Ultrastrong steel via minimal lattice misfit and high-density nanoprecipitation

Abstract

Next-generation high-performance structural materials are required for lightweight design strategies and advanced energy applications. Maraging steels, combining a martensite matrix with nanoprecipitates, are a class of high-strength materials with the potential for matching these demands. Their outstanding strength originates from semi-coherent precipitates, which unavoidably exhibit a heterogeneous distribution that creates large coherency strains, which in turn may promote crack initiation under load. Here we report a counterintuitive strategy for the design of ultrastrong steel alloys by high-density nanoprecipitation with minimal lattice misfit. We found that these highly dispersed, fully coherent precipitates (that is, the crystal lattice of the precipitates is almost the same as that of the surrounding matrix), showing very low lattice misfit with the matrix and high anti-phase boundary energy, strengthen alloys without sacrificing ductility. Such low lattice misfit (0.03 ± 0.04 per cent) decreases the nucleation barrier for precipitation, thus enabling and stabilizing nanoprecipitates with an extremely high number density (more than 10 per cubic metre) and small size (about 2.7 ± 0.2 nanometres). The minimized elastic misfit strain around the particles does not contribute much to the dislocation interaction, which is typically needed for strength increase. Instead, our strengthening mechanism exploits the chemical ordering effect that creates backstresses (the forces opposing deformation) when precipitates are cut by dislocations. We create a class of steels, strengthened by Ni(Al,Fe) precipitates, with a strength of up to 2.2 gigapascals and good ductility (about 8.2 per cent). The chemical composition of the precipitates enables a substantial reduction in cost compared to conventional maraging steels owing to the replacement of the essential but high-cost alloying elements cobalt and titanium with inexpensive and lightweight aluminium. Strengthening of this class of steel alloy is based on minimal lattice misfit to achieve maximal precipitate dispersion and high cutting stress (the stress required for dislocations to cut through coherent precipitates and thus produce plastic deformation), and we envisage that this lattice misfit design concept may be applied to many other metallic alloys.

A Review on Nano-Scale Precipitation in Steels

“…Finally, the paper is wrapped up with the challenges and the future directions of nano-scale precipitate strengthened steels. [28] (Fe,Mn) 3 AlC 0.38 [6], (Fe,Mn) 3 AlC 0.51 [6] M 2 C carbides Hexagonal [18] (Mo,Cr) 2 C [17]; (Mo,Cr,V) 2 C [30] Co-located with Cu on lath boundaries and dislocations [17] Rod (coherent) [17]; Irregular (incoherent) [17] Ni 3 Ti Hexagonal (DO 24 ) [36,37] → L1 2 [37] → FCC [37] (Ni,Fe,Co) 3 (Ti, Mo) [14] Homogeneously throughout matrix, heterogeneous nucleation on interphases, dislocations and grain boundaries [36] Disc and rod [14,36] NiAl B2 [3,15] Ni, Al, Mn, Fe, Cu [15,35]; Ni, Al, Fe, Mn [2]; Ni(Al,Fe) [3]; Ni, Al, Fe, Cr, Mo [38] Co-located with Cu [15]; Mostly homogeneously distributed in matrix and some elongated particles on dislocations [3] Spherical [2,3] [7] Cu and (Ti,Mo)C Fe-1.53Mn-1.17Cu-0.34Si-0.21Mo-0.09Ti-0.04Al-0.07C 732 13 [16] * Monzen et al [21] argued that the B2 to 9R transformation depends on the surrounding temperature. At 500 • C, the transformation occurs at 12 nm instead of 4 nm.…”

“…Among them, nano-scale Copper (Cu)-rich precipitates are able to transform martensitically from the Body Centered Cubic (BCC) to Face Centered Cubic (FCC) phase during aging [19][20][21][22] or deformation [23,24]. Ultrafine nanometer sized-Nickel Aluminium (NiAl) precipitates also display unusual mechanical properties [1][2][3]. NiAl is a brittle intermetallic compound and incorporating NiAl precipitates in steels can yield a marked strength increment, but often cause a dramatic drop of ductility to less than 1%, leading steels to low engineering uses [25].…”

“…Some recently investigated precipitates [2][3][4]7,[14][15][16][17][18] in steels from year 1987 to 2017 are listed in Table 2. These nano-scale precipitates are metastable.…”

“…Precipitation also plays a very important role in stabilizing the microstructure, inhibiting grain growth, and suppressing the recrystallization of steels with ultrafine grain size down to several nanometers [5]. The common processing techniques to incorporate precipitates in steels include precipitation through high temperature solutionization, followed by quenching and further aging at a relatively lower temperature (as depicted in Figure 1 and Table 1) [1][2][3][4]6,7], diffusion reaction techniques, such as nitriding and carburizing [8,9], and mechanical alloying [10,11]. The incorporation of precipitates through diffusion reaction techniques and mechanical alloying will not be discussed in this paper, as these techniques require long processing time [9,12] and can sometimes involve expensive processing routes [13].…”

“…Various precipitates [1][2][3][4] have been used to produce high grade steels with strength that is greater than 1 GPa. Precipitation also plays a very important role in stabilizing the microstructure, inhibiting grain growth, and suppressing the recrystallization of steels with ultrafine grain size down to several nanometers [5].…”