CIVE Construction Materials Phase Diagrams A phase diagram is

1/16/2008 CIVE 202 – Construction Materials Phase Diagrams A phase diagram is a map which shows the relationship between phases in i equilibrium i a system at various t ilib i in t t i temperatures, pressures, and t d compositions. A phase is a physically-distinct, chemically-homogeneous and mechanically-separable region of a system. The components of a system refer to the chemical species that comprise the system. They may be elements, ions or compounds. CIVE 202 – Construction Materials Phase Diagrams Phase Diagram for Water Single Component Phase Diagram 1 1/16/2008 CIVE 202 – Construction Materials Phase Diagrams The phase rule determines the number of independent variables (i.e. degrees of freedom) needed to describe the system at any given point. point It is also known as Gibb’s Equation: F=C–P+2 Degrees of g Freedom Number of Components Number of Phases Ph The following examples refer to the equilibrium diagram for water. CIVE 202 – Construction Materials Phase Diagrams Example 1 – Liquid Region F=C–P+2=1–1+2=2 Water Liquid Phase Diagram for Water 2 1/16/2008 CIVE 202 – Construction Materials Phase Diagrams In a single phase region (two degrees of freedom), both temperature and pressure can be varied independently within the limits of the phase boundaries without changing the phase. phase Phase Diagram for Water CIVE 202 – Construction Materials Phase Diagrams Example 2 – Freezing Line F=C–P+2=1–2+2=1 Water Liquid, Solid Phase Diagram for Water 3 1/16/2008 CIVE 202 – Construction Materials Phase Diagrams When two phases are in equilibrium (one degree of freedom), either temperature or pressure can vary independently, but not both. both Phase Diagram for Water CIVE 202 – Construction Materials Phase Diagrams Example 3 – Triple Point F=C–P+2=1–3+2=0 Water Liquid, Solid, Gas Phase Diagram for Water 4 1/16/2008 CIVE 202 – Construction Materials Phase Diagrams At the triple point (three phase equilibrium) there are no degrees of freedom. Therefore, in this situation, neither temperature or pressure may be varied without altering one or two of the phases phases. Phase Diagram for Water CIVE 202 – Construction Materials Phase Diagrams Two component systems are better described by binary phase diagrams. In these equilibrium diagrams we have a composition variable in addition to the usual temperature and pressure variables. We therefore need a three dimensional diagram to plot all three variables. To simplify the relationships on paper, binary phase diagrams are usually drawn at atmospheric pressure, showing variations of temperature and composition only. 5 1/16/2008 CIVE 202 – Construction Materials Phase Diagrams Phase diagram for a binary system exhibiting complete mutual solubility. CIVE 202 – Construction Materials Phase Diagrams In binary phase diagrams there will always be a two phase region which separates the single phase regions. The phase boundary between the liquid and the two phase region is called the liquidus. The boundary between the solid and the two phase region is called the solidus. 6 1/16/2008 CIVE 202 – Construction Materials Phase Diagrams Phase Diagram for Nickel and Copper CIVE 202 – Construction Materials Phase Diagrams When the solid solubility is limited and the melting points of the components are not vastly different, a eutectic phase diagram different results. When the solid solubility is limited, it is indicated by the phase boundary called the solvus. One example is the lead-tin equilibrium diagram. 7 1/16/2008 CIVE 202 – Construction Materials Phase Diagrams Phase Diagram for Lead and Tin CIVE 202 – Construction Materials Phase Diagrams When the melting points of the components are vastly different from each other a peritectic phase diagram is formed. other, formed One example is the silver-platinum equilibrium diagram. 8 1/16/2008 CIVE 202 – Construction Materials Phase Diagrams Phase Diagram for Platinum and Silver L+α→β CIVE 202 – Construction Materials Phase Diagrams Tertiary Phase Diagram for CaO-Al2O3-SiO2 9 1/16/2008 CIVE 202 – Construction Materials Phase Diagrams Variation of some properties in an isomorphous system: CIVE 202 – Construction Materials Phase Diagrams Variation of some properties in a eutectic system: 10 1/16/2008 CIVE 202 – Construction Materials Phase Diagrams 11