Recap 3.40/22.71 (Summary of 11/26/2013)

Recap of 11/26/2013 3.14/3.40J/22.71J Physical Metallurgy

12/03/2013

Intak JeonDepartment of Materials Science and EngineeringMassachusetts Institute of Technology12No size, shapeinformation

Ostwald ripeningTotal thermodynamic driving force for phase transformation

Martensitic microstructure in CuZnAl (M. Morin, INSA de Lyon)(+ Kinetics)This is the total thermodynamic driving force for phase transformationFrom the free energy, bulk solution thermodynamics is all about X, T, P. There is nothing in bulk solution thermo that defines shape/orientation (or size, for that matter, as long as it is large enough).Three factors (a) capillary energy, (b) elastic energy, (c) kinetics. control shapes and sizes of microstructures / nanostructures:23Capillary energy effectsHow is the second-phase shape determined?the precipitate and matrix are strain free.

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A zone with no misfit (-Al, -Ag)Fully coherent precipitatesPartially coherent precipitates

David A. Porter,Kenneth E. Easterling, Phase Transformations in Metals and Alloys,From an interfacial energy standpoint Surrounded by low-energy coherent interfaces

Different crystal structures difficultby choosing the correct orientation relationship low-energy coherent or semicoherent interfaceOr bounded by high-energy incoherent interfaces. If , have the same structure & a similar lattice parameterTwo lattices are in a parallel orientation relationshipHappens during early stage of many precipitation hardeningGood match can have any shape spherical

GP Zone in Al Ag Alloys0.7% negligible contribution to the total free energy

Triangular, square, or hexagonal plate shapesLow energyCoherentHigh energyIncoherentCapillary energy effects4

F. R. N. Nabarro, Proc. Phys. Soc. 52 90 (1940)5The elastic energy E of a particle of precipitate as a function of its shape a is the equatorial diameter, c is the polar diameterElastic energy effectsThe elastic strain energy for a homogeneous incompressible inclusion in an isotropic matrix 6Diffusionless transformationBody centered tetragonal (BCT) crystal structureBCT if C0 > 0.15 wt% CBCT few slip planes hard, brittle% transformation depends only on T of rapid cooling/10103105time (s)10-1400600800T(C)Austenite (stable)200PBTE0%100%50%AAM + AM + AM + A0%50%90%

Martensitic transformationIsothermal Transformation DiagramDiffusionless shear-dominant phase transformation Martensitic transformation

Martensite needlesAustenite7Martensite formation rarely goes to completion because of the strain associated with the product that leads to back stresses in the parent phase.

- Each colony of martensite plates consists of a stack of different variants. - This allows large shears to be accommodated with minimal macroscopic shear.Martensitic transformationFig. Twins in martensite may be self-accommodating and reduce energy by having alternate regions of the austenite undergo the Bain strain along different axes

Maki, T., and C. M. Wayman,Metallurgical Transactions A 7 (1976)

8Martensitic transformationA movie of martensitic transformation in Fe0.18C0.2Si0.9Mn2.9Ni1.5Cr0.4Mo wt% steel, using confocal laser microscopy.by Professor Toshihiko Koseki of The University of Tokyo9

(a) Initial specimen with length L0 (b, c, d) Formation of martensite and growth by glissile motion of interfaces under increasing compressive stresses.

(e) Unloading of specimen.

(f) Heating of specimen with reverse transformation.

(g) Corresponding stressstrain curve with different stages indicated.Martensitic transformation: Shape memory alloy10Thank you