Strengthening mechanisms of materials

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.

[citation needed]For this strengthening mechanism, solute atoms of one element are added to another, resulting in either substitutional or interstitial point defects in the crystal (see Figure on the right).

TRIP steels also use C and Mn, along with heat treatment, in order to retain small amounts of austenite and bainite in a ferrite matrix.

Polymers fracture via breaking of inter- and intra molecular bonds; hence, the chemical structure of these materials plays a huge role in increasing strength.

For polymers consisting of chains which easily slide past each other, chemical and physical cross linking can be used to increase rigidity and yield strength.

These cross-links are particularly helpful in improving tensile strength of materials which contain much free volume prone to crazing, typically glassy brittle polymers.

Fillers such as clay, silica, and carbon network materials have been extensively researched and used in polymer composites in part due to their effect on mechanical properties.

For example, the high strength of Kevlar arises from a stacked multilayer macrostructure where aromatic polymer layers are rotated with respect to their neighbors.

[8] Mixing polymers is another method of increasing strength, particularly with materials that show crazing preceding brittle fracture such as atactic polystyrene (APS).

For example, by forming a 50/50 mixture of APS with polyphenylene oxide (PPO), this embrittling tendency can be almost completely suppressed, substantially increasing the fracture strength.

The former variant is exemplified by fiberglass which contains very strong but delicate glass fibers embedded in a softer plastic matrix resilient to fracture.

For a composite containing aligned, stiff fibers which span the length of the material and a soft, ductile matrix, the following descriptions provide a rough model.

In order to have stronger buildings and bridges, one must have a strong frame that can support high tensile or compressive load and resist plastic deformation.

[16] Previous studies observed inconsistent grain size dependence of the strength of graphene at the length scale of nm and the conclusions remained unclear.

Based on the arrangement and energetics of simulated particles, the inverse pseudo Hall-Petch behavior can be attributed to the creation of stress concentration sites due to the increase in the density of grain boundary junctions.

This study explains the previous inconsistent experimental observations and provides an in-depth understanding of the grain boundary strengthening mechanism of nanocrystalline graphene, which cannot be easily obtained from either in-situ or ex-situ experiments.

Shim et al. applied MD simulations to study the precipitate strengthening effects of nanosized body-centered-cubic (bcc) Cu on face-centered-cubic (fcc) Fe.

Thus, Shim et al. simulated coherent bcc Cu precipitates with diameters ranging from 1 to 4 nm embedded in the fcc Fe matrix.

In addition, it also has been observed that the screw dislocation detachment mechanism with the larger, transformed precipitates involves annihilation-and-renucleation and Orowan looping in the twinning and anti-twinning direction, respectively.

[19] Zhang et al. took a step further to combine the first-principle DFT calculations with MD to study the influence of stacking fault energy (SFE) on strengthening, as partial dislocations can easily form in this material structure.

To obtain accurate MD simulation results, it is essential to build a model that properly describes the interatomic potential based on bonding.

To further extend the scale of simulation time, it is common to apply a bias potential that changes the barrier height, therefore, accelerating the dynamics.

Based on the mechanism of strengthening discussed in the previous contents, nowadays people are also working on enhancing the strength by purposely fabricating nanostructures in materials.

Here we introduce several representative methods, including hierarchical nanotwined structures, pushing the limit of grain size for strengthening and dislocation engineering.

Nanoscale twins – crystalline regions related by symmetry have the ability to effectively block the dislocation motion due to the microstructure change at the interface.

However, many researchers have found that the nanocrystalline materials will soften when the grain size decreases to the critical point, which is called the inverse Hall-Petch effect.

The exceptional strength is resulted from the appearance of low-angle grain boundaries, which have low-energy states efficient for enhancing structure stability.

Rupert et al.[26] conducted first-principles simulations to study the impact of the addition of common nonmetallic impurities on Σ5 (310) grain boundary energy in Cu.

Unlike the traditional interstitial strengthening, the introduction of the ordered oxygen complexes enhanced the strength of the alloy without the sacrifice of ductility.

This is a schematic illustrating how the lattice is strained by the addition of interstitial solute. Notice the strain in the lattice that the solute atoms cause. The interstitial solute could be carbon in iron for example. The carbon atoms in the interstitial sites of the lattice creates a stress field that impedes dislocation movement.
This is a schematic illustrating how the lattice is strained by the addition of substitutional solute. Notice the strain in the lattice that the solute atom causes.
Figure 2: A schematic illustrating how the dislocations can interact with a particle. It can either cut through the particle or bow around the particle and create a dislocation loop as it moves over the particle.
Figure 3: A schematic roughly illustrating the concept of dislocation pile up and how it effects the strength of the material. A material with larger grain size is able to have more dislocation to pile up leading to a bigger driving force for dislocations to move from one grain to another. Thus, less force need be applied to move a dislocation from a larger, than from a smaller grain, leading materials with smaller grains to exhibit higher yield stress.