Designing Ultrahigh Strength Steels with Good Ductility by Combining Transformation Induced Plasticity and Martensite Aging

 Steels with a high ultimate tensile strength (UTS) above 1 GPa and good ductility (total elongation (TE) of 15-20% in a tensile test) are of greatest relevance for lightweight engineering design strategies and corresponding CO2 savings. In this project we work on a novel design approach for precipitation hardened ductile high strength martensitic and austenitic-martensitic steels (up to 1.5 GPa strength).

Dierk Raabe, Dirk Ponge, Olga Dmitrieva, Benedikt Sander

 Abstract

Steels with a high ultimate tensile strength (UTS) above 1 GPa and good ductility (total elongation (TE) of 15-20% in a tensile test) are of greatest relevance for lightweight engineering design strategies and corresponding CO2 savings.

In this project we work on a novel design approach for precipitation hardened ductile high strength martensitic and austenitic-martensitic steels (up to 1.5 GPa strength). The alloys are characterized by a low carbon content (0.01 wt.% C), 9-15 wt.% Mn to obtain different levels of austenite stability, and minor additions of Ni, Ti, and Mo (1-2 wt.%). The latter are required for creating precipitates during aging heat treatment.

Steels with a high ultimate tensile strength (UTS) above 1 GPa and good ductility (total elongation (TE) of 15-20% in a tensile test) are of paramount relevance for lightweight engineering design strategies and corresponding CO2 savings.

Figure 1: Overview of the typical strength-ductility profiles of different types of steels. The strength is expressed in terms of the ultimate tensile strength measured during tensile testing and the ductility is expressed in terms of the total sample elongation. The data represent regimes such as published in the references given below. TRIP: transformation-induced plasticity; TWIP: twinning-induced plasticity; Complex phase: multiphase steels (e.g. austenitic-martensitic steels which may contain also bainite); maraging TRIP: new steel concept that includes hardening mechanisms based on transformation induced plasticity and the formation of intermetallic nanoparticles in the martensite during aging. The approach leads to an unexpected simultaneous increase in both strength and total elongation (green area) enhancing the regime of formable ultrahigh strength steels by 0.5 GPa.

Here we report about our novel design approach for precipitation hardened ductile high strength martensitic and austenitic-martensitic steels (up to 1.5 GPa strength). The alloys are characterized by a low carbon content (0.01 wt.% C), 9-15 wt.% Mn to obtain different levels of austenite stability, and minor additions of Ni, Ti, and Mo (1-2 wt.%). The latter are required for creating precipitates during aging heat treatment.

Hardening in these materials is realized by combining the TRIP effect with a maraging treatment (TRIP: transformation-induced plasticity; maraging: martensite aging through thermally stimulated precipitation of particles). The TRIP mechanism is based on the deformation-stimulated athermal transformation of metastable austenite (face centered cubic Fe-Mn phase) into martensite (metastable body centered cubic or orthorhombic Fe-Mn phase) and the resulting matrix and martensite plasticity required to accommodate the transformation misfit. The maraging treatment is based on hardening the heavily strained martensite through the formation of small intermetallic precipitates (of the order of several nanometers). These particles act as highly efficient obstacles against dislocation motion through the Orowan and Fine-Kelly mechanisms enhancing the strength of the material.

While both types of alloys, i.e. TRIP steels  and maraging steels have been well investigated, the combination of the two mechanisms in the form of a set of simple Fe-Mn alloys as suggested in this work, namely, the precipitation hardening of transformation-induced martensite by intermetallic nanoparticles, opens a novel and lean alloy path to the development of ultrahigh strength steels that has not been much explored in the past.

Figure 3: Microstructure results for the four alloys obtained via EBSD analysis (100 nm step size). Top row: phase distribution of a0-martensite (green) and of retained austenite (red). Bottom row: microtexture in terms of the {hkl} Miller triple color coding and high angle grain boundaries (black, >15 8). (a) Conventional 17 wt% Ni-11 wt% Co maraging steel; (b) Fe–Mn steel with 9wt% Mn; c) Fe–Mn steel with 12 wt% Mn; (d) Fe–Mn steel with 15 wt% Mn, Table 1.
Figure 4: High resolution EBSD analysis of the 15 wt% Mn steel showing details of the differentiation between retained austenite (red), a0-martensite (green), e-martensite (blue) (50 nm step size).
Figure 6: ATP measurements (IMAGO LEAP 3000x HR) conducted on the maraging alloy with 9wt% Mn. (a) Atomic distribution of the Ni content (green). The shaded zones indicate atomic Ni concentrations above 20 at.%. (b) Distribution of the Ni (green), Al (yellow), and Ti (magenta) clusters together with Al atoms (yellow). The shaded zones indicate an atomic Ni (green) concentration above 20 at.%, an atomic Al (yellow) concentration above 10 at.%, and an atomic Ti (magenta) concentration above 5 at.%. (a) (b)
Figure 7: High resolution EBSD maps taken on the 12 wt%Mn sample in the as-quenched plus aged state (a), and the same microstructure at uniform elongation in the deformed flat tensile specimen (b). The top row shows the phase content. The second row shows the texture in terms of the {hkl} Miller indices parallel to the normal direction. The third row shows the texture in terms of the Miller indices parallel to the rolling direction. The bottom row shows the average local misorientation (50 nm step size).
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