# Midterm Material Notes

Metallic bonding: Found in metals and alloys, 1, 2, or 3 valence electrons that are not bound to any particular atom in the solid. They drift throughout the entire metal. This results in a sea of electrons. These free electrons result in good electrical and heat conductivity. Coordination number is 8 or 12 (number of nearest neighbors). Ionic Bonding

Found in compounds that are composed of both metallic and nonmetallic elements. Metallic element gives up valence electrons to the nonmetallic elements. Attractive forces from positive and negative ions

Ceramic materials: high melting temperatures, hard, poor conductors Covalent Bonding

Electrons are shared between adjacent atoms resulting in stable electron configurations. Many nonmetallic elemental molecules are covalently bonded. Polymeric materials have this bond. Crystal Structure

Describe the crystal structure using an array of points or lattice. The unit cell is the smallest number of atoms that describes the crystal structure. FCC: aluminum, copper, gold, lead, nickel, silver.

Coordination number, number of nearest neighbors is 12.

Atomic Packing Factor

APF = volume of atoms in a unit cell

Total unit cell volume

Crystallographic Planes and Directions

Crystallographic Directions: a vector with coordinates in the xyz axis system. The length of the vector is the smallest integer values. They are written [uvw] i.e. [111] This defines a direction. Crystallographic Planes: Miller indicies are (hkl). Three numbers that define the orientation of a plane with respect to a given coordinate system.

h = a/x

k = a/y

l = a/z

Defining a Plane

The plane cannot go through the origin of the cell.

The plane either intersects or parallels each of the three axes. The length of the planar intercept for each axis is determined in terms of the lattice parameters a,b,c The reciprocals of these numbers are taken. A plane that parallels an axis may be considered to have an infinite intercept, and a zero index. These numbers are changed to the set of smallest integers.

The integer indices, not separated by commas, are enclosed within parentheses. Dislocations

Edge dislocations, Screw dislocations.

Dislocation movement and how this effects mechanical properties. Phase Diagrams

Determine phases present.

Determine amount of each phase.

Determine composition of each phase.

Phase Transformations

Continuous Cooling Curves.

Time Temperature Transformations.

Non Equilibrium phases.

Class 2

Mechanical Properties

Tensile Properties

Stress = Load /Area

Strain = Change in length / initial length

Modulus = Stress / Strain

Tensile deformation of ductile metal

Ductile vs. brittle

Stress Strain Curve

Modulus

Yield Strength

Ultimate Tensile Strength

Modulus of Elasticity

Would you expect MgO or magnesium to have a higher modulus of elasticity? Dislocations

Extra half plane

Edge dislocation

Screw dislocation

Burgers vector

Perpendicular to edge dislocation

Parallel to screw dislocation

Strengthening Mechanisms

Strength is inversely related to dislocation mobility.

Crystal structure determines the number and type of slip systems. Grain size determines the amount of grain boundary area and thus dislocation mobility. Finer grain size results in higher strength.

Grain Boundary Strengthening

Hall – Petch Relationship

Stress = Friction stress + constant/(grain diameter)1/2

Friction Stress = stress to move unlocked dislocation along slip plane This says the smaller the grain size, the higher the strength. This is true if comparing two annealed materials with different grain size. If cold work is present, this can alter relationship.

Grain-Size Measurement

Number of grains per square inch @ 100X = N.

Grain size number = n

N = 2 (n-1)

The smaller the grain size, the larger the grain size number. Yield-Point Phenomenon

Upper and lower yield point occurs in low-carbon steel.

This results in luders bands or stretcher...

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