Introduction à la Science des Matériaux

Introduction to Materials Science

• The presentation, given by Gael Sattonnay, focuses on the plastic behavior of metals.
• The goal is to understand and model the plastic behavior in calculations, using Finite Element Analysis (FEA), for accurate results.
• Current calculations often assume elastic linear behavior, but this is not always accurate.

Plasticity

• General aspects:
  • Macro-plasticity, focusing on tensile testing.
  • Conventional/rational tensile curve.
  • Necking (striction) during plastic deformation.
  • Plasticity origin: dislocations.
  • Mechanisms of hardening.
  • Constitutive laws.
  • Plasticity criteria: Tresca and Von Mises.
  • Load surface and work hardening.
  • Creep.

Importance of Plastic Behavior

• This is crucial for analyzing structures (e.g., car components, railway axles), predicting deformations and failures.
• It is used in manufacturing processes (e.g., metal forming).

Dislocations

• Defects in crystalline structure.
• Edge and screw.
• Mixed type of dislocations.
• Burgers vector.
• Line vector.
• Obstacle to plastic deformation.

Dislocation Density

• A significant factor influencing material properties.
• Metal before deformation (recrystallized metal): ~ 10⁶ to 10⁸ cm^-2.
• Highly deformed metal: ~ 10¹¹ to 10¹² cm^-2.

Plastic Deformation

• Occurs on atomic plans.
• Dislocations move on slip systems (crystallographic planes/directions).
• Creates strain.
• Formation of slip bands.

Plastic Behavior

• The behavior of monocrystals.
• The formation of slip systems and bands.
• Motion of dislocations.
• The role of dislocation density.
• The presence of local rupture for dislocation movement.
• Importance of stress on dislocation motion.

Hardenening Mechanisms

• Intrinsic:
  • Work hardening.
  • Resistance of grain boundaries.
• Extrinsic:
  • Precipitation hardening.
  • Solid solution hardening.
• Effect of lattice resistance (peirls-nabarro force).

Hardenening: Solution Strengthening

• Differences in atomic radii between solute and solvent creates stress fields.
• Impedes dislocation movement.
• Increases yield strength.

Hardenening: Grain Size Refinement

• Smaller grain size increases strength according to Hall–Petch relationship.
• Barrier effect.
• Enhanced resistance to dislocation motion.

Hardenening: Precipitation

• Formation of small, hard particles impedes dislocation movement.
• Important for many alloys.

Constitutive Laws

• Mathematical expressions describing the relationship between stress and strain, considering both elastic and plastic behavior.
• Examples: Ramberg-Osgood equation, Ludwik equation.

Plasticity Criteria

•  Tresca criterion (maximum shear stress).
• Von Mises criterion (equivalent stress).

Load Surface and Hardening

• Describes the boundaries of elastic and plastic deformation.

Creep

• Time-dependent deformation under constant load and elevated temperature.
• Primary, secondary, and tertiary stages.
• Affected by temperature and applied stress.

Diffusion in Solids

• Atomic movement in solids.
• Atomic migration.
• Influences the rate of creep.

Diffusion Mechanisms

• Vacancy diffusion.
• Interstitial diffusion.
• Correlation to creep.