Phase Diagrams - Chapter 10 - Solubility and Cu-Ni Alloy Phase Diagram

Oct 29, 2026

Phase diagrams explain equilibrium states in binary systems by linking temperature, composition, and the number, type, and proportions of coexisting phases. The material defines components versus phases, introduces the solubility limit and its dependence on temperature, and shows how changing temperature or overall composition moves the alloy among phase regions. Using the Cu-Ni system, rules demonstrate how T and composition Co determine which phases form, and tie-line intersections provide phase compositions in two-phase regions, enabling prediction of phase transformations during cooling.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages41

Reading Time1 h 43 min

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Components and Phases

The Solubility Limit

Effect of T and Composition

Phase Diagrams: Number and Types of Phases Present

Phase Diagrams: Composition of Phases

Cu-Ni Phase Diagram Examples

CHAPTER 9: PHASE DIAGRAMS ISSUES TO ADDRESS... • When we combine two elements... what equilibrium state do we get? • In particular, if we specify...--a composition (e.g. , wt%Cu-wt%Ni), and--a temperature (T) then... How many phases do we get? What is the composition of each phase? How much of each phase do we get? Phase A Phase B Nickel atom Copper atom Some Definitions • An alloy is a combination, either in solution or compound, of two or more components (elements), at least one of which is a metal. • An alloy with two components is called a binary alloy; one with three is a ternary alloy; one with four is a quaternary alloy. • The result is a material with properties different from those of its components. The microstructure ofFe-C alloy COMPONENTS AND PHASES • Components: The elements or compounds which are mixed initially (e.g., Al and Cu) • Phases: The physically and chemically distinct material regions that result (e.g., a and b) . AluminumCopper Alloy β (lighter phase) α (darker phase) THE SOLUBILITY LIMIT • Solubility Limit: Max concentration for which only a solution occurs (remember HUME ROTHERY RULES) . Example: Water – Sugar System Question: What is the solubility limit at 20C? Answer: 65wt% sugar. If Co < 65wt% sugar: syrup If Co > 65wt% sugar: syrup + sugar. 100 Temperature (°C) 80 60 40 20 Solubility Limit L L (liquid) (liquid solution + i.e. , syrup) S (solid ~~ ~~ sugar) 65 0 20 40 60 80 100 Composition (wt% sugar) Pure Water Pure Sugar • Solubility limit increases with T: e.g. at T=99°C, solubility limit is ~ 80wt% EFFECT ofT and COMPOSITION • Changing T can change number of phases: path A to B. • Changing Co can change number of phases: path B to D. A(70, 20) 2 phases Water-Sugar (C 12H22O 11) system B(100,70) D(100,90) 1 phase 2 phases Phase Diagrams • A phase diagram shows what phases are present and where the process boundaries are within the composition space. • Equilibrium phase diagrams represents relations between temperature, pressure, compositions and quantities of phases at equilibrium. • Phase diagrams allows to predict phase transformations which occur during temperature change (e.g. upon cooling) . The following type of binary (contains only two component) systems will be discussed below: -complete solubility: isomorphous -eutectic -with intermediate phases or compounds -involving eutectoid and peritectic reactions Special attentions will be paid on the iron-iron carbide system Binary Isomorphous Systems • Isomporhous system is characterized by complete liquid and solid solubility of the components • For this course:--binary systems: just 2 components. --independent variables: T and Co (P = 1atm is always used) . • Phase Diagram for Cu-Ni system T(°C) 1600 1500 14001300 12001100 10000 Phase boundaries: liquidus and solidus lines • C –L (homogeneous liquid solution) • B- α and L • A- α (FCC solid solution) 20 40 60 80 100wt% Ni • C L (liquid) • B α (FCC solid solution) • A liquidus solidus L + α PHASE DIAGRAMS: Number and Types of Phases Present • Rule 1: If we know T and Co, then we know: -the number and types of phases present. • Examples: A(1100 ° C, 60 wt%): 1 phase: α B (1250, 35): 2 phases: L +α 1600 1500 1400 1300 1200 1100 1000 T(°C) L (liquid) 0 20 40 60 80 100 Cu-Ni phase diagram wt% Ni PHASE DIAGRAMS: Composition of Phases • Rule 2: If we know T and Co, then we know: the composition of each phase • Examples: CO== 35wt%Ni • at TC=1350 C, only one, Liquid phase exists with composition: 35 wt % Ni – 65 % Cu T(°C) • at TA=1175 C, again only one, solid phase exists with composition: 35 wt % Ni – 65 % Cu Cu-Ni system TC 1300 TB 1200 TA | | | --- | |

L + α

liquidus
L + α solidus | 20 30 CL Co 40 Cα

FAQs

What are components and phases in the context of phase diagrams?

Components are the initially mixed elements or compounds, while phases are physically and chemically distinct regions that result in the material.

How does the solubility limit depend on temperature?

The solubility limit increases with temperature; for example, the text notes that at about 99°C the solubility limit is roughly 80 wt%.

How are phase compositions determined in a two-phase region of a Cu-Ni phase diagram?

When two phases exist, a tie line (an isotherm) intersects phase boundary lines; those intersection points give the compositions of the corresponding phases (e.g., liquid and solid solutions).

Oct 29, 2026

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