Lecture 6 Strengthening Mechanisms 2

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Transcript of Lecture 6 Strengthening Mechanisms 2

Precipitation Hardening Particles impede dislocations. Things that slow down/hinder/impede dislocation movement will increase, y and TS And also other phases - especially v. 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 Some alloy and stainless steels

Outline Precipitation Hardening Dispersion Strengthening Martensite Mechanical Properties of Steel Effect of Pearlite

Many Al-alloys are precipitation hardenable Al - Cu is best known alloy, e.g. Al - 4%CuDr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/1 Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/2

Precipitation HardeningProcedure: Heat to To ; Hold until only - phase present. Quench (rapid cooling) to T1 ; because rapid, no diffusion occurs - SSSS Super-saturated solid solution of formed. B atoms trapped in . Not thermodynamically stable. Reheat to T2 ; diffusion can occur, small precipitates of -phase form.M = Max. solubility of metal B in metal A. Solid solubility decreases to N as T

Precipitation Hardening Ex: Al-Cu system Procedure: - Pt A: solution heat treat (get a solid solution) - Pt B: quench to room temp. - Pt C: reheat to nucleate small crystals within crystals.T(C)700 600 L +L

A C

+L

CuAl2

500 400

+20 30 40 50

(Al)

300 0 B 10

composition range needed for precipitation hardening

wt%Cu

+ Heat (~550oC) Quench (0oC) (ssss) Heat/age (~150oC) + pptTemp. Pt A (soln heat treat) Pt C (precipitate ) TimeMECH 321 lecture 6/4

Pt BDr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/3 Dr. M. Medraj Mech. Eng. Dept. - Concordia University

Precipitation HardeningSingle phase - (SSSS)

Precipitation Hardening + pptMax. strength/hardness

+ ppt

Initially have a supersaturated solid solution of Cu in Al

- form various zones or small Cu clusters - upon further aging form '' particles

Can no longer match up incoherent with each other (.) - less resistance to dislocation movement

Formation & growth of precipitates. These are very small (5 x 10-9m) initially but grow with time.

Too long at temperature and precipitates get too large and softening occurs.

Lattices are trying to match up with each coherent other (stay .) - large lattice strain high resistance to dislocation movementDr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/5

is the optimum for strengthening is overaged due the precipitate becoming incoherent with the Al matrixDr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/6

no so n-eq l id u il so . lut ion ma pre ny s ci p m a ita ll ag tes ed

Precipitate Effect On TS and %Eltensile strength (MPa)

Effect of Precipitate Size on Dislocation Movementfe pre wer ov cip larg era itat e ge es d

TS peaks with precipitation time. accelerates Increasing T process. %EL reaches minimum with precipitation time. Higher strength achieved at lower ageing temperatures but for longer times Consequently, if cold-working is to be done, it should be carried out after quenching ageing . and before ..

500 400 300 200

avg. particle size = 64b

149 C 204C

1min 1h 1day 1mo 1yr precipitation heat treat time (h)

%EL (2in sample)

30 20 10 0 149C

avg. particle size = 361b204C

1min 1h 1day 1mo 1yr precipitation heat treat time (h)MECH 321 lecture 6/7 Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/8

Dr. M. Medraj

Mech. Eng. Dept. - Concordia University

Precipitation Hardening2014 Al Alloy: Natural aging occurs at room temperature- so will start aging at RT unless stored at low temperature - freezer! E.g. some Al-alloys for rivets

Dispersion Strengthening Adding very small foreign particles (hard + inert/non-reactive) into matrix metal. Particles may be metals/non-metals but oxides are often used Thorium oxide (Thoria/Th02) Like precipitation hardening, particles interfere with dislocation movement But not as large strengthening effect, however, not greatly affected long term high temp. by temperature - so good for .., applications. Examples: Nickel + 3 vol% Thoria (TD Nickel) SAP (sintered Al. powder)MECH 321 lecture 6/9 Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/10

Artificial aging occurs at elevated temperature and is accelerated Aluminum alloys which show age hardening are: Al-Cu Al-Mg Al-Mg-Zn combinations of aboveDr. M. Medraj Mech. Eng. Dept. - Concordia University

Martensite Austenite dissolves ~2%C whereas ferrite only dissolves 0.022% max. If a medium to high carbon steel is quenched then C remains in solid solution forming a body-centred tetragonal phase (BCT) called martensitecarbon sites

Martensite Microstructure Diffusionless transformation: -martensite (very fast) platelike or needle-like appearance microstructure always contains retained austenite microstructural development of martensite and bainite defined by transformation curves, e.g.: time, temperature, transformation (TTT)

martensite

Retained austenite

metastable martensite is and does not appear on the phase diagram

alloying element affect the ease of martensite formationDr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/12

Dr. M. Medraj

Mech. Eng. Dept. - Concordia University

MECH 321 lecture 6/11

BainiteHow does the bainite look like?

Martensite

FIGURE 10.8 Transmission electron micrograph (TEM) showing the structure of bainite. A grain of bainite passes from lower left to upper right-hand corners, which consists of elongated and needle-shaped particles of Fe3C within a ferrite matrix. The phase surrounding the bainite is martensite.

Note: need to cool faster than 140oC/s to get 100% martensite!

Eutectoid stealDr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/13 Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/14

Martensite

Mechanical Properties of Steel

Effect of Fe3C and pearlite Alloy steel; + Ni, Cr, Mo. Get martensite at slower cooling rates.

Depending on microstructure, obtain different mech. properties increases amount of Fe3C (hard phase) in microstructure the hardness and strength decrease whereas, the ductility and impact resistance Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/15 Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/16

Effect of Pearlite Fine pearlite is stronger and harder than coarse pearlite cementite adheres strongly to ferrite and restricts its deformation (reinforces) plastic deformation (dislocations) cannot cross -Fe3C phase boundary

Spheroidite least boundary area/unit volume soft, low strength and ductile structure (easy to machine) spheroidized carbide is a low stress raiserSpheroidite Coarse Pearlite Fine Pearlite

Boundary area/unit volume is much greater in the case of fine pearlite3000X

Fe3C

1000X

3000X

Spheroidite vs. fine and coarse pearliteDr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/17 Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/18

Martensite Hardness of martensite is dependant on the carbon content up to ~0.6%C plastic deformation (dislocation motion) is restricted by interstitial carbon high carbon martensite is the hardest most brittle microstructure in steel there is an increase in volume upon quenching which can cause cracking

Tempered Martensite Tempering (heating) at 250650oC allows diffusion of carbon out of supersaturated martensite: -martensite -ferrite + Fe3C (bct) (bcc) Microstructure is tempered martensite Cementite precipitates (hard) are very fine and dispersed Continuous (ductile) phase Very large boundary area/unit volume This microstructure makes TM ductile and relatively tough

9300X

Cementite precipitates When both TM structure and pearlite structure have the same strength, the fracture toughness of TM structure will greater be much .. than that for pearlite structure.MECH 321 lecture 6/20

Dr. M. Medraj

Mech. Eng. Dept. - Concordia University

MECH 321 lecture 6/19

Dr. M. Medraj

Mech. Eng. Dept. - Concordia University

Effect of Tempering on Mechanical Properties of Steel

Summary: Processing OptionsAustenite ( )slow cooling moderate cooling rapid quench

The mechanical properties are affected by the precipitate size increases Precipitate size .. with tempering time and temperature up to the eutectoid composition (carbon diffusion)The microstructure will become Spheriodite

( + Fe3 C layers + a proeutectoid phase)

Pearlite

( + Fe3 C plates/needles)

Bainite

Martensite (BCT phasediffusionless transformation)

Note high yield strength: Maximum with fine pearlite was 500MPa. 1 hour tempered 4340 steelDr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/21 Dr. M. Medraj

Martensite T Martensite bainite fine pearlite coarse pearlite spheroidite General Trends

reheat Ductility Tempered Martensite

Strength

( + very fine Fe 3 C particles)

Mech. Eng. Dept. - Concordia University

MECH 321 lecture 6/22

Summary

The versatility of steel arises because of the wide range of microstructures that can be achieved, and the corresponding wide range of mechanical properties. In general, steel microstructures in which carbon is more finely dispersed have .. strength and .. fracture toughness.

Martensite T Martensite bainite fine pearlite coarse pearlite spheroidite General Trends

Strength

Ductility

Next topic:

Composites

Dr. M. Medraj

Mech. Eng. Dept. - Concordia University

MECH 321 lecture 6/23

Dr. M. Medraj

Mech. Eng. Dept. - Concordia University

MECH 321 lecture 6/24