unit 4 and 5

• Crystallization

  The formation of crystalline solid from the uniform liquid. A first-order thermodynamic phase transition where the temperature remains nearly equal to the melting point due to slow removal of heat.

  
• Metals can be produced in the glassy or amorphous solid state, but only with the utmost difficulty. In practice, 'solid' is almost synonymous with 'crystalline' in metals.
• Crystallization ideally is a sharp change of state from the disordered atomic arrangement of the liquid to the ordered arrangement of the crystal, at a single temperature (ideal freezing point = ideal melting point).

  
• The regularity of the liquid is different from the crystallinity of the solid.
• Nucleation


  The step wherein the molecules start to gather into clusters on the nanometer scale, arranging in a defined and periodic manner that defines the crystal structure.
• Crystal growth


  The subsequent growth of the nuclei that succeed in achieving the critical cluster size.
• The ability to use heat to produce, melt, and cast metals such as copper, bronze, and steel is regarded as an important hallmark in the development of mankind.
• Solidification is still considered one of the most important manufacturing processes.
• The solidification process is used to manufacture specific components (e.g., aluminum alloys for automotive wheels) and to produce metallic slabs or ingots as a primary processing step.
• Solidification also is applied when joining metallic materials using techniques such as welding, brazing, and soldering.
• Almost all metals and alloys are produced from liquids by solidification.
• For both castings and wrought products, the solidification process has a major influence on both the microstructure and mechanical properties of the final product.
• Solidification


  The transformation of liquid into solid below the melting point.

  
• At the melting point, liquid gets converted into solid during cooling.

  
• Thermodynamically, both liquid and solid have equal energy at the melting point and both are equally stable at the melting point. No solidification or melting will take place at the melting point.
• Cooling curve


  A graphical plot of the changes in temperature with time for a material over the entire temperature range through which it cools.

  
• Solidification in alloys takes place over a range of temperatures rather than at a constant temperature.
• Under equilibrium conditions, all metals exhibit a definite melting or freezing point, which is shown as a horizontal line in the cooling curve for a pure metal.

  
• In the case of alloys, the solidification does not take place at a constant temperature, but rather in a range of temperatures.

  
• Nucleation


  The formation of the first nano crystallites from molten material.
• Growth


  The increase in size of the solid nuclei as more atoms become attached to the solid surface.
• Types of Nucleation

  • Homogeneous Nucleation: Nuclei of the new phase form uniformly throughout the parent phase
  • Heterogeneous Nucleation: Nuclei form preferentially at structural inhomogeneities, insoluble impurities, grain boundaries, dislocations, and so on
• Dendrites


  Crystals that form in the liquid during freezing, following a pattern consisting of a main branch with many appendages, resembling a pine tree.
• The formation of dendrites occurs because crystals grow in defined planes due to the crystal lattice they create.
• During freezing of a polycrystalline material, many dendritic crystals form and grow until they eventually become large enough to impinge upon each other.

  
• The original dendritic pattern may not be apparent when examining the microstructure of a material, but the dendrites can often be seen in solidification voids that sometimes occur in castings or welds.
• The understanding of phase diagrams for alloy systems is extremely important because there is a strong correlation between microstructure and mechanical properties, and the development of microstructure of an alloy is related to the characteristics of its phase diagram
• Phase diagrams provide valuable information about melting, casting, crystallization, and other phenomena

  
• Phase diagram

  A diagram that conveniently and concisely displays the control of the phase structure of a particular system
• Phase


  • A portion, including the whole, of a system which is physically homogeneous within itself and bounded by a surface that separates it from any other portions
  • The same structure or atomic arrangement throughout
  • Roughly the same composition and properties throughout
  • A definite interface between the phase and any surrounding or adjoining phases

  
• Types of phase diagrams

  • Unary (single component)
  • Binary
  • Ternary

  
• Unary phase diagram


  Used mainly for carbon and pure metal, with very limited practical utilities plotted between temperature and pressure axis

  
• Binary phase diagram


  Used for a system of two components, classified based on the number of phases present (e.g. isomorphous, containing two phases)

  
• Ternary phase diagram


  Used for a system of three components, common in polymeric and ceramic systems

  
• Binary phase diagram


  • Represents the relationships between temperature, compositions, and quantities of phases at equilibrium, which influence the microstructure of an alloy
  • Helpful in predicting phase transformations and resulting microstructures, which may have equilibrium or non-equilibrium character

  
• Liquidus line


  Separates the all melt phase from the melt + crystal phase

  
• Solidus line


  Separates the melt + crystal phase from the all crystal phase
• Eutectic


  The point at which all three phases (A, B, and melt) can exist simultaneously
• The solidus and liquidus lines are experimental, determined by melting and cooling many melts at different percent compositions

  
• Crystallization from an equilibrium melt with 50/50 eutectic


  1. Cool melt to liquidus line, only B crystals form

  2. Removing B changes melt composition, melt composition migrates to left toward eutectic point

  3. At eutectic point, A and B crystallize out together at 50/50 ratio until all melt is gone

  4. After all melt is gone, A+B crystals can leave the eutectic