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.