Phase Diagrams

• mixture

  more than one phase
• tie line

  also sometimes called an isotherm
• eutectoid

  one solid phase transforms to two other

  solid phases
• important phase diagram phase transformations include

  eutectic, eutectoid, and peritectic!!


  phase diagrams are useful tools to determine:

  • the number and types of phases present,
  • the composition of each phase,
  • the weight fraction of each phase given the temperature and composition of the system
• determine microstructure of alloy- alloying elements present; their concentration; heat treatment of alloy
• coring- concentration gradients exist across grains in polycrystalline alloy; higher concentrations of the component have lower melting temperature at grain boundaries; unable to achieve equilibrium due to rapid cooling
• gibbs phase rule:

  number of phases (P) + non-compositional variable (N) = components (C) + number of degrees of freedom (F)
• microconstituent

  distinct component or structure within a material observed under a microscope
• eutectoid ferrite

  one of phases of pearlite; forms below eutectoid temperature
• hypereutectoid

  carbon concentration above eutectoid composition
• microconstituent

  specific component within region which may consist of more than one phase
• phase

  region with uniform properties
• incongruent phase transformation

  at least one of phases experience change in composition

• congruent phase transformation

  no compositional alterations
• consequence of coring- loss in mechanical integrity of alloy; grain boundaries melt first at temperature below equilibrium phase boundary
• metastability

  system not at equilibrium; very slight imperceptible changes of phase characteristics over time
• phase equilibrium

  completely stable system
• condition for equilibrium- minimum free energy under same specified combination of temperature; pressure, and composition
• solubility limit

  maximum concentration for which only a single phase solution exists
• phases

  physically and chemically distinct material regions that form (e.g., a and b)
• high mutual solubility

  same crystal structure; similar electronegativities and atomic radii (W. Hume – Rothery rules)
• copper- FCC; 1.8 electronegativity; 0.1278 nm atomic radius
• binary systems

  just 2 components
• isomorphous

  complete solubility of one component in another
• lever rule

  determine the fraction of each phase
• slow rate of cooling

  equilibrium structure
• eutectic

  has a special composition with a minimum melting temperature
• eutectic

  liquid transforms to two solid phases
• peritectic

  liquid and one solid phase transform to a second solid phase
• equilibrium structure

  homogeneous size/ structure
• copper and silver have limited solubility
• eutectic microstructure

  lamellar structure
• hypo

  left; before eutectic point
• copper and zinc

  eutectoid or peritectic
• equilibrium phase diagrams

  convenient and concise way of representing the most stable relationships between phases and alloy systems
• microstructural characteristics important for multiphase alloys:

  • number of phases present
  • relative proportion of phases
  • manner in which phases are arranged
• one-component (unary) diagram

  pressure vs temperature; solid/liquid/vapor phase
• binary system

  temperature vs composition; external pressure held constant
• isomorphous diagram

  complete solubility in solid phase (Cu-Ni)
• for alloy of specific composition, at known temperature, and at equilibrium, phases present, phase composition, and phase mass fraction can be determined