17-12-2012, 12:29 PM
PHASE DIAGRAMS
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Mixtures –Solutions –Phases
Almost all materials have more than one phase in them. Thus engineering materials attain their special properties.
Macroscopic basic unit of a material is called component. It refers to independent chemical species. The components of a system may be elements, ions or compounds.
A phase can be defined as a homogeneous portion of a system that has uniform physical and chemical characteristics i.e. it is a physically distinct from other phases, chemically homogeneous and mechanically separable portion of a system.
A component can exist in many phases.
E.g.: Water exists as ice, liquid water, and water vapour.
Carbon exists as graphite and diamond.
Solid solution
The solid solutions comprise of atoms of almost the same atomic radius , and they tend to form a single phase and the elements are soluble in both their solid and liquid states .
The key feature of a solid solution is that the metals retain their homogeneity and hence their solubility after their transformation to a solid crystalline state.
Types of solid solutions
Substitutional Solid Solutions have a direct substitution of one type of atom for another so that solute atoms (Cu) enter the crystal to take positions normally occupied by solvent atoms (eg . Ni atoms).
Disordered Substitutional Solid Solutions are those type of solid solutions in which the solute atoms are randomly distributed in the solvent lattice structure , hence disordered.
Substitutional Solid Solution
Factors that affect the solubility in alloy systems.
Crystal-structure factor: Complete solid solubility of two elements is never attained unless the elements have the same type of crystal lattice structure.
Relative size factor: Difference in atomic radii less than 15 percent favors solid solution formation.
Difference grater than 15 percent limits the solid solution formation.
Intermediate phases
In many binary alloy systems, when the chemical affinity of metals is great, their mutual solubility becomes limited or reduced and a so called “intermediate phase” is formed.
The intermediate phase may have either narrow or wide ranges of homogeneity and may or may not include a composition having a simple chemical formula.
Gibbs phase rule
In a system under a set of conditions, number of phases (P) exist can be related to the number of components © and degrees of freedom (F) by Gibbs phase rule.
Degrees of freedom is the number of independent variables (e.g., pressure ,temperature) that can be varied individually to effect the changes in a system.
[attachment=44007]
Mixtures –Solutions –Phases
Almost all materials have more than one phase in them. Thus engineering materials attain their special properties.
Macroscopic basic unit of a material is called component. It refers to independent chemical species. The components of a system may be elements, ions or compounds.
A phase can be defined as a homogeneous portion of a system that has uniform physical and chemical characteristics i.e. it is a physically distinct from other phases, chemically homogeneous and mechanically separable portion of a system.
A component can exist in many phases.
E.g.: Water exists as ice, liquid water, and water vapour.
Carbon exists as graphite and diamond.
Solid solution
The solid solutions comprise of atoms of almost the same atomic radius , and they tend to form a single phase and the elements are soluble in both their solid and liquid states .
The key feature of a solid solution is that the metals retain their homogeneity and hence their solubility after their transformation to a solid crystalline state.
Types of solid solutions
Substitutional Solid Solutions have a direct substitution of one type of atom for another so that solute atoms (Cu) enter the crystal to take positions normally occupied by solvent atoms (eg . Ni atoms).
Disordered Substitutional Solid Solutions are those type of solid solutions in which the solute atoms are randomly distributed in the solvent lattice structure , hence disordered.
Substitutional Solid Solution
Factors that affect the solubility in alloy systems.
Crystal-structure factor: Complete solid solubility of two elements is never attained unless the elements have the same type of crystal lattice structure.
Relative size factor: Difference in atomic radii less than 15 percent favors solid solution formation.
Difference grater than 15 percent limits the solid solution formation.
Intermediate phases
In many binary alloy systems, when the chemical affinity of metals is great, their mutual solubility becomes limited or reduced and a so called “intermediate phase” is formed.
The intermediate phase may have either narrow or wide ranges of homogeneity and may or may not include a composition having a simple chemical formula.
Gibbs phase rule
In a system under a set of conditions, number of phases (P) exist can be related to the number of components © and degrees of freedom (F) by Gibbs phase rule.
Degrees of freedom is the number of independent variables (e.g., pressure ,temperature) that can be varied individually to effect the changes in a system.