H9

Definition: A Phase is a homogeneous portion of a system that has uniform physical and chemical characteristics. Objectives • Understand binary phase diagrams: • Sketch a phase diagram • label liquidus, solidus and solvus lines. • Use phase diagrams to calculate phase constitution • what phases are present, the composition of each phase, and the mass fraction of each phase given temperature and alloy composition. • Use phase diagrams to determine the microstructure of an alloy at a given temperature and alloying composition. • Define and write the reactions for eutectic, eutectoid, and peritectic phase transformations. A single-phase material has the same composition and structure at every point throughout the material Poly-phase materials: Many materials contain two or more coexisting phases. Such materials can be quite complex, and their properties depend upon: •the number of phases present •the relative amounts of the phases •the composition & structure of each phase •the size and spatial distribution of the phases What is a Phase Diagram? A phase diagram is a graphical representation of the relationships between environmental constraints (e.g.; temperature), composition, and regions of phase stability under equilibrium conditions. Other courses: • Composition fixed, Temperature & pressure vary This course: • Pressure fixed (1atm), composition & temperature vary. H9-1 PHASE DIAGRAMS • Tell us about phases as function of Temperature (T), Composition (Co) . (Note Pressure = 1atm is always used.) • Phase Diagrams are EQUILIBRIUM diagrams! They are based on the free energy of the system at a given composition & temperature. Kinetics may affect the phases present in reality Topic 9! • For this course: • binary systems: just 2 components, for example, Cu- Ni, Cu-Al, Fe-C. • independent variables: T and Co. Understanding Phases H9-2 Determining Phase Composition • For single phase regions the composition of the phase is the same as the overall composition of the alloy. • For 2 phase region the composition of the two phase is determined by the composition of the single phase region on either side of the two phase region at that temperature. Determining Phase Composition in 2 phase regions 1. Draw a horizontal line at the temperature of interest through the two- phase region --> this is called a Tie Line. 2. The composition at the intersection of the tie line and the phase boundaries on either side of the line are noted, these are the compositions of the two phases. Definition – Tie Line A horizontal line constructed across a two phase region of a binary phase diagram; its intersections with the phase boundaries on either end of represents the equilibrium compositions of the respective phases at the temperature in question. The Lever Rule • A mathematical expression whereby the relative phase amounts in a two phase alloy at equilibrium may be computed. • Basically a geometric interpretation of the conservation of Mass. H9-3 Definition - Eutectic • Eutectic means easily melted (from Greek) • Eutectic phase diagram have a point on them where there is an equilibrium between a 2 phase solid region and a liquid. • Liquid starts to form at the Eutectic temperature. • When does ice form if we add 23.3wt% NaCl to pure water? • What happens to the ice formation temperature if we add more than 23.3% NaCl? If we add less than 23.3% NaCl? Important Regions on a binary phase diagram • Solvus line- The line on a phase diagram representing the limit of solid solubility as a function of temperature. • Solidus line- The line on a phase diagram at which solidification is complete upon equilibrium cooling, or at which melting begins upon heating. • Liquidus line-The line indicating above which melting of a solid is complete upon heating or at which solidification begins upon cooling. • Invariant point-a point on a phase diagram at which three phases are in equilibrium (e.g.; eutectic point) H9-4