ME2733 - Final Exam - Dr. Wang

CHAPTER 1

CHAPTER 1

Natural Materials

Cotton, silk

Synthetic Materials

Nylon, plastics

Advanced Materials

Semiconductor for electronics, nanomaterials, smart materials, energy materials

What is the core of MSE?

-Triangle relationships:

-1) Processing

-2) Structure (the arrangement of internal components in a material)

-3) Properties

What are the different length scales of structure?

-1) Subatomic structure

-2) Atomic structure

-3) Nanostructure

-4) Microstructure

-5) Macrostructure

Subatomic Structure

This involves the electrons within the individual atoms, their energies, and their interactions with the nuclei

Atomic Structure

This describes the organization of atoms which will yield molecules or crystals

Nanostructure

This describes the aggregates of atoms that form particles (nanoparticles) which have nanoscale dimensions (less than about 100 nm)

Microstructure

Describing structural elements that can be directly observed using a microscope (structural features measuring between 100 nm and several mm)

Macrostructure

Describing structural elements that may be viewed with the naked eye (scale range between several mm and on the order of a meter)

What are the categories of materials?

-1) Metals

-2) Ceramics

-3) Polymers

-4) Composites

Metals

-Composed of atleast one metallic elements and commonly small amounts of nonmetals (carbon, nitrogen, oxygen)

-Ex) Platinum, silver, copper, iron, steel, titanium, aluminum, magnesium, gold,

Ceramics

-Compounds between metallic and nonmetallic elements

-Glass, concrete, ceramics, aluminum oxide, silicon dioxide, silicon carbide, silicon nitride, porcelain, clay mineral composition

Polymers

-Very large molecular structures and often organic compounds

-Light, not stiff or strong

-Plastic, nylon, Rubber, polyethylene, silicon rubber, other poly plastics

Composites

-Two or more of either a metal, ceramic, or polymer to pull the best characteristics of each group into one compound

-Wood

Order of density of materials

Metals > Ceramics > Polymers ~= Composites

Advanced Materials

-Semiconductors: electrical properties between conductors and insulators

-Biomaterials: nonliving materials that can be inserted in the body

-Smart (intelligent) materials: they can respond to and detect changes in their environment

-Nanomaterials: when atleast one dimension of the material is less than 100 nm.

What does a greater surface area to volume ratio cause?

-An increase in surface atom broken bonds, which cause chemical reactions and physical processes

CHAPTER 2

CHAPTER 2

Atomic Number

the number of protons and electrons in an atom

Atomic Mass Unit (AMU)

What atomic mass (or weight) is measured in

Valence Electrons

-Electrons in the unfilled outershells

-They are more unstable, therefore bond easier

-They usually determine an atom's chemical properties

Electronegativity Periodic Table Trend

-Rightmost elements are the most electronegative and readily gain electrons

-Leftmost elements are the most electropositive and readily give up electrons

Ionic Bonding

-Bonding between elements of dissimilar electronegativities

-Bonding between metals and nonmetals

-Nonmetals gain electrons while metals donate them

-Ionic bonding is predominant in salts and ceramics

-Ex) NaCl, MgO, CaF2, CsCl

-High melting temperature

-Large activation energy

-Small thermal expansion coefficient

Covalent Bonding

-Bonding between elements of similar electronegativities and electrons are shared

-Involves Valence electrons

-Bonding between nonmetals

-High melting temperature

-Large activation energy

-Small thermal expansion coefficient

Metallic Bonding

-Bonding where electrons are delocalized to form an electron cloud

-Bonding between metals

-Moderate melting temperature

-Moderate activation energy

-Moderate thermal expansion coefficient

Secondary Bonding

-AKA molecular forces

-Bonds between molecules

-These are much weaker than primary bonding

-The three molecular forces include: Van Der Waals Forces, dipole-dipole interactions, and hydrogen bonds

-Lowest melting temperature

-Smallest activation energy

-Lowest thermal expansion coefficient

Van Der Waals Forces

-This occurs between all molecules

-It is the weakest of all molecular bonds

-Nonpolar molecules

Dipole-Dipole

-Polar molecules

-Stronger than Van Der Waals

Hydrogen Bonding

-Strongest of all intermolecular forces

-When hydrogen atoms bond to nitrogen, oxygen, or fluorine atoms

CHAPTER 3

CHAPTER 3

CHAPTER 7

CHAPTER 7

Briefly describe what is "necking" during plastic deformation of a material when it is under tensile testing.

Necking is the significant shrinkage of the cross-section area of the sample during tensile testing. (It usually happens between the maximal point and the fracture point in the stress vs. strain plot.)

Slip

Plastic deformation that occurs by motion of dislocations (edge, screw, mixed). Deformation occurs through a caterpillar-like locomotion. Atomic bonds are broken and reformed along the slip place.

Edge Dislocation

Dislocation moves in the direction of the applied shear stress.

Screw Dislocation

Dislocation moves perpendicular to the direction of applied shear stress.

Slip System

A combination of slip plane and slip direction

Slip Plane

The crystallographic plane where slip occurs most easily. This is the plane with high planar density.

Slip Direction

The crystallographic direction where slip occurs most easily. This is the direction with high linear density.

FCC Crystal Structure Slip System

-Slip system is {111}

How many independent slip systems are there for FCC crystal structure?

12

Does reducing dislocation mobility strengthen/harden metals?

yes

What is related to the mobility of dislocations?

Strength and hardness

What are the three strengthening mechanisms for metals?

1) Grain size reduction

2) Solid solution strengthening

3) Strain hardening (cold working)

Mechanism 1: Reduce Grain Size

-Reducing the grain size increases the grain boundary area which means there are more barriers for dislocation motion which increases yield strength, tensile strength, and hardness

-Grain boundaries act as barriers to dislocation motions

-Slip planes change directions at boundaries and there is discontinuity of slip planes

Mechanism 2: Solid-Solution Strengthening

-High shear stress is required to cause dislocation motion

-When edge dislocation is located above the slip line, impurity atoms introduce lattice strain interactions with strains

-When edge dislocation is located below the slip line, large substitutional impurities introduce compressive strains

Mechanism 3: Strain Hardening

-Where metals at room temperature are plastically deformed

-Also known as Cold Working-Often times, a cross-sectional areas is reduced through deformation, (e.g. rolling)

-Dislocation density is increased though Cold Working from new dislocations being formed.

-Dislocation motion is hindered by the presence of other dislocations

Annealing (Heat Treatment)

-Annealing nullifies the effects of cold work on metals

-Tensile strength is decreased

-There are three Annealing stages (Recovery, Recrystallization, Grain Growth)

Stage 1: Recovery

-There is a reduction in dislocation density, as dislocations are annihilated

-Scenario 1: atoms are split into two planes, atoms diffuse to regions of tension, and planes come back together

OR

-Scenario 2: dislocations of opposite sign meet and annihilate (stair stepping)

Stage 2: Recrystallization

-This is where new grains form that have low dislocation densities, are small, and consume and replace parent cold-working grains.

Recrystallization Temperature

-The temperature at which recrystallization just reaches completion in one hour.

-Recrystallization temperature decreases as purity increases

-Recrystallization temperature decreases as %CW increases

Deformation above recrystallization temperature

hot working

Deformation below recrystallization temperature

cold working

Stage 3: Grain Growth

-Grain growth occurs as heat treatment continues-The average grain size increases, small grains shrink, and large grains continue to grow

CHAPTER 8

CHAPTER 8

Simple Fracture

The separation of a body into two or more pieces in response to a static stress

What are the two types of fracture?

Ductile and Brittle

Ductile

-Slow crack propagation

-Accompanied by significant plastic deformation

-Fails with warning

Brittle

-Rapid crack propagation

-Little or no plastic deformation

-Fails without warning

Which fracture is generally more desirable?

Ductile

Stages of Moderately Ductile Failure

-Ductile failure starts with necking and ends with a fracture

A brittle surface fracture displays what shape?

A "V" or chevron shape

-These "V" features point to the crack initiation site

Intergranular crack propagation

-This is cracks between grains on brittle fracture surfaces

Transgranular crack propagation

-This cracks through grains on brittle fracture surfaces

Why is the measured fracture strengths of most materials much lower than what is predicted by theory?

-Microscopic flaws always exist in materials-The magnitude of applied tensile stress is amplified at the tips of cracks

Fracture Toughness

The measure of material's resistance to brittle fracture when a crack is already present

Brittle Failure Test for Ductile Materials

-Testing conditions favor a brittle fracture occurring:

-High strain rate

-Deformation at low temps

-Presence of a notch

Fatigue

-Failure under a long period of repeated stress or strain cycling

-Can cause part failure and is responsible for about 90% of mechanical engineering failures

What function does stress vary with?

Time

How is fatigue data plotted?

stress amplitude (S) versus number of cycles to failure (N)

What are the two types of fatigue behavior observed?

1) Fatigue limit (Sfat)

2) Fatigue Life (Nf)

Fatigue Limit (Sfat)

-There is no fatigue if S < Sfat

-Some materials have no fatigue limit

Fatigue Life (Nf)

The total number of stress cycles to cause fatigue failure at a specified stress amplitude

What are the three general techniques to improve fatigue life?

1) Reducing magnitude of mean stress

2) Surface treatments: surface compression is increased by 2 different methods to increase surface hardness

-1) Shot Peening: surface compressive stress due to plastic deformation in outer surface layer

-2) Carburizing: surface compressive stress due to carbon atoms diffusing into outer surface layer

3) Design Changes: remove stress concentrators such as sharp corners (filet them instead)

Creep

The measure of deformation (strain) at a constant stress with respect to time

What are the three stages of Creep?

-1) Primary Creep: deformation (strain) decreases exponentially

-2) Secondary Creep: deformation (strain) stays constant with respect to time; steady-state

-3) Tertiary Creep: deformation (strain) increases exponentially with respect to time

What is the steady-state creep rate directly proportional to?

Increasing temperature and stress

What causes a rupture lifetime to decrease?

Increasing temperature and stress

CHAPTER 9

CHAPTER 9

Component

-An alloy that is composed of a pure metal or compound

-Also defined as the elements or compounds that are present in an alloy

System

Could be a material or a series of alloys of that consist of the same components (although this does not mean they have the same alloy composition)

Phase

-A homogeneous portion of a system (so uniform physical and chemical characteristics)

-Also defined as physically and chemically distinct material regions that form

Homogeneous

A single phase system

Heterogeneous

-AKA mixtures: a system composed of two or more phases

What are the three externally controlled temperatures that affect phase structure?

temperature, pressure, composition

Solution

- a single phase solution, so either a solid, liquid, or gas

Mixture

-A mixture with more than one phase involved

Solubility Limit

-The maximum concentration where a solution exists in a single phase, usually at a specified temperature

What are the two variables that can change the number of phases?

-Temperature and composition

Cored Structure

-Fast rate of cooling, components are not uniform throughout layers

Equilibrium Structure

-Slow rate of cooling, components are uniform throughout layers

Binary-Eutectic Systems

Binary-Eutectic Systems

Lamellar Eutectic Structure

-A structure where elements are not mixed in, but are layered on top on each other

Eutectic

Eutectic

Eutectoid

Where one solid phase transforms into two other solid phases

Peritectic

Where one liquid and one solid phase transform into a second solid phase

Gibbs Phase Rule

P + F = C + N

-P: Number of phases

-F: Number of degree of freedom

-C: Number of components

-N: Number of non-compositional variables

CHAPTER 10

CHAPTER 10

What are the three types of phase transformation?

1) Diffusion dependent where there is no change in number or composition of the phases

2) Diffusion dependent where there is some change in the number and composition of the phases

3) Diffusionless where a metastable phase is produced