Mechanics in Steels through Microscopy

The goal of the study consolidated in this thesis is to understand the mechanics in steels using microscopy. In particular, the mechanical response of Transformation Induced Plasticity (TRIP) steels is correlated with their microstructures. Chapter 1 introduces the current state of the art of TRIP steels, highlighting the importance of microstructure - mechanical properties - applications relationships. In Chapter 2 the material properties and material processing are described into more detail, and an overview is presented of the characterization techniques used in this thesis. For the purpose of understanding the structure-property relationships, two TRIP steels (Al alloyed TRIP 700 and Si alloyed TRIP 800) and one TRIP assisted Multi Phase or TAMP (Nb microalloyed TAMP 800) steel were investigated in this study. The numbers (700, 800) designate the ultimate tensile strength of the materials. Despite the fact that steels are amongst the most extensively investigated materials, many features of the austenite-martensite transformation process during deformation remain unclarified to date. Deformation induced transformation behavior of retained austenite grains is assessed in Chapter 3 with the help of Electron Back Scattered Diffraction (EBSD) and X-Ray Diffraction (XRD) techniques. The transformation tendency of retained austenite grains at the surface and inside the bulk as a result of uni-axial tension was investigated on TRIP 800 steel sheets. The experiments showed that the retained austenite grains at the surface have a stronger tendency to transform than the retained austenite grains in the bulk of the material. Local defects, such as twins, lower the stability of austenite grains and thus transform at the earlier stages of the deformation process. Austenite grains positioned at grain boundaries between multiple ferrite grains were amongst the first to transform, whereas austenite grains that are completely embedded in larger ferrite grains were often found to undergo rotations. These differences in strain-induced behavior are likely associated with different stress states and grain rotation capabilities of the austenite grains upon deformation. The rotation angles of embedded austenite grains were found to depend on the crystallographic orientation of the grains with respect to the straining direction. A lowering of Schmid factors of embedded austenite grains is seen to be the reason for the delayed transformation of these grains during the deformation process. The current findings suggest that grain rotation is an additional mechanism contributing to the high ductility of these steels and may be as important as the contribution of the austenite–martensite transformation. Furthermore, EBSD experiments were carried out on TRIP 700 steels which confirmed that the grain rotation behavior is consistent in both TRIP steels. In addition to the austenite-to-martensite phase transformation, the role of alloying elements such as C, Mn, Si, and Al, and their effect on transformations during deformation, are poorly understood and there is an increasing need to gain fundamental insight into this topic. Consequently, in Chapter 4, a combination of Electron Back Scattered Diffraction (EBSD) and high-sensitivity Electron Probe Microanalysis (EPMA) was used to correlate the changes in microstructural features of metastable austenite grains upon deformation along with their respective local chemical compositions in TRIP 700 and TRIP 800 steels. In contrast to current metallurgical beliefs, we found direct evidence that austenite grains with higher Mn content transformed earlier than the grains with lower Mn contents. Various microstructural factors such as the local carbon concentrations, grain size, morphology, crystal lattice orientation in relation to strain direction, location of the grain in relation to its surrounding grains, and the presence of local defects (twins) were investigated. Analysis performed on several twinned austenite grains showed higher enrichment of Mn contents and an early transformation during deformation. At a very local scale, the austenite grains with high Mn contents have an inhomogeneous Mn distribution which can result in local strain acting as a trigger for early transformation. The local variation in manganese content therefore allows for an overall gradual transformation of the retained austenite grains, providing another tunable parameter for deformation induced austenite-to-martensite transformation. Multiphase steels designed with composite strengthening may be further strengthened by grain refinement or precipitation by the addition of microalloying elements. In Chapter 5 we have therefore investigated microstructures and precipitates in a Nb microalloyed TRIP assissted multiphase (TAMP) steel containing ferrite, bainite, martensite and retained austenite. Precipitates were observed in the ferrite, martensite and bainite phases, but not in the austenite phase. Two kinds of precipitates were found in this steel: NbC and novel (Nb,Ti)N phases. The (Nb,Ti)N precipitates were found in different sizes, ranging from 10-150 nm, whereby the larger precipitates have a Ti/Nb atomic ratio of approximately 2.83 - 0.81 and were faceted in shape. Smaller NbC precipitates were found in a size range of 5-20 nm. The (Nb,Ti)N precipitates have a well-defined Nishiyama Wasserman (N-W) orientation relationship with the ferrite matrix, while the NbC precipitates have random orientations. Both NbC and (Nb,Ti)N carbide precipitates are expected to take part in strengthening of these steels. The comparison of calculated lattice parameters and experimental lattice parameters indicates that both precipitates are deficient in carbon and nitrogen contents. In Chapter 6 the investigations on nano-sized precipitates in Al alloyed TRIP 700 steel with Ti micro additions are described. Two types of Ti(N) precipitates were observed with cubic and faceted morphologies. The faceted precipitates were found in a size range of 40-70 nm and the cubic precipitates in a size range of 70-120 nm. The cubic precipitates had higher nitrogen concentrations and exhibited slightly larger lattice parameters than the faceted TiN precipitates. Additionally, spherical shaped Ti2CS precipitates were found with sizes ranging from 20 to 100 nm. Both the cubic and faceted Ti(N) precipitates and Ti2CS precipitates were found in random orientations with respect to the ferrite matrix. A novel iron carbide precipitate was observed in all ferrite grains. These Fe(C) precipitates have sizes ranging from 2 to 5 nm and exhibited disc-shaped morphologies. All the Fe(C) precipitates showed a well-defined Pitsch Schrader (P-S) orientation relationship with the ferrite matrix. From the precipitate size, inter-particle spacing, and lattice misfit calculations, the newly observed Fe(C) precipitates are considered to possess significant potential in enhancing the strength of these steels. Electron microscopy experiments were also carried out on Si alloyed TRIP 800 steels which confirmed that ultrafine Fe(C) precipitates exist in both TRIP counterparts. The elements Chromium (Cr) and Manganese (Mn) are well known in technologically important iron and steel alloys. In Chapter 7 it is revealed for the first time that Cr and Mn have combined to form two novel ternary carbides. These novel ternary carbide phases were identified in TRIP-assisted multi phase (TAMP) steel. The carbides were characterized using transmission electron microscopy (TEM), electron diffraction and density functional theory (DFT) calculations. Electron diffraction analysis revealed that the Orthorhombic carbide has lattice parameters a = 5.09 Å, b = 6.98 Å, and c = 4.55 Å, with a Pnma space group, while the Monoclinic carbide has lattice parameters a = 6.83 Å, b = 4.54 Å, c = 5.00 Å, and ? = 92.2° with a P21/c space group. Excellent agreement was found between calculation and experiment on the lattice parameters and relative atomic positions. Remarkable Mn-Cr alloying leads to these ternary phases having high thermodynamic stabilities. The calculations have shown that introduction of carbon atoms strongly induce magnetism within these structures and that these carbides are stable also at higher pressures.Kavli Institute of Nanoscience DelftApplied Science