Strengthening Mechanisms

how do dislocation density reduce at high temperatures

both atomic diffusion and vacancy concentration are dependent on high temperatures. The increase in both atomic diffusion and vacancy concentration will combine and reduce the dislocation density

what is happening during the grain growth stage of annealing

as time passes, these smaller grains will shrink and disappear as larger grains continue to grow

why are alloys stronger than pure metals

because impurities reduces dislocation motion

where do small and large impurities concentrate at the host material

smaller impurities will concentrate at regions of compressive strains and larger impurities will concentrate at regions of tensile strains

What is happening during the recrystallization stage of annealing

smaller new grains that have low dislocation density will start to consume and eventually replace the cold-worked grains as time passes

Define solid solution strengthening

the impurity atoms added into material will distort the lattice and generate lattice strains. these strains can act as barriers to dislocation motion

what will happen if dislocation motion is being shut down completely

the material will become brittle like ceramics

what is recrystallization temperature

the temperature at which recrystallization just reaches completion in 1 hour

how does strain/work hardening reduce dislocation motion

when a material is cold worked, its dislocation will multiply and entangle one another, making dislocation motion difficult

how to calculate % cold worked of a material

((original area - deformed area) / original area) * 100%

three method to strengthen (hinder dislocation movement) the material

1) grain boundary strengthening - by reducing the grain size 2) solid solution strengthening - adding impurities to material 3) strain/work hardening - cold and hot working

three stages of annealing

1) recovery (thermal softening process) 2) recrystallization 3) Grain growth

explain why polycrystals such as ceramics have higher strength than single crystals like pure metals

At temperature below 0.5 melting point, grain boundaries act as barriers to dislocation motion. The reason why polycrystalline materials have higher strength single crystals is that they have multiple grain boundaries at different orientations. and a high amount of stress is required for dislocation motion cross grain boundaries while single crystals have grain boundaries all at the same orientation, requiring less stress to cross the grain boundaries

what is Hall-Petch relation

It is the linear relation of the yield strength and the grain size of the material

Why do we want to strengthen materials

Some materials have ideal properties for certain applications but they have low strength. That is why we increase the yield strength of the material

how do we increase the yield strength of the material

increasing its intrinsic resistance to dislocation motion

how do substitutional impurities distort the lattice of the host material

both the smaller and larger substitutional impurity will distort the lattice of the host materials due to their size. and the difference in size will result in an increase of attractive potential energy and repulsion potential energy respectively generating local stress to its adjacent atoms and opposing the motion of dislocation.

how do strain/work hardening works?

by plastically deforming the material at room temperature, reducing its cross sectional area

what is the difference between cold working and hot working

cold working is a process that deform materials at a temperature below recrystallization temperature, Tr hot working process deforms materials at temperature above recrystallization temperature, Tr

partial cancellation of dislocation due to compressive and tensile strains reduces

dislocation mobility

what is the relation of dislocation mobility and yield strength

dislocation motion causes the metal to yield(plastically deform). Therefore, the easier it is for dislocation to move, the lower the strength of the material

what property is sacrificed for hindering dislocation motion

ductility is being sacrificed for the increase in strength of the material.

what is the process of intermittent annealing

heating the cold worked metal to high temperature and holding it for a pre-determined period of time to restore its ductility

what is the temperature range for recrystallization to complete in 1 hour

in between 0.3 melting point and 0.6 melting point 0.3Tm < Tr < 0.6Tm

How do we reduce a large pure metal into a smaller desired dimension by cold working without it fracturing

it is impossible to to reduce the size of a material drastically by just one cycle of cold working. The material has to be cold worked several times to achieve its ideal size but every time the material is cold worked, it becomes brittle. To prevent it from failing during the cold working process, we put the material under a process called intermittent annealing to restore ductility.

define grain boundary strengthening

it is to the reduce the size of the grain boundary by hindering or slowing down dislocation motion as grain boundaries acts as barriers. More barriers means more stress is needed to for dislocation motion. hence higher strength

what happens to the material during the recovery stage of annealing

it will reduce dislocation density by annihilation

what are the two scenarios for reduction of dislocation density by annihilation

results from diffusion and climbing

are small-grained or large-grained metals stronger

small-grained metals as they have more grain boundaries(barriers) and it requires more stress for dislocation to move across the grain boundary.