Deformation‐Induced Martensite: A New Paradigm for Exceptional Steels

Abstract

process. Here, we present the surprising discovery that Fe-C martensite can also be formed inside a pearlitic steel, i.e., a ferrite-cementite composite without any austenite, by a new and unexpected route: severe mechanical deformation. [ 10 ] Current developments for improving the mechanical properties of metals, including steels, aim at producing nanostructured materials by severe plastic deformation (SPD). [ 11,12 ] To this end, cold-drawn pearlitic-steel wires have been very successful, reaching an ultrahigh tensile strength of up to ca. 7 GPa, [ 13 ] making them one of the world's strongest bulk materials. Pearlitic steels are used, for example, in large constructions such as suspension bridges. The high strength is associated with the refi nement of the originally lamellar eutectoid body-centeredcubic (bcc) α-Fe (ferrite) + Fe 3 C (cementite) structure of the pearlite, which leads to a nanocomposite that is stabilized by carbon segregation to the α-Fe grain boundaries. [ 13,14 ] With ongoing structure refi nement, more and more carbon must be accommodated inside the ferrite due to the dissolution of the cementite. [ 15,16 ] As a consequence, the concentration of carbon in the α-Fe exceeds the equilibrium solubility limit by far. It is generally assumed that a high density of vacancies and dislocations [ 17 ] accommodate the excess carbon. [18][19][20][21] A recent study by Taniyama et al., [ 22 ] revealed a tetragonal distortion of the ferrite lattice in heavily drawn pearlitic steel, which could be a consequence of the carbon supersaturation of the Fe matrix. Despite many studies on this topic, the accommodation of carbon in ferrite and a possible phase transformation are still controversial.In this study, we have combined atom-probe tomography (APT) and synchrotron X-ray diffraction (XRD) to study the carbon supersaturation of ferrite for two pearlitic steel-wire compositions -eutectoid and hypereutectoid. Knowledge of the carbon accommodation in the ferrite provides control to design the strength and ductility of nanostructured pearlitic steels. True drawing strains, ε , of 0 to 6.52 were analyzed, exceeding by far the drawing strains studied previously. [ 22 ] The two compositions, the high strains, the combination of advanced chemical and structural characterization methods, and a supporting ab-initio-based theoretical description show that a new mechanism of martensite formation is triggered under the extreme deformation conditions that occur in the SPD-induced structural refi nement of ultrahigh strength pearlitic steels. Deformation-driven nanoscale phase transformation provides a new way to tailor the mechanical properties of nanostructured steels and steel surfaces.The synchrotron XRD results of the cold-drawn pearliticsteel wires reveal signifi cant microstructural changes induced by SPD. We focus on the evolution of the diffraction peaks of ferrite with drawing strain rather than on cementite decomposition. The four major diffraction peaks, {110}, {200}, {211} and Steel is the dominant structu...

Stress-controlled carbon diffusion channeling in bct-iron: A mean-field theory

“…lt is only significant for nanocrystals. In the limit of large crystals, we will use the simplified form (12) This formula involves the tensor of activation volume of the jump av = S(P T -P 0 ). In the case of pure ferrite under pressure p, P vanishes, and the enthalpy of migration takes the usual form H m = H/rpav, where av= v Tv 0 is the activation volume.…”

“…Various models of order-disorder transition during low temperature ageing [7][8][9][10][11] or severe plastic deformation [12,13) have been proposed in the literature. Sorne of them include the effect of particular mechanical loadings [7,14,15).…”

Carbon concentration measurements by atom probe tomography in the ferritic phase of high-silicon steels

“…Based on Zener's considerations, Kurdjumov and Khachaturyan [57] later developed a model for the order-disorder transition using their microscopic elasticity theory. In their recent, work Djaziri and Liet al [58]have developed a full thermodynamic approach of Zener coupling which also considers any type of internal or external lattice strains, such as from applied loads or microstructural strains of second or third order. These effects have been shown to change the critical transition temperature for Zener ordering in a direction to similar to the works of Kurdjumov and Khachaturyan, extending the stability region of bct [58].…”

Carbon nanotube fibers with martensite and austenite Fe residual catalyst: room temperature ferromagnetism and implications for CVD growth

“…Finally, it is interesting to enquire into the stabilization of FCC Fe and martensite in the CVD growth of CNTs. Both phases form in the bulk under severe quenching and martensite can form under severe plastic deformation of perlitic steel [32]. In the CVD method used in this work nanoparticles are expected to cool down rapidly by radiative losses on account of their large surface/volume ratio, certainly much faster that the cooling rate extracted from the temperature profile of gases in the reaction.…”