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Physical Metallurgy 16 th Lecture. MS&E 410 D.Ast dast@ccmr.cornell.edu 255 4140. Solid State Phase Transformations 1. Homogenization 2. Precipitation 3.Coarsening - akin to Oswald Ripening 4. Eutectoid transformation (Eutectic Separ. In Solids)
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Physical Metallurgy16 th Lecture MS&E 410 D.Ast dast@ccmr.cornell.edu 255 4140
Solid State Phase Transformations 1. Homogenization 2. Precipitation 3.Coarsening - akin to Oswald Ripening 4. Eutectoid transformation (Eutectic Separ. In Solids) 5. Twinning, - akin to Deformation Twining ..and I would like to add :-) 6. Martensitic transformation
1. Homogenization Key idea: Get it into one phase field and cool rapidly 1. Heat 2. Wait 3. Cool Dissolve second phase particles Form small grains Reduce composition gradients
How long to wait 1. Depends on how far you want to dissolve particle x(t) = Ko - K D t 2. Useful to keep particles if you want to pin existing grains and prevent grain growth at solution temperature.
Homogenization : Super Alloys (turbine blades) Two phase field narrows at high T (you should know by now why) hence heating up the green composition will homogenize it - cooling will generate new -if done right bimodal - gg’ structure
2. Precipitation • a => a + b • Two steps • 1. Nucleation - best at low T => high driving force • DG = K DT • 2. Growth - best at high T => largest rate • v = vo exp( - DGg/kT) • Classic approach is a two step heat treatment • Quench (air, oil, water)* • Age (Anneal) * Can induce residual stresses that warp specimen. Residual stresses increase with quench rate and scale with d2T/dx2 (self test why?)
Discovery - misguided attempt to mimic quenching steel Precipitation (age) hardening was discovered by Alfred Wilm in Germany in 1906. He attempted to harden an alloy of essentially aluminum-2 atom percent copper in an analogous way to steels by a quenching treatment. The specimen was initially soft, but the hardness increased with time at room temperature after the quench. Later, explained being due to precipitation. The seminal event in the development of of precipitation-hardened alloys.
Guinier Preston Zones BF TEM Al + 3.5 % Cu, aged 100 days
Schematic 3-D view HAADF image. Bright is copper. Lighter area with asterics, GP-I zones seen plan view. GP zones parallel to {100} planes
Rate depend on • Are all nuclei formed at start vs continuing nucleation • If not, how does depletion of a by diffusion to existing nuclei reduce driving force for new nuclei • Nucleation geometry (spheres, plates...
Nucleation • Homogeneous • Heterogeneous • Point defects, nucleation agents • Dislocations (G-P zones) • Grain Boundaries (common) • Nucleation at Grain Boundaries ( M23C) pins G.B
Example: Use if vacancies to precipitate oxygen in Silicon • Lure vacancies into areas where O should precipitate • Precipitate oxygen United States Patent 6849119 A process for heat-treating a single crystal silicon wafer to influence the precipitation behavior of oxygen in the wafer in a subsequent thermal processing step. The wafer has a front surface, a back surface, and a central plane between the front and back surfaces. In the process, the wafer is subjected to a heat-treatment to form crystal lattice vacancies, the vacancies being formed in the bulk of the silicon. The wafer is then cooled from the temperature of said heat treatment at a rate which allows some, but not all, of the crystal lattice vacancies to diffuse to the front surface to produce a wafer having a vacancy concentration profile in which the peak density is at or near the central plane with the concentration generally decreasing in the direction of the front surface of the wafer.
HW 16-1 Google “MDZ wafer” 1. What does MDZ stands for ? 2. What stands BMD stands for ? 3. Optional for sharp MS&E scientists: What is the relation of BMD to MDZ ?????
TTT diagram of Steel (from Haasen)
Useful, wanted precipitation: • IC industry => impurity gettering at O precipitates • Metals • Hardening => dislocation obstacles • Locking GB => Precipitates in GB :-) • Ductility (spherical graphite in Steel with Mg) • Lubrication (graphite in steel) • Improved wear (Si precipitation in Al => automotive) • Useless, unwanted, detrimental precipitation • Brittleness (Carbides in GB)* • …. Sensitizing GB in SS (Stainless Steel) remember ? • …..Overaging Al alloys (coherent to incoherent precip transform * we went of sy vs K1c before. Generally inverse related but dual phase steels try to work around it. See previous lectures
To control wanted and suppress unwanted precipitation, industry employs complex heat treatments.
Industrial heat treatments are complex because two conflicting requirements need to be balanced: • Minimize distortion when quenching and annealing • Maximize throughput, i.e. use fast heat treatment methods. • Quenching induces distortion because different parts of the specimen cool at different rates. The different rate of thermal contraction then warps the specimen. Quenched and annealed , steel can only machined by grinding, a slow and expensive method. • Lowering the temperature slowly below the transformation and hold it (austempering) minimizes distortion but, because of the low driving force for transformation, requires a long holding times in a furnace. This a) requires energy and b) lowers throughput.
Same steel, error in one of previous heat treat cycles The right sample was not correctly heat treated permitting Carbon to form carbide precipitates at grain boundaries. Note that the fracture toughness decreased by factor 5
Unwanted GB precipitation: Case I SS sensitization
Unwanted G.B. precipitation: Case II MnS precipitation at grain boundary. SEM of fracture surface showing precipitate protruding. Initiator of (near) brittle failure. A scanning electron micrograph showing a fractured MnS particle protruding edge-on from the fracture surface
Sea temperature -2 C SEM from Steel recovered from Titanic. Toughness ~ 4J For comparison: ASTM A 36 a modern low cost construction steel with 0.20% C has ten times the toughness.
Ductile-brittle transition temperature at impact energy of 20 joules • -27°C for ASTM A36, • 32°C longitudinal specimen,Titanic, hull plate • 56°C transverse specimen, Titanic hull plate Lives lost: 1,523
Historic progress in controlling MnS precipitation in G.B. of steels. The lock gate steel is from 1912, same age as Titanic The chemical analysis of the steel from the hull is given in Table II. The first item noted is the very low nitrogen content. This indicates that the steel was not made by the Bessemer process; such steel would have a high nitrogen content that would have made it very brittle, particularly at low temperatures. In the early 20th century, the only other method for making structural steel was the open-hearth process. The fairly high oxygen and low silicon content means that the steel has only been partially deoxidized, yielding a semikilled steel. The phosphorus content is slightly higher than normal, while the sulfur content is quite high, accompanied by a low manganese content. This yielded a Mn:S ratio of 6.8:1—a very low ratio by modern standards. The presence of relatively high amounts of phosphorous, oxygen, and sulfur has a tendency to embrittle the steel at low temperatures. Davies7 has shown that at the time the Titanic was constructed about two-thirds of the open-hearth steel produced in the United Kingdom was done in furnaces having acid linings. There is a high probability that the steel used in the Titanic was made in an acid-lined open-hearth furnace, which accounts for the fairly high phosphorus and high sulfur content. The lining of the basic open-hearth furnace will react with phosphorus and sulfur to help remove these two impurities from the steel. It is likely that all or most of the steel came from Glasgow, Scotland.
Coarsening (Oswald Ripening) • Diffusion limited (bulk transport limited • (less common Kinetic limited (surface reaction)) • Driving force • Difference in curvature • p (energy per volume) = 2g/r • Kinetic • A is a constant, D/kT the mobility (Einstein Relation) v= m F • r is average radius
Volume fraction of precipitates stays constant. • Size distribution changes • If precipitates grow in 3 dimension, log-log plot will have a slope of 1/3 The microstructure and precipitation kinetics of a cast aluminium alloy D. Ovono Ovonoa, I. Guillotb and D. Massinonc
Eutectoid Transformation • Like Eutectic, but operate from solid state starting phase By far the most famous is the eutectoid transformation of g iron FCC => higher solubility for C
Pearlite spacing: Lamella spacing inversely proportional to DT 1/l = K1DT DT, temperature difference between hold T, and eutectoid T
Hypoeutectoid Steel 0.1 wt% C Slow air cool. Most of Austenite transformed to ferrite, small islands of pearlite.
Eutectoid Steel 0.8 wt% C Pearlite (a + Fe3C) Slow cooled, coarse pearlite. Outlines of austenite grains visible
Formation by nucleation and growth X is fraction transformed K is a constant n depends on conditions
Example: Recrystallization kinetics of cold rolled Al, slope ~ 2
Twinning is a common deformation mechanism at low temperatures. Rules of deformation High T=> low coordination => single point defect motion => bulk and G.B. creep controlled Medium T => Medium coordination => kink movement => dislocation controlled deformation Low T => highly coordinated => deformation twinning F. Twins and twinning
Growth twins Easily formed, as twin boundary energy is ~ 1/10 to 1/5th of other GB. In Si twin boundary energy is 30 ergs/cm2 compared to 1000 to 2000 ergs/cm2 surface Deformation twin Crystal lattice “flips over” in highly coordinated movement Geometry compatible with movement of partial dislocations but speed (~ sound) is not.
Twin Geometry HW-16-2 Read reading material in Baker 16, Quiz is possible If you deform a circle into an ellipse there are two planes that are invariant. These are A* - B* and C*-D* These planes rotate (move) from A-B and C-D but do not change length.
Twin Shearing a circle. All vectors in the red planes are invariants Vectors not on the red planes get extended (hatched) or compressed (white)
G. Martensitic Transformation Note the periodic appearance of misfit dislocations. Although here pictures as edge dislocations they can just as easily be screw dislocations. Since any plane containing the screw is a glide plane, a boundary made of screws is highly mobile
The austenite - martensite boundary is highly mobile because the screw dislocations can move in any directions. Note that the plane surface of austenite transforms into a zig-zag up down in the martensite
The boundary between austenite and martensite is the habit plane of the twin
Fcc Iron => bcc martensite a ) look at fcc as b.c tetragonal with c/a = 1.412 b) now take the b.c tetragonal (fig b) and compress it to 83% of its height and expand it horizontally, x and y wise, to 112% c) The C atom in the center of the fcc austenite cell is now on the corner of the tetragonal cell, where is wants to be, as we discussed before, because of long range stress cancellation !
HW 16-3 1) What is the volume change in the austenite martensite transformation. Answer in %, with + for increasing volume and - for decreasing volume 2) What is the habit plane of martensite ? (you may google it, or if you are a born metallurgist figure it out yourself, in which case you will earn 5 bonus points in my “instructors discretion” book !)
