Uniaxial straining experiments were performed on a rolled and annealed Si-alloyed TRIP (transformation-induced plasticity) steel sheet in order to assess the role of its microstructure on the mechanical stability of austenite grains with respect to martensitic transformation. The transformation behavior of individual metastable austenite grains was studied both at the surface and inside the bulk of the material using electron back-scattered diffraction (EBSD) and X-ray diffraction (XRD) by deforming the samples to different strain levels up to about 20%. A comparison of XRD-and EBSD-results revealed that the retained austenite grains at the surface have a stronger tendency to transform than the austenite grains in the bulk of the material.The deformation-induced changes of individual austenite grains before and after straining were monitored with EBSD. Three different types of austenite grains can be distinguished that have different transformation behaviors: austenite grains at the grain boundaries between ferrite grains, twinned austenite grains, and embedded austenite grains that are completely surrounded by a single ferrite grain. It was found that twinned austenite grains and the austenite grains present at the grain boundaries between larger ferrite grains typically transform first, i.e. are less stable, in contrast to austenite grains that are completely embedded in a larger ferrite grain. In the latter case, straining leads to rotations of the harder austenite grain within the softer ferrite matrix before the austenite transforms into martensite. The analysis suggests that austenite grain rotation behavior is also a significant contributing factor for enhancement of the ductility.

Multiphase steels utilising composite strengthening may be further strengthened via grain refinement or precipitation by the addition of microalloying elements. In this study a Nb microalloyed steel comprising martensite, bainite and retained austenite has been studied. By means of transmission electron microscopy (TEM), we have investigated the size distribution and the structural properties of (Nb,Ti)N and NbC precipitates, their occurrence in the various steel phases, and their relationship with the Fe matrix. (Nb,Ti)N precipitates were found in ferrite, martensite, and bainite, while NbC precipitates were found only in ferrite. All NbC precipitates were found to be small (5 -20 nm in size) and to have a face centred cubic crystal structure with lattice parameter a = 4.36 ± 0.05 Å. In contrast, the (Nb,Ti)N precipitates were found in a broader size range (5 -150 nm) and to have a face centred cubic crystal structure with lattice parameter a = 8.09 ± 0.05 Å. While the NbC precipitates were found to be randomly oriented, the (Nb,Ti)N precipitates have a well-defined Nishiyama Wasserman (N-W) orientation relationship with the ferrite matrix. An analysis of the lattice mismatch suggests that the latter precipitates have a high potential for effective strengthening. DFT calculations were performed for various stoichiometries of NbC X and Nb X Ti Y N Z phases and the comparison with experimental data indicates that both the carbides and nitrides are deficient in C and N content.

A transmission electron microscopy (TEM) study was conducted on nanoprecipitates formed in Ti microalloyed TRIP assisted steels, revealing the presence of Ti(N), Ti 2 CS, and a novel type of ultra-fine Fe(C) precipitates. The matrix/precipitate orientation relationships, sizes and shapes were investigated in detail. The ultrafine, disc-shaped Fe(C) precipitates have sizes of 2-5 nm and possess a hexagonal close packed (hcp) crystal structure with lattice parameters a = 5.73 ± 0.05 Å, c = 12.06 ± 0.05 Å. They are in a well-defined Pitsch Schrader (P-S) orientation relationship with the basal plane of the precipitate parallel to the [110] habit plane of the surrounding body centred cubic (BCC) ferritic matrix. Detailed analysis of precipitate distribution, orientation relationship, lattice mismatch, and inter-particle spacing suggest that these ultrafine precipitates contribute considerably to the strengthening of these steels.

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Chromium (Cr), Manganese (Mn), and Carbon (C) are well known alloying elements used in technologically important alloy steels and advanced high strength steels. It is known that binary CrC x and MnC x carbides can be formed in steels, but in this study we reveal for the first time that Cr and Mn were found combined in novel ternary cementite type (Cr,Mn)C carbides. Electron diffraction experiments showed that Cr, Mn, and C have formed two distinct carbide phases possessing orthorhombic and monoclinic crystal structures. Density functional theory (DFT) calculations were performed on these phases and excellent agreement was found between calculations and experiments on the lattice parameters and relative atomic positions. The calculations showed that the combination of Mn and Cr has resulted in a very high thermodynamic stability of the (Cr,Mn)C carbides, and that local structural relaxations are associated with carbon additions. Possible implications of these ternary carbides for novel applications in steel design and manufacturing are discussed.

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