Phase diagram

• Phase diagram


  Illustrates the phase behavior of a single substance as a function of temperature and pressure
• One-component phase diagram


  • Illustrates the phase behavior of a single substance as a function of temperature and pressure
• System


  Any portion of nature isolated from the rest. Two meanings are possible: (1) a specific body of material being considered, and (2) a series of possible alloys consisting of the same components
• Phase


  A homogeneous portion of a system that has uniform physical and chemical characteristics. Every pure material is considered to be a phase; so also is every solid, liquid, and gaseous solution.
• Phases


  • Solid
  • Liquid
  • Gas
• Component


  The smallest number variable chemical constituents necessary to express composition of each phase
• H2O is a component, not (H and O)
• Intensive variables


  The number of variables which must be arbitrarily fixed in order that the conditions of a system may be defined
• Intensive variables


  • Temperature
  • Pressure
• Degree of freedom


  The number of variables that can be adjusted while keeping the system in equilibrium
• Gibbs phase rule

  F + P = C + N, where F = Degree of freedom, P = # of phases, C = # of Composition, N = # of Intensive Variables
• Phase diagram


  • Shows the phases present and their compositions at various conditions of temperature and pressure

  
• Solubility limit


  The maximum concentration of solute atoms that may dissolve in the solvent to form a solid solution
• Solubility


  The ability of a solvent to dissolve a solute
• The solubility of sugar (C12H22O11) in a sugar-water syrup is 65% sugar and 35% water

  
• Phase equilibria


  A system is at equilibrium if its free energy is at a minimum under some specified combination of temperature, pressure, and composition
• Change in temperature, pressure, and/or composition for a system in equilibrium

  Results in an increase in the free energy and in a possible spontaneous change to another state by which the free energy is lowered
• Binary isomorphous system


  A binary system in which the two components are completely soluble in each other in the solid state, forming a single-phase solid solution over the entire composition range
• In a binary isomorphous system, the components are Cu and Ni
• Interpretation of phase diagram

  1. Determine phases present

  2. Determine phase composition

  3. Determine phase amounts
• At point A (T=1100, Co=60% Ni), the phase present is solid
• At point B (T=1250, Co=35% Ni), the phases present are liquid and solid
• At point A (T=1100, Co=60% Ni), the composition of the liquid phase is CL = Co (60 wt% of Ni), CL = Co (40 wt% of Cu)

  
• If we know the temperature and Composition, then we will know the phase present.
• Phase present at point B:

  • Liquid
  • Solid
• Determination of Phase Composition

  1. Construct tie line across two-phase region

  2. Note intersections of tie line and phase boundaries

  3. Drop perpendiculars to composition axis to read phase compositions
• DEVELOPMENT OF MICROSTRUCURE IN ISOMORPHOUS ALLOYS

  • In isomorphous alloys, the microstructure develops through the solidification process
  • Atoms of both components arrange randomly within crystal lattice, resulting in homogeneous microstructure
• MICROSTRUCURE


  • The physical properties and mechanical behavior depend on the microstructure
  • Important characteristics: number of phases, relative proportions, arrangement of phases
  • Affected by: alloying elements present, their concentrations, heat treatment
• Equilibrium Cooling


  Slow process allowing material to uniformly adjust temperature and reach thermodynamic equilibrium
• Nonequilibrium Cooling


  Rapid cooling preventing material from achieving thermodynamic equilibrium, leading to metastable structures
• MECHANICAL PROPERTIES OF ISOMORPHOUS ALLOYS

  • Single solid phase below melting temperature of lowest melting component
  • Solid-solution strengthening: increase in strength and hardness by additions of other component