Module 6

Module 6: Dislocations and Strengthening Mechanisms

Contents

• Dislocations & Plastic Deformation
  • Mechanisms of plastic deformation in metals.
• Strengthening Mechanisms in Metals.
• Recovery, Recrystallization, and Grain Growth.

Plastic Deformation – Dislocations

• Permanent plastic deformation is due to:
  • Shear processes where atoms change their neighbors.
  • Inter-atomic forces and crystal structure play significant roles.
• Cumulative movement of dislocations leads to gross plastic deformation.
  • Edge dislocations move by slip and climb.
  • Screw dislocations move by slip and cross-slip.
• Dislocation interactions are complex due to multiple dislocations moving across different slip systems.

Dislocation Interactions

• Dislocations moving on parallel planes can annihilate each other, leading to:
  • Vacancies or interstitials.
• Dislocations on non-parallel planes hinder each other's movement, causing sharp breaks:
  • Jog: Break out of slip plane.
  • Kink: Break in slip plane.
• Other hindrances to dislocation motion include:
  • Interstitial and substitutional atoms.
  • Foreign particles and grain boundaries.
  • External grain surfaces and phase change structures.
• Material strength can be increased by arresting dislocation motion.

Plastic Deformation Mechanisms – Slip

• Two main mechanisms: Slip and Twinning.
  • Slip is more prominent, involving sliding of crystal blocks along slip planes.
  • Occurs when shear stress surpasses a critical value.
  • Slip occurs in specific directions (slip directions) on certain crystallographic planes.
  • A combination of a slip plane and a slip direction forms a Slip System.
• During slip, each atom moves an integral number of atomic distances along the slip plane.

Factors Affecting Slip

• Extent of slip is influenced by:
  • External load and resulting shear stress.
  • Crystal structure and slip planes’ orientation relative to shear stress directions.
  • Critical shear stress defined for single crystals by Schmid.

Slip in Polycrystalline Materials

• In polycrystalline aggregates:
  • Individual grains mutually constrain each other, preventing plastic deformation at lower stresses.
  • Slip involves the generation, movement, and rearrangement of dislocations, maintaining mechanical integrity at grain boundaries.
  • Need at least five independent slip systems for ductility and grain boundary integrity (von Mises).
  • Twinning also contributes to crystal deformation.

Slip Systems Overview

Plastic Deformation Mechanisms – Twinning

• Twinning involves a portion of crystal changing orientation in a symmetrical way relative to the rest.
• Twinning changes plane orientation, allowing further slip to take place and occurs on specific planes and directions for each crystal structure.

Slip vs. Twinning Comparison

Strengthening Mechanisms

• Increasing material strength by hindering dislocation motion leads to various strengthening mechanisms:
  • Single-phase materials:
  • Grain size reduction.
  • Solid solution strengthening.
  • Strain hardening.
  • Multi-phase materials:
  • Precipitation strengthening.
  • Dispersion strengthening.
  • Fiber strengthening.
  • Martensite strengthening.

Strengthening by Grain Size Reduction

• Dislocations face hindrances when moving from one grain to another due to orientation changes.
• Smaller grain sizes increase hindrances, increasing yield strength.
• Yield strength is related to grain size through the Hall-Petch relation:
  • (\sigmay = \sigma0 + k d^{-1/2})
• Can be controlled through cooling or plastic deformation followed by heat treatment.

Effects of Grain Size Reduction

• Enhances both strength and toughness.
• Grain size can be quantified by comparing with standard charts.
• ASTM grain size number (Z) relates to grain diameter (D) as:
  • (D = \frac{1}{Z})

Solid Solution Strengthening

• Impurities introduce lattice strains which anchor dislocations.
• Effectiveness is determined by:
  • Size difference of solute and base material.
  • Volume fraction of solute.
• Various interactions include elastic, modulus, stacking-fault, and electrical interactions.

Yield Point Phenomenon

• Upper Yield Point (UYP) marks a transition from elastic to plastic deformation.
• Lower Yield Point (LYP) requires higher stress for initiation of plastic flow compared to sustaining it.
• Lüders bands occur during this phenomenon, aligning at approximately 45 degrees to the tensile axis.

Causes of Yield Point Phenomenon

• Associated with small amounts of interstitial or substitutional impurities.
• Could involve unlocking of dislocations or generation of new dislocations.
• Magnitude is influenced by the interaction energy between solute atoms and dislocations.

Strain Hardening

• Involves increasing strength and hardness through plastic deformation.
  • Strain hardening occurs primarily at low temperatures.
• Increased dislocation density results from deformation, enhancing yield stress correlation:
  • (\tauT = \tau0 + A\sqrt{p})
• Notable changes in physical properties include:
  • Decreased density and conductivity.
  • Increased thermal expansion and reactivity.
• Recovery through annealing restores original properties in three stages: recovery, recrystallization, and grain growth.

Precipitation & Dispersion Hardening

• Foreign particles obstruct dislocation movement, enhancing material strength.
• Precipitation hardening (age hardening) requires a soluble second phase at high temperatures that precipitates upon cooling.
• Aging can be natural (room temperature) or artificial (heated).
• In dispersion hardening, fine particles, like oxides, are added and must have low solubility.

Fiber Strengthening

• Incorporates high-strength fibers into a ductile matrix for improved performance.
• Requires high strength and modulus for fibers and ductility for the matrix.
• Strengthening mechanisms involve load distribution and require exceeding a critical fiber volume.

Martensite Strengthening

• Formation of martensitic phase induces strengthening from the original high-temperature phase.
• Characterized by unique growth patterns and twin structures; occurs in various alloy systems.
• Strength attributed to dislocation density and carbon involvement in Fe-C systems.

Recovery, Recrystallization, and Grain Growth

• Recovery:
  • Initial annealing stage, reducing dislocation density and stress without altering microstructure.
• Recrystallization:
  • Occurs post-recovery; formation of new strain-free equiaxed grains; influenced by stored energy and purity.
• Grain Growth:
  • Follows complete recrystallization, driven by grain boundary energy reduction. Effectively retarded by impurities.