3, MSE 791 Mechanical Properties of Nanostructured Materials

[Pages:17]3, MSE 791 Mechanical Properties of Nanostructured Materials

Module 3: Fundamental Physics and Materials Design

Lecture 1

1. What is strain (work) hardening? What is the mechanism for strain hardening? Strain hardening is the flow stress increase with increasing strain. Its magnitude is !",

!"

where is the true stress and ! is the true strain. The strain hardening is caused by the accumulation of crystalline defects: primarily dislocations, and also stacking faults and twins in some cases.

2. What are the factors that determine the strength of a metal? List some factors that can enhance the strength. Strength is determined by dislocation obstacles that block dislocation slip. Factors that can increase the strength include: a) smaller grain size (follows the Hall-Petch relationship: ! = !!+kd-1/2, b) solute atoms (solution hardening), c) second phase-particles, d) twins, e) stacking faults, f) entangled dislocations, g) phase boundaries

3. What are the factors that determine the ductility of a metal? List two mechanical factors that can enhance the ductility. 1) Strain hardening rate (primary); 2) Strain rate sensitivity (secondary). High steain hardening rate and high strain rate sensitivity will increase the uniform elongation (i.e. ductility).

4. Nanostructured metals usually have low ductility. Explain why in terms of mechanical behavior and deformation mechanisms. Nanostructurd metals usually have low ductility because they have low strain hardening rate. The low strain hardening rate is caused by low and even zero accumulation of dislocations and other defects. When the grain size becomes very small, e.g. ................
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