Dispersion Strengthening by Heat Treatment Chapter 11a – 4 th Edition Chapter 12a- 5 th Edition

In the last two chapters… We looked at some fairly simple 2 component phase diagrams in some detail Copper and Nickel benefit from solid solution strengthening to improve properties

Of course some were more complicated than others The lead-tin phase diagram demonstrates a eutectic composition, which makes dispersion strengthening possible

We also introduced more complicated two component phase diagrams The copper-zinc phase diagram includes a number of 3 phase transformation points

We did not examine in detail how the phase change from one solid phase to another or others occurs We just accepted that it does

Precipitation What we want to do now is examine the precipitation process as we go from one solid to two solids, much as we examined the solidification process from a liquid to a solid

Solid State Reactions The change from one solid to another has a lot in common with the solidification process (from liquid to solid) It does not happen instantly Need time for nucleation Need time for nucleation Need time for growth Need time for growth

Recall For solidification  G = 4/3  r 3  G v +4  r 2   G = 4/3  r 3  G v +4  r 2  Volume free energy + surface energy Volume free energy + surface energy For one solid phase changing to another  G = 4/3  r 3  G v +4  r 2  + 4/3  r 3   G = 4/3  r 3  G v +4  r 2  + 4/3  r 3  Volume energy + surface energy + strain energy Volume energy + surface energy + strain energy Because the new solid does not take up the same volume as the old solid Because the new solid does not take up the same volume as the old solid

In solids it is easier to start a grain growing than from liquids There is already an existing crystal, which functions a lot like a seed crystal in heterogeneous nucleation from a liquid

Nucleation of Carbon Dioxide bubbles

Let’s look at the two factors controlling the growth of new crystals NucleationGrowth

Nucleation Nucleation usually occurs at grain boundaries – because that’s where there’s room! Unlike solidification, it isn’t too hard to get a nucleus going The nucleation rate increases as the temperature goes down – just like in solidification from a melt

Growth The nucleus grows as material diffuses through the surrounding solid material to the site Diffusion is a function of temperature If you cool the material off immediately, it is hard for diffusion to occur Supersaturated non-equilibrium structures can occur

Kinetics The combination of Nucleation and growth determine how fast the transformation will occur. At a constant temperature the transformation is described by the Avrami relationship f=1-exp(-ct n ) f is the fraction converted f is the fraction converted t is time t is time c and n are constants for a given temperature c and n are constants for a given temperature

Avrami Plot Fraction Converted Time (sec) Conversion is 50% Complete  is the time required for 50% conversion | Incubation Time |

Growth Rate Often expressed as 1/  min    he growth rate is a function of temperature Often, the higher the temperature, the faster the solid transforms Why? Diffusion dominates in many systems because diffusion is hard (slow) and nucleation is easy (fast)

Growth Dominated Phase Change is common for many metals, especially during recrystallization after cold work For these metals nucleation occurs readily The only factor that changes with temperature then is the growth rate – which is diffusion controlled For these metals, the solid to solid phase change always occurs faster at higher temperatures

Effect of Temperature on Copper Recrystallization after Cold Work Fraction Transformed Time 135 C 120 C 80 C t is the time required for 50% conversion Higher temperature equals faster conversion

Growth rate follows an Arrhenius Relationship Growth rate = 1/ t Growth rate is proportional to overall transformation rate Growth rate = A exp(-Q/RT)

What happens if nucleation controls the phase change? Nucleation happens more readily at LOWER temperatures That means the overall transformation goes UP as the temperature goes down However… eventually as the temperature gets colder and colder diffusion slows to the point where it plays a factor

Effect of Temperature on Phase Transformation Growth Rate Nucleation Rate Overall Transformation Rate Temperature Rate Equilibrium transformation temperature

Effect of Temperature on Phase Transformation Temperature Time Time for 50% Transformation Minimum Time required for Transformation

C-curve Typical of many metals, ceramics, glasses and polymers Ex. Iron changes phase this way

Aluminum Copper Phase Diagram Intermetallic Compound

Partial Al – Cu Phase Diagram Al % Cu Many Aluminum Alloys are strengthened with copper

How Does the Solid Form? Liquid L +    This is what we would like to happen Al % Cu

How Does the Solid Form? Liquid L +     This is what typically happens We want to avoid this structure, which is caused by slow cooling

Age hardening or Precipitation Hardening A treatment used on non optimum alloy structures Produces a uniform dispersion of Fine Fine Hard Hard Coherent Precipitate Coherent Precipitate In a softer, more ductile matrix

#1 Solution Treatment Reheat the alloy up to a temperature where only one solid phase exists (above the solvus) This dissolves the second solid phase (  for example) into the primary phase (which may take significant time) Don’t exceed the eutectic temperature L    L AlCu Temperature Time

#2 Quench Rapidly cool to room temperature or below This results in a supersaturated – nonequilibrium structure The second phase does not form, because diffusion is so slow!! L    L AlCu Temperature

#3 Aging Reheat to a temperature below the solvus Diffusion is still slow, so the atoms can only diffuse a short distance Results in a fine precipitate There is an optimum aging time L    L AlCu Temperature

Coherent vs. Non-Coherent Precipitate ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Non-CoherentCoherent

Coherent Precipitates Form First Eventually grow until they snap out of solution Produce more hardening If you over age – the strength goes down because Precipitate goes from coherent to noncoherent Precipitate goes from coherent to noncoherent

Aging See the animations on the CD Artificial aging – elevated temperatures Natural aging – room temperature Not suitable for use at high temperature Why? Why? Problems with welding

Requirements for Age Hardening Must have a phase diagram that exhibits a change from a single solid phase to two solid phases (  Matrix should be soft and ductile Precipitate should be hard and strong Must be quenchable Must have a coherent precipitate

Aluminum Copper Phase Diagram Intermetallic Compound Does the Aluminum Copper system meet the requirements for Age Hardening (Precipitation Hardening)?

Aluminum - Copper Aging    #1 Solution Treatment #2 Quench ssss #3 Aging  

What About the Lead Tin System? There is no intermetallic phase, so it is probably not a good candidate

What about compositions between 22 wt% and 36 wt% Al in the Titanium-Aluminum system? Ti

What about other compositions in the Titanium-Aluminum system? Ti