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

1. Recap of 11/26/2013 3.14/3.40J/22.71J Physical Metallurgy 12/03/2013 IntakJeon Department of Materials Science and Engineering Massachusetts Institute of Technology

2. Total thermodynamic driving force for phase transformation (+ Kinetics) No size, shape information For small Dominant Coarsen For large Dominant Refine/split/pattern Martensitic microstructure in CuZnAl (M. Morin, INSA de Lyon) Ostwald ripening

3. Capillary energy effects -matrix In a two-phase microstructure one of the phases is dispersed within the other, for example -precipitates in an -matrix. -precipitate How is the second-phase shape determined? The lowest total interfacial free energy () when the shape of the precipitate and its orientation relationship to the matrix are optimized to give the lowest total interfacial free energy

4. Capillary energy effects Fully coherent precipitates Partially coherent precipitates Low energy Coherent High energy Incoherent A zone with no misfit (○-Al, ●-Ag) GP Zone in Al – Ag Alloys precipitates in an Al-4 atomic % Ag alloy • From an interfacial energy standpoint •  Surroundedby low-energy coherent interfaces • Different crystal structures – difficult • by choosing the correct orientation relationship • low-energy coherent or semicoherentinterface • Or bounded by high-energy incoherent interfaces. • If α, β have the same structure & a similar lattice parameter • Two lattices are in a parallel orientation relationship • Happens during early stage of many precipitation hardening • Good match can have any shape spherical 0.7% → negligible contribution to the total free energy Triangular, square, or hexagonal plate shapes David A. Porter,Kenneth E. Easterling, Phase Transformations in Metals and Alloys,

5. Elastic energy effects The elastic strain energy for a homogeneous incompressible inclusion in an isotropic matrix The elastic energy E of a particle of precipitate as a function of its shape a is the equatorial diameter, c is the polar diameter F. R. N. Nabarro, Proc. Phys. Soc. 52 90 (1940)

6. 800 Austenite (stable) TE T(°C) A P 600 B 400 A 100% 50% 0% 0% 200 M + A 50% M + A 90% M + A time (s) -1 10 3 5 10 10 10 Martensitic transformation Isothermal Transformation Diagram (martensite) Martensite needles • Diffusionless transformation • Body centered tetragonal (BCT) crystal structure • BCT if C0 > 0.15 wt% C • BCT  few slip planes  hard, brittle • % transformation depends only on T of rapid cooling/ Austenite Diffusionlessshear-dominant phase transformation  Martensitic transformation

7. Martensitic transformation - Each colony of martensite plates consists of a stack of different variants. - This allows large shears to be accommodated with minimal macroscopic shear. Fig. 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) Martensiteformation rarely goes to completion because of the strain associated with the product that leads to back stresses in the parent phase.

8. Martensitic transformation A 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 Tokyo

9. Martensitic transformation: Shape memory alloy (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 stress–strain curve with different stages indicated.

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