MAE 381 Materials: Chapter 7 - Dislocations and Strengthening Mechanisms

Vocabulary practice flashcards covering dislocation motion, slip systems, and metal strengthening mechanisms from MAE 381 Exam 3 Study Guide.

Plastic Deformation

The motion of a large number of dislocations; a metal's ability to deform depends directly on dislocation mobility.

Theoretical Shear Stress for Deformation ($\tau$)

Estimated by the equation $\tau = G\gamma$, where $\tau$ is the shear stress required for dislocation, $G$ is the shear modulus, and $\gamma$ is strain.

Slip

Plastic deformation that results from dislocation motion.

Slip Plane

The crystallographic plane where two opposite shear forces meet to cause slip; it corresponds to the preferred plane possessing the highest planar density.

Slip Direction

The preferred crystallographic direction within the slip plane along which dislocation motion occurs, characterized by having the highest linear density.

Slip System

The combination of a preferred slip plane and a preferred slip direction ($\text{Slip system} = \text{Slip plane} + \text{slip direction}$).

Edge Dislocation Motion

Dislocation motion that occurs parallel to the direction of the applied shear stress.

Screw Dislocation Motion

Dislocation motion that occurs perpendicular to the direction of the applied shear stress.

FCC Slip Systems

Consist of the $\{111\}\langle 110\rangle$ family, with $4$ unique non-parallel $\{111\}$ planes and $3$ independent $\langle 110\rangle$ directions per plane, yielding a total of $12$ slip systems.

BCC Slip Systems

Consist of the $\{110\}\langle 111\rangle$ family, with $6$ unique planes and $2$ independent directions per plane, yielding a total of $12$ slip systems.

Burgers Vector ($b$)

A vector corresponding to a dislocation's slip direction whose magnitude equals the unit slip distance (e.g., $b(\text{FCC}) = \frac{1}{2}\langle 110\rangle$, $b(\text{BCC}) = \frac{1}{2}\langle 111\rangle$, and $b(\text{HCP}) = \frac{1}{3}\langle 11\bar{2}0\rangle$).

Peierls-Nabarro Stress

The stress required to move a dislocation; it is minimized by small Burgers vectors ($b$) along close-packed directions and large interplanar spacing ($d$) along close-packed planes.

Critical Resolved Shear Stress

The shear stress resolved within a slip plane and slip direction required to initiate slip in a single crystal, reaching its minimum when $\phi = \lambda = 45^\circ$.

Creep

Plastic deformation that occurs under the yield stress when a load is applied for a specific amount of time, especially at high temperatures.

Hall-Petch Equation

An equation that relates yield strength to grain size, though it is not applicable for extremely large or very small grains.

Strengthening by Grain Size Reduction

A strengthening mechanism where grain boundaries act as barriers to dislocation motion due to differing crystallographic orientations and slip plane discontinuities; finer grains yield higher strength.

Solid Solution Strengthening

Hardening and strengthening a metal by adding interstitial or substitutional impurity atoms that generate lattice strain (tensile for smaller atoms, compressive for larger atoms) and segregate around dislocations.

Strain Hardening

Strengthening caused by plastic deformation multiplying dislocations, which increases dislocation density and causes mutually repulsive interactions that require higher energy for further motion.

Percent Cold Work

A quantitative parameter expressing the degree of plastic deformation imparted to a metal during strain hardening.

Recrystallization

The formation of a new set of strain-free and equiaxed grains through heat treatment, restoring pre-cold-worked mechanical properties by making the metal softer and more ductile.

Recrystallization Temperature

The minimum temperature required to completely recrystallize a material within $1\text{ hour}$.

Grain Growth

An increase in the average grain size following recrystallization at elevated temperatures, driven by a reduction in total grain boundary surface area and overall energy.