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Phase Diagrams for Metallic Systems

INTRODUCTION

Alloy Phase Diagrams are useful to metallurgists, materials engineers, and materials

scientists in four major areas: (1) development of new alloys for specific applications, (2)fabrication of alloys into useful configurations, (3) design and control of heat treatmentprocedures for specific alloys (to produce the required mechanical, physical, and chemicalproperties), and (4) solving problems from the performance of specific alloys in commercialapplications, that improves product predictability. Explicitly seen, using the phase diagramsallows research, development, and production to be done more efficiently and costeffectively.In addition, data from phase diagrams is essential in the design and development ofalloys. More so, it is reasonably accessible for binary systems.

Definitions and Basic Concepts

Component : Pure metal or compound (e.g., Cu, Zn in Cu-Zn alloy, sugar, water)

Solvent : Host or major component in solution

Solute : Dissolved, minor component in solution

System : Set of possible alloys from same component (e.g., iron-carbon system.)

Solubility Limit : Maximum solute concentration that can be dissolved at a giventemperature

Phase : Part with homogeneous physical and chemical characteristics

Phase Diagrams : Show the relationships between the various phases that appear withinthe system under equilibrium conditions to be able to record andvisualize the results of studying the effects of state variables on a system

Binary Diagrams : Are composed of two components [(e.g, two metals - Cu and Ni), or a

metal and a compound ( Fe and Fe3C ), or two compounds (Al2O3 andSi2O3) ]. Usually they show variations in temperature and compositiononly. Additionally, they consist only one liquid phase.

Isomorphous Sys.: Show one complete liquid and solid solubility

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DISCUSSION

Phase diagram, also known as equilibrium or constitutional diagram, depicts theexistence of different phases of a system under equilibrium.

Equilibrium phase diagrams represent the relationships between temperature and thecompositions and the quantities of phases at equilibrium. In general practice, it is sufficient toconsider only solid and liquid phases, thus pressure is assumed to be constant (1 atm.) in mostapplications.

A phase diagram is a collection of solubility limit curves. The phase fields in equilibriumdiagrams depend on the particular systems being depicted. Set of solubility curves thatrepresents locus of temperatures above which all compositions are liquid are called liquidus ,while solidus represents set of solubility curves that denotes the locus of temperatures belowwhich all compositions are solid. Every phase diagram for two or more components must showa liquidus and a solidus, and an intervening freezing range, except for pure system, as meltingof a phase occurs over a range of temperature.

There are certain locations on the phase diagram where the liquidus and solidus meet,whether the components are metals or nonmetals. For a pure component, a contact point liesat the edge of the diagram. The liquidus and solidus also meet at the other invariant positions

on the diagram.

For almost all alloy systems, at a specific temperature, a maximum of solute atoms candissolve in solvent phase to form a solid solution, which is known as solubility limit . Generally,solubility limit changes with temperature. If solute available is more than the solubility limit, itmay lead to formation of either a solid solution or compound. Phase equilibrium is the set ofconditions where more than one phase may exist. It can be reflected by constancy with time inthe phase characteristics of a system. In most metallurgical and materials systems, phaseequilibrium involves just solid phases. However, the state of equilibrium is never completelyachieved due to very slow rate of approach of equilibrium in solid systems, that leads to non-equilibrium or meta-stable state, which may persist indefinitely and has more practicalsignificance than equilibrium phases. An equilibrium state of solid system can be reflected interms of characteristics of the microstructure, phases present and their compositions, relativephase amounts and their spatial arrangement or distribution.

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Phase diagrams are classified based on the number of components in the system. Singlecomponent systems have unary diagrams, two-component systems have binary diagrams, andthree-component systems have ternary diagrams, and so on. Having more than twocomponents in the system makes the diagrams to be complicated and difficult to represent.

Phase Rule

The Phase Rule, introduced by J. Willard Gibbs in 1876, relates the physical stateof amixture to the number of constituents in the system and to its conditions. Gibbs was also thefirst one who each homogeneous region in a system by the term "phase." When pressure andtemperature are the state variables, the rule can be written as follows:

P + F=C + 2

where f is the number of independent variables(called degrees of freedom), c is the

number ofcomponents, and p is the number of stable phasesin the system.

Binary Eutectic Systems

Binary Eutectic Systems obtain phases present, concentration of phases and theirfraction (%).

Solvus line is the limit of solubility, Eutectic or invariant point. Liquid and two solidphases exist in equilibrium at the eutectic composition and at the eutectic temperature.

*Note: the melting point of the eutectic alloy is lower than that of the components. At most two phases can be in equilibrium within a phase field. Two-phase regions separate single-phaseregions.

Some sets of invariant reactions that may occur in binary systems are:

1. Eutectic Reaction a liquid transforms into two solid phases

1a. Eutectoid Reaction the solid state-analog of eutectic reaction wherein onesolid phase with eutectoid composition turns into two different solid phases

2. Peritectic Reaction a solid phase reacts with a liquid phase to produce a new solidphase

2a.Perictoid Reaction two solid phases react to form a new solid phase

*Note: peritectic and peritectoid reactions do not give rise to micro-constituents as the eutecticand eutectoid reactions do

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3. Monotectic Reaction a liquid phase transforms into a solid phase and a liquid phaseof different composition (wherein two liquids are immiscible e.g,. oil and water).

The Iron Iron Carbide (Fe Fe3C) Phase Diagram

Iron-carbon system is useful in many aspects such as (1) steels constitute greatestamount of metallic materials used by man and (2) solid state transformations that occur insteels are varied and interesting.

This is one of the most important alloys for structural applications. The diagram Fe C issimplified at low carbon concentrations by assuming it is the Fe Fe3C diagram. Concentrationsare usually given in weight percent. The possible phases are:

a-ferrite (BCC) Fe-C solution g-austenite (FCC) Fe-C solution d-ferrite (BCC) Fe-C solution liquid Fe-C solution Fe3C (iron carbide) or cementite. An intermetallic compound.

The maximum solubility of C in a- ferrite is 0.022 wt%. D-ferrite is only stable at hightemperatures. Austenite has a maximum C concentration of 2.14 wt %. It is not stable belowthe eutectic temperature (727 C) unless cooled rapidly. Cementite is metastable, decomposinginto a-Fe and C when heated for several years between 650 and 770 C.

*Note: Ferrite is soft and ductile; Cementite is hard and brittle. Thus, combining these twophases in solution, an alloy can be obtained with intermediate properties.

STATEMENT OF THE PROBLEM

The equilibrium state that can be obtained in combining two elements. Invariant reactions that may occur in binary systems and the phases of matter produced

in each reaction.

METHODOLOGY

Data for this report were gathered from 9 September to 15 September 2014. The topicof Phase Diagram for Metallic Systems was researched online and by the in-depth study of the

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Book Materials Science and Engineering, and then a series of reading was conducted to collectdata for the given topic. Analysis of the data revealed the interesting part of the topic and thecompilation was made. This report is supported by a PowerPoint presentation and a video forvisual purposes.

CONCLUSION

In conclusion, the phase diagram is essential to understanding the materials, itsstructure, composition, and specially the phase changes. On metals, it is very important tofamiliarize the phase diagram so that we can determine the amount of heat and pressure tomake a metal or alloys to change its phase. A phase diagram will tell you what phase or state abinary alloy will be in at any given temperature or composition. One thing to remember is thatphases do not necessarily mean gas, liquid, solid, vapor. in the case of the Fe-C example, it canbe seen that iron and carbon can have several different solid states at the same temperaturedepending on the composition. Also, phase diagrams literally shows how much heat andpressure a metal or alloy can withstand before changing its phase since metals are used asstrongholds materials. This is commonly studied for enhancing the durability of metallicmaterials.