Minggu Ke-8 Mekanisme Penguatan.pdf

Mekanisme Penguatan (Strengthening Mechanisms)

Strengthening • The ability of a metal to deform plastically depends on the ability of dislocations to move. • Hardness and strength are related to how easily a metal plastically deforms, so, by reducing dislocation movement, the mechanical strength can be improved. • If dislocation movement is easy (unhindered), the metal will be soft, easy to deform. Program Studi Teknik Pengelasan - PPNS

Introduction • Engineering alloys are designed to have maximum strength with some ductility and toughness. • Plastic deformation depends on the ability of dislocations to move. • All strengthening mechanisms rely on restricting the motion of dislocations. • Mechanisms of strengthening are: •Grain boundary strengthening •Solid Solution strengthening •Strain (Work) hardening •Precipitation Strengthening •Steel Alloys Strengthening

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Dislocation Motion Dislocations & plastic deformation Cubic & hexagonal metals - plastic deformation is by plastic shear or slip where one plane of atoms slides over adjacent plane by defect motion (dislocations).

If dislocations don't move, deformation doesn't occur! Program Studi Teknik Pengelasan - PPNS

STRENGTHENING BY GRAIN SIZE REDUCTION • Grain boundaries are barriers to slip.

• Barrier "strength" increases with Increasing angle of misorientation. Adapted from Fig. 7.14, Callister 7e. (Fig. 7.14 is from A Textbook of Materials Technology, by Van Vlack, Pearson Education, Inc., Upper Saddle River, NJ.)

• Smaller grain size: more barriers to slip.

• Hall-Petch Equation:

yield  o  k y d 1/ 2

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STRENGTHENING BY GRAIN SIZE REDUCTION Hall-Petch equation:

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STRENGTHENING BY GRAIN SIZE REDUCTION Grain Size Reduction Techniques: • Increase Rate of solidification from the liquid phase.

• Perform Plastic deformation followed by an appropriate heat treatment. • Adding grain refiner (e.g: TiB) during melting. The grain refiner acts as 'nuclei' for solidification, resulting in finer grains. • Thermomechanical treatment (TMT): severe hot deformation in the austenitic condition followed by controlled cooling, so that g → a transformation occurs with minimum grain growth. Program Studi Teknik Pengelasan - PPNS

SOLID-SOLUTION STRENGTHENING  Impurity atoms distort the lattice & generate stress.  Stress can produce a barrier to dislocation motion. • Smaller substitutional impurity

• Larger substitutional impurity

A

C

B

Impurity generates local stress at A and B that opposes dislocation motion to the right. Program Studi Teknik Pengelasan - PPNS

D

Impurity generates local stress at C and D that opposes dislocation motion to the right.

SOLID-SOLUTION STRENGTHENING

Adapted from Fig. 7.4, Callister 7e.

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SOLID-SOLUTION STRENGTHENING small impurities tend to concentrate at dislocations on the “Tensile stress” side reduce mobility of dislocation  increase strength

Adapted from Fig. 7.17, Callister 7e.

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SOLID-SOLUTION STRENGTHENING Large impurities concentrate at dislocations on “Compressive Stress” side – pinning dislocation

Adapted from Fig. 7.18, Callister 7e.

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SOLID-SOLUTION STRENGTHENING Ex: Solid Solution Strengthening in Copper

400 300

200 0 10 20 30 40 50

Yield strength (MPa)

Tensile strength (MPa)

• Tensile strength & yield strength increase with wt% Ni.

wt.% Ni, (Concentration C)

• Alloying increases y and TS. Program Studi Teknik Pengelasan - PPNS

180 120 60

0 10 20 30 40 50 wt.%Ni, (Concentration C)

Adapted from Fig. 7.16 (a) and (b), Callister 7e.

Strain hardening (work hardening) • Strain hardening (work hardening) is where a material becomes less ductile, harder and stronger with plastic deformation. • Encountered during cold working • Percentage cold work can be expressed as:

( A0  Ad ) %CW  x100% A0 Ao = original cross-sectional area Ad = deformed cross-sectional area Program Studi Teknik Pengelasan - PPNS

Strain hardening (work hardening) COLD WORK (%CW) • Room temperature deformation. • Common forming operations change the cross sectional area: force -Forging

die Ao blank

-Drawing die Ao die

Ad

Adapted from Fig. 11.7, Callister 6e.

force Ad

-Rolling

-Extrusion tensile force

Ao  Ad %CW  x100 Ao

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Strain hardening (work hardening) ANISOTROPY IN yield • Can be induced by rolling a polycrystalline metal

-before rolling

-after rolling Adapted from Fig. 7.11, Callister 6e. (Fig. 7.11 is from W.G. Moffatt, G.W. Pearsall, and J. Wulff, The

Structure and Properties of Materials, Vol. I, Structure, p. 140, John Wiley and Sons, New York, 1964.)

rolling direction

235 mm

-isotropic

-anisotropic

since grains are approx. spherical & randomly oriented.

since rolling affects grain orientation and shape.

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Strain hardening (work hardening) During Cold Work • Ti alloy after cold working: • Dislocations entangle and multiply • Thus, Dislocation motion becomes more difficult.

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Adapted from Fig. 4.6, Callister 7e. (Fig. 4.6 is courtesy of M.R. Plichta, Michigan Technological University.)

Strain hardening (work hardening) Impact of Cold Work As cold work is increased • Yield strength (y) increases. • Tensile strength (TS) increases. • Ductility (%EL or %AR) decreases.

Low-Carbon Steel! Adapted from Fig. 7.20, Callister 7e.

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Strain hardening (work hardening) Cold Work Analysis • What is the tensile strength & ductility after cold working? Copper Cold Work

D d =12.2mm

D o =15.2mm

%CW 

ro2  rd2 ro2

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x 100  35.6%

Strain hardening (work hardening) Cold Work Analysis • What is the tensile strength & ductility after cold working to 35.6%? yield strength (MPa)

tensile strength (MPa)

700

800

500

600

300 100 0

Cu 20

40

% Cold Work

60

60 40 20

400 340MPa 200 0

YS = 300 MPa

ductility (%EL)

20

Cu 40

60

% Cold Work TS = 340MPa

Cu

7%

00

20

40

60

% Cold Work %EL = 7%

Adapted from Fig. 7.19, Callister 7e. (Fig. 7.19 is adapted from Metals Handbook: Properties and Selection: Iron and Steels, Vol. 1, 9th ed., B. Bardes (Ed.), American Society for Metals, 1978, p. 226; and Metals Handbook: Properties and Selection: Nonferrous Alloys and Pure Metals, Vol. 2, 9th ed., H. Baker (Managing Ed.), American Society for Metals, 1979, p. 276 and 327.)

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Strain hardening (work hardening) - e Behavior vs. Temperature • Results for polycrystalline iron:

Adapted from Fig. 6.14, Callister 7e.

Stress (MPa)

800

-200C

600

-100C

400

25C

200 0

0

0.1

0.2

0.3

0.4

Strain • y and TS decrease with increasing test temperature. • %EL increases with increasing test temperature. 3. disl. glides past obstacle • Why? Vacancies 2. vacancies help dislocations replace move past obstacles. atoms on the obstacle disl. half plane

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1. disl. trapped by obstacle

0.5

Strain hardening (work hardening) Annealing

• Process where material is heated to above the recrystallization temperature of the sample and then cooled down. • Main purpose is to improve Cold work properties by increasing ductility and retaining most of the hardness. • There are 3 steps involved with annealing: recovery, recrystallization and grain growth.

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Strain hardening (work hardening) Effect of Heating After %CW

• 1 hour treatment at Tanneal... decreases TS and increases %EL. • Effects of cold work are reversed!

100 200 300 400 500 600 700 600 60

tensile strength 50 500 40 400

30

ductility 300

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20

ductility (%EL)

tensile strength (MPa)

annealing temperature (ºC)

• 3 Annealing stages to discuss... Adapted from Fig. 7.22, Callister 7e. (Fig. 7.22 is adapted from G. Sachs and K.R. van Horn, Practical Metallurgy, Applied Metallurgy, and the Industrial Processing of Ferrous and Nonferrous Metals and Alloys, American Society for Metals, 1940, p. 139.)

Strain hardening (work hardening)

Recovery • Occurs during heating at elevated temperatures below the recrystallization temperature. • Dislocations reconfigure due to diffusion and relieve the lattice strain energy. • Electrical and thermal properties are recovered to their pre-cold worked state.

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Strain hardening (work hardening) Recovery Annihilation reduces dislocation density. • Scenario 1 extra half-plane Results from diffusion

• Scenario 2

of atoms atoms diffuse to regions of tension extra half-plane of atoms

3. “Climbed” disl. can now move on new slip plane 2. grey atoms leave by vacancy diffusion allowing disl. to “climb” 1. dislocation blocked; can’t move to the right Program Studi Teknik Pengelasan - PPNS

Dislocations annihilate and form a perfect atomic plane.

tR 4. opposite dislocations meet and annihilate Obstacle dislocation

Strain hardening (work hardening) Recrystallization

• New grains are formed that: -- have a low dislocation density -- are small -- consume cold-worked grains. 0.6 mm

0.6 mm Adapted from Fig. 7.21 (a),(b), Callister 7e. (Fig. 7.21 (a),(b) are courtesy of J.E. Burke, General Electric Company.)

33% cold worked brass Program Studi Teknik Pengelasan - PPNS

New crystals nucleate after 3 sec. at 580C.

Strain hardening (work hardening) Further Recrystallization • All cold-worked grains are consumed. 0.6 mm

0.6 mm

Adapted from Fig. 7.21 (c),(d), Callister 7e. (Fig. 7.21 (c),(d) are courtesy of J.E. Burke, General Electric Company.)

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After 8 seconds

Strain hardening (work hardening) Grain Growth • At longer times, larger grains consume smaller ones. • Why? Grain boundary area (and therefore energy) is reduced.

0.6 mm

After 8 s, 580ºC

0.6 mm

After 15 min, 580ºC

• Empirical Relation:

exponent typ. ~ 2 grain dia. At time t. n

d  d  Kt

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n o

Adapted from Fig. 7.21 (d),(e), Callister 7e. (Fig. 7.21 (d),(e) are courtesy of J.E. Burke, General Electric Company.)

coefficient dependent on material & Temp. elapsed time

º

Strain hardening (work hardening)

TR = recrystallization temperature TR

Adapted from Fig. 7.22, Callister 7e.

º

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º

Strain hardening (work hardening) Recrystallization Temperature, TR TR = recrystallization temperature = point of highest rate of property change 1. TR  0.3-0.6 Tm 2. Due to diffusion  annealing time shorter annealing time => higher TR 3. Pure metals lower TR due to dislocation movements Easier to move in pure metals => lower TR

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º

Precipitation Strengthening • Particles impede dislocations. • Things that slow down/hinder/impede dislocation movement will increase, σy and σTS. • And also other phases -especially very small, well dispersed particles. • In some alloys can get small, uniform particles to precipitate out of (solid) solution. Hence name “precipitation hardening”, also known as "AGE" -hardening. Examples include: • Al-Cu • Cu-Be • Cu S • Mg-Al • Stainless steels • Many Al-alloys are precipitation hardenable. • Al -Cu is best known alloy, e.g. Al-4%Cu Program Studi Teknik Pengelasan - PPNS

º

Precipitation Strengthening Application: Precipitation Strengthening

• Internal wing structure on Boeing 767 Adapted from chapteropening photograph, Chapter 11, Callister 5e. (courtesy of G.H. Narayanan and A.G. Miller, Boeing Commercial Airplane Company.)

• Aluminum is strengthened with precipitates formed by alloying & H.T.

Adapted from Fig. 11.26, Callister 7e. (Fig. 11.26 is courtesy of G.H. Narayanan and A.G. Miller, Boeing Commercial Airplane Company.)

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