UNIT-IIUNIT-II

• TOPICS COVEREDTOPICS COVERED

Learning objectives Learning objectives

Constitution of alloys:Constitution of alloys:»Necessity of alloyingNecessity of alloying»Types of solid solutionsTypes of solid solutions»Hume Rothery’s rulesHume Rothery’s rules» Intermediate alloy phases Intermediate alloy phases »Electron compounds Electron compounds

PURE METAL AND ALLOYPURE METAL AND ALLOY

• Pure metal only one metal constituent no presence of foreign metal.

• An Alloy is a combination of two or more metals

WHAT ARE THE APPLICATIONS OF WHAT ARE THE APPLICATIONS OF PURE METALS ?PURE METALS ?

•With our experience we observed the pure metals are mainly used for

•Electrical & thermal conductors e.g Copper and Aluminum .

•Corrosion resistance widely used in plating (a coating) of metals over another metal e.g Tin , Nickel and Chromium.

• For Heating elements e.g Molybdenum and Tungsten

WHAT ARE THE REQUIREMENTS OF INDUSTRY/ENGINEERING WORLD? -1.High strength materials - designers and engineers to withstand high loads.-2. Modern applications demand properties are not obtainable from pure metals.-3. Special applications like Lightness and strength of aircrafts and space vehicles e.g Al.-4. High strengths for bridges, alloy steels etc.-5. Good high temperature strengths for turbines, -6. For re-entry of space vehicles into the atmosphere.- 7. Industrial corrosion & Erosion resistance etc .

LET US COMPARISION OF MECHANICAL PROPERTIES OF PURE AND ALLOYS

Alloying is one of the method to meet the requirement of current /present engineering/Industrial to withstand higher loads. How to achieve?

•By mixing different proportion of metals and prepare an alloy • Suitable combination. Ex: Cu-C ?•We do not have liberty to mix them indiscriminately. •Certain rules are framed for alloy preparation.

WHAT IS THE ROLE OF ALLOY ELEMENT?

• The addition of second element to a metal, distorts the parent crystal lattice by occupying parent metal lattice site, such situation we observe in substitutional solid solutions there by increase strength of the metal alloy.

• Alloy elements offer frictional resistance to the motion of dislocations in the crystal and hence strength increases this is particularly when interstitial elements are added..

SOLID SOLUTION STRENGTHENING

EX:COPPER ALLOYS

SOLID SOLUTION STRENGHTENING

PURPOSE OF ALLOYING?

• To get high strength metal alloys for meeting the requirements of designer /Engineer.

• Special applications like corrosion resistance (Resistance to atmospheric e.g Cr in Stainless steels )

• Corrosion resistance at High temperature e.g Inconel (Nickel based alloys) etc.,

BASIC INFORMATION

• Solvent is a major (% by weight) constituent in a solution.

• Solute is a minor(% by weight) constituent in a solution.

• For example we consider Copper-Nickel

If Cu - 70% by weight – Solvent

Ni - 30% by weight – Solute

END OF THIS INSTRUCTION

INTRODUCTION TO SOLID SOLUTIONS

SOLID SOLUTION

• A solid solution is a A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single Homogeneous phase.

SOLID SOLUTION

• The solute may incorporate into the The solute may incorporate into the solvent crystal lattice solvent crystal lattice substitutionallysubstitutionally, , by replacing a solvent particle in the by replacing a solvent particle in the lattice, or lattice, or interstitiallyinterstitially, by fitting into , by fitting into the space between solvent particles. the space between solvent particles. Both of these types of solid solution Both of these types of solid solution affect the properties of the material by affect the properties of the material by distorting the crystal lattice and distorting the crystal lattice and disrupting the physical and electrical disrupting the physical and electrical homogeneity of the solvent material. homogeneity of the solvent material.

SOLID SOLUTIONS

• A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase.

SOLID SOLUTIONS

• This often happens when the two elements (generally metals) involved are close together on the periodic table; conversely, a chemical compound is generally a result of the non proximity of the two metals involved on the periodic table

HUME ROTHERYS RULES FOR SOLID SOLUTIONS

• A set of basic rules describing the conditions under which an element could dissolve in a metal forming a solid solution.

• And the restriction of solubility of solvent

metal in solvent metal etc.,

RULES FOR FORMATION OF SUBSTITUTIONAL SOLID SOLUTION • For substitutional solid solutions, the Hume-Rothery

rules are:– 1. The atomic radii of the solute and solvent atoms must differ by

no more than 15%: – 2. The crystal structures of solute and solvent must match. – 3. Maximum solubility occurs when the solvent and solute

have the same valency. Metals with lower valency will tend to dissolve in metals with higher valency.

– 4. The solute and solvent should have similar electro negativity. If the electro negativity difference is too great, the metals will tend to form intermetallic compounds instead of solid solutions.

RULES FOR THE FORMATION OF INTERSTITIAL SOLID SOLUTION

• For interstitial solid solutions, the Hume-Rothery rules are:– 1. Solute atoms must be smaller than the

pores in the solvent lattice. – 2. The solute and solvent should have

similar electro negativity.

INTERMEDIATE ALLOY PHASES

• Intermediate phases normally form because of the two metals (components) have chemical affinity for each other.

• The chemical composition of these phases intermediate between the two pure metals, but crystal structures are different from pure metals, which distinguish them from primary solid solutions.

ELECTRON COMPOUNDS• These compounds are formed at

specific electron to (e/a) ratio.

• e/a ratio controls the formation of intermediate phases are called as electron compounds.

• The e/a ratios corresponding to specific crystal structure in an alloy system are 3/2,21,13 and 7/4.

• Example β brass(CuZn) B.C.C structure e/a ratio is 3/2

SUMMERY

• Pure metals are limited application.

• Alloys are stronger than pure metals as per the strength comparison table.

• Hume – Rothery rules

Same crystal structure & Atomic ratio of two metals <15% is the favorable condition for the formation substitional solid solutions..

SUMMERY•Hume – Rothery rules contd……………………….Solute atoms must be smaller than the pores in the solvent lattice and should have similar electro negativity.

• Intermediate alloys should have chemical affinity each other.•Electron compounds Electron and atom ratio

End of Unit-IIEnd of Unit-II

ADDITIONAL INFORMATION

SOLID SOLUTIONS

• An alloy is a partial or complete solid solution of one or more elements in a metallic matrix. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may be homogeneous in distribution depending on thermal (heat treatment) history. Alloys usually have different properties from those of the component elements.

ALLOY

• Alloying one metal with other metal(s) or non metal(s) often enhances its properties. For instance, steel is stronger than iron, its primary element.

Steel is a metal alloy whose major component is iron, with carbon content between 0.02% and 2.14% by mass

DISCUSSION-NO NEED

• All solutions are characterized by interactions between the solvent phase and solute molecules or ions that result in a net decrease in free energy. Under such a definition, gases typically cannot function as solvents, since in the gas phase interactions between molecules are minimal due to the large distances between the molecules. This lack of interaction is the reason gases can expand freely and the presence of these interactions is the reason liquids do not expand.

SOLUTIONS In a liquid solution , if the solution consists

of two different elements we define the two a solvent and solute

-1 Solvent a major constituent of solution

-2 Solute a minor constituent of solution.

The same rule applies to solids we call it as solid solution.

e.g Steel (In steel Iron is major constituent and carbon is minor constituent ).

Alloying• Given a metal (with only 1 type of atom) refined to

99.9999% purity, there would still exist 1022 to 1023 impurity atoms in 1 cubic meter of material.

• Most metals are alloys. Alloying is done to improve strength, corrosion resistance, ductility, lower melting T.

• For example, sterling silver is an alloy of 92.5% silver, 7.5% copper. At room temperature, “pure” silver is highly corrosion resistant, but also very soft. The addition of copper improves the strength and maintains good corrosion behavior.

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Solid Solution• The addition of impurity atoms to a metal

results in the formation of a solid solution.

• The solvent represents the element that is present in the greatest amount (the host atoms). (For example, in Lab Precipitation Hardening of

Aluminum), aluminum is the solvent and copper is the solute(present in minor concentration ).

• Solid solutions form when the solute atoms (Cu) are added to the solvent (Al), assuming the crystal structure is maintained and no new structures are formed. 36

Solid Solution - continued

• A solid solution is a homogenous composition throughout.

• The impurity atoms (Cu) are randomly and uniformly dispersed within the solid.

• The impurity defects in the solid solution are either substitutional or interstitial.

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What are the outcomes if impurity (B) is added to host (A)?• Solid solution of B in A (random distribution of point defects)

• Solid solution of B in A plus particles of a new phase (usually for a larger amount of B)

OR

Substitutional solid solution.(e.g., Cu in Ni)

Interstitial solid solution.(e.g., C in Fe)

Second phase particle-- different composition-- often different structure.

Imperfections in Metals

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Hume - Rothery RulesThe Hume-Rothery rules are basic conditions for an element to dissolve in a metal, forming a substitutional solid solution.

1. The atomic radius of the solute and solvent atoms must differ by no more than 15% (r < 15%).

2. The solute and solvent should have similar electronegativities.

3. Same crystal structure for “pure” metals.

4. Maximum solubility occurs when the solvent and solute have the same valence. Metals with lower valence will tend to dissolve metals with higher valence.

Substitutional Solid Solution Example: Copper and Nickel

Element AtomicCrystal Electro- ValenceRadius Structure nega-

(nm) tivity

Cu 0.1278 FCC 1.9 +2C 0.071 2.5H 0.046O 0.060Ag 0.1445 FCC 1.9 +1Al 0.1431 FCC 1.5 +3Co 0.1253 HCP 1.8 +2Cr 0.1249 BCC 1.6 +3Fe 0.1241 BCC 1.8 +2Ni 0.1246 FCC 1.8 +2Pd 0.1376 FCC 2.2 +2Zn 0.1332 HCP 1.6 +2

• Carbon forms an interstitial solid solution when added to iron; the maximum concentration of carbon that can be added is roughly 2%.

• The atomic radius of the carbon atom is much less than that of iron (0.071nm vs 0.124 nm).

• For interstitial solid solutions, the Hume-Rothery rules are:– 1. Solute atoms must be smaller than the

pores in the solvent lattice. – 2. The solute and solvent should have

similar electronegativity.41

INTERSTITIAL SOLID SOLUTION

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• Since there are both anions and cations in ceramics, a substitutional impurity will replace the host ion most similar in terms of charge. •Charge balance must be maintained when impurities are present.

• Ex: NaCl

IMPERFECTIONS IN CERAMICS

Na+ Cl-

• Substitutional cation impurity

without impurity Ca2+ impurity with impurity

Ca2+

Na+

Na+Ca2+

cation vacancy

• Substitutional anion impurity

without impurity O2- impurity

O2-

Cl-

anion vacancy

Cl-

with impurity

ADDING SODIUM CHLORIDE TO WATER

SOLVENT AND SOLUTE