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

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 paramount relevance for lightweight engineering design strategies and corresponding CO 2 savings, Figure 1. [1,2] In this work, we report about 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.Hardening in these materials is realized by combining the TRIP effect with a maraging treatment [transformationinduced plasticity (TRIP); 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. [3][4][5][6][7][8][9][10][11] 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. [12][13][14][15][16][17] While both types of alloys, i.e., TRIP steels [3][4][5][6][7][8][9][10][11] and maraging steels [12][13][14][15][16][17] 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. [18,19] We refer to these alloys as maraging TRIP steels.Related steel design trends that are based on small-scaled second phase precipitates are also pursued by using oxide, [20][21][22][23] nitride, [24,25] or Cu nanosized particles. [26] Other pathways to the design of ultrahigh strength steels have been realized in ultra fine grained materials obtained by advanced thermomechanical processing [27][28][29][30][31][32][33][34] or by accumulative roll bonding. [35] The joint maraging TRIP approach justifies a more detailed study since it opens up a new approach to increase the UTS of conventional steels at relatively lean alloying costs, i.e., without large quantities of expensive alloying elements, owing to the small volume fraction of precipitates required in maraging steels. [12][13][14][15][16][17] The negative side of such strategies lies often in the fact that an increase in tensile strength is typically accompanied by a corresponding dro...

Characterisation of L21-ordered Ni2TiAl precipitates in Fe Mn maraging steels

“…Recently, a group of newly-developed 9-12 wt.% Mn lean maraging TRIP (transformation induced plasticity) steels with minor additions of Ni, Ti and Mo was reported to possess an excellent combination of strength and ductility (e.g. 12 wt.% Mn alloy: ultimate tensile strength (UTS): ~1.3 GPa, total elongation (TE): ~21%) [4] [5]. The authors attributed the simultaneous increase of strength and ductility upon aging to the joint effect of precipitation strengthening mechanism and TRIP mechanism of retained austenite.…”

“…However, the characterisation of precipitates in these Fe-Mn maraging steels is a matter of debate. Initially, both atom probe tomography (APT) and local energydispersive X-ray spectroscopy (EDS) analyses revealed the precipitates were enriched in Ni, Al and Ti and it was speculated that the precipitates might be -Ni 3 (Ti,Al) phase which is the most common precipitates in conventional 18Ni maraging steels [4] [5]. However, later…”

Predicting microstructure and strength of maraging steels: Elemental optimisation

“…It is assumed that the reverted austenite has no effect in the 4 Since the growth and coarsening equations in 9 and 13 evolve according to ∼t 1/2 and ∼t 1/3 , respectively, a sharp variation in r p can be observed at the transition when .…”

“…Schober et al [53] observed in Fe Ni Al Mo the formation of -NiAl. 2 Small additions of Ti partition into NiAl forming Ni 2 AlTi in Fe Ni Mn Al Ti when nickel content is low (≤4 wt%) [4,25,54], whilst very low amounts of Mn atoms partition into the precipitates. No Mo partitioning to NiAl and its variants in 1 Although BCC-Cu is not an intermetallic phase, the same evolution descriptions will be adopted for all precipitates.…”

“…Systematic experimental studies have been performed to explore optimal compositions for improving the mechanical properties of maraging steels [3][4][5][6][7], whilst optimisation algorithms have been postulated to link alloying additions with strength variations [8][9][10]. Although the optimisation methods show good correlation with experiments, no detailed microstructural information is predicted; this lim * * Corresponding author.…”