Chapter1 Basic Principles Crystalline Structures

8/10/2019 Chapter1 Basic Principles Crystalline Structures

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metallic materialsbasic principles: crystalline structures WELS

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basic principles of materials technology


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basic principles of materials technology crystalline structures properties phase transformation thermally activated processes constitutional diagrams

production metal minning smelting and solidification deformation

alloys and properties iron - steels non iron metalls ceramics


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crystalline structures

basic principles of materials technology crystalline structure

- crystalline amorphous- chemical bonds of anorganic materials- ideal crystals- real materials


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crystalline - amorphouscrystalline structure:

the atoms are arranged regulary metalls, ceramics, some plastics (partly crystalline),

amorphous structure:the atoms have no clear arrangement liquids, glasses, some plastics,

amorphous crystalline


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chemical bonds of anorganic materials

atom consists of a nucleus and electrons

atoms are reactiv

more stabil when the outer shell is filled up with electronsto reach this they have to take in or dispense electrons

the chemical bonds are the result of the electromagnetic forcesbetween the positive nuclei and the negativ electrons


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chemical bonds of anorganic materialsmetallic bond: metalls dispense the outer electrons and become positive ions

the elelctron stay unbond and are free movable (electron sea)good electric and thermal conductivity

the positive ions are hold together by the electrons around themand they are interchangeable

good plastic deformability

positive metall ions

free electrons


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chemical bonds of anorganic materialsionic bond: one atoms gives away an electron, another one takes it

no free electronsbad electric and thermal conductivity

the positive anions attract the negative cations a change of places leads to distruction

no plastic deformability


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chemical bonds of anorganic materialscovalent bond:

different atoms share pairs of electrons (e.g. gases, semiconductors: Si, Ge)

bad electric and thermal conductivityno plastic deformability


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ideal crystals unit cells describe the lattice of crystals

the main important unit cells:

cubic (based on a cube) hexagonal (based on a hexagon)

cubic primitiv hexagonal primitiv


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ideal crystalsunit cells and lattice parameters of industrial important metalls [nm]

1 nm = 10

Fe and Ti change their lattice at higher temperatures

bcc fcc hdp


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real materials

real metalls have many imperfections

lattice defects help us to influence the properties on a wide scale

defects:- punctiform- one-dimensional- two-dimensional

- three-dimensional


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real materials punctiform defects (point defects)

vacancies: missing atoms in the latticevacany:

density (number) of vacancies in equilibrium with temperature:- at room temperature 10 -12 (one in a trillion)- at melting point 10 -4 (one in ten thousend)


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real materials punctiform defects (point defects)solid solutions: foreign atoms in the crystalsubstitution solid solution: interstitial solid solution:

substitution atom: interstitial atom:

foreign atoms in a metal:- lead to tensions in the lattice- increase the strength of a material

solid solution strengthening


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real materialsone-dimensional defects (line defects): dislocations

edge dislocation screw dislocationedge of a lattice plain that ends in a crystal lattice is helical skewed

real dislocation lines: a mixture of both types

dislocations end at the edge of a crystal or they are closed loops


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real materialsone-dimensional defects (line defects): dislocations

density of dislocations: measured in length within a certain volume- annealed metal: around 10 6cm/cm (10m in 1 mm)

- deformed material: around 10 12cm/cm (10 000km in 1mm)

deformation increases the dislocation density dramatically- they start to hinder the movement of each other

increases the strengthof the material

force to move dislocations through a crystal is comparatively lowgood plastic deformation(change of shape: rolling, forging) of metals


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real materialstwo-dimensional defects (planar defects):

grain boundaries:low angel grain boundaries LAGB high angel grain boundaries HAGBsmall variation of the crystal orientation big variation of the crystal orientation

twin boundaries: crystals symmetric to the border plain


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real materialspolycrystalline materials: grain boundaries

polycristalline materials:ferrite: bcc-iron austenite: fcc-iron with twin boundaries

bcc-materials (ferritic iron): free of twin boundaries

fcc-materials (austenitic iron): many twin boundaries


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real materials

lattice with different crystal orientations:(isotropic behaviour)

isotropic anisotropic behaviour

single crystal: anisotropic properties (different behaviour in different directions)

polycrystalline materials:- many anisotropic crystals with different lattice orientations

- the bulk material has isotropic behaviour (the same properties in all different directions)


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summary metals and ceramics have a crystalline structure

metallic bond: free electrons: good conductivity and deformability ionic bond/covalent bond: no free electrons: bad conductivity and deformability

main important unit cells:- cubic: bcc body centered cubic, fcc face centered cubic- hexagonal: hdp hexagonal dense packed

real metals have defects:- vacancies, solid solution (substitutional and interstitial)- dislocations- grain boundaries and twin boundaries- segregations, impurities

Chapter1 Basic Principles Crystalline Structures

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