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

2 No size, shape information Ostwald ripening Total thermodynamic driving force for phase transformation Martensitic microstructure in CuZnAl (M. Morin, INSA de Lyon) (+ Kinetics)

3 Capillary energy effects How is the second-phase shape determined?

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 – difficult  by choosing the correct orientation relationship  low-energy coherent or semicoherent interface  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 GP Zone in Al – Ag Alloys Low energy Coherent High energy Incoherent Capillary energy effects

F. R. N. Nabarro, Proc. Phys. Soc (1940) 5 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 Elastic energy effects The elastic strain energy for a homogeneous incompressible inclusion in an isotropic matrix

6 1.Diffusionless transformation 2.Body centered tetragonal (BCT) crystal structure 3.BCT if C 0 > 0.15 wt% C 4.BCT  few slip planes  hard, brittle 5.% transformation depends only on T of rapid cooling/ time (s) T(°C) Austenite (stable) 200 P B TETE 0% 100% 50% A A M + A 0% 50% 90% Martensitic transformation Isothermal Transformation Diagram Diffusionless shear-dominant phase transformation  Martensitic transformation Martensite needles Austenite

7 Martensite 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 transformation 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)

8 Martensitic transformation A movie of martensitic transformation in Fe 0.18 C 0.2 Si 0.9 Mn 2.9 Ni 1.5 Cr 0.4 Mo wt% steel, using confocal laser microscopy. by Professor Toshihiko Koseki of The University of Tokyo

9 (a) Initial specimen with length L 0 (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. Martensitic transformation: Shape memory alloy

10 Thank you