che 333 class 17 strengthening of metals.. strengthening at cold temperatures metals – basically...

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CHE 333 Class 17 Strengthening of Metals.

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Page 1: CHE 333 Class 17 Strengthening of Metals.. Strengthening at COLD temperatures Metals – basically all work in same way which is to block dislocations or

CHE 333 Class 17

Strengthening of Metals.

Page 2: CHE 333 Class 17 Strengthening of Metals.. Strengthening at COLD temperatures Metals – basically all work in same way which is to block dislocations or

Strengthening at COLD temperatures

Metals – basically all work in same way

which is to block dislocations or retard

Them.

Remember – COLD is less than 0.3 Tm in

Kelvin

Page 3: CHE 333 Class 17 Strengthening of Metals.. Strengthening at COLD temperatures Metals – basically all work in same way which is to block dislocations or

Strengthening MechanismsTo optimize properties of metals, greater strengths can be achieved by several techniques:-

1. Cold Working.

2. Grain Size Control

3. Solution Strengthening

4. Second Phases.

5. New Phases.

These are the engineering alloys that are used for structural applications. Pure materials are used for electronic and electrical applications or chemical applications.

Page 4: CHE 333 Class 17 Strengthening of Metals.. Strengthening at COLD temperatures Metals – basically all work in same way which is to block dislocations or

Cold Working

As the number of dislocations increase they interact and block each other. The first dislocations will bethe ones nearest 450 to the applied stress where theresolved shear stress is greatest. Therefore whenanother slip system needs to be activated, the applied stress must be increased to reach the critical resolved shear stress on a new slip plane. So to increase strain, the stress must be increased. Plastic deformationresults and so work hardening occurs and the yield stress effectively increased, strengtheningthe metal.

Page 5: CHE 333 Class 17 Strengthening of Metals.. Strengthening at COLD temperatures Metals – basically all work in same way which is to block dislocations or

Dislocation Interactions1

2

2

3

After 3, another slip system needs to be activatedby increasing the applied stress and so meeting the critical resolve shear stress on a new slip system.

Page 6: CHE 333 Class 17 Strengthening of Metals.. Strengthening at COLD temperatures Metals – basically all work in same way which is to block dislocations or

Grain Size Control.Grain boundaries block dislocation motionas they change the orientation of slip planeswith respect to the applied stress. As the firstslip system activated will be the one nearest 45o then all others will need more applied stressto reach the critical resolved shear stresseson planes which are not near 450 to the applied stress.In the figure, a dislocation is blocked by the grainboundary as the (111) planes in the next grainare not at 450 to the stress applied. This will leadto a “dislocation pile up” where many dislocationsget blocked on the slip plane. This “pile up” creates a stress build up in the next grain, addingto the applied stress and so initiating slip in thenext grain. The smaller the grain size, the fewerdislocations in the pile up and the higher theapplied stress to cause further slip. So the smallerthe grain size the higher the mechanical strength

(111)

s

45o

30o

Page 7: CHE 333 Class 17 Strengthening of Metals.. Strengthening at COLD temperatures Metals – basically all work in same way which is to block dislocations or

Hall Petch EquationEmpirical Equation relating yield

strength to grain size.

sy = so + kd -1/2

sy = yield stress for polycrystaline

so = yield stress single crystal

k = constant

d = grain size.

The smaller the grain size the higher

the yield stress. Grain size can be controlled

by recrystallization and other techniques.

Page 8: CHE 333 Class 17 Strengthening of Metals.. Strengthening at COLD temperatures Metals – basically all work in same way which is to block dislocations or

Solution Strengthening.Add a solute to the metal, such as zinc to copper to create brass. The zinc atoms are

a different size and so affect dislocations.

Around a dislocations stress and

strain fields exist, compressive

above the slip plans and tensile

below it. A small atom can reduce

the compressive stress field

while a large atom can reduce

the tensile stress field. The applied

stress to move a dislocation will

therefore increase if the internal

stress field is decreased. The limit for

this is the Hume Rothery rules which

limit the amount of solute before

second phases form.

Compressive

Tensile

Page 9: CHE 333 Class 17 Strengthening of Metals.. Strengthening at COLD temperatures Metals – basically all work in same way which is to block dislocations or

Dislocation Locking

For steels, there is an upper andlower yield point. This is due to carbon in interstitial sites “locking”the dislocations in place. The smallatoms reduce the strain energy of adislocation. It then requires moreexternal energy in the form of stressto move the dislocation.Once the dislocation is free from thethe local carbon atom, less stressis required to move it.

Page 10: CHE 333 Class 17 Strengthening of Metals.. Strengthening at COLD temperatures Metals – basically all work in same way which is to block dislocations or

Dislocation FrictionSolute atoms have strainfields associated withthem. As a result, asdislocations move past solute atoms, the energyof the dislocation islowered and more stressis required to keep it in motion. This increasesthe UTS of a material but not the yield stress.

Stress

Dislocation friction raises plastic deformation curve

Strain

Page 11: CHE 333 Class 17 Strengthening of Metals.. Strengthening at COLD temperatures Metals – basically all work in same way which is to block dislocations or

Second Phases.The presence of second phases will strengthena material by blocking dislocation motion, and requiring increased applied stress to produce strain.Second phases all work in the same mannerby blocking dislocation, their effectivenessdepends on the second phase distribution.The spheroidal structure will be much weakerthan the eutectoid structure. The strength of theeutectoid is a function of cooling rate, faster cooling the plates are narrower and the strength is higher than slow cooling rateswith wider plate spacing.Aluminum alloys – age hardening producesoptimum properties – small particles whichinteract with dislocations very effectively.

Page 12: CHE 333 Class 17 Strengthening of Metals.. Strengthening at COLD temperatures Metals – basically all work in same way which is to block dislocations or

Second Phases

Second Phase ParticleDislocation mobile

Dislocation pinned by particle

t

t = (Gb)/R

t = shear stress to keep dislocation movingR = radius of curvature of dislocationsAs R decreases, t increases so strengthening the material.R decreases as the particles are closer together, so the distribution is important

R

Page 13: CHE 333 Class 17 Strengthening of Metals.. Strengthening at COLD temperatures Metals – basically all work in same way which is to block dislocations or

Dislocation Motion Large Particle Spacing – overaged.

t

Small spacing R smallt will be large - peak age

Large spacing, R smaller, and dislocation can bend around particles and rejoin.

Dislocation loop after dislocation passes.

Underage – GP zones areFCC, like matrix so don’t block dislocations- solutionstrengthening only.

Page 14: CHE 333 Class 17 Strengthening of Metals.. Strengthening at COLD temperatures Metals – basically all work in same way which is to block dislocations or

New Phases.Best example would be steel transformation to martensite. Other alloy systems are also

capable of this type of diffusionless transfer such as titanium alloys Ti-6Al-4V, Fe-Ni

alloys. In this case the crystal structure is one that has a very high critical resolved shear

stress such as body centered tetragonal.

Other structures can produce high strength such as “amorphous” or “glassy” metals.

These metal alloys systems are quenched very rapidly, at a rate of several thousand

degrees per second. In this case, the resulting structures are not crystalline and so

have few dislocations and behave elastically to higher yield stresses.

Ni-Ti systems are a good example of these. They are metastable, and cannot be used at

temperature otherwise they gradually revert to their equilibrium crystal structure.

Page 15: CHE 333 Class 17 Strengthening of Metals.. Strengthening at COLD temperatures Metals – basically all work in same way which is to block dislocations or

Homework

• It the temperature is increased from 0.5 to 0.8 of Tm (K) what effect does this have on the recrystallization process?

• For aluminum estimate the shear stress required to move a dislocation when R =20nm – use half the elastic modulus as the shear modulus. What is the effect of decreasing the radius of curvature of the dislocation?