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MEE 312 Final Study

What are some general characteristics of metals and alloys? ferrous and non-ferrous Moderate melting temperature good ductility moderate strength good electrical/thermal conductivity
What are some general characteristics of ceramics? Crystalline poor ductility high strength brittle
What are some general characteristics of polymers? low melting temp poor electrical/thermal conductivity low strength thermoplastics, thermosets, elastomers
What are some general characteristics of composites? engineered properties two or more materials
mechanical properties response to an applied force, microstructure sensitive (strength, ductility)
physical properties materials response to applied field, microstructure insensitive (modulus, CTE, conductivity)
Primary bonds (types) covalent ionic and metallic
Secondary bonds (types) Van der Waals
What are covalent bonds? sharing of valence electrons, strong bond (high strength, low CTE, high melting temp, high E); no free electrons, so low conductivity; examples: ceramics, bonds within molecular chains
What are ionic bonds? strong electrostatic attraction b/w anion and cation, strong bond (same characteristics of covalent bond) example: ceramics
What are metallic bonds? outermost electrons given up to form electron cloud, relatively weak bond (lower strength, high CTE, low melting temp, low E); many free electrons (good conductivity)
What are secondary bonds? weak electrostatic attraction that results from polarity in molecules
amorphous no long range order
crystalline long range order
three most common types of unit cells FCC, BCC, HCP
FCC characteristics low strength high ductility ex: aluminum
BCC characteristics moderate strength moderate ductility ex: steel
HCP characteristics high strength low ductility example: some titanium
close packed direction where atoms are all touching
BCC close packed planes (BCC) BCC does not have any close packed planes
How do point defects affect the material's strength and ductility? they disrupt the perfect arrangements of atoms in the crystal and act as a barrier to slip
How and why do surface defects affect a material's strength and ductility? they disrupt the perfect arrangements of atoms in the crystal and act as a barrier to slip
How can some defects be controlled? Inoculation: smaller grains and more grain boundaries, therefore more barriers for slip faster cooling rate: smaller grains, dispersion strengthening phase boundaries
What are the two different mechanisms for diffusion? interstitial and vacancy
Interstitial diffusion smaller species, more sites, lower activation energy (fast)
Vacancy diffusion fewer sites, but increase exp with temperature, larger species, higher activation energy (slow)
Factors that affect diffusion in materials type and mechanism of diffusion temp crystal structure (the more open, the faster the rate) bonding (strong bonds have slower rate)
What are the types and rates of diffusion? Surface (fastest) Grain boundary (moderate) Volume (slowest)
How does diffusion relate to grain growth? The factors that affect diffusion will affect all diffusion controlled phenomenon such as creep, heat treating, and processing
How does hardness data relate to tensile data? As hardness increases so does strength
Two types of hardness tests Rockwell and Brinell
How can we determine the toughness based on a stress strain curve? Area under the curve
What is cold working? Plastically deforming a material, simultaneously shaping and hardening it
What is the effect of strain hardening? Creates more dislocations which are line defects, and these defects disrupt perfect arrangement of atoms
What is the difference associated with the strain hardening of polymers and metals? For polymers, we are aligning chains, NOT increasing number of dislocations (no dislocations in amorphous materials)
What are the three stages of annealing? Recovery, Recrystallization, Grain growth
recovery low temperature treatment rearranges dislocations to a polygonized subgrain structure strength does not change electrical conductivity restored residual stresses eliminated
recrystallization new grains nucleate on boundaries of dislocations strength decreases ductility restored dislocation decreases
grain growth new grains grow fewer grain boundaries decrease in strength
How does hot working compare to cold working? Hot working is done above the recrystallization temp, so the material is annealed as it is deformed. There is no change in properties
What are the two steps with solidification? nucleation and growth
homogeneous nucleation spontaneous clustering of atoms forms a nucleus that is large enough to grow only in lab conditions
heterogeneous nucleation formation of a nucleus on the surface of an impurity, occurs most frequently
What is solid solution strengthening? addition of point defects, forms a solid solution
What is dispersion strengthening? solubility limit is exceeded so that a second phase forms, phase boundaries act as a very effective block to slip increase in strength (non coherent precipitate)
What type of materials are used in age hardening? mostly non ferrous metals and stainless steels
How and why does age hardening affect a material's properties? tricks the material into forming a coherent precipitate, more effective at blocking slip significant strengthening
What are the three steps in age hardening? Solution treatment Quench Age
Solution treatment heating a material to a single phase solid solution
Quench cool rapidly to prevent diffusion of atoms to nucleation sites, so the second phase cannot form (supersaturated solid solution)
Age naturally - at room temp artificially - at temp above room temp but above the solvus allows diffusion so that a coherent precipitate of a second phase forms
overaging occurs during artificial aging, coherent precipitate breaks free from parent lattice, becomes non coherent and a decrease in strength occurs
Three major types of microconstituents pearlite bainite martensite
pearlite slow cooling rate lamellar structure of ferrite and cementite lower strength, good ductility
bainite moderate cooling rate fine layered structure of ferrite and cementite moderate strength, moderate ductility
martensite fast cooling rate forms BCT structure through a twinning process very strong but brittle it is tempered to allow diffusion to occur and form a fine microstructure of ferrite and cementite with high strength and useable ductility
Created by: flyerbud



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