Chapter 7 Flashcards
Strength and hardness are a measure of ____.
A material’s resistance to deformation.
Ref: Pg 181
On a microscopic scale, plastic deformation corresponds to ___.
A net movement of large numbers of atoms in response to applied stress.
Ref: Pg 181
True or False
During plastic deformation, atomic bonds are fractured then reformed.
True
Ref: Pg 181
In crystalline solids, plastic deformation most often involves the motion of ____.
Dislocations (linear crystalline defects).
Ref: Pg 181
What are the two fundamental dislocation types?
Edge and screw
Ref: Pg 182
Describe an edge dislocation.
A half-plane of atoms is inserted into a lattice (a piece of paper inserted between bread slices but only halfway).
Book Def: A localized lattice distortion exists along the end of an extra half-plane of atoms which also defines the dislocation line. See fig 4.4 in the book.
Ref: Pg 182
Describe a screw dislocation.
A dislocation that is the result of shear distortions; its dislocation line passes through the center of a spiral, atomic plate ramp (See fig 4.5 in the book).
Ref: Pg 182
What are mixed dislocations?
A combination of screw and edge dislocations.
Ref: Pg 182
True or False
Plastic deformation corresponds to the motion of large numbers of dislocations.
True
Ref: Pg 182
True or False
Before and after the movement of a dislocation through some particular region of the crystal, the atomic arrangement is ordered and perfect; it is only during the passage of the extra half-plane that the lattice structure is disrupted.
True
Ref: Pg 182
Define “slip.”
The process by which plastic deformation is produced by dislocation motion.
Ref: Pg 182
Define “slip plane.”
The crystallographic plane along which the dislocation line traverses.
Ref: Pg 182
True or False
With respect to the motion of a screw dislocation in response to applied shear stress, the direction of movement is perpendicular to the stress direction.
True
Ref: Pg 183
Define “dislocation density,” and give its units.
The total dislocation length per unit volume
or
The number of dislocations that intersect a unit area of a random section.
Units: mm of dislocation per cubic millimeter
Ref: Pg 183
A heavily deformed material will have a ____ dislocation density.
High
Ref: Pg 183
True or False
When metals are plastically deformed, some fraction of the deformation energy is retained internally; the remainder is dissipated as heat.
True
Ref: Pg 184
What are littice strains?
Strains that occur simply because there is a deformation present regardless of applied shear stress.
Ref: pg 184
Two dislocations of the same slip plane that have the same sign are ____.
Repulsive.
Ref: Pg 184
Two dislocations of the same slip planes but are opposite in sign will ____.
Attract one another.
Extra Notes: See figure 7.5 in the book
Ref: Pg 184
True or False
Dislocations can move with the same degree of ease on all crystallographic planes of atoms and in all crystallographic directions.
False. There are variations between how difficult it is to move in certain directions or against different planes of atoms. Typically there is a preferred plane that defines which directions along which dislocation motion occurs.
Ref: Pg 185
Define the slip plane.
The preferred plane that defines which directions along which dislocation motion occurs.
Extra Notes: The direction of motion is called the slip direction.
Ref: Pg 185
What is the slip system?
The combination of the slip plane and the slip direction.
Ref: Pg 185
For a particular crystal structure, the slip plane is the plane that has ___.
The densest atomic packing (greatest planar density–see section 3.11)
Ref: Pg 185
For a particular crystal structure, the slip direction corresponds to ___.
The direction in this plane that is most closely packed with atoms (highest linear density, see section 3.11)
Ref: Pg 185
True or False
Only one slip system can exist for a single structure.
False, several slip systems may exist for a particular crystal structure; the number of independent slip systems represents the different possible combinations of slip planes and directions.
Ref: Pg 186
With regard to the process of a slip, a Burger vector’s direction corresponds to ____, whereas it’s magnitude is equal to _____.
a dislocation’s slip direction
the unit slip distance
Ref: Pg 186
True or False
Plastic Deformation is permanent.
True
Ref: Pg 181
True or False
During Plastic Deformation, atomic bonds are broken and then reformed.
True
Ref: Pg 181
An edge dislocation moves in response to shear stress applied in a direction ____ to its line.
Perpendicular
Ref: Pg 182
True or False
All metals and alloys contain some dislocation that were introduced during solidification, during plastic deformation, and as a consequence of thermal stresses that result from rapid cooling.
True
Ref: Pg 183
True or False
Heating a deformed metal will have no effect on dislocation density.
False, Heating a deformed metal can diminish the deformation density to on the order of 10^5 to 10^6 mm^-2.
Ref: Pg 183
Edge, screw, and mixed dislocations move in response to _______________.
shear stresses applied along a slip plane and in a slip direction.
Ref: Pg 187
True or False
Even though an applied stress may be pure tensile (or compressive), shear components exist at all but parallel or perpendicular alignments to the stress direction.
True
Ref: Pg 187
Even though an applied stress may be pure tensile (or compressive), shear components exist at all but parallel or perpendicular alignments to the stress direction. These are termed ______.
Resolved shear stresses.
Ref: Pg 187
The magnitude of resolved shear stresses depends on ___.
Applied shear stress and the orientation of the slip plane
Ref: Pg 187
Define “critical resolved shear stress.”
The value that represents the minimum shear stress required to initiate a slip and is a property of the material that determines when yielding occurs.
Ref: Pg 188
When does a single crystal plastic deform or yield?
When the max resolved shear is equal to the critical resolved shear stress.
Ref: pg 188
The minimum stress necessary to introduce yielding occurs when a single crystal s orientated such that ____.
Both phi and lambda are equal to 45 degrees.
Ref: Pg 188
Slip lines occur when ____.
The movement of a large number of dislocations along the same slip plane takes place.
Ref: Pg 188
Why is slip in polycrystalline materials more complex than single crystals?
Because of the random crystallographic orientations of the numerous grains, the direction of slip will vary from one grain to another.
Ref: Pg 190
Ture or False
With respect to plastic deformation in polycrystalline materials, during deformation, mechanical integrity and coherency are maintained along the grain boundaries.
True
Ref: Pg 190
Before deformation in polycrystalline materials, grain are ___, or have approximately the ____.
Equaxed
Same dimensions in all directions.
Ref: Pg 190
True or False
Polycrystalline metals are stronger than their single-crystal equivalents, which means that greater stresses are required to initiate slip and the attendant yielding.
True
Ref: Pg 191
Describe how a twin forms.
A shear force can produce atomic displacements such that on one side of a plane (the twin boundary), atoms are located in mirror image positions of atoms, on the other side.
Ref: Pg 192
True or False
Grain are barriers to dislocation motion?
True
Ref: CH7 Lecture Slides
True or False
Slip plane and directions (phi and lamda) do not change from one crystal to another.
False, they do change from one crystal to another.
Ref: CH7 Lecture Slides
True or False
Dislocation motion is a result of shear forces.
True
Ref: CH7 Lecture Slides
Resolved shear stress results from ____.
Applied tensile stresses.
Ref: CH7 Lecture Slides
Lambda is the angle between _____.
Applied tensile stress and the slip direction.
Ref: CH7 Lecture Slides
Phi is the angle between _____.
tensile stress and normal to slip plane.
Ref: CH7 Lecture Slides
True or False
The orientation of a slip system (crystal orientation) can make it easy or hard to move dislocations on that system.
True
Ref: CH7 Lecture Slides
True or False
The crystal with the largest resolved shear stress will yield first. Other grains (less favorable orientated ) will yield later.
True
Ref: CH7 Lecture Slides
How can anisotropy be induced in a polycrystalline metal?
By rolling. Rolling affects grain orientation and shape.
Ref: CH7 Lecture Slides
True or False
Typically some ductility is sacrificed when a material is strengthened.
True
Ref: Pg 193
The ability of a metal to deform plastically depends on the ability of dislocations to __________.
Move.
Ref: Pg 193
True or False
The size of the grains, or average grain diameter, in a polycrystalline metal influence the mechanical properties of that metal.
True
Ref: Pg 193
What are the two reasons why a grain boundary acts as a barrier to dislocation motion.
- Because the two grains are different. A dislocation passing from grain A to B must change its direction of motion; this becomes more difficult as the crystallographic misorientation increases.
- The atomic disorder within a grain boundary region results in a discontinuity of slip planes.
Ref: Pg 193
True or False
A fine-grained material (one that has small grains) is harder and stronger than one that is coarse-grained because the former has a greater total grain boundary area to impede dislocation motion.
True
Ref: Pg 194
For many materials, the yield strength varies with grain size according to ____.
The Hall-Petch equation.
How can grain size be controlled in a metal alloy?
- The rate of solidification from the liquid phase
- Performing a plastic deformation followed by appropriate heat treatment.
Ref: Pg 194
True or False
Grain size reduction improves strength but not toughness.
False, it improves both.
Ref: Pg 194
True or False
Small grain boundaries are not effective in interfering with the sipping process because of the slight crystallographic misalignment across the boundary.
True
Ref: Pg 194
What is “solid solution strengthening?”
Adding impurity atoms that go into either substitutional or interstitial solid solutions.
Ref: Pg 194.
Why are alloys stronger than pure metals?
Because impurity atoms that go into solid solutions typically impose lattice strains on the surrounding host atoms. Lattice strain field interactions between dislocation and these impurity atoms results and consequently dislocation movement is restricted.
Ref: Pg 194 and 195
What is strain hardening?
The phenomenon by which a ductile metal becomes harder and stronger as it is plastically deformed. This is also referred to as “cold working.”
Ref: Pg 186
How does cold working work on a microscopic scale?
The dislocations density in a metal increases with cold work because of dislocation multiplication or the formation of new dislocations. Consequently the average distance of separation between dislocations decreases. The net result is that the motion of dislocation is hindered by the presence of other dislocations.
Ref: Pg 198
How can strain hardening be removed?
By an annealing heat treatment. See chapter 11.8.
Ref: Pg 198
