MIME 260 lecture 6

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

1. Nanosized materials, completely avoid dislocations, however very expensive and unpractical

2. 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