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Role of Retained Austenite Homogeneity in Suppressing Flow Discontinuities in a Medium-Mn Steel
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DOI：10.1016/j.mtla.2026.102828.png)
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Abstract
En 中文
Medium-Mn steels (MMnS) achieve attractive strength–ductility combinations through retained-austenite-assisted deformation, but their plastic flow can be interrupted by yield-point phenomena and dynamic strain aging. Although retained austenite heterogeneity is often considered beneficial for progressive transformation-induced plasticity, its role in controlling flow stability remains unclear. Here, a cold-rolled Fe–0.4C–6Mn–2Al–1Si–0.05Nb MMnS was subjected to a two-step intercritical annealing route to systematically vary the fraction, stability, and spatial distribution of retained austenite. The first annealing step produced a ferritic matrix containing retained austenite, part of which transformed to strain induced martensite during subsequent cold rolling. During the second annealing step, deformation-inherited austenite ( ) and newly formed austenite ( ) developed in different fractions. X-ray diffraction, electron microscopy, tensile testing, and quasi in-situ magnetic measurements show that the tensile response is governed not only by retained austenite fraction, but also by its stability and spatial distribution. Heterogeneous, relatively stable retained austenite containing both and delays strain-induced martensitic transformation and promotes discontinuous yielding. In contrast, a more homogeneous and less stable -dominated microstructure activates transformation-induced plasticity earlier and more gradually, leading to continuous yielding and stable plastic flow while maintaining approximately 850 MPa yield strength, 1 GPa ultimate tensile strength, and 30% total elongation. At excessive effective annealing severity, achieved either by higher annealing temperature or longer holding at intermediate temperatures, the least mechanically stable retained austenite, together with fresh martensite, promotes serrations in the stress-strain curve associated with dynamic strain aging. These results demonstrate that mitigating flow discontinuities in MMnS requires a balanced retained-austenite architecture, combining sufficient retained-austenite fraction with controlled stability and chemical–morphological homogeneity.
Keywords:
Intercritical annealing
Retained austenite
Flow discontinuity
Mechanical properties
Medium-Mn Steels
APT
atom probe tomography
ARΩS
Aarhus Rapid Ohmic Sintering furnace
BCC
body centered cubic
BCT
body centered tetragonal
DSA
dynamic strain aging
EBSD
electron backscatter diffraction
EDS-TEM
energy dispersive X-ray spectroscopy in TEM
FCC
face-centered cubic
FIB
focused ion beam
FM
fresh martensite
FWHM
full width at half maximum
HAGB
high angle grain boundary
HEXRD
high energy X-ray diffraction
IA
intercritical annealing
IA1
first intercritical annealing
IA2
second intercritical annealing
IPF
inverse pole figure
IQ
image quality
LAGB
low angle grain boundary
MMnS
medium-Mn steels
PLC
Portevin–Le Chatelier
RA
retained austenite
SAED
selected-area electron diffraction
SFE
stacking fault energy
SHR
strain hardening rate
SIM
strain induced martensite
STEM
scanning transmission electron microscopy
TKD
transmission Kikuchi diffraction
TRIP
transformation-induced plasticity
TWIP
twinning-induced plasticity
UTS
ultimate tensile strength
XRD
X-ray diffraction
YPE
yield-point elongation
YS
yield strength
newly formed austenite
pre-existing austenite
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