Three stabilization mechanisms—the shortage of nuclei, the partitioning of alloying elements, and the fine grain size—of the remaining metastable austenite in transformation-induced plasticity (TRIP) steels have been studied by choosing a model alloy Fe-0.2C-1.5Mn-1.5Si. An examination of the nucleus density required for an athermal nucleation mechanism indicates that such a mechanism needs a nucleus density as large as 2.5 · 1017 m−3 when the dispersed austenite grain size is down to 1 µm. Whether the random nucleation on various heterogeneities is likely to dominate the reaction kinetics depends on the heterogeneous embryo density. Chemical stabilization due to the enrichment of carbon in the retained austenite is the most important operational mechanism for the austenite retention. Based on the analysis of 57 engineering steels and some systematic experimental results, an exponential equation describing the influence of carbon concentration on the martensite start (Ms) temperature has been determined to be Ms (K)=273+545.8 · e−1.362wc(mass pct). A function describing the Ms temperature and the energy change of the system has been found, which has been used to study the influence of the grain size on the Ms temperature. The decrease in the grain size of the dispersed residual austenite gives rise to a significant decrease in the Ms temperature when the grain size is as small as 0.1 µm. It is concluded that the influence of the grain size of the retained austenite can become an important factor in decreasing the Ms temperature with respect to the TRIP steels.

Stabilization mechanisms of retained austenite in transformation-induced plasticity steel

Precise measurement of strain accommodation in austenite matrix surrounding martensite in ferrous alloys by electron backscatter diffraction analysis

Hot forming of boron steels using heated and cooled tooling for tailored properties

Exceptionally high cavitation erosion and corrosion resistance of a high entropy alloy.

A strain rate sensitive constitutive model for quenched boron steel with tailored properties

The development and mechanical performances of new type air-cooled Mn series bainitie steels including granular bainitie steels, FGBA/BG duplex steels, CFB/M duplex steels, medium carbon bainite/martensite steels, cast bainitie steels invented by the authors are summarized. The novel series of bainitie steels are alloyed with Mn, and several series bainitie duplex microstructures can be easily obtained under the condition of air cooling through unique composition design. The invented idea, the principle of alloying design, the strengthening mechanism, and the evolution of the microstructure of new type air-cooled Mn series bainitie steels are presented. Furthermore, the applications in different fields of these Mn series air-cooled bainitie steels with different strength level are also introduced. It is suggested that the significance of the development of the air-cooled Mn series bainitie steel can be summarized as follows: reducing costs of both raw materials and production; good combination of strength and toughness; self-hardening with high bainitie hardenability by air cooling from hot working without additional quenching-tempering treatment or quenching procedure; large savings in energy resources; and reduced environmental pollution.

Creation of Air-Cooled Mn Series Bainitic Steels

The failure models induced by white layers during impact wear

Effect of an electric field during solution treatment of 2091 Al-Li alloy

Scanning tunneling microscopy (STM), together with light optical microscopy (LOM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), of ultra fine structure of bainitic ferrite (α) shows that subunits inside a bainitic plate consist of sub-subunits, and STM of the surgace relief effect accompanying bainitic structure further confirms the existence of sub-subunits. The multi-layer structural constitution of bainite exhibits the typical self-embedded characteristics, and the fractal multi-dimension has been determined to be 2.74. Moreover, STM shows that an individual relief corresponding to a bainitic plate is composed of a sheaf of miniature reliefs which seem to arise from the formation of bainitic platelets, such as subplates, subunits and/or sub-subunits. The discovery of bainitic sub-subunits is helpful to give insight into transformation mechanism and interpret the complex and variable morphology of bainite. Based on the findings regarding the multi-layer structure of bainite, it is proposed that bainite forms via ledgewise growth which is essentially accompanied by sympathetic nucleation, i.e., sympathetic-nucleation-ledgewise-growth (SNLG). In this paper the SNLG mechanism will be theoretically discussed and practically applied to explain the multi-layer structure of bainite.

Fine structure and formation mechanism of bainite in steels

We studied the surface relief induced by martensite in an Fe-Ni-C alloy with atomic-force microscopy (AFM) quantitatively. The phenomenological theory of martensitic crystallography (PTMC) is applied to calculate the surface relief angle and compare with that obtained by AFM, and it seems to conform with the experimental result, indicating a reasonable mechanism of martensitic transformation proposed by PTMC.

Surface relief accompanying martensitic transitions in an Fe-Ni-C alloy by atomic-force microscopy and phenomenological theory of martensitic crystallography.

Yan-Kang Zheng

Papers

Creation of Air-Cooled Mn Series Bainitic Steels

The failure models induced by white layers during impact wear

Effect of an electric field during solution treatment of 2091 Al-Li alloy

Fine structure and formation mechanism of bainite in steels

Surface relief accompanying martensitic transitions in an Fe-Ni-C alloy by atomic-force microscopy and phenomenological theory of martensitic crystallography.