Chapter 7 - Dislocations and Strengthening Mechanisms

Question 1

fundamental dislocation types (2)

Answer 1

1. edge: extra half-plane of atoms is inserted, motion is

2. screw: extends alonga line that passes through the center of a spiral

Question 2

slip

Answer 2

the process by which plastic deformation is produced by dislocation motion, along the plane with the highest density

Question 3

dislocation density

Answer 3

the number of dislocations, total dislocations per unit volume

Question 4

lattice strains

Answer 4

compressive, tesile, shear, imposed by dislocations, in screw dislocations there are only shear strains

Question 5

slip plan

Answer 5

the plane that the dislocations prefer to move on

Question 6

slip direction

Answer 6

the direction of dislocation movement

Question 7

slip system

Answer 7

the slip plane and slip direction, depends on the crystal structure of the metal, wants the minimum atomic distortion

Question 8

b(FCC)

Answer 8

a/2<110>

Question 9

b(BCC)

Answer 9

a/2<111>

Question 10

b(HCP)

Answer 10

a/3<11-20>

Question 11

resolved shear stresses

Answer 11

the shear components that exist at all but parallel or perpendicular alignments

Question 12

critical resolved shear stress

Answer 12

the minimum shear stress required to initiate slip, property of the material that determines when yielding occurs

Question 13

gross plastic deformation

Answer 13

corresponds to the comparable distortion of the individual grains by means of slip

Question 14

grain boundary reaction

Answer 14

mechanical integrity and coherency are maintained along the grain boundaries, do not usually come apart or open up

Question 15

grain size reduction

Answer 15

the smaller the grains, the harder/stronger the material

Question 16

solid-solution strengthening

Answer 16

technique to strengthen and harden materials, alloying metals with impurity atoms, resistance to slip is greater where impurity atoms are present

Question 17

smaller substitutional impurity atom

Answer 17

exerts tensile strain, located where the tensile strain will partially nullify some of the dislocation’s compressive strain

Question 18

larger substitutional impurity atom

Answer 18

imposes compressive strains

Question 19

strain hardening

Answer 19

where ductile metals become harder and stronger as they are plastically deformed, aka work hardening or cold working

Question 20

percent cold work trends

Answer 20

increased yield and tensile strengths, decreased ductility

Question 21

strain hardening exponent

Answer 21

n, measure of the ability of a metal to strain harden, the larger the magnitude the greater the strain hardening for a given amount of plastic strain

Question 22

mechanisms to strengthen and harden single-phase metal alloys (3)

Answer 22

1. strengthening by grain size reduction
2. solid-solution strengthening
3. strain hardening

Question 23

annealing treatment

Answer 23

heat treatment that reverts properties to precold-worked states, recovery or recrystallization (followed by grain growth)

Question 24

recovery

Answer 24

some of the stored internal strain energy is relieved by virtue of the dislocation motion, electrical and thermal conductivities are recovered to their precold-worked states

Question 25

recrystallization

Answer 25

the formation of a new set of strain-free and equilaxed grains that have low dislocation densities, material becomes more ductile but softer and weaker

Question 26

recrystallization temperature

Answer 26

the temperature at which recrystallization just reaches completion in one hour, typically it is between one-third and one-half of the absolute melting temperature