Strain Partitioning & Mechanical Stability of Retained Austenite

J. H. Ryu a D. I. Kim b H. S. Kim c

H. K. D. H. Bhadeshia a,d D. W. Suh a

a Graduate Institute of Ferrous Technology, POSTECH, Pohang 790-784, KoreabKorea Institute of Science and Technology, Seoul, 136–791, Korea

cMaterials Science and Engineering, POSTECH, Pohang 790-784, KoreadMaterials Science and Metallurgy, University of Cambridge, CB2 3QZ, U.K.

C Mn Si Al

L–Al 0.12 4.6 0.55 1.1

H–Al 0.12 5.8 0.47 3.1

What makes the difference?

• Size effect

: similar grain size

• Chemical effect

: difference in Mn content

only by 1wt%

• Microstructural effect

: strain partitioning

Un-deformed Deformed

Strain analysis of SEM images with

Digital Image Correlation (DIC)

A B

Alloys and mechanical behaviors

Experimental method and results

FE-SEM (2000X) equipped with EBSD (70nm) and

in situ tensile-test attachment (crosshead speed of 5㎛s-1)

Before deformation

L-Al

H-Al

9.6%

10.8%

Conclusion

The partitioning of strain between the different phases present in TRIP–assisted steels can lead to a misleading impression of the mechanical stability of

retained austenite when comparisons are made between steels with substantially different microstructures, or when attempts are made to explain the stability

in terms of chemical composition alone

Abstract

The mechanical stability of austenite in steels which rely on transformation–induced plasticity is usually attributed to its chemical composition, size and shape.

We demonstrate here that another factor, the partitioning of strain between phases with different mechanical properties, can dramatically influence the stability.

19.1%

L-Al H-Al H-Al

[D. W. Suh et al, Metall. Mater. Trans, 41A (2010) 397.]

After deformation

0.26m

L-Al

0.29m

H-Al

7.2 0.4 wt% (24 grains)

6.9

7.1

5.6 6.1

5.7

8.1

7.5

6.96.56.9

6.8

6.5

Grain size of austenite

6.2 0.8 wt% (15 grains)

Mn content in austenite grains Stability – measured with Ms

Chemical compositions

L-Al, 720 oC

H-Al, 780 oC

Tem

p.

Time

reheating

hot rolling

~4.5mm

cold rolling

~1.0mm

Ae3

annealing for

2min.

10oC/s 10oC/s

Heat treatment