Material Science Flashcards
What is a phase?
A phase as seen under the microscope is a physically distinct, chemically homogenous and mechanically separable portion of a material.
A phase is an easily seen part of a microstructure (i.e. different colour, grain shape and grain size to other grains).
A phase has the same composition throughout the grain.
What is an alloy?
An alloy is made up of two or more elements, one of which must be a metal and the alloy itself must show metallic characteristics (i.e. good conductor, luster, tough, ductile).
The base metal of an alloy is the metal present in the greatest proportion, while other metallic or non-metallic elements present are known as alloying elements.
What is a solution?
All solutions are homogenous mixtures in which the atoms or molecules of the solute material are more or less uniformly dispersed throughout those of the solvent material.
Note; the solute atoms consist of the smaller volume of solution material and the solvent atoms are the greater volume of solution material.
What is an interstitial solution?
Interstitial solid solutions are formed when the solute atoms are very small in comparison with the solvent atoms. The solute atoms will occupy some of the interstices (spaces ) present in the crystal lattice of the solvent.
What is a substitutional solution?
Substitutional solid solutions form by the operation whereby the solute atoms take up positions normally occupied by atoms of the solvent material. Meaning the atoms ‘swap’ or substitute themselves in the crystal lattice. In this case, solid solutions form readily if solute and solvent atoms are of about the same atomic size.
Example: Copper and nickel form a continuous series of Substitutional solid solutions for all percentages of copper and nickel.
What are the three systems (structures of metals) into which the important metal will crystallise?
Body centered cubic (BCC)
Face centered cubic (FCC)
Close packed hexagonal (CPH)
What is a solid solution?
A solid solution consists of atoms of at least two different types; the solute atoms occupy either substitutional or interstitial positions in the solvent lattice, and the crystal structure of the solvent is maintained
How are phase diagrams constructed?
Thermal equilibrium diagrams or Phase Diagrams are constructed by the data collected from cooling curves.
Cooling curves are obtained by plotting falling temperature against time for small samples of selected alloys.
The selected alloys MUST be cooled under equilibrium conditions.
Cooling under equilibrium conditions means that the alloy must be cooled at such a rate that all reactions occurring within it, both while it is solid and when it is liquid are allowed to reach equilibrium.
What can be observed on a cooling curve?
When a metal cools the atoms gradually slow down, which releases heat energy. During a change of phase (e.g. liquid to solid) a dramatic decrease in energy occurs, (latent heat), and the temperature does not change until the phase change is complete. This shows up on the temperature – time graph (cooling curve) as an arrestment point.
What is a solubility limit?
For many alloy systems and at some specific temperature, there is a maximum concentration of solute atoms that may dissolve in the solvent to form a solid solution; this is called a solubility limit.
What does composition represent in a phase diagram?
Compositions of phases are expressed in terms of weight percents of the components (e.g., wt% Cu, wt% Ni). For any alloy consisting of a single phase, the composition of that phase is the same as the total alloy composition. If two phases are present, the tie line must be employed, the extremities of which determine the compositions of the respective phases.
What does the fractional phase amount (Concentration) represent in a phase diagram?
With regard to fractional phase amounts (e.g., mass fraction of the α or liquid phase), when a single phase exists, the alloy is completely that phase. For a two-phase alloy, on the other hand, the lever rule is utilized, in which a ratio of tie line segment lengths is taken.
What is complete solid insolubility?
If complete solid insolubility exists in the solid state, then the two metals will not mix and thus will be quite separate in the final structure.
This occurs in a eutectic reaction which results in alternating bands or layers of metal A and metal B.
What is a eutectic alloy?
A mixture of metals having a melting point lower than that of any of its components.
What is a eutectoid reaction?
Eutectoid is describing the phase-change reaction of an alloy in which, on cooling, a single solid phase transforms into two other solid phases.
