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MEE 312 Final Study
Question | Answer |
---|---|
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 |