Question

for materials class

Manipulating strength

a. Using words and visuals, briefly describe 4 strategies to increase the strength of a metal.

b. What do these strategies have in common in terms of their mechanism?

c. Use a plot to discuss the trade-off between strength and ductility in metals. Explain why this trade off occurs based on the mechanism of plastic deformation in metals.

Answer 1

strengthening of a metal is that , preventing dislocation motion and propagapropagation.Preventing dislocation motion renders a material harder and stronger .

There are four strategies to increase the strength of metal they are

1) grain size reduction

2)solid solution alloying

3)strain hardening (cold working)

4) annealing

Grain size reduction

The grain size of a polycrystal line metal influence the mechanical properties.Ajacent grains normally have different crystallographic orientations and a common grain boundary.

The grain boundary act as a barrier to dislocation motion for two reasons:

1) since the two grains are of different orientations,a dislocation passing into grain B must change its direction of motion .This becomes more difficult as the crystallographic misorientation increase.

2)The atomic disorder with in a grain boundary region results in a discontinuity of slip plane from one grain into the other.

for many materials the yield strength sigma(y) varies with grain size according to

sigma (y)= sigma(0)+k(y)d^-1/2

this expression is termed as Hall Perch equation.where d is the average grain diameter

small angle grain boundaries are not effective in interfering with the slip process because bof the slight crystallographic misalignment across the boundary.2) solid solution alloying

This is a technique to strengthen and harden metal with impurity atoms that go into either substitutional or interstitial solid solution.

Alloys are stronger than pure metals because impurity atoms that go into solid solution typically impose lattice strain in the surrounding host atoms.Lattice strain field interaction between dislocation and these impurity atoms result and consequently dislocation motion is restricted.The resistance to slip is greater when impurity atoms are present because the overall lattice strain must increase if a dislocation is torn away from them

3) strain hardening

It is a phenomenon by which a ductile metal becomes harder and stronger as it is plastically deformed.it is also called work hardening because the temperature at which deformation take place is cold relative to absolute melting temperature of the metal,cold working.

Percent cold work is defined as

%CW= ((A0-Ad)÷Ad)×100

The dislocation density in a metal increase with cold working because of dislocation multiplication.4) Annealing

It is process in which materials is heated to above the recrystallization temperature of the sample and then cooled down.

It involves 3steps :

recovery

Recrystallization

Grain growth

During recovery some of the stored internal strain energy is relieved by virtue of dislocation motion as a result of the enhanced atomic diffusion at the elevated temperature. Recrystallization is the formation of a new set of strain free and equiaxed grain that have low dislocation densities and are characteristics of the precold worked condition. After recrystallization is complete the strain free grains will continue to grow if the metal specimen is left at the elevated temperature this phenomenon is called grain growth.

b) restricting or hindering dislocation motion renders a material harder and stronger.

c)