3.40/22.71
Summary of 11/29/2012
Sergio Castellanos
Mechanical Engineering Department
Massachusetts Institute of Technology, Cambridge, MA (USA)
Fe-C Phase Diagram
δ
(Ferrite)
Liquid
BCC
1394 ºC
γ+Liquid
Solid
1130 ºC
2.11%
γ
(Austenite)
FCC
γ+Cementite
912 ºC
α+γ
α
(ferrite)
0
Not to scale
0.022%
α+Cementite
Carbon (% by Weight)
Cementite
(Fe3C)
723 ºC
6.67
Martensite Transformation
Austenite
Martensite
x
x
Large Cooling Rate
dT

dt
BCT*
FCC
Scalar
Manufacturing, Engineering & Technology, Fifth Edition, by Serope Kalpakjian and Steven R. Schmid.
ISBN 0-13-148965-8. © 2006
Vectorial
(BCC)
Martensite Transformation
 Displacive: C diffusion is restricted
 Bain Transformation:
tr ε =~ 0
Constant Volume
Twinning Strain
(Simple Shear)
[1] Lieberman, Acta Metallurgia 6 (1958)
[2] H. Ledbetter, M.L. Dunn, Materials Science and Engineering A273-275 (1999) 222-225
Twinning
plate  w   v1  (1  w)   v 2
Energy: Least if formed into disk
But all at once?
ΔG

Still incur in expense:
Interface Energy
Good thing:
w proportions cancel
strain fields
Coherent TB
Wechsler-Lieberman-Read
(WLR) Theory
A.L. Roitburd and G.V. Kurdjumov, Mater. Sci. & Eng, 39 (1979), 141.
a
Elastic Energy vs. Interfacial Energy
…..
Steels: 10’s nm bi-layer
Importance of Martensitic Transformation
Utilization:
total  applied  residual
Tempering:

Hardness
(Rockwell)
200 ºC
600 ºC
t
Difussion process
Cementite in Ferrite matrix
Image credit: http://courses.washington.edu/mse170/labs/HeatTreatment/HeatTreatment_micrographs.shtml
Shape-Memory Alloys

Detwinning

Cooling
Heating/Recovery
T
Image Credits: [1] Texas A&M – Active Materials Laboratory
[2] http://beckause.blogspot.com/2008/02/accordions-are-sexy.html
Thank you
Image credit: http://www.lassp.cornell.edu/sethna/Tweed/What_Are_Martensites.html
Appendix
 Martensite-Austenite
interface structure