53 terms

Material Science Chapter 8

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Strength and Hardness in materials are defined as
The amount of a material's resistance to deformation.
Plastic deformation
Involves the motion of dislocations. They are linear crystalline defects such as edge and screw.
Slip
The process by which plastic deformation is produced by dislocation motion. Atoms on the same side of the slip plane move equal distance and leaves a series of steps.
Screw dislocation
may be thought of as being formed by a shear stress that is applied to produce the distortion. The direction of movement is perpendicular to the stress direction.
Edge dislocation
is a linear defect that has an extra half-plane of atoms inserted in a crystal structure, which also defines the dislocation line. The motion of the dislocation line is parallel to the shear stress. The applied stress is perpendicular to the dislocation line.
Dislocation density
is the amount of dislocations per unit volume of a crystalline material, or the number of dislocations that intersect a unit area of a random section.
lattice strains
are regions in which compressive, tensile, and shear strains that are imposed on the neighboring atoms.
Slip system
combination of the slip plane and the slip direction.
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Dislocation motion (Slip) is easier along
a close-packed plane.
slip direction
The directions in this plane that has the highest linear density of atoms.
slip plane
is the plane that has the greatest planar density.
Slip for an edge dislocated crystal structure
only requires local atoms to break and reform to achieve plastic deformation.
Dislocation motion for a Perfect crystal structure
Plastic deformation requires breaking and reforming of a whole array of atoms along the shear direction.
Why is dislocation motion easier along a Close-Packed Planes (lines)?
Less energy or lower shear stress is required for slip along the most densely packed plane and direction.
Does slip only occur along the most dense packing directions and plane.
No slip can occur along any plane or direction.
How many major slip systems,unique slip planes and slip directions does a FCC structure have.
12 slip systems, 4 unique planes, and 3 directions per plane.
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How many major slip systems, unique slip planes and slip directions does a BCC have.
12 slip systems, 6 unique planes, and 2 directions per plane.
How many major slip systems, unique slip planes and slip directions does a HCP have.
3 slip systems, 1 unique plane, and 3 directions.
Resolved shear stresses
shear components exist at all but parallel or perpendicular alignments to the stress direction. The magnitudes depend not only on the applied stress, but also on the orientation of both the slip plane and direction within that plane. reaches maximum at φ=λ=45.
Critical resolved shear stress
the minimum stress required for slip to occur. The minimum stress necessary to introduce yielding occurs when a single crystal is oriented such that φ=λ=45.
Slip in polycrystalline metals
because of the random crystallographic orientations of the numerous grains, the direction of slip varies from one grain to another. Each grain has its own slip systems. Grain boundaries act as barrier for dislocation motions within each grains and slip systems
become discontinuous.
Polycrystalline materials versus single crystal form.
Polycrystalline materials are much stronger than their single crystal form because greater stresses are required to initiate slip.
DEFORMATION BY TWINNING
can reorient crystal grains in a direction more favorable to slip allowing plastic deformation and are more important in HCP metals.
Define Strengthening mechanisms of metals.
The ability of a metal to deform plastically depends on the dislocation motion. Restricting or hindering dislocation motion makes a material harder and stronger.
Dislocation motion may be restricted by
Impurity atoms, Grain boundaries and the increased dislocation density.
Strengthening by impurity atoms / solid solution strengthening.
Metals may be strengthened by adding impurities, both substitutional and interstitial.
How does impurity defects strengthen materials?
Impurity atoms can reduce lattice strain
around a dislocation due to the atomic size difference. The decreased lattice strain leads to a restriction in dislocation motion that is caused by atoms being "pinned," Impurity atoms accumulate around dislocations and form lower energy configurations.
Twinning Deformation
atomic displacement increases with distance from the twinning planes. It leaves small but well defined regions of crystal deformations.
Strain hardening
As a ductile metal is plastically deformed its yield strength increases . It creates internal lattice stress/strain from movement of atoms and increases dislocations density. After elastic recovery the material has a restructured crystal grains.
How does strain hardening work?
The increased strength is a result of a decrease in dislocation motion that is caused by an increased in dislocation density.
How does the dislocation density increase in strain hardening?
The density of dislocations increase because the cross sectional area of the material decreases, and new dislocation forest form, and some repulsive interactions prevent motion.
Why does the increase in dislocation density restrict dislocation mobility?
Dislocation combination can occurs due to attractive interaction between dislocations, sometimes causing annihilation of the dislocation. There are repulsive interactions that also prevents dislocation motion.
Cold work Stain hardening for metals.
Deformation at room temperature for most metals. A common forming operations is to reduce the cross sectional area.
Impact of cold work
Increase the yield strength, and tensile strength (TS)
but Ductility (%EL or %AR) decreases.
Can a crystalline ceramic materials be strain-hardened at low temp?
No because plastic deformation does not occur for ceramics. The ceramic material will usually fracture at the yield strength of the material.
Strengthening by grain boundaries
When dislocations encounters the grain boundary, the movements of the dislocations are hindered.
What are two reasons that GB hinder the dislocation motion across two neighboring grains?
The slip planes will be in different orientations, causing the dislocation to change direction.
The slip planes may be discontinuous between the two grains.
Which type smaller grain or larger grains, resist dislocations motion more?
Smaller grains resist dislocation more because there is a greater area of grain boundaries in small grain polycrystalline materials. This results that smaller grain polycrystalline materials have a higher yield strength.
What happens at high angular misalignments of grain boundaries?
dislocation may not traverse the grain boundary. Instead, the dislocation density increases at the grain boundary and the resulting lattice stress creates new dislocations across the boundary.
Can strain hardening effects be reversed?
By annealing / heat treatment, the strain hardening effects can be reversed.
Recrystallization
The formation of new grains. New grains nucleate within existing grains and consume the parent material. The process is time and temperature dependent.
What is the driving force of recrystallization during annealing?
The residual internal stress from strain hardening causes the high energy state to become a lower energy state. The greater the cw% the lower the temp need for recrystallization.
Grain Growth
After recrystallization, grains will continue to grow if the temperature is high enough. The large grains grow at the expense of smaller grains to reduce surface energy. Atoms can diffuse across a grain boundary and increased temperature leads to faster grain growth.
Does grain growth require prior strain hardening to be produced?
Grain growth can occur at high temperatures for any polycrystalline material and does not require prior strain hardening.
Can grain sizes be increased and reduced?
Yes an increase grain size occurs during grain growth at high temperatures. The decrease grain size occurs during cold work of materials.
Metals having small grains
relatively strong and tough at low temperatures.
Metals having large grains
good creep resistance at relatively high temperatures.
Plastic Deformation for crystalline ceramics
Ionic bonding prevents dislocation motion, therefore slip is very difficult in ceramics. Making them have high hardness and brittleness. Plastic deformation becomes easier at elevated temperatures.
Plastic Deformation for noncrystalline ceramics
With no regular atomic structure in the material, deformation occurs via viscous flow. The atoms slide past one another by breaking and reforming bonds.
Noncrystalline ceramics have very high viscosities, as compared to liquids, the viscous flow becomes easier at elevated temperatures.
Can brittle polymers and elastomers plasticly deform?
No brittle polymers will fracture before plastic deformation and elastomers will always return to original structure or fracture.
Can thermoplastics plasticly deform?
Yes semicrystalline polymers display significant plastic deformation. Furthermore, drawing and annealing are commonly used to strengthen thermoplastics = semicrystalline.
Drawing semicrystalline polymers
stretches the polymer prior to use and aligns the chains in the stretching direction. This increases the elastic modulus (E) in the stretching direction, increases the tensile strength (TS) in the stretching direction and decreases ductility (%EL).
Effects of Annealing on Mechanical Properties of Semicrystalline polymers.
Annealing after drawing decreases chain alignment
and reverses the effects of drawing (reduces E and
TS, enhances %EL). Annealing before drawing increases the amount of crystallinity, which causes an increase in E, TS, and reduced % EL.
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