MEEN 222 A7

ductility is sacrificed when an alloy is strengthened

the ability of a metal to deform plastically depends on the ability of dislocations to move

Where do dislocations move the easiest since they do not move the same degree of ease for each one; what are the specific directions they can move

the direction it can move is the slip direction, a regular plane be called a plane,The slip direction corresponds to the direction in this plane that is most closely packed with atoms that is, has the highest linear density.

What if its both?

Many dislocations in crystalline materials have both edge and screw components; these are mixed dislocations

What is a screw dislocation?

A screw dislocation may be thought of as resulting from shear distortion; its dislocation line passes through the center of a spiral, atomic plane ramp

resolved shear stresses

An applied tensile or compressive stress resolved into a shear component along a specific plane and direction within that plane. Their magnitudes depend not only on the applied stress, but also on the orientation of both the slip plane and direction within that plane.

onsequently, the average distance of separation between dislocations decreases—the dislocations are positioned closer together

As the dislocation density increases, this resistance to dislocation motion by other dislocations becomes more pronounced. Thus, the imposed stress necessary to deform a metal increases with increasing cold work.

The grain boundary acts as a barrier to dislocation motion for two reasons

Because the two grains are of different orientations, a dislocation passing into grain B must change its direction of motion; this becomes more difficult as the crystallographic misorientation increases. 2. The atomic disorder within a grain boundary region results in a discontinuity of slip planes from one grain into the other.

Recovery 2

During recovery: There is some relief of internal strain energy by dislocation motion. Dislocation density decreases, and dislocations assume low-energy configurations. Some material properties revert back to their precold-worked values.

recrystallization temperature

For a particular alloy, the minimum temperature at which complete recrystallization occurs within approximately 1 h.

Plastic deformation of Polycrystalline Materials

For polycrystalline metals, slip occurs within each grain along those slip systems that are most favorably oriented with the applied stress. Furthermore, during deformation, grains change shape and extend in those directions in which there is gross plastic deformation.

Strength by Grain Size reduction

Grain boundaries are barriers to dislocation motion for two reasons: When crossing a grain boundary, a dislocation's direction of motion must change. There is a discontinuity of slip planes within the vicinity of a grain boundary. • A metal that has small grains is stronger than one with large grains because the former has more grain boundary area and, thus, more barriers to dislocation motion. • For most metals, yield strength depends on average grain diameter according to the Hall-Petch equation, Equation 7.7.

Grain Growth

Grain growth is the increase in average grain size of polycrystalline materials, which proceeds by grain boundary motion. • The driving force for grain growth is the reduction in total grain boundary energy. • The time dependence of grain size is represented by Equation 7.9.

solid-solution strengthening

Hardening and strengthening of metals that result from alloying in which a solid solution is formed. The presence of impurity atoms restricts dislocation mobility.

What is an edge dislocation?

In an edge dislocation, localized lattice distortion exists along the end of an extra half-plane of atoms, which also defines the dislocation line

Briefly explain why some metals (e.g., lead, tin) do not strain harden when deformed at room temperature.

Metals such as lead and tin do not strain harden at room temperature because their recrystallization temperatures lie below room temperature

Would you expect a crystalline ceramic material to strain harden at room temperature? Why or why not?

No, it would not be expected. In order for a material to strain harden it must be plastically deformed; because ceramic materials are brittle at room temperature, they will fracture before any plastic deformation takes place.

Would you expect it to be possible for ceramic materials to experience recrystallization? Why or why not?

No, recrystallization is not expected in ceramic materials. In order to experience recrystallization, a material must first be plastically deformed, and ceramic materials are too brittle to plastically deform.

basic concepts

On a microscopic level, plastic deformation corresponds to the motion of dislocations in response to an externally applied shear stress. An edge dislocation moves by the successive and repeated breaking of atomic bonds and shifting by interatomic distances of half planes of atoms. • For edge dislocations, dislocation line motion and direction of the applied shear stress are parallel; for screw dislocations, these directions are perpendicular. • Dislocation density is the total dislocation length per unit volume of material. Its units are per square millimeter. • For an edge dislocation, tensile, compressive, and shear strains exist in the vicinity of the dislocation line. Shear lattice strains only are found for pure screw dislocations.

What is a slip

Plastic deformation as the result of dislocation motion; also, the shear displacement of two adjacent planes of atoms. Like the caterpillar

Explain the difference between resolved shear stress and critical resolved shear stress.

Resolved shear stress is the shear component of an applied tensile (or compressive) stress resolved along a slip plane that is other than perpendicular or parallel to the stress axis. The critical resolved shear stress is the value of resolved shear stress at which yielding begins; it is a property of the material.

slip in single crystals

Resolved shear stress is the shear stress resulting from an applied tensile stress that is resolved onto a plane that is neither parallel nor perpendicular to the stress direction. Its value is dependent on the applied stress and orientations of plane and direction according to Equation 7.2. • Critical resolved shear stress is the minimum resolved shear stress required to initiate dislocation motion (or slip) and depends on yield strength and orientation of slip components per Equation 7.4. • For a single crystal that is pulled in tension, small steps form on the surface that are parallel and loop around the circumference of the specimen.

Lattice Strain

Slight displacements of atoms relative to their normal lattice positions, normally imposed by crystalline defects such as dislocations, and interstitial and impurity atoms.

cold working

The plastic deformation of a metal at a temperature below that at which it recrystallizes.

Strain Hardening

Strain hardening is the enhancement in strength (and decrease of ductility) of a metal as it is deformed plastically. • Degree of plastic deformation may be expressed as percent cold work, which depends on original and deformed cross-sectional areas as described by Equation 7.8. • Yield strength, tensile strength, and hardness of a metal increase with increasing percent cold work (Figures 7.19a and 7.19b); ductility decreases (Figure 7.19c). • During plastic deformation, dislocation density increases, the average distance between adjacent dislocations decreases, and—because dislocation-dislocation strain field interactions, are, on average, repulsive—dislocation mobility becomes more restricted; thus, the metal becomes harder and stronger.

Slip System

The combination of a crystallographic plane and, within that plane, a crystallographic direction along which slip (i.e., dislocation motion) occurs.

slip systems

The motion of dislocations in response to an externally applied shear stress is termed slip. • Slip occurs on specific crystallographic planes, and within these planes only in certain directions. A slip system represents a slip plane-slip direction combination. • Operable slip systems depend on the crystal structure of the material. The slip plane is that plane that has the densest atomic packing, and the slip direction is the direction within this plane that is most closely packed with atoms. • The slip system for the FCC crystal structure is ; for BCC, several are possible: , , and .

Reccrystallization

The driving force for recrystallization is the difference in internal energy between strained and recrystallized material. • For a cold-worked metal that experiences recrystallization, as temperature increases (at constant heat-treating time), tensile strength decreases and ductility increases (per Figure 7.22). • The recrystallization temperature of a metal alloy is that temperature at which recrystallization reaches completion in 1 h. • Two factors that influence the recrystallization temperature are percent cold work and impurity content. Recrystallization temperature decreases with increasing percent cold work. It rises with increasing concentrations of impurities. • Plastic deformation of a metal above its recrystallization temperature is hot working; deformation below its recrystallization temperature is termed cold working.

Mechanisms of Strengthening in metals

The ease with which a metal is capable of plastic deformation is a function of dislocation mobility—that is, restricting dislocation motion leads to increased hardness and strength.

Recrystallization

The formation of a new set of strain-free grains within a previously cold-worked material; normally, an annealing heat treatment is necessary.

When making hardness measurements, what will be the effect of making an indentation very close to a preexisting indentation? Why?

The hardness measured from an indentation that is positioned very close to a preexisting indentation will be too large. The material in this vicinity was cold-worked when the first indentation was made.

grain growth

The increase in average grain size of a polycrystalline material; for most materials, an elevated-temperature heat treatment is necessary. Grain growth does not need to be preceded by recovery and recrystallization; it may occur in all polycrystalline materials, metals and ceramics alike.

strain hardening

The increase in hardness and strength of a ductile metal as it is plastically deformed below its recrystallization temperature.

Plastic deformation operations are often carried out at temperatures above the recrystallization temperature in a process termed hot working

The material remains relatively soft and ductile during deformation because it does not strain harden, and thus large deformations are possible.

Recovery

The relief of some of the internal strain energy of a previously cold-worked metal, usually by heat treatment.

critical resolved shear stress

The shear stress, resolved within a slip plane and direction, required to initiate slip. It represents the minimum shear stress required to initiate slip and is a property of the material that determines when yielding occurs.

Restricting or hindering dislocation motion renders a material harder and stronger.

The size of the grains, or average grain diameter, in a polycrystalline metal influences the mechanical properties.

Which of the following is the slip system for the simple cubic crystal structure? Why?

The slip system for some crystal structure corresponds to the most densely packed crystallographic plane, and, in that plane, the most closely packed crystallographic direction. For simple cubic, the most densely packed atomic plane is the -type plane; the most densely packed direction within this plane is a type direction. Therefore, the slip system for simple cubic is

Solid Solution Strengthening

The strength and hardness of a metal increase with increase of concentration of impurity atoms that go into solid solution (both substitutional and interstitial). • Solid-solution strengthening results from lattice strain interactions between impurity atoms and dislocations; these interactions produce a decrease in dislocation mobility.

What is dislocation density?

The total dislocation length per unit volume of material; alternatively, the number of dislocations that intersect a unit area of a random surface section. All metals and alloys contain some dislocations that were introduced during solidification, during plastic deformation, and as a consequence of thermal stresses that result from rapid cooling

Metals with FCC or BCC crystal structures have a relatively large number of slip systems (at least 12).

These metals are quite ductile because extensive plastic deformation is normally possible along the various systems. Conversely, HCP metals, having few active slip systems, are normally quite brittle.

Deformation by Twinning

Under some circumstances, limited plastic deformation may occur in BCC and HCP metals by mechanical twinning. The application of a shear force produces slight atomic displacements such that on one side of a plane (i.e., a twin boundary), atoms are located in mirror-image positions of atoms on the other side.

What different strains are present when having an extra half plane in the structure?

With half plane feels a compressive strain, without half plane feels a tension force

twinning??

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, In addition, slip occurs in distinct atomic spacing multiples, whereas the atomic displacement for twinning is less than the interatomic separation, read 7.07 on twinning

How is dislocation related to strength?

a theory of dislocations has evolved that explains many of the physical and mechanical phenomena in metals [as well as crystalline ceramics

What happens when a shear force is applied?

pushed the half plane of atoms to one side and creates an excess plane sticking out of the figure.