mee1005 materials engineering and technology l11-12

DEVAPRAKASAM DEIVASAGAYAMProfessor of Mechanical Engineering

Room:11, LW, 2nd FloorSchool of Mechanical and Building Sciences

Email: [email protected], [email protected]

MEE1005: Materials Engineering and Technology (2:0:0:2)

Devaprakasam D, Email: [email protected], Ph: +91 9786553933

mailto:[email protected],

mailto:[email protected]



MEE1005 MATERIALS ENGINEERING AND TECHNOLOGY


Purpose of Engineering /Education/Research


There are two important purposes and driving force behind the Engineering/Education/Research:

1. Minimum consumption of Energy and Materials without sacrificing the efficiency and functionality.

2. Maximum conversion of Energy from one form to the other, to identify or design highly efficient process or system.


Purpose of Engineering /Education/Research


UNIT-I : Structure of Constitution of Alloys

Mechanism of Crystallization- Nucleation-Homogeneous andHeterogeneous Nucleation- Growth of crystals- Planargrowth – dendritic growth – Cooling curves - Diffusion -Construction of Phase diagram -Binary alloy phase diagram –Cu-Ni alloy; Cu-Zn alloy and Pb-Sn alloy; Iron-Iron carbidephase diagram – Invariant reactions – microstructuralchanges of hypo and hyper-eutectoid steel- TTT and CCTdiagram



It has been mentioned that noncrystalline solids lack a systematic and regular arrangementof atoms over relatively large atomic distances. Sometimes such materials are also calledamorphous (meaning literally “without form”), or supercooled liquids, inasmuch as theiratomic structure resembles that of a liquid.

Two-dimensional schemes of the structure of (a) crystalline silicon dioxide and (b) noncrystalline silicon dioxide.

Most crystalline solids are composed of a collection of many small crystals or grains; such materials are termed polycrystalline.



• Polymorphous substances have similar chemical properties but different physical properties.

Existence of substance into more than one crystalline forms is known as "POLYMORPHISM".In other words: Under different conditions of temperature and pressure, a substance can form more than one type of crystals. This phenomenon is called Polymorphism and different crystalline forms are known as ‘POLYMORPHICS’

"Existence of an element into more than one physical forms is known as ALLOTROPY "Under different conditions of temperature and pressure an element can exist in more

than one physical forms. This phenomenon is known as Allotropy and different forms are known as "Allotropes"

Example:Coal, lamp black, coke, Diamond, graphite etc. are all allotropic forms of carbon.



Some metals, as well as nonmetals, may have more than one crystal structure, aphenomenon known as polymorphism. When found in elemental solids, the condition isoften termed allotropy. The prevailing crystal structure depends on both the temperatureand the external pressure. One familiar example is found in carbon: graphite is the stablepolymorph at ambient conditions, whereas diamond is formed at extremely high pressures.Also, pure iron has a BCC crystal structure at room temperature, which changes to FCC iron at912C (1674F). Most often a modification of the density and other physical propertiesaccompanies a polymorphic transformation.

Existence of different substances in one crystalline form is known as "ISOMORPHISM"Or Different substances may exist in identical crystalline forms. This phenomenon is called as Isomorphism and these substances are known as ‘Isomorphous’.

1) Isomorphic substances have same atomic ratio2) Empirical formula of isomorphic substances is same3) They have different chemical & physical properties.4) When their solutions are mixed, they form mixed type of crystals.



There are two types of nucleation: homogeneousand heterogeneous.The distinction betweenthem is made according to the site at whichnucleating events occur. For the homogeneoustype, nuclei of the new phase form uniformlythroughout the parent phase, whereas for theheterogeneous type, nuclei form preferentially atstructural inhomogeneities, such as containersurfaces, insoluble impurities, grain boundaries,dislocations, and so on.



(a) Schematic curves for volume free energy and surface free energy contributions to the total free energy change attending the formation of a spherical embryo/nucleus during solidification. (b) Schematic plot of free energy versus embryo/nucleus radius, on which is shown the critical free energy change (G*) and the critical nucleus radius (r*).



where ΔHf is the latent heat of fusion (i.e., the heat given up during solidification), and Tm and the temperature T are in Kelvin.



It is apparent that during the cooling of a liquid, an appreciable nucleation rate (i.e.,solidification) will begin only after the temperature has been lowered to below the equilibriumsolidification (or melting) temperature (Tm). This phenomenon is termed supercooling (orundercooling), and the degree of supercooling for homogeneous nucleation may be significant(on the order of several hundred degrees Kelvin) for some systems. Table 10.1 shows, for severalmaterials, typical degrees of supercooling for homogeneous nucleation.



Although levels of super cooling for homogeneous nucleation may be significant (on occasion several hundred degrees Celsius), in practical situations they are oftenon the order of only several degrees Celsius. The reason for this is that the activation energy (i.e., energy barrier) for nucleation (G* of Equation 10.4) is lowered when nuclei form on pre-existing surfaces or interfaces, because the surface free energy ( of Equation 10.4) is reduced. In other words, it is easier for nucleation to occur at surfaces and interfaces than at other sites. Again, this type of nucleation is termed heterogeneous

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The growth step in a phase transformation begins once an embryo has exceededthe critical size, r*, and becomes a stable nucleus. Note that nucleation will continue to occur simultaneously with growth of the new phase particles; of course, nucleation cannot occur in regions that have already transformed to the new phase.
