Studied by 4 people
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plastic deformation achieved by…
dislocation motion
straight dislocation
line direction remains unchanged along dislocation line, full edge dislocation or full screw dislocation,(not mixed dislocation).
slip system
combination of slip plane and slip direction
slip plane
crystallographic plane on which dislocation motion occurs; highest planar density, most widely spaced planes to preserve bonding environment
slip direction
crystallographic direction along which dislocation moves; highest linear density to avoid high dislocation energy, by choosing smaller burgers vector
rules for slip system
depends on crystal structure, chosen such that atomic distortion accompanied by dislocation motion is minimized
why does HCP have less slip systems than BCC and FCC
because HCP is much more brittle
what is needed to move dislocations
shear stress
critical resolved shear stress
minimum shear stress required to begin plastic deformation or slip
single crystal
only favorite slip system is activated, unidirectional slip deformation
polycrystal
different slip systems are activated in different grains, slip deformation in all directions, causes necking
why strengthen materials?
to reduce material usage which is more energy efficient
What is strength?
resistance to plastic deformation
How to strengthen materials?
restrict dislocation formation and motion
Methods to strengthen
Nanosized materials, completely avoid dislocations, however very expensive and unpractical
engineering microstructures, barriers/resistance to dislocation activities, like bulk nanomaterials
Grain boundary engineering
grain size reduction, smaller grain size, more barriers to slip/dislocation motion. greater degree of misalignment, more effective resistance to slip
small impurities concentrate at dislocation…
compressive stress side
large impurities concentrate at dislocations…
tensile stress side
alloying increases…
UTS and yield strength
solid solution strengthening pros
increases yield strength without significant decrease of ductility
solid solution strengthening cons
limited by solubility of alloying element and by difference of atomic radius
Strain hardening/Cold working
deformation at room temp, common forming operations reduce cross section area
dislocation structure changes during cold working:
dislocations become entangled, making motion more difficult
impact of cold working
yield strength increases, UTS increases, ductility decreases
Precipitate/particle strengthening
hard precipitates are difficult to shear, ceramics in metals
effect of aging
can obtain optimum precipitate size and number to maximize strength
Aging
maintain at elevated temperature for a given time before fully cooling to room temp
aging effect on precipitates
precipitates want to merge together to decrease surface energy, so over time precipitates increase in size but decrease in number
