MSE 200 Final Exam NCSU

Which of the following defects can occur in an ionic ceramic

substitutional impurity cation

substitutional impurity anion

a single missing positive ion and an interstitial positive ion

a interstitial anion

a missing anion and a missing cation

Which is larger cation or anion

Anion is larger because the elctron cloud

If a ceramic material is denoted as an AX structure what does this mean

AX specifies the stoichiometry

SI coordination in silicate materials would be best described as

Tetrahedral, because SI bonds to 4 oxygens

Tempering Glass

puts the glass under compressive forces, making it stronger

Which ideal structures contain only carbon

graphene

fullerene

diamond

In a covalently bonded ceramic material, which effect is most likely to determine the structure?

bond hybridization

Forming cross-links between amorphous polymer chains

causes a decrease in the elastic modulus

Elastomer properties

moderate amount of cross linking between chains

a very low modulus of elasticity

high recoverable elastic strains

A semi crystalline polymer will exhibit

a glass transition temp

a melting temp

supercooled liquid or amorphous region

the deformation behavior of a thermoplastic is increasingly brittle as

temperature decreases or strain rate increases

composite classifications

particle reinforced, fiber reinforced, structural

an example of a particle reinforced composite is

cement

The purpose of the matrix is to

transfer stress to other phases,

protect dispersed phase from outer elements

Composite properties are dependent upon

constituent phases

amounts of each phase

the geometry of each phase

σ (Engineering stress)

F / Ao (force/cross sectional area before loading)

ε (Engineering strain)

ΔL/lo (change in length / original length)

elastic deformation

non permanent & reversible

Modulus of Elasticity

stress/strain

High modulus of elasticity

rigid or stiff material

Poisson's Ratio

-εx/εz = -εy/εz

εx (lateral strain)

Δd/ d

γ (shear strain)

Δx/y = tanΘ

Δl

Flo/(EAo) (force initial length / elasticmodulus initial cross sectional area)

plastic deformation

permanent deformation caused by strain when stress exceeds a certain value

yield strength

stress at which noticeable plastic deformation has occurred

Tensile strength (TS)

maximum stress on engineering stress-strain curve

Ductility

amount of plastic deformation at failure

Reslience

ability of a material to absorb energy during elastic deformation

Toughness

amount of energy absorbed before fracture = area under stress-strain curver

brittle fracture

small toughness, low ductility

ductile fracture

high toughness, high ductility

True stress

The instantaneous applied load divided by the instantaneous cross-sectional area of a specimen

true strain

the elongation of the specimen in increments of instantaneous change in length

hardness

measure of resistance to surface plastic deformation

edge dislocation

Burgers vector perpendicular to dislocation line

screw dislocation

Burgers vector is parallel to the dislocation line

dislocation motion

movement of extra half-plane of atoms by breaking and reforming of interatomic bonds

slip plane

the crystallographic plane along which the dislocation line traverses

slip direction

The direction in the crystal in which the dislocation moves. The slip direction is the same as the direction of the Burgers vector.

slip system

combination of slip plane and slip direction

most slip systems

FCC

critical resolved shear stress

the shear stress required to cause a dislocation to move and cause slip

λ

angle between force and slip direction

Φ

angle between force and slip plane normal

When does shear stress occur?

If the applied load is not perpendicular nor parallel to the stress direction

Strengthening Mechanisms

1. reduce the grain size

2. solid solution alloying

3. strain hardening (cold working)

grain size reduction strengthening

increases # grain boundaries thus making it harder for slip to occur (the dislocation motion must change direction at each grain boundary)

dislocation in ceramics

much less likely then in metals due to their ionic and covalent bonds, slip motion is much harder

resolved shear stress

σcosλcosΦ (stress cos(angle)cos(angle))

Solid Solution Strengthening

makes a metal harder by adding impurity atoms, these atoms cause stress fields which thus increases the materials σy and σTS (yield & tensile strength)

Strain hardening (cold working)

The increase in hardness and strength of a ductile metal as it is plastically deformed below its recrystallization temperature.

if % CW increases? (percent coldwork)

then σy and σTS increase, but %EL goes down (ductility)

Annealing

reverses effects of cold working, causes recover recrystallization and grain growth

intergranular fracture

crack propagation is along grain boundaries

transular fracture

fracture cracks pass through the grains

σm

stress at crack tip

σo

applied stress

Pt

radius of curvature

a

crack length (for surface cracks length = a, for internal cracks length = 2a)

For calculating stress/strain/elastic modulus

use units in N, m, and MPa

Pascals

force in N / area in m^2

MegaPascals

force in N / area in mm^2

(d^n)-(do^n)

kt

maximum crack length

(2E*SE)/(σ²π) (2 times modulus of elasticity times surface energy) / (stress squared times pi)

when does creep become an issue

at 0.4 * Tm (absolute melting temp)

composition in wt% of α phase

just draw a tie line and see where it intersects all the phases

mass fraction of α phase

use lever rule

Which has a lower activation energy? Homogeneous or Heterogeneous nucleation?

Heterogeneous nucleation has a lower activation energy than homogeneous nucleation due to the presence of surfaces and interfaces.

Which is faster heterogeneous or homogeneous

heterogeneous is because of its lower activation energy

Eutectic Point

liquid transforms to two solids

eutectoid

one solid phase transforms to two other solid phases

Peritectic

liquid and one solid phase transform to a second solid phase

austenite

gamma phase region

cementite

Fe3C (Iron Carbide)

ferrite

alpha region

pearlite

alpha + Fe3C (ferrite + cementite)

bainite

-Elongated Fe3C particles in alpha-ferrite matrix

spherodite

Fe3C particles in alpha-ferrite matrix, requires diffusion

martensite

A Nonequilibrium Transformation Product

How do you form pearlite

slowly cool austenite (gamma)

How do you form bainite?

moderately cool austenite

How do you form martensite?

rapidly quench austenite before any transformation

how do you form tempered martensite

reheate martensite

rank in terms of strength:

spherodite, bainite, coarse pearlite, fine pearlite, Tempered martensite, martensite

M, tm, B , fp, cp, s

rank in terms of ductility:

spherodite, bainite, coarse pearlite, fine pearlite, Tempered martensite, martensite

s, cp, fp, b, tm, m

primary creep

slope (creep rate) decreases with time

secondary creep

steady state, constant slope

tertiary creep

slope (creep rate) increases with time, i.e. acceleration of rate

instantaneous deformation

the minimum strain which the creep-time plot begins

rupture

at the end of creep graph

fatigure

failure under lengthy periods of repeated stress strain cycling

Measures that can be taken to reduce fatigue

(1) Polish the surface to remove stress amplification sites.

(2) Reduce the number of internal defects (pores, etc.) by means of altering processing and fabrication

techniques.

(3) Modify the design to eliminate notches and sudden contour changes.

(4) Harden the outer surface of the structure by case hardening (carburizing, nitriding) or shot peening

Cite three variables that determine the microstructure of an alloy.

(1) the alloying elements present, (2) the

concentrations of these alloying elements, and (3) the heat treatment of the alloy.

Name the two stages involved in the formation of particles of a new phase. Briefly describe each.

nucleation and growth

The nucleation process involves the formation of normally very small particles of the new phase(s), which are stable and

capable of continued growth. The growth stage is simply the increase in size of the new phase particles.

Superheating

Heating to above a phase transition temperature without the occurrence of the transformation.

What are the differences between bainite, pearlite, and spherodite

The microstructures of pearlite, bainite, and spheroidite all consist of α-ferrite and cementite phases.

For pearlite, the two phases exist as layers which alternate with one another.

Bainite consists of very fine and parallel

needle-shaped particles of cementite that are surrounded an α-ferrite matrix. For spheroidite, the matrix is ferrite,

and the cementite phase is in the shape of sphere-shaped particles.

Bainite is harder and stronger than pearlite, which, in turn, is harder and stronger than spheroidite.

What are 0D defects?

point defects - vacancies, interstitials, substitutional impurity atoms

What are 1D defects?

line defects - edge, screw, mixed dislocations

What are 2D defects?

Area defects - grain boundaries, twin boundaries, stacking faults