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Lecture 4 Notes (BME 296)

Lecture Overview

  • Lecture 4: Focus on microstructure and defects in metals used in biomedical engineering (BME 296).

Practice Questions

  • Miller Indices:

    • Sketch planes to illustrate Miller indices.

      • (124)

      • (012)

Review Topics

  • Importance of Structures:

    • Face-Centered Cubic (FCC) and Body-Centered Cubic (BCC): Important for understanding mechanical properties.

    • Miller Index: Key for determining crystal orientations.

    • Types of Defects:

      • Vacancy: Absence of an atom in a lattice position.

      • Self-Interstitial: Atoms from the lattice occupy interstitial sites.

Atomic Packing Factor (APF)

  • Highest APF: 0.74

    • Indicates the efficiency of packing spheres in a structure.

    • Equivalence in equal volume spheres to unequal volume spheres affects properties.

Biomaterials Manufacturing Methods

  • Various techniques for creating biomaterials in different shapes:

    • Methods:

      • Imprinting

      • 3D Printing

      • Microcontact Printing

      • Other advanced techniques for different structures.

Metal Manufacturing Process

  • Steps of Manufacturing:

    • Cooling molten metal followed by shaping.

    • Shaping involves pouring into molds.

    • Cooling rate affects mechanical properties due to crystal formation.

Crystal Formation and Grain Boundary Processes

  • Nucleation: Formation of the first unit cell in a crystal.

  • Growth Phase: Additional unit cells attach to existing ones, forming structures.

  • Grain Boundaries: Areas where different crystal orientations meet, influencing material properties.

Defects in Materials

  • Point defects (0D): Such as substitutions and vacancies.

  • Line defects (1D): Includes edge dislocations.

  • Planar defects (2D) and volume defects (3D): Affect larger areas and material properties.

Line Defects

  • Characteristics:

    • Edge dislocations can arise from growth accidents.

    • Defined by the line of extra half-plane in the crystal structure.

Differences Between Defects

  • Point Defect: Isolated defect at a single lattice point.

  • Line Defect: Underlies a continuous plane of atoms.

Plastic vs Elastic Deformation

  • Plastic Deformation: Involves slip of dislocations through adjacent planes.

  • Elastic Deformation: Temporary change in shape that recovers upon load removal.

Effect of Line Dislocations on Material Properties

  • Plastic Deformation: Facilitated by movement of dislocations; impacts mechanical behavior of materials.

Energy Considerations in Defects

  • Atoms at surfaces have higher energy (surface free energy) due to fewer bonds with neighbors.

  • Higher energy states enhance chemical reactivity.

Planar Defects and Grain Boundaries

  • Grain Boundaries: Define interfaces between crystallites; higher energy states lead to increased reactivity.

  • Types of boundaries: small-angle and high-angle grain boundaries, defined by misorientation.

Volume Defects in Crystals

  • Volume Defects: 3D regions where crystal order is disrupted (e.g., voids and precipitates).

  • Applications in Biomaterials: Control of voids to influence biological responses.

Recap of Defects

  • Summary of defect characteristics and their influence on properties:

    • Point defects can enhance stiffness; line defects improve plasticity.

    • Grain boundaries change surface energy dynamics and susceptibility to corrosion.

Diffusion Processes in Solids

  • Solid-state Diffusion: Movement of atoms from high to low concentration areas.

  • Requirements for Diffusion: Vacancy availability and sufficient energy to overcome bond restrictions.

Mechanisms of Diffusion

  • Types:

    • Vacancy Diffusion: Atoms move into adjacent vacancies.

    • Interstitial Diffusion: Small atoms migrate through interstitial spaces (e.g., hydrogen, carbon).

Problem-Solving in Material Properties

  • Interfacial Energy: Smaller grain structures lead to higher energy in grain boundaries.

  • Corrosive Attack: More likely at grain boundaries due to higher energy states compared to grain centers.

Methods for Strengthening Metals

  • Strain Hardening: Increasing dislocation density by deforming materials below melting point.

  • Annealing: Heating treatment that aids in dislocation movement, reducing strain energy and dislocation density.