04 Metals and Ceramics 1

How is a crystalline material defined?

• Material whose molecules are arranged in a highly ordered, periodic structure over large atomic distances

• The arrangement within a period is called “unit cell“

• All metals and ceramics form crystalline structures under normal solidification conditions

How is non-crystalline material defined?

• (Also amorphous) material which lacks a systematic and regular arrangement of molecules

How is lattice defined?

• 3D array of points coinciding with atom positions (sphere centers)

What are the common lattice structures? (3)

• Body centered cube (BCC)

• Face centered cube (FCC)

• Hexagonal closed packed (HCP)

What does BCC mean? Give examples (Hint: apparently harder)

• Body centered cube

• Cr

• Alpha-Fe

• Mo

• Ta

• W

What does FCC mean? Give examples (Hint: apparently most ductile)

• Face centered cube

• Al

• Gamma-Fe

• Cu

• Au

• Pb

• Ag

What does HCP mean? Give examples (Hint: apparently somewhere inbetween)

• Mg

• Ti

• Zn

How are ceramics formed?

• The whole crystall has to be electrically neutral

• Out of electrically charged ions instead of atoms like for metals

• Radii of Cations / Radii of Anions < 1 (a cation surrounded by many anions)

What are lattice defects?

• Imperfections on form or composition within the lattice structure of a material

• Can be you used to “tailor“ the material

Which types of lattice defects are there? (4)

• 0-dimensional: point defects

• 1-dimensional: linear defects/dislocations

• 2-dimensional: interfacial defects, stacking faults, grain boundaries

• 3-dimensional: precipitations, impurities, voids

When do point defects occur? (4)

• An atom form the crystal is crwoded into an interstitial site (self-interstitial atom (same species))

• Foreign atoms crowded in lattice (substitutional atoms) or in interstices (interstitial atoms (interstitial atom))

• Lattice sites no ocupied (vacancies)

• Solid solution (larger amount of a second species dissolved in the lattice) (alloys)

• HINT: extra atoms, atoms missing, foreign atoms

What kind of defects are alloys?

• 0-dimensional (point defects), namely solid solutions

Which types of 1-dimensional defects are there? (2)

• Edge dislocation

• Screw dislocation

What are edge dislocations?

• 1-dimensional defect around which some atoms are misaligned

What are screw dislocations?

• 1-dimensional defect caused by a shear stress, deforming the lattice structure

How can linear defects be recognized?

• They star and end at the surface of the crystal, or

• form a loop within the crystal

Why can linear defects be useful?

• Dislocations can be created with little enrgy inputs, giving metals their ductility

• Dislocation interference strengthens the material (strain hardening)

How can interfacial defects be recognized?

• Separate regions of the material with different crystal structures or crystallographic orientations

• Include external surfaces, grain boundaries, stackin faults, phase boundaries

What is a grain boundary?

• Meeting point of areas of material with different orientations of the lattice structure

How are interfacial defects formed? (2)

• Different crystallization speed leads to different size and form of grains

• Different grains touch eventually, stopping the crystallization

Why can grain boundaries be useful?

• They block dislocation migration → better strength behavior

What are 3-dimensional defects?

• Bulk defects/macroscopic defects like pores, cracks or inclusions

• Normally introduced during processing and fabrication

Which types of 3-dimensional defect are there? (3)

• Coherent, partially coherent, incoherent

What is an elastic deformation?

• Temporary deformation due to application of stress, after which the material returns to its original shape

What is a plastic deformation?

• Deformation in which rupture and reformation of interatomic bonds lead to permanent deformation

Def: slip, slip plane, slip steps

• Process by which plastic deformation is produced by a dislocation motion

• Slip plane: crystallographic plane traversed by the slip

• Slip steps: result of slip on the slip steps

What is a slip system?

• Combination of slip plane and slip direction

What is the relation between the number of slip systems and the ductility of a meterial?

• Increased number of slip planes leads to a greater deformability

How are unit cells ordered from less ductile to more ductile?

• HCP (Ti, Zn) < BCC (alpha-Iron) < FCC (Cu, Al, Au)

Why is FCC more ductile than BCC?

• Despite having the same amount of slip systems, FCC slip systems are more densely packed than BCC

What happens during a dislocation? (2)

• Regions in tensile, compressive and shear stress arise

• The overall sum of dislocations still forms a perfect crystall

What happens during a tensile test wrt dislocation formation?

• They are created in the elastic region, migrate in the Lüders region and pile up towards the ultimate strength (stagnation of the curve)

• Dislocation interference hindered deformation until failure of the material

What strenghtening mechanisms are there? (3)

• Solid-solution strengthening

• Strengthening by grain size reduction

• Strain hardening

What is Solid-solution strengthening?

• A strengthening mechanism

• Foreign atoms are introduced in the lattice, hindering dislocation migration

• For both metal and cereamics

• Basically, an alloy

What is Strengthening by grain size reduction?

• Large number of grains via size reduction

• Grain boundaries hinder dislocation migration by

  • a) direction change of dislocation at grain boundary

  • b) discontinuity of slip planes

What is Strain Hardening?

• Ductile materials become harder via plastic deformation

• Already created deformations hinder new dislocation migration

• Achieved through cold working

SQ: What is the difference in molecular structure between crystalline and non-crystalline materials?

• Crystalline materials: periodically ordered and repetitive over large atomic distances

• Non-crystalline materials: lack systematic and regular arrangement of molecules

SQ: Draw unit cells for FCC, BCC, and HCP crystal structures.

• FCC: cube, atom in each corner, atom in the center of each face

• BCC: cube, atom in each corner, atom in the very center of the cube

• HCP: hexagonal prism, atom in each corner, atom in the center of each hexagonal face, atom traingle in the middle of the prism

SQ: What is the relationship between the unit cell edge and the atomic radius of a BCC structure?

• a = 4R/√3

SQ: What is the difference between metallic lattices and ceramic lattices?

• Metal lattice composed of atoms, ceramic lattice composed of cations and anions (r_C / r_A < 1

• Ceramic lattice more complex

• Ceramic crystall must be electrically neutral

SQ: What are the lattice defects? Why are they important?

• Various imperfections present in all solids, classified into 0, 1, 2, 3 dimensional defects

• They enable material tailoring by changing material properties such as conductivity, ductility, hardness, etc.

SQ: Describe edge and screw dislocation motion from an atomic perspective

• Edge dislocation: slip along slip plane, leading to misalignment of atoms

• Screw dislocation: deformation caused by a shear stress, expressed in terms of the Burgers Vector

SQ: How can materials be plastically deformed?

• Applied stress leads to rupture and reformation of atomic bonds, so that material gets permanently deformed

SQ: What are slip planes and slip systems?

• Slip planes: crystallographic plane traversed by a dislocation. Plane in which atoms displace in a plastic deformation

• Slips system: combination of a slip plane and a slip direction

SQ: How many slip planes do the different lattices have?

• FCC: 12

• BCC: 12

• HCP: 3

SQ: What are the different types of strengthening mechanisms and how do they work?

• Solid-Solution Strengthening: inclusion of foreign atomic species in the material to hinder dislocation propagations

• Strengthening by reduction of grain size: reduction of grain size leads increased amount of grains, which hinder migration of dislocations since grain boundaries induce a change of direction in propagation of the dislocation and discontinuity of the slip planes

• Strain hardening: already existing dislocations hinder migration of new ones. Usually achieved through cold working