Material Science: Strengthening Mechanisms and Fracture

1)SOLID SOLUTION HARDENING: addition of strange elements inthelattice produces high distortions and reduce the mobilityofdislocations.

2)PRECIPITATION HARDENING: we introduce precipitates in the material.If the part are very hard,dislocations cant break and form new bonds, so they stop moving.

3)STRAIN: when a material is plastically deformed,the dislocation density increases,making it harder.

4)GRAIN SIZE EFFECT:the smaller the grain size, the tougher the mat is.Grain limits act as barriers to cracks propagation.

1)HOMOGENEOUS NUCLEATION:it takes place when the system doesnt pose any heterogeneity where nucleation can occur in a preferential way.

2)HETEROGEOUS:the nucleation process can suffer a catalytic effect driven by the pressure of heterogeneities.HET NUCL is faster and easier than...And occurs at smaller deviations form equilibrium. It is much more common in the real world than the homogeneous nucleation.

DUCTILE TO BRITTLE TRANS TEMP: minimum temp in which a given mat has the ability to absorb a specific amount of energy without fracturing. As temperature decreases,the material ability to deform in a ductile matter decreases.

PHASE TRANSFORMATION: NUCLEATION and GROWTH (of the new phase(s)). HIGH T--> low nucleation and high growth rates(large grains). LOW T--> high nucleation and low growth rates (small grains).

Some materials react to the external loads creating new surfaces --> BREAKING. Different ways to react to applied loads: ELASTIC RESPONSE(reversible),PLASTIC(dissipating energy),FRACTURE(new surfaces)

TOUGHNESS is the ability of materials to absorbe energy before breaking. Tough mat(deform plastically,metals). BRITTLE materials(none or little plastic deform). Gc--> FRACTURE ENERGY[J/m2]

When the energy release rate exceeds the fracture energy of the material, cracks propagate.

If they broke in different number of pieces that is because they have different fracture energy.

YIELD CRITERION: is a hypothesis defining the limit of elasticity in a material and the onset of plastic deformation under any possible combination of stress (TRESCA,VON MISES)

TRESCA --> Material yiels when the maximun shear stress exceeds a critical value. 

VON MISES --> Material yields when a particular function on the applied stresses exceeds a critical value.

The ductile-fragile transition occurs because the mobility of the dislocations and the resistance to plastic deformation depends on the temperature. At low temperatures, dislocations have low movility, so the material has a high resistance to plastic deformation. However, at high temp the mobility of the dislocatons increase so the material begins to deforming plastically more easily, making the beahaviour ductile (it absorbs a lot of energy by plastic deformation before breaking).

BRAG´S LAW --> consttructive interference --> diffracted waves are in phase.