Strain Hardening (Work Hardening)

Strain Hardening (Work Hardening)

Methods have been devised to modify the yield strength, ductility, and toughness of both crystalline and amorphous materials. These strengthening mechanisms give engineers the ability to tailor the mechanical properties of materials to suit a variety of different applications. For example, the favorable properties of steel result from interstitial incorporation of carbon into the iron lattice. Brass, a binary alloy of copper and zinc, has superior mechanical properties compared to its constituent metals due to solution strengthening. Work hardening (such as beating a red-hot piece of metal on anvil) has also been used for centuries by blacksmiths to introduce dislocations into materials, increasing their yield strengths. The ability of a metal to plastically deform depends on the ability of dislocation to move.

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

Phenomenon where ductile metals become stronger and harder when they are deformed plastically is called strain hardening or work hardening. Increasing temperature lowers the rate of strain hardening. Hence materials are strain hardened at low temperatures, thus also called cold working. During plastic deformation, dislocation density increases.  And thus their interaction with each other, resulting in increase in yield stress.

Dislocation density (ρ) and shear stress (τ) are related as follows:

During strain hardening, in addition to mechanical properties physical properties also change:

-a small decrease in density

-an appreciable decrease in electrical conductivity

-small increase in thermal coefficient of expansion

-increased chemical reactivity (decrease in corrosion resistance).

Deleterious effects of cold work can be removed by heating the material to suitable temperatures–Annealing. It restores the original properties into material. It consists of three stages–recovery, re-crystallization and grain growth. In industry, alternate cycles of strain hardening and annealing are used to deform most metals to a very great extent

Precipitation & Dispersion hardening

Foreign particles can also obstruct movement of dislocations i.e. increases the strength of the material. Foreign particles can be introduced in two ways – precipitation and mixing -and- consolidation technique. Precipitation hardening is also called age hardening because strength increases with time.  Requisite for precipitation hardening is that second phase must  be soluble at an elevated temperature but precipitates up on quenching and aging at a lower temperature.

E.g.:Al-alloys ,Cu-Be alloys, Mg-Al alloys, Cu-Sn alloys

If aging occurs at room temperature–Natural aging; If material need to be heated during aging– Artificial aging. In dispersion hardening, fine second particles are mixed with matrix powder, consolidated, and pressed in powder metallurgy techniques. For dispersion hardening, second phase need to have very low solubility at all temperatures.

E.g.: oxides, carbides, nitrides, borides etc.

Dislocation moving through matrix embedded with foreign particles can either cut through the particles or bend around and bypass them. Cutting of particles is easier for small particles which can be considered as segregated solute atoms. Effective strengthening is achieved in the bending process, when the particles are submicroscopic in size

Fiber strengthening

Second phase can be introduced into matrix in fiber form too.

Requisite for fiber strengthening:

Fiber material – high strength and high modulus

Matrix material – ductile and non-reactive with fiber material

E.g.: fiber material – Al2O3, boron, graphite, metal, glass, etc;    matrix material – metals, polymers

Mechanism of strengthening is different from other methods.

Higher modulus fibers carry load, ductile matrix distributes load to fibers. Interface between matrix and fibers thus play an important role. Strengthening analysis involves application of continuum, not dislocation concepts as in other methods of strengthening

Cold work will lead to:

•       Increase of Yielding Strength

•       Increase of Tensile Strength

•       Reduction of Elongation

•       Material becomes stronger but more brittle

Plastic deformation occurs when large numbers of dislocations move and multiply so as to result in macroscopic deformation. In other words, it is the movement of dislocations in the material which allows for deformation. If we want to enhance a material's mechanical properties (i.e. increase the yield and tensile strength), we simply need to introduce a mechanism which prohibits the mobility of these dislocations. Whatever the mechanism may be, (work hardening, grain size reduction, etc.) they all hinder dislocation motion and render the material stronger than previously.

The stress required to cause dislocation motion is orders of magnitude lower than the theoretical stress required to shift an entire plane of atoms, so this mode of stress relief is energetically favorable. Hence, the hardness and strength (both yield and tensile) critically depend on the ease with which dislocations move. Pinning points, or locations in the crystal that oppose the motion of dislocations, can be introduced into the lattice to reduce dislocation mobility, thereby increasing mechanical strength.

Dislocations may be pinned due to stress field interactions with other dislocations and solute particles, creating physical barriers from second phase precipitates forming along grain boundaries. There are four main strengthening mechanisms for metals, each is a method to prevent dislocation motion and propagation, or make it energetically unfavorable for the dislocation to move. For a material that has been strengthened, by some processing method, the amount of force required to start irreversible (plastic) deformation is greater than it was for the original material.

In amorphous materials such as polymers, amorphous ceramics (glass), and amorphous metals, the lack of long range order leads to yielding via mechanisms such as brittle fracture, crazing, and shear band formation. In these systems, strengthening mechanisms do not involve dislocations, but rather consist of modifications to the chemical structure and processing of the constituent material.

The strength of materials cannot infinitely increase. Each of the mechanisms explained below involves some trade-off by which other material properties are compromised in the process of strengthening.