Lecture7-MTE271-LN

CENGAGE THE SCIENCE AND ENGINEERING OF MATERIALS

• Chapter 10: Enhanced Seventh Edition

• Solid Solutions and Phase Equilibrium

• Reading Assignment: Lecture 10 (textbook) and Chapter 10

• Authors: Donald R. Askeland, Wendelin J. Wright

Key Topics Overview

• Atomic Bonds: Includes metallic, covalent, ionic, and secondary bonds.

• Crystal Structures: Types include hcp (hexagonal close-packed), bcc (body-centered cubic), fcc (face-centered cubic), and their Miller indices, directions, and planes.

• Phase Diagrams: Understanding nucleation, TTT (time-temperature-transformation) diagrams.

• Material Properties: Mechanical, optical, thermal, magnetic, and corrosion resistance.

• Types of Materials and Applications: Metals, ceramics, polymers, composites.

• Solidification Process: Overview of Lectures 1-5, 6-9, 10-12, 13-23 covering solidification and defects.

Learning Objectives

• Apply Gibbs phase rule to phase systems.

• Conditions for solid solutions formation.

• Explain mechanical property effects from solid solutions in metallic materials.

• Draw and analyze isomorphous phase diagrams and compute phase composition.

Phases & Phase Diagrams

• Definition of Phase: Homogeneous physical portions of a system differentiated by boundaries, characterized by uniform structure and properties.

• Gibbs Phase Rule: Connects the number of phases to the number of species and degrees of freedom.

  • Equation: [ P + F = C + 2 ] where P = phases, F = degrees of freedom, C = components.

• Unary Phase Diagram: Represents single-component systems, relating pressure and temperature.

Solubility and Solid Solutions

• Unlimited Solubility: Continuous phase formation—e.g., copper and nickel.

• Limited Solubility: One species can form a single-phase solution until a composition limit is reached, after which a second phase forms, e.g., salt in water.

• Polymeric Systems: Copolymers result from mixing different soluble polymers.

Conditions for Unlimited Solid Solubility (Hume-Rothery Rules)

• Size Factor: Atom/ion size difference < 15%.

• Crystal Structure: Same crystal structure is necessary.

• Valence: Same valence preferred to avoid compound formation.

• Electronegativity: Similar electronegativities discourage compound formation.

Solid Solution Strengthening

• Mechanism: Increases resistance to dislocation movement.

• Factors Affecting Strengthening:

  • Atomic size difference: Greater differences increase strength.

  • Amount of alloying element: More increases strengthening effects.

• Property Effects: Increased yield strength, tensile strength, and hardness; reduced ductility; decreased electrical conductivity; improved resistance to creep.

Solidification Concepts

• Solidification Dynamics: Differences in solidification for alloys vs. pure metals.

  • Requires both nucleation and growth processes.

  • Cooling rates influence structure and properties (segregation can occur with rapid cooling).

• Cored Structures: Non-uniform composition due to fast cooling rates during solidification.

  • Equilibrium Structure: Achieved with slow cooling rates.

Nonequilibrium Solidification & Segregation

• Microsegregation: Occurs over small distances in solid phases, leading to poorer casting properties.

• Macrosegration: Large distance variations, difficult to eliminate, often reduced by hot working.

• Homogenization: Heat treatment to reduce composition differences in segregated structures.

• Rapid Solidified Powders: Minimization of segregation through rapid cooling during processing.

Example Case Studies in Solid Solutions

• Cu-Ni System: Demonstrates solid solution principles and properties affected by phase and composition.

• Phase Diagrams of Cu-Ni: Facilitates understanding of phases present based on temperature and composition, key for process control.

• It is essential to understand the complex interactions between the composition, structure, and mechanical properties of materials for effective application in engineering and design.