MATERIAL SCIENCE MIDTERM #2

Imperfections within metallic crystal structures may be any but which of the following?

1. Lattice vacancies and extra interstitial atoms

2. Displacements of atoms to interstitial sites

3. Lineal (linear) defects of dislocations caused by shear

4. Ion pairs missing in ionic crystals

ion pairs missing in ionic crystals

Which of the following might be found on its own in a ceramic?

(A) Frenklel defect
(B) substitutional solute
(C) interstitial solute
(D) vacancy

vacancy

Which of the following is not an example of a bulk defect

(A) crack
(B) vacancy
(C) void
(D) 2^nd phase particle

vacancy

Which of the following is not an effect of grain boundaries?

(A) they are a short circuit path for diffusion
(B) they are the primary mechanism for plastic deformation in alloys
(C) they are preferential sites for corrosion
(D) they tend to collect impurities

they are the primary mechanism for plastic deformation in alloys

molecular size

the amount of monomers that are linked together

molecular shape

the extent to which a polymer chain has folded into a ball

molecular structure

how polymer chains branch or are linked together

molecular configuration

the arrangement of various elements or groups along the length of a polymer chain

What is the predominant form of bonds between carbon in a polymeric macromolecule?

(A) Metallic

(B) Ionic

(C) Covalent

(D) Van der Waals

covalent

What does resolved shear stress characterize?

the ability of an applied load to cause slip in a specific plane and direction 

Gibbs Free Energy was used to explain the tradeoffs that cause an equilibrium vacancy rate. What two important thermodynamical values does Gibbs Free Energy consider?   Briefly explain what these two values represent in the context of a solid material.

enthalpy: represents the energy required to form a vacancy by breaking bonds in the solid 

entropy: a measure of disorder in a system

Why do we often use engineering stress when we know that true stress is more accurate?

it is simple, standardized, and sufficient for most design work, even though true stress is more accurate in describing the actual material behavior during large deformation 

What is the difference between a void and a vacancy?

void: hole in the material

vacancy: missing 1 atom from a lattice site

What condition(s) would lead to a material not developing a crystalline structure during solidification?

when atoms do not have enough time or mobility to arrange into an ordered lattice, due to rapid cooling, complex molecular structure, high viscosity 

What happens during the recovery portion of annealing?  b) what is the driving force behind this behavior? 

recovery: reduces internal stresses and reorganizes dislocations 

driving force: stored elastic energy from prior deformation 

 What happens to the structure of thermoset polymers upon curing?  What does this do to the mechanical properties?

curing: converts thermosets into a rigid, cross-linked network 

mechanical effect: stronger, stiffer, heat-resistant, but more brittle

Explain why reducing grain size can strengthen an alloy.

increases the number of grain boundaries, which block dislocation motion, making the alloy stronger and harder 

difference between addition polymerization and condensation polymerization

addition polymerization: links monomer units end-to-end 

condensation polymerization: involves more than 1 monomer species and results in by-products 

similarities and differences between Poisson Effect and necking

poisson effect: uniform, elastic behavior occurring along the length evenly

necking: localized plastic deformation 

similarities: both are related to material deformation under stress 

how charpy results are affected by temperature

high temp —> ductile, dull surface, shear fracture

low temp —> brittle, shiny surface, cleavage fracture

3 conditions that make ductile steel more brittle

low temp, high strain rate, and stress concentrators

similarities and differences between creep and relaxation

creep: increase in strain resulting from sustained stress

relaxation: decrease in stress from sustained strain

similarities: can lead to permanent change in the material’s shape and predicts long-term performance of the material

explain why diffusion rates can increase along grain boundaries

the structure becomes more disoriented, with more free volume and weaker bonding 

what is the definition of dislocation density and how is this value measured?

dislocation density: the amount of dislocations in a body

D.D. = total length of dislocations / volume sampled

what is the trend that is described by the Hall-Petch equation

as yield stress increase, grain size decreases

describe the behavior and interaction between macromolecules during the deformation 

• first stretch elastically, then slide and orient as stress increases 

• fracture occurs when chains disentangle and covalent bonds break 

• high temp —> ductile 

• low temp —> brittle 

explain how solid solution can strengthen an alloy

creates stress field that block dislocation motion, more mismatch in size or bonding between solute and solvent atoms, the more stronger the alloy becomes 

why are self-interstitial defects rare in metallic alloys?

they are too large to fit into interstitial sites without causing too much distortion

explain the difference between a thermoplastic and thermoset polymer

thermoplastic: bonds between macromolecules that reversibly soften upon heating 

thermoset: material permanently cross links during synthesis, heating, or curing 

what is the term used to describe the mechanical behaviors observed during creep of metals and ceramics or recovery of polymers,

viscoelastic behavior