Hardening of strain hardening is a situation where a material resists further deformation. Hardness, by definition is resistance to plastic deformation.

Before addressing the strain hardening mechanism, I would like to clear some broad misconceptions.

For most conventional materials, it is claimed, often incorrectly, that hardness and brittleness go hand in hand. But hardness does not necessarily imply brittleness. Remember that both of them have very specific definitions. A solid is considered brittle when it offers no intrinsic resistance to crack propagation. However, hardness is simply resistance to plastic deformation. Although a very stubborn resistance to plastic deformation does lead to brittle behavior, the process of hardening is actually quite important for structural reliability. Any material which has higher hardenability, is actually more ductile. For example, metals that undergo strain hardening are more ductile than those which strain soften. Because, if materials tested in tension strain soften or are perfectly plastic (flow stress is constant) will start undergoing necking and fail prematurely. This is because the reduction in cross section area elevates the stress level in the specimen and since the material cannot strain harden, it cannot sustain the higher stress levels. Therefore, strain hardening is necessary for improving ductility. For instance, pure Cu undergoes strain hardening when loaded in tension and this is the reason why it can be drawn into wires.

This whole process should not be confused with the age old materials science adage that ductility and yield strength have an inverse relationship.

The adage specifically refers to the materials like ceramics and glasses which have a high yield strength but are highly brittle. Note that it does not refer to the hardenability or strain hardening ability of a material. In ceramics, the yield strength is higher than that of metals because the Pierels stress required to move dislocations is very high. Remember that one of the first requirements for plastic flow is that dislocations should be mobile. Moreover, most ceramics have covalent bonds which are intrinsically directional. The directional rigidity of the bonds prevents a dislocation from treading smoothly in the lattice as it involves a lot of distortion. Hence, the inability to move existing dislocations makes ceramics really hard (resistance to plastic flow).

On straining such a ceramic material beyond its yield point it fractures in a brittle fashion. To understand this, note that all materials, including metals and ceramics, will have some finite sized flaws or stress concentrators (or inclusions or pores) within them. These flaws raise the stress levels in their vicinity. If dislocations in the material had the ability to move, their motion could alleviate the local rise in stresses and strains and help in dissipating the crack driving force. However, since the dislocations in ceramics are immobile, the crack becomes critical and runs catastrophically through the material.

In a ductile metal, cracks cannot propagate easily because the area ahead of the crack tip allows dislocation motion and the material can plastically flow. However, there are two possibilities now. If the material strain hardens due to some barrier for dislocation motion, the area around the crack tip strain- hardens. As strain hardened material can bear more load, the envelope over which plasticity can be sustained will increase. This facilitates more fracture energy dissipation in the material. Hence the whole strain hardening process and dislocation motion in this zone absorbs the crack driving force and arrests its propagation. On the other hand, strain softening materials allow strains to localize ahead of the crack tip, which is not good as the fracture energy is not dissipated in a wider region. Such a crack will soon accumulate enough strain and propagate catastrophically (assuming that this solid follows the critical strain based fracture criterion). Similarly, when a strain hardening material cannot undergo further hardening, the material cannot flow and dissipate fracture energy. This again leads to strain accumulation mediated crack propagation.

The purpose of this digression was to impress upon people that fact that strain hardening ability of materials is actually important for improving the ductility of a material.

Now I will address how and why strain hardening takes place.

It must be clear by now that strengthening or hardening can be achieved by resisting plastic flow, Since plastic flow is mediated by dislocation slip, to facilitate strain hardening, dislocation motion must be resisted as much as possible. Strain hardening mainly arises from Dislocation- Dislocation interactions that reduces their mobility. Since each dislocation is associated with stress fields around them, they can either repel or attract each other. Dislocations on the same plane with the same Burgers vector repel each other and serve as mutual barriers for their individual motion. When a material is deformed, numerous dislocations are emitted from a source and are forced to interact with each other. A higher dislocation density in a metal or alloy implies that it has already undergone strain hardening. Sometimes a higher dislocation content is not good as dislocations of opposite signs can attract each other and mutually annihilate. This leads to recovery or softening of the material. Recovery is typically observed in cold-worked copper.

There are other strengthening mechanisms such as grain boundary strengthening, particulate strengthening, solid-solution strengthening, all of which involves different ways to impede the motion of dislocations. However, strain hardening per se is facilitated by dislocation-dislocation interactions.

Hi,

Work hardening, also known as strain hardening, is the strengthening of a metal or polymer by plastic deformation. This strengthening occurs because of dislocation movements and dislocation generation within the crystal structure of the material.

Please, refer to below book, for finding the clear idea and understanding the dislocation movements phenomenon.

Book: Mechanical Metallurgy

Author: E. Dieter,

Bests,

When one applies a plastic deformation to a metal, the plastic deformation causes the dislocations in the metal start moving. Movement of the dislocations causes them to accumulate and finally trapped at different sites. Once the dislocations are trapped, they cannot move anymore leading the metal to behave stiffer. If all dislocations are trapped and they cannot move at all, at that point metals behaves like a brittle materials.

You may wanna take a look at this website:

http://www.virginia.edu/bohr/mse209/chapter7.htm

Best,

Reza.

The dislocations tangle together, therefore, accumulate in different spots.

Hardening of strain hardening is a situation where a material resists further deformation. Hardness, by definition is resistance to plastic deformation.

Before addressing the strain hardening mechanism, I would like to clear some broad misconceptions.

For most conventional materials, it is claimed, often incorrectly, that hardness and brittleness go hand in hand. But hardness does not necessarily imply brittleness. Remember that both of them have very specific definitions. A solid is considered brittle when it offers no intrinsic resistance to crack propagation. However, hardness is simply resistance to plastic deformation. Although a very stubborn resistance to plastic deformation does lead to brittle behavior, the process of hardening is actually quite important for structural reliability. Any material which has higher hardenability, is actually more ductile. For example, metals that undergo strain hardening are more ductile than those which strain soften. Because, if materials tested in tension strain soften or are perfectly plastic (flow stress is constant) will start undergoing necking and fail prematurely. This is because the reduction in cross section area elevates the stress level in the specimen and since the material cannot strain harden, it cannot sustain the higher stress levels. Therefore, strain hardening is necessary for improving ductility. For instance, pure Cu undergoes strain hardening when loaded in tension and this is the reason why it can be drawn into wires.

This whole process should not be confused with the age old materials science adage that ductility and yield strength have an inverse relationship.

The adage specifically refers to the materials like ceramics and glasses which have a high yield strength but are highly brittle. Note that it does not refer to the hardenability or strain hardening ability of a material. In ceramics, the yield strength is higher than that of metals because the Pierels stress required to move dislocations is very high. Remember that one of the first requirements for plastic flow is that dislocations should be mobile. Moreover, most ceramics have covalent bonds which are intrinsically directional. The directional rigidity of the bonds prevents a dislocation from treading smoothly in the lattice as it involves a lot of distortion. Hence, the inability to move existing dislocations makes ceramics really hard (resistance to plastic flow).

On straining such a ceramic material beyond its yield point it fractures in a brittle fashion. To understand this, note that all materials, including metals and ceramics, will have some finite sized flaws or stress concentrators (or inclusions or pores) within them. These flaws raise the stress levels in their vicinity. If dislocations in the material had the ability to move, their motion could alleviate the local rise in stresses and strains and help in dissipating the crack driving force. However, since the dislocations in ceramics are immobile, the crack becomes critical and runs catastrophically through the material.

In a ductile metal, cracks cannot propagate easily because the area ahead of the crack tip allows dislocation motion and the material can plastically flow. However, there are two possibilities now. If the material strain hardens due to some barrier for dislocation motion, the area around the crack tip strain- hardens. As strain hardened material can bear more load, the envelope over which plasticity can be sustained will increase. This facilitates more fracture energy dissipation in the material. Hence the whole strain hardening process and dislocation motion in this zone absorbs the crack driving force and arrests its propagation. On the other hand, strain softening materials allow strains to localize ahead of the crack tip, which is not good as the fracture energy is not dissipated in a wider region. Such a crack will soon accumulate enough strain and propagate catastrophically (assuming that this solid follows the critical strain based fracture criterion). Similarly, when a strain hardening material cannot undergo further hardening, the material cannot flow and dissipate fracture energy. This again leads to strain accumulation mediated crack propagation.

The purpose of this digression was to impress upon people that fact that strain hardening ability of materials is actually important for improving the ductility of a material.

Now I will address how and why strain hardening takes place.

It must be clear by now that strengthening or hardening can be achieved by resisting plastic flow, Since plastic flow is mediated by dislocation slip, to facilitate strain hardening, dislocation motion must be resisted as much as possible. Strain hardening mainly arises from Dislocation- Dislocation interactions that reduces their mobility. Since each dislocation is associated with stress fields around them, they can either repel or attract each other. Dislocations on the same plane with the same Burgers vector repel each other and serve as mutual barriers for their individual motion. When a material is deformed, numerous dislocations are emitted from a source and are forced to interact with each other. A higher dislocation density in a metal or alloy implies that it has already undergone strain hardening. Sometimes a higher dislocation content is not good as dislocations of opposite signs can attract each other and mutually annihilate. This leads to recovery or softening of the material. Recovery is typically observed in cold-worked copper.

There are other strengthening mechanisms such as grain boundary strengthening, particulate strengthening, solid-solution strengthening, all of which involves different ways to impede the motion of dislocations. However, strain hardening per se is facilitated by dislocation-dislocation interactions.

It is a very interested question

I agree with all above answer especially Narayan's answer

The hardness of the material is related to the ability of a material to deform elastically or plastically, more a material become harde, the more difficult it is to deform and it causes dislocation

Hardness is a materials resistance to "localized" plastic deformation (wear, scuffs, scratches, dings, etc.). The concept, and the Mohs Scale, predates the bulk of our understanding of material microstructure. Don't confuse this with resistance to gross plastic deformation, which is yield. Hardness and yield strength are related. They both tend to go up/down together, but very hard materials fail in crack propagation before yield.

As far as I understand, strain hardening increases hardness and strength, together, by several mechanisms:

1. Dislocation multiplication and entanglement (as stated)

2. Dislocations become trapped against grain boundaries

3. Dislocations are pinned around impurities that cause internal lattice stress

4. Dislocations are blocked by precipitates

All of these happen beyond the yield stress, and the stress-strain curve for a given material must show increased stress after yield for effective hardening as the yield stress is being increased for the next loading cycle.

Hardening due to mechanical working is primarily a process of movement of dislocations. These dislocations may be blocked at the points of obstructions or may be entangled or accumulated at grain boundaries. Since the density of dislocation has increased at grain boundaries and stacking of these dislocations is fairly random, the poly crystalline metallic material becomes stiffer.

Further, fast rate of cooling do not allow diffusion to take place in totality and equilibrium is not attained. It results in distortion and straining in the crystal lattice bringing hardness and strength. Conversion from austenite to martensite is a good example in case of iron.

Heat treatment processes such as tempering, normalizing or annealing as required may be utilized to restore the ductility as per the need.

The fast neutron irradiation of the pressure vessel of many types of nuclear reactors leads to the hardening and embrittlement of the steel used for manifacturing the vessel. The reason is the dislocation. (The words in bold are taken from the question.)

A good handbookwith the topical information (published in 2017) can be downloaded from the IAEA (International Atomic Energy Agency of UN). It contains the description of the effects of the irradiation procedure, as part of the ageing, together with thermal &c procedures. The treatment of those is described, as well.

https://www-pub.iaea.org/MTCD/Publications/PDF/P1738_web.pdf