5. ALLOY, PHASE DIAGRAM & STEELS - 2017. 8. 24.¢  Module 2 5. Alloy, Phase Diagram & Steels Pruthvi

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Text of 5. ALLOY, PHASE DIAGRAM & STEELS - 2017. 8. 24.¢  Module 2 5. Alloy, Phase Diagram...

  • Pruthvi Loy, Chiranth B. P. 1 SJEC, Mangaluru

    5. ALLOY, PHASE DIAGRAM & STEELS

     Introduction

    - Solid solution: substitutional and interstitial solid solutions

    - Hume Rothary rule

    - Intermediate phases

    - Phase diagrams

    - Gibbs phase rule

     Binary phase diagrams

    - Types of binary phase diagram

    - Construction of a binary phase diagram

    - Interpretation of a phase diagram; lever rule

    - Invariant reactions (Eutectic and Eutectoid)

    - Iron carbon equilibrium diagram

     Steels

    - Classification of steels

    - Specification

    - Alloying additives

    5.1 INTRODUCTION

    Definitions – Phase, Solution, Mixture

    Any material or a substance (system) is composed of a number of independent chemical species

    (constituent); thus a material can be of a single constituent or a multiple constituent type. The

    constituents of a system could be an element or a compound which can exist as different phases.

    A phase may be defined as a physically distinct, chemically homogeneous and mechanically

    separable portion of a system.

    Example: Water is a single constituent material which can exist in different phases (ice, liquid

    water, and water vapour)

    A single phase material having one or more constituents is called a solution (liquid or solid);

    when more than one phase exists, then it is called a mixture. When two phases are present in a

    system, it is not necessary that there be a difference in both physical and chemical properties; a

    disparity in one or the other set of properties is sufficient.

    Example: Water is a solution; on adding sugar crystals to it they dissolve forming a sugar syrup

    which is still a solution (single phase) but when sugar is added beyond its solubility limit in

    water it will not dissolve and forms a mixture with two phases.

  • Module 2 5. Alloy, Phase Diagram & Steels

    Pruthvi Loy, Chiranth B. P. 2 SJEC, Mangaluru

    5.1.1 Solid Solution

    The properties of materials are greatly influenced by changing its composition due to

    modification of its microstructure this is known as alloying. An alloy is a substance that has

    metallic properties and is composed of two or more chemical elements of which at least one is a

    metal. The alloy constituents of a system can be either two metals (Cu and Ni), or a metal and a

    compound (Fe and Fe3C), or two compounds (Al2O3 and Si2O3), etc. These constituents when

    mixed can form a solid solution phase entirely or an intermediate phase.

    a) Solid Solution

    When two metals are mixed in their liquid states, they form a homogeneous solution, if this

    homogeneity is maintained after solidification, then such a solid is known as a solid solution

    (that is, a solution in the solid state consisting of two types of atoms in one type of space lattice).

    In a solid solution the metal in major proportion is called the solvent and the one in minor

    proportion is called the solute.

    Types of Solid Solution:

    The solid solution formed can be of two types;

    Substitutional Solid Solution: In this type of solid solution the solute atoms substitute the atoms

    of the solvent in the crystal structure of the solvent. Example: Cu-Ni system.

    Interstitial Solid Solution: These are formed when atoms of small atomic radii fit into the

    interstitial spaces of the larger solvent atoms. Example: carbon in γ-iron

    Hume-Rothary rule:

    These are the rules governing the formation of substitutional solid solutions;

    1) Crystal structure factor: for complete solid solubility of two elements, they should have

    the same type of crystal structure. For example, copper atoms may substitute for nickel

    atoms without disturbing the FCC structure of nickel.

    2) Relative size factor: the atoms of the solute and the solvent should be approximately of

    the same size (difference in radii should be less than 15%). For example, both silver and

    lead have FCC structure but the relative size factor is about 20 %. Therefore, they have

    poor solubility. Whereas, silver and palladium are completely soluble in each other as

    they have the same type of crystal structure (FCC) and differ in atomic radii by about 5%.

    3) Chemical affinity: the two metals should have very less chemical affinity. Generally, if

    the two metals are separated in the periodic table widely then they possess greater

    chemical affinity and are likely to form some compound instead of solid solution.

  • Module 2 5. Alloy, Phase Diagram & Steels

    Pruthvi Loy, Chiranth B. P. 3 SJEC, Mangaluru

    4) Electronegativity (tendency to acquire electrons): the two metals should have less

    electronegativity; higher the electronegativity of two elements greater will be the chance

    of forming an intermediate phase.

    5) Relative valence factor: among the metals, the one with the lower valency tends to

    dissolve more of a metal of higher valency than vice versa. For example, in a Ni-Al

    system; Ni has valency of 2 which dissolves 5% Al, but Al has valancy of 3 and dissolves

    only 0.04% Ni.

    b) Intermediate phase

    An intermediate phase is a compound made up of two or more elements of which at least one of

    them is metal. If the constituent elements forming an intermediate phase are exclusively metal-

    metal systems, then they are called intermetallic compounds. When an intermediate phase is

    formed the elements lose their individual identity and properties to a great extent and the

    compound will have its own characteristic physical, chemical and mechanical properties.

    Example: H2O, NaCl, etc.

    5.1.2 Phase Diagram

    A phase diagram is a graphical representation of the various phases of a substance and the

    conditions at which thermodynamically those distinct phases can exist at equilibrium. A phase

    diagram is also called as equilibrium or constitutional diagram.

    Phase diagrams are classified as:

    a) Unary phase diagram or one component phase diagram

    b) Binary phase diagram or two component phase diagram

    c) Ternary phase diagram or three component phase diagram

    a) Unary Phase Diagram

    In single component systems the composition remains same for any variation of temperature and

    pressure but it may undergo change of phase (solid, liquid and gaseous phase). A unary phase

    diagram is a plot of pressure (P) vs. temperature (T) as shown in Figure 5.1. The lines on the

    diagram represent conditions (T & P) at which a phase change occurs under equilibrium. That is,

    at a point on a line, it is possible for two (or three) phases to coexist at equilibrium. At the triple-

    point, three phases can coexist at equilibrium. In other regions of the plot, only one phase exists

    at equilibrium.

    Example: Water exists as ice, liquid water, and water vapour.

  • Module 2 5. Alloy, Phase Diagram & Steels

    Pruthvi Loy, Chiranth B. P. 4 SJEC, Mangaluru

    Figure 5.1: Unary phase diagram

    Beyond certain value of temperature and pressure it is not possible to distinguish between the gas

    and liquid phase. This is known as critical point and the corresponding temperature and pressure

    are known as the critical temperature (Tc) and critical pressure (Pc) respectively.

    b) Binary Phase Diagram

    A binary phase is a two component system; it is the most commonly used phase diagram in alloy

    designing. For most systems, pressure is constant and independently variable parameters are –

    temperature and composition. Thus a binary phase diagram is a plot of temperature vs.

    composition.

    Example: The simplest binary system is the Cu-Ni which exhibits complete solubility in liquid

    and solid state.

    Figure 5.2: Binary Phase diagram for a Cu-Ni System

  • Module 2 5. Alloy, Phase Diagram & Steels

    Pruthvi Loy, Chiranth B. P. 5 SJEC, Mangaluru

    Figure 5.3: Unary phase diagram for water

     The line above which the solid solution is liquid is called as a liquidus line.

     The line below which solidification completes is called as solidus line, here only α-solid

    solution exists at any temperature below the solidus line (for complete solubility).

     The line between these two regions is called as an intermediate phase; where both liquid

    and solid co-exist.

     It can be noted that the two metals are soluble in each other in the entire range of

    compositions in both liquid and solid state. This kind of system is known as

    ‘Isomorphous’ system.

     Liquids line separates liquid from liquid + solid, solidus line separates solid from liquid +

    solid

    c) Ternary Phase Diagram

    A ternary phase diagram has three components. The three components are usually compositions

    of elements, but may include temperature or pressure also. This type of diagram is three-

    dimensional but is illustrated in two-dimensions for ease of drawing and reading. Ternary phase

    diagrams are needed so that three components can be compared at once.

    5.1.3 Gibbs Phase rule

    It states that,

    P + F = C + 2

    Where,