D. Raabe - Alloy Design, Combinatorial Synthesis, and Microstructure Property Relations for Low-Density Fe-Mn-Al-C Austenitic Steels.pdf

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JOM, Vol. 66, No. 9, 2014

DOI: 10.1007/s11837-014-1032-x

2014 The Minerals, Metals & Materials Society

Alloy Design, Combinatorial Synthesis, and Microstructure–

Property Relations for Low-Density Fe-Mn-Al-C Austenitic

Steels

D. RAABE,

1,3

H. SPRINGER,

I. GUTIERREZ-URRUTIA,

1,4

F. ROTERS,

M. BAUSCH,

J. -B. SEOL,

M. KOYAMA,

P. -P. CHOI,

and K. TSUZAKI

1.—Max-Planck-Institut fur Eisenforschung, 40237 Dusseldorf, Germany. 2.—Department of

Mechanical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, Fukuoka 819-0395,

Japan. 3.—e-mail: d.raabe@mpie.de. 4.—e-mail: i.gutierrez@mpie.de

We present recent developments in the eld of austenitic steels with up to 18%

reduced mass density. The alloys are based on the Fe-Mn-Al-C system. Here,

two steel types are addressed. The rst one is a class of low-density twinning-

induced plasticity or single phase austenitic TWIP (SIMPLEX) steels with

25–30 wt.% Mn and

<4–5

wt.% Al or even

wt.% Al when naturally aged.

The second one is a class of

j-carbide

strengthened austenitic steels with even

higher Al content. Here,

j-carbides

form either at 500–600°C or even during

quenching for

>10

wt.% Al. Three topics are addressed in more detail,

namely, the combinatorial bulk high-throughput design of a wide range of

corresponding alloy variants, the development of microstructure–property

relations for such steels, and their susceptibility to hydrogen embrittlement.

INTRODUCTION TO LOW DENSITY STEELS

Reducing energy consumption in conjunction with

improving safety standards is a paramount target in

modern mobility concepts. Hence, the development

of strong, tough, and ductile steels for automo-

tive applications is an essential topic in steel

research.

1–15

In this context, twinning-induced

plasticity (TWIP) steels with up to 30 wt.% Mn and

>0.4

wt.% C content have shown an excellent

combination of ductility and strength.

16–34

Increas-

ingly, the reduction in mass density of TWIP steels

becomes an additional challenge. Two effects enable

such efforts. The rst one is that Mn increases the

face-centered cubic (fcc) lattice parameter. The sec-

ond one is that very high Mn and C alloying stabi-

lizes the austenite, so that it can tolerate Al

additions up to about 10 wt.% without becoming

unstable, i.e., transforming into body-centered cubic

(bcc)-ferrite.

Such an alloy concept sustains many advantages

associated with TWIP steels, e.g., mechanical

twinning and very high strain hardening;

16,19,24

yet,

it enables density reductions of up to 18% (Fig.

1).

Hence, alloys based on the quaternary system

Fe-Mn-Al-C are specically promising for the design

of low-density TWIP steels.

36–47

Regarding the excellent mechanical properties of

TWIP steels, which are characterized by the tran-

sition from dislocation and cell hardening to mas-

sive mechanical twinning, it has to be considered

that Al increases the stacking fault energy

(SFE).

17,18

This means that the overall strain-

hardening behavior and the onset of mechanical

twinning in density-reduced TWIP grades

may

differ from those observed in conventional TWIP

steels.

17,19,24

However, alloys based on the Fe-Mn-Al-C system

offer an even larger variety in deformation and

strain-hardening mechanisms than those associated

with the TWIP effect alone. This is due to the

characteristic dislocation substructures and the

higher number of phases present in the Fe-Mn-Al-C

system, namely, fcc-austenite, bcc-ferrite, and

ordered structures such as DO

and L’1

-type car-

bides. Depending on composition, low-density steels

can assume austenitic structure for the composition

regime Mn: 15–30 wt.%, Al: 2–12 wt.%, and C: 0.5–

1.2 wt.%. To combine the advantages of the TWIP

mechanisms with the reduction in specic weight,

(Published online June 24, 2014)

1845

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1846

Raabe, Springer, Gutierrez-Urrutia, Roters, Bausch, Seol, Koyama, Choi, and Tsuzaki

Fig. 1. Density reduction in Fe-Mn-Al-C TWIP steels as a function of

Al content.

Fig. 2. Specic energy absorption values of selected Fe-Mn-Al-C

steels in comparison with different conventional deep-drawing steels

(Color gure online).

this alloy range is hence the most promising one.

When increasing the Al content to a range

>6–8

wt.%, strain hardening in these steels is less

dominated by the TWIP effect but instead by the

formation of nanosized L’1

-type carbides, so-called

j-carbides.

44,45,48

Density-reduced steels with ferritic structure have

compositions in the range Mn

8 wt.%, Al: 5–8

wt.%, and C

0.3 wt.%. Corresponding complex

grades, consisting of austenite and ferrite, can be

synthesized by using compositions Mn: 5–30 wt.%,

Al: 3–10 wt.%, and C: 0.1–0.7 wt.%.

Besides these

compositions, ordered D0

structures, i.e., near-fer-

ritic Fe-Al-Cr alloys without Mn, have also been

addressed in the past in the context of density-re-

duced alloy design.

When comparing the synthesis and properties

among the different classes of weight reduced steels,

alloys based on the austenitic Fe-Mn-Al-C system

are most attractive due to their superior strain

hardening, high energy absorption (Fig.

2),

high-

density reduction and robust response to minor

changes in composition and processing.

37–47

Even

thin-strip casting with associated in-line hot rolling

has been successfully conducted in our group as a

pathway for efcient small-scale manufacturing of

such grades.

Recent publications on austenitic Fe-Mn-Al-C

alloys have reported yield strength values of 0.5–

1.0 GPa, elongations to fracture in the range 30–

80%, and ultimate tensile strength in the range of

1.0–1.5 GPa.

37–47

When blended with an Al content below 5 wt.%, a

single austenite phase prevails at room tempera-

ture, showing excellent strain hardening, which was

attributed to the hierarchical evolution of the

deformation substructure.

Al also promotes the

formation of nanoprecipitates upon aging with L’1

structure and approximate stoichiometry of (Fe,

Mn)

AlC.

44,45,48

These phases are referred to as

j-carbides.

They belong to the group of non-oxide

perovskites. Because of their ordered fcc structure,

j-carbides

have a lattice mismatch below 3% with

respect to an austenitic Fe-Mn-C matrix phase and

can hence form cuboidal nanoprecipitates.

44,48

When embedded in a ferritic matrix, the lattice

mismatch can be as large as

$6%,

45,48

which leads

to semicoherent interfaces and, hence, different

precipitate morphologies.

This article provides a concise introduction to

some recent developments in the eld of low-density

Fe-Mn-Al-C TWIP steels placing attention on alloy

design, synthesis routes, and microstructure–prop-

erty relations. We also provide a brief outlook on

pending questions associated with the role of

j-carbides

on strain hardening and hydrogen

embrittlement.

BULK COMBINATORIAL DESIGN

OF LOW-DENSITY AUSTENITIC STEELS

Here, we use a combinatorial approach for rapid

trend screening and alloy maturation of metallur-

gically melted and processed Fe-Mn-Al-C low-den-

sity TWIP and

j-carbide

hardened steels. The

approach is referred to as rapid alloy prototyping

(RAP).

We apply it here to one group of Fe-30Mn-

1.2C-xAl (wt.%) TWIP steels and to a second group

of Fe-20Mn-0.4C-xAl TWIP steels, both with vary-

ing Al content (x) and different aging conditions.

In both cases, the samples were synthesized by

melting and casting in a vacuum-induction melting

(VIM) furnace under 400 mbar Ar pressure. The

system was modied to enable synthesis of ve dif-

ferent alloys in one operation for each alloy system.

We used ve Cu molds, which could be moved

stepwise inside the furnace. They were successively

lled with melt from a 4-kg ingot. After each cast,

the remaining melt composition in the ingot was

adjusted by charging Al through an air lock. After

cooling and cutting, the 10

130-mm

-sized

blocks were hot rolled at 1100°C into 2

0.1-mm-

Author's personal copy

Alloy Design, Combinatorial Synthesis, and Microstructure–Property Relations

for Low-Density Fe-Mn-Al-C Austenitic Steels

1847

thick and

$500-mm-long

sheets. These were

reheated to 1100°C, water quenched, and cut per-

pendicular to the rolling direction into sets of nine

segments with dimensions 2

55 mm

for

each alloy composition. Homogenization was per-

formed at 1100°C for 2 h under Ar, followed by

water quenching. Aging was conducted in air at

450°C, 500°C, 550°C, and 600°C for 0 h, 1 h, and

24 h at each temperature, followed by oil quenching.

This results in a matrix of 45 different sample con-

ditions. Scales were removed from the surfaces by

low-pressure, ne-grit sandblasting after the heat

treatments. Samples for mechanical testing and

microstructure investigation were prepared from

the segments by package spark erosion. Tensile

testing was conducted at room temperature with an

initial strain rate of 10

À3

À1

. All values plotted

represent averages of three measurements for every

material state. Cross-sectional areas of selected

samples were prepared in the plane perpendicular

to the rolling direction by grinding and polishing

with standard metallographic techniques. X-ray

diffraction (XRD) analysis was performed on the

rolling plane of samples ground to a thickness of

1 mm. Further details of the method are explained

in Ref.

49.

The RAP method enabled us to screen two dif-

ferent sets of ve Fe-Mn-C-based weight-reduced

Al-containing compositions each exposed to nine

respective heat treatments within 35 h. For each

alloy base set, synthesis, processing, mechanical

screening, and phase characterization are included.

The metallographic analysis showed no cracks,

pores, or macrosegregations in the nal materials.

The as-cast samples had a coarse dendritic micro-

structure. Hot rolling and water quenching resulted

in a fully recrystallized microstructure with a grain

size of

$20

with some retained microsegrega-

tions of Mn.

Figure

shows the mechanical properties of 45

different material conditions obtained for the Fe-

30Mn-1.2C base composition (i.e., in total 135 ten-

sile tests and hardness measurements) in terms of

the yield strength (YS, Fig.

3a),

ultimate tensile

strength (UTS, Fig.

3b),

total elongation (TE,

Fig.

3c),

and hardness (Fig.

3d).

The results are

plotted according to the systematically varied Al

content. Color-coding reects individual aging con-

ditions. The data are reproduced from an earlier

publication.

They show a clear dependence of the

mechanical behavior on both composition and heat

treatment.

For the reference material (no Al addition, i.e.,

ternary Fe-30Mn-1.2C alloy), the best mechanical

behavior is found for the as-homogenized state,

namely, 360 MPa YS, high work hardening

(830 MPa UTS), and high ductility (77% TE). Aging

of the Fe-30Mn-1.2C alloy leaves the YS virtually

unchanged and increases the hardness slightly.

However, it reduces UTS and TE. Embrittlement

becomes most apparent for long aging times (24 h)

and higher temperatures (>500°C). For the alloy

Fe-30Mn-1.2C-8Al, i.e., the material with the high-

est Al content and lowest density, the opposite trend

applies. Without aging, the mechanical response is

similar to that of the Al-free alloy. Only a minor

change in YS (increase), UTS, and TE (decrease) is

found. Aging for 1 h leads to an increase in YS,

UTS, and hardness (increasing with temperature)

and only to a minor reduction in TE. Aging of the

alloy Fe-30Mn-1.2C-8Al for 24 h further increases

YS, UTS, and hardness to levels twice as high as in

the as-homogenized state and drastically reduces

ductility. The mechanical data for the alloys with

intermediate Al contents fall between the two dif-

ferent behaviors described above, i.e., alloys with 2–

6 wt.% Al have properties between the respective

values from materials without Al and with 8 wt.%

Al. Especially the alloys with Al additions of 4 wt.%

and 6 wt.% are only very weakly affected by the

applied aging treatments in terms of their

mechanical data compared with the alloys Fe-30Mn-

1.2C (weakening/embrittlement) and Fe-30Mn-

1.2C-8Al (strengthening). To better extract the

mechanical trends for the investigated compositions

and heat treatments, selected results are summa-

rized in Fig.

By using the RAP approach, we nd for the

30 wt.% Mn system that high amounts of Al

($8 wt.%) result in pronounced strengthening dur-

ing aging, depending on time and temperature

(Fig.

4).

The observed effects are attributed to the

formation of nm-sized

j-carbides

during aging. The

intermediate alloy variants with Al concentrations

of 2–6 wt.% do not show equivalent mechanical

properties compared to the

wt.% Al variant.

Also, a much smaller inuence of aging on the ten-

sile behavior can be observed in the 2–6 wt.% Al

cases. A detailed and systematic screening of the

stoichiometry, interface structure, lattice mist,

and mechanical effects of the nm-sized

j-carbides

still pending, but rst results suggest that they can

occur in a wide compositional existence range

(Fig.

5).

Also it is observed that they are thermally

very stable at 600°C and even prevail after 100 h

heat treatment with only modest coarsening

(Fig.

5).

For the second alloy class investigated, namely,

the Fe-20Mn-0.4C-xAl TWIP steels, a more homo-

geneous trend of the mechanical properties as a

function of the Al content is observed (Fig.

6).

While

yield strength and ultimate tensile strength both

increase, the tensile elongation drops as a function

of the Al content in the range between 2 wt.% and

11 wt.% Al. As observed by XRD screening of the

corresponding samples, the increasing ferrite con-

tent seems to be more relevant for this trend than

the presence of

carbides or

e-martensite.

Author's personal copy

1848

Raabe, Springer, Gutierrez-Urrutia, Roters, Bausch, Seol, Koyama, Choi, and Tsuzaki

Fig. 3. Overview of the obtained mechanical properties from RAP experiments as a function of alloy composition and applied aging treatment:

(a) yield stress (YS), (b) ultimate tensile stress (UTS), (c) total elongation (TE), (d) hardness. (RAP: Rapid Alloy Prototyping). Data and gures

are reproduced from an earlier publication

(Color gure online).

MICROSTRUCTURE–PROPERTY

RELATIONS FOR LOW-DENSITY

AUSTENITIC STEELS

When deriving microstructure–property relation-

ships for austenitic low-density steels, two scenarios

must be considered: For compositions with

25–30 wt.% Mn and up to 4–5 wt.% Al or even

8 wt.% Al, when naturally aged, the TWIP effect

prevails as deformation mechanism.

41,48

Thus,

dislocation-based strain hardening, which is fol-

lowed at higher loads by mechanical twinning, is

the primary strain-hardening mechanism. Such

low-density TWIP alloys are referred to as single

phase austenitic TWIP (SIMPLEX) steels.

After

aging at 500–600°C or for alloys with higher Al

content, twinning is reduced and strain hardening

is essentially associated to

j-carbide

and solid-

solution strengthening (Figs.

and

6).

If the Al content exceeds 10 wt.%, then

j-carbides

can form during quenching. The exact strain-hard-

ening mechanism for the latter case is still under

investigation. In a recent study, Gutierrez-Urrutia

and Raabe

suggested that the prevalent deformation

Fig. 4. Selected mechanical testing results from RAP experiments:

Tensile strength and total elongation for alloys Fe-30Mn-1.2C

through Fe-30Mn-1.2C-8Al. Some data were taken from Ref.

(Color gure online).

Author's personal copy

Alloy Design, Combinatorial Synthesis, and Microstructure–Property Relations

for Low-Density Fe-Mn-Al-C Austenitic Steels

1849

Fig. 5. (a) Joint atom probe tomographic and (b) Transmission Electron Microscopy (TEM) analysis of

j-carbides

in a Fe-30Mn-1.2C-8Al low-

density steel after 24 h at 600°C. This sample was synthesized after identifying suited compositions by the RAP process (Color gure online).

mechanisms in austenitic steels that are strengthened

by regularly arranged nanosized

j-carbides,

such as

those shown in Fig.

consist in Orowan bypassing of

longitudinal rods of

j-carbides

and subsequent

expansion of dislocation loops, which is assisted by

dislocation cross-slip and, to a minor extent, shearing

j-carbides.

They further suggest that the higher dislocation

densities observed within the dislocation bundles are

caused by minor topological differences in the spac-

ing between the

j-carbide

rods, so that dislocations

follow preferential soft paths within the widest

channels. In certain cases, these dislocation accu-

mulations can lead to stress values that are high

enough to shear

j-carbide

interfaces. Although these

complex nanoscale interactions are still subject to

further analysis, we give below some suggestions

how to incorporate such effects into a mean eld

dislocation density-based model of strain hardening

j-carbide

containing Fe-Mn-Al-C steels.

A general form of a temperature-sensitive consti-

tutive model for fcc metals with low stacking fault

energy and mechanical twinning, depending on

chemical composition, deformation rate, and tem-

perature, was introduced by Steinmetz et al.

extends the three-internal-variable model of Roters

et al.

with a physical description of twin nucleation.

The model is based upon experimental observations

performed on Fe-Mn-C TWIP steels (without

j-car-

bides) by electron channeling contrast imaging,

Transmission Electron Microscopy (TEM), and

electron backscatter diffraction (EBSD).

19–34

These

works revealed that the important microstructural

internal state variables in such alloys include dislo-

cations, grain size, mechanical twins, and dislocation

cells. In this regard, this model provides a quantita-

tive description of strain-hardening behavior of low-

density TWIP steels, i.e., SIMPLEX steels. Following

the comments made above, strain hardening in

carbide-containing steels can be described in a

seamless fashion where the activation barrier for

twinning can vary as a function of the Al content and

a carbide-rod-dependent Orowan loop mechanism

can be introduced in the form of a corresponding

stress term in the kinetic equation of state. The latter

formulation is not yet presented here.

In this section, we provide a summarized

description of the temperature-sensitive constitu-

tive model of strain hardening for single-phase low-

density TWIP steels, i.e., SIMPLEX steels. The

model uses three different dislocation densities (q

, and

) and the volume fraction of mechanical

twins (f

) as state variables. The three dislocation

densities are those in the cell interior, in the cell

walls, and dipoles. The evolutions of these state

variables represent the microstructural changes

that occur during plastic deformation. The evolution

of the dislocation densities is given by the following

equations:

1 2d

(1)

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