Phases PDF

**Phases** Seperate and identifiable state of matter in which a substance exist. A region is chemically and physically unfiform and can seperate from one another, ex. Ice and water - A consituent phase has a state of lowest energy - Phases can exist in meta-stable condition, often favour in...

**Phases** Seperate and identifiable state of matter in which a substance exist. A region is chemically and physically unfiform and can seperate from one another, ex. Ice and water - A consituent phase has a state of lowest energy - Phases can exist in meta-stable condition, often favour in industry - Two or more phases can coexist Stable phase = Lowest energy Water: T=-5C results in lowest energy is solid. T=20C results in lowest energy is liquid Composition, temperature and pressure determined energy state Ternary phase diagram: 4-phase eq: P=4, C=3 -\> F=1 3-phase eq: 2-phase **Equilibrium:** State in which the free energy of two phases is equal. The composition of the solid and liquid will be the same at any given time within this range. **Enthalphy:** Total energy of a system plus the product of it's pressure and volume **Entropy**: Measure of disorder within system. (liquid has higher entropy than solid) **Kelvin:** Absolute 0: −273.15 °C **Free energy (Energy necessary to achieve phase transformation)** **First law:** Energy can't be created or destroyed, only transformed **Second law:** Entropy of a system can never decrease, tends to increase (Your room tend to get messier over time, never cleaner) Ideal solid solution: Mixing of components without change in enthalpy. Components behave similarly = uniform distribution. Gibbs free energy is negative. Ex. Copper and Nickel Non-ideal solid solution: Components do not behave uniformly which leads to deviation from expected (ideal) behaviour. Ex. Metalls and ceramic. **Nucleation phase** First step of new thermodynamic phase or structure. Material becomes unstable due to pressure or temperature and clusters of atoms/molecules start to form. - Homogeneous: Spontaneous where new phases take form uniformly throughout the material. - Heterogeneous: Formation of new phase occurs at specific sites like inbetween materials or on particles/dust/dirt. Easier to form onto, less random. **Dendrites** Start to form when the temperature is below materials solidifcation temperature. Tiny, solid particles (nuclei) start forming, becoming tree like shapes that brances out. The shape, size and speed of which they grow affects the properties of the material. **Segregation during freezing** When a liquid freezes it can contain other substances that doesn't freeze at the same rate. This can result in them gathering together and can cause changes to physical properties like strength and ductility. **Cored structures** The material is cooled down to quickly which leads to uneven solidifcation troughout the structure, leaving the core with another crystal structure than the edges. **Equilibrium structures** The liquid is cooled down slowly enough for the material to solidy uniformly. The homogenous structure makes the material more predicable. **Chill casting** Preheated permanent mold. The mold acts as heat sink, extracting heat from molten metal. The rapid cooling results in finer grainstructure which enhances strength. Can lead to brittle surface and temperature tension if cooled down to quickly. **Continous casting** Molten metal is poured into cooled mold which extracs the heat rapidly. Eutectoid: **Quiz:** Suppose the maximum solubility of Pb in Sn is 5% Pb. If 3% Pb is added to tin, **We excpect to find a single-phase alloy, since 3% is lower than 5% it doesn't exceed the solubility limit.** Which one of the following is NOT a condition for unlimited solid solubility? **Only metals, and not ceramics, can display complete soluability.** The temperature at which an alloy begins to melt during heating is called the **Solidus** The temperature at which an alloy is completely solid during cooling is called the **Solidus** The temperature difference between the liquidus and solidus temperatures is called the **Freezing range** Between the liquidus and solidus temperatures, we expect to find **Two phases, each having the same composition. Why?** Suppose that you heat three alloys, each having a different composition, to the same temperature. In all three alloys, a mixture of liquid and solid form **The composition of the solid phase in all three alloys is the same. When they are all heated to a specific temperature that falls within the two-phase region, the solid phase that forms will have a composition determined by the phasae diagram at that temperature.** During nonequilibrium solidification, we expect that **The last solid to form contains a larger amount of the low melting point elements** In order to eliminate the effects of microsegregation, we might **Perform a homogenization heattreatment** The cooling rate of a casting is slowest **Between the liquidus and solidus temperatures. Latent heat must be removed for the material to solidify.** If we plan to heat treat or hot work a casting that has been produced by nonequilibrium solidification, we should  **Be sure to keep the maximum temperature well below solidus.** - **Prevent re-melting** - **Maintaining microstructure** - For a Fr-60 Gd alloy, the liquidus temperature is **600** The freezing range for a Fr-20% Gd alloy is about **160** A Fr-50% Gd alloy is heated to 400°C. The composition of the alpha phase is about **60%** A Fr-50% Gd alloy is heated to 400°C. The amount of liquid phase is about **25** A Fr-Gd alloy is heated to 500°C and a mixture of liquid and α is produced. The composition of the liquid is **40** A cooling curve is shown for a Fr-Gd alloy. The composition of the alloy from which the curve was obtained is about **40** The local solidification time of the casting from which the cooling curve was obtained is about **400s**