25/2008 for the course MATSCI 104 taught by Professor Lan during the Spring '08 term at UCLA.

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Lecture12 - Strengthening Mechanisms The Million Dollar...

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