Austenite in Transformation-Induced Plasticity Steel Subjected to Multiple Isothermal Heat Treatments

The thermodynamic limit to the progress of the bainite reaction in steels containing a cementite inhibitor often leaves large quantities of thermally or mechanically unstable austenite. Such austenite is not effective in delaying the onset of plastic instabilities during the course of deformation. In such circumstances, it is useful to conduct isothermal transformation at a high temperature where the rate of reaction is relatively rapid, followed by a lower temperature step that permits more bainite to be generated. This in turn increases the stability of the refined austenite, which then transforms gently over a large range of strain during a tensile test. A significant corollary is that the two-step heat treatments are unnecessary in low-carbon steels, where the bainite reaction is able to proceed to a greater extent before reaching the thermodynamic limit. Furthermore, the two-step process can be counterproductive in low carbon steel, because the austenite content is reduced to a level below which it does not enhance the mechanical properties. Other circumstances in which multiple heat treatments are necessary are also discussed.The authors are grateful to POSCO for support through Steel Innovation Programme, and to the World Class University Programme of the National Research Foundation of Korea, Ministry of Education, Science and Technology, project number R32-2008-000-10147.This is the accepted manuscript version. The final published version is available from Springer at http://link.springer.com/article/10.1007%2Fs11661-014-2405-z

Finding gene regulatory network candidates using the gene expression knowledge base

BACKGROUND: Network-based approaches for the analysis of large-scale genomics data have become well established. Biological networks provide a knowledge scaffold against which the patterns and dynamics of ‘omics’ data can be interpreted. The background information required for the construction of such networks is often dispersed across a multitude of knowledge bases in a variety of formats. The seamless integration of this information is one of the main challenges in bioinformatics. The Semantic Web offers powerful technologies for the assembly of integrated knowledge bases that are computationally comprehensible, thereby providing a potentially powerful resource for constructing biological networks and network-based analysis. RESULTS: We have developed the Gene eXpression Knowledge Base (GeXKB), a semantic web technology based resource that contains integrated knowledge about gene expression regulation. To affirm the utility of GeXKB we demonstrate how this resource can be exploited for the identification of candidate regulatory network proteins. We present four use cases that were designed from a biological perspective in order to find candidate members relevant for the gastrin hormone signaling network model. We show how a combination of specific query definitions and additional selection criteria derived from gene expression data and prior knowledge concerning candidate proteins can be used to retrieve a set of proteins that constitute valid candidates for regulatory network extensions. CONCLUSIONS: Semantic web technologies provide the means for processing and integrating various heterogeneous information sources. The GeXKB offers biologists such an integrated knowledge resource, allowing them to address complex biological questions pertaining to gene expression. This work illustrates how GeXKB can be used in combination with gene expression results and literature information to identify new potential candidates that may be considered for extending a gene regulatory network. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12859-014-0386-y) contains supplementary material, which is available to authorized users

Austenite stability in TRIP steels studied by synchrotron radiation

TRIP steel is a material providing great mechanical properties. Such steels show a good balance between high-strength and ductility, not only as a result of the fine microstructure, but also because of the well-known TRIP effect. The Transformation Induced-Plasticity (TRIP) phenomenon is the transformation of the soft metastable austenite phase to the hard martensite phase due to a mechanical or a thermal stimulus. Already developed in the 1980s, these materials gained new interest since their fabrication has been achieved via a considerable reduction of relevant alloying element concentrations. The mechanical response of such steels, intended to be used in the automotive industry to decrease the gas emission of vehicles, remains difficult to predict. The reason is that the stability of austenite simultaneously depends on a number of intrinsic and extrinsic parameters. As a result, limited in-depth information can be obtained using conventional characterization techniques. Some progress has been made by means of micromechanical models developed to tailor the TRIP effect. These advanced multi-scale models that consider both the macrostructure and microstructure are valuable tools, but have so far been based on limited experimental input for validation and are likely to be not correct. Improved physical characterization methods based on synchrotron radiation make it possible to study the transformation behaviour of the metastable phase in-situ during mechanical or thermal deformation both macroscopically and at the level of individual grains. The aim of this thesis is to quantify the microstructural parameters controlling the mechanical and thermal stability of austenite at an individual grain level using synchrotron X-ray diffraction. This thesis presents the results from experiments probing the transformation behaviour at a macroscopic scale, at a grain level scale and even at a sub-grain scale. In Chapter 3 I report in-situ magnetization and high-energy X-ray diffraction measurements on two aluminum-based TRIP steels during cooling from room temperature down to 100 K in order to evaluate amount and stability of the retained austenite for different heat treatment conditions. I found that the bainitic holding temperature affects the initial fraction of retained austenite at room temperature but does not influence significantly the rate of transformation upon cooling. In Chapter 4 the stability of the retained austenite has been studied in-situ in low-alloyed TRIP steels using high-energy X-ray diffraction during tensile tests at variable temperatures down to 153 K. A detailed powder diffraction analysis has been performed to probe the austenite to martensite transformation by characterizing the evolution of the phase fraction, load partitioning and texture of the constituent phases simultaneously. Our results show that at lower temperatures the mechanically induced austenite transformation is significantly enhanced and extends over a wider deformation range, resulting in a higher elongation at fracture. Low carbon content grains transform first, leading to an apparent increase in average carbon concentration of the remaining austenite. At higher deformation levels the average carbon content saturates while the austenite still continues to transform. In the elastic regime the probed {hkl} planes develop different strains reflecting the elastic anisotropy of the constituent phases. The observed texture evolution indicates that the austenite grains oriented with the {200} along the loading direction are transformed preferentially as they experience the highest resolved shear stress. For increasing degrees of plastic deformation the combined preferential transformation and grain rotation results in the standard deformation texture for austenite with the {111} component along the loading direction. The mechanical stability of retained austenite in TRIP steel is found to be a complex interplay between carbon concentration in the austenite, grain orientation, load partitioning and temperature. In Chapter 5 the microstructure evolution during shear loading of a low-alloyed TRIP steel with different amounts of the metastable austenite phase and its equivalent Dual Phase (DP) grade has been studied by in-situ high-energy X-ray diffraction methods. A detailed powder diffraction analysis has been performed to probe the austenite-to-martensite transformation by characterizing simultaneously the evolution of the austenite phase fraction and its carbon concentration, the load partitioning between the austenite and the ferritic matrix and the texture evolution of the constituent phases. My results show that for shear deformation conditions the TRIP effect extends over a significantly wider deformation range than for simple uniaxial loading. A clear increase in average carbon content during the mechanically-induced transformation proves that austenite grains with a low carbon concentration are least stable during shear loading. The observed texture evolution indicates that under shear loading the orientation dependence of the austenite stability is relatively weak. Earlier work had shown that under a tensile load the {110} component transforms preferentially. The mechanical stability of retained austenite in TRIP steel is found to be a complex interplay between the interstitial carbon concentration in the austenite, the grain orientation and the load partitioning. Chapter 6 focuses on the determination of the local retained austenite-to-martensite transformation behaviour in an inhomogeneous yet carefully controlled shear loaded region of double notched TRIP and DP steel samples. A detailed powder analysis has been performed to simultaneously monitor the evolution of the phase fraction and carbon enrichment of metastable austenite and the local strain components in the constituent phases as a function of the macroscopic stress and location with respect to the shear band. The metastable retained austenite showed a mechanically-induced martensitic transformation in the localized shear zone, which is accompanied by an apparent carbon enrichment in the remaining austenite. At the later deformation stages the geometry of the shear test samples results in the development of an additional tensile component. The experimental strain field within the probed sample area is in good agreement with the predictions from finite-element calculations. The strain development observed in the low-alloyed TRIP steel with metastable austenite is compared to that of steels with the same chemical composition yet containing either no austenite (a DP grade) or a stable (i.e. non-transforming) retained austenite fraction (a TRIP grade produced at a long bainitic holding time). The transformation of metastable austenite under shear load is a complex interplay between the local microstructure and the evolving strain fields. In Chapter 7 the stability of individual metastable austenite grains during tensile loading has been studied in-situ. A new analysis method based on Friedel diffraction pairs has been developed to correlate the macroscopic behaviour of the material to the microstructural parameters of individual grains. The carbon concentration, grain volume and orientation have been determined from single peaks of the diffraction pattern. My results show that these three parameters control the mechanical stability, while the grain volume was found to be the dominant parameter. The orientation-dependent stability of the austenite grains with respect to the tensile axis shows a transformation sequence that is in line with their Schmid factor. It has been observed that for increasing tensile load most austenite grains transform into martensite in one step. In Chapter 8 the martensitic transformation behaviour of the meta-stable austenite phase in low-alloyed TRIP steels during deformation has been studied in more detail. The stability of austenite has been studied at different length scales during tensile tests. A powder diffraction analysis has been performed to correlate the macroscopic behaviour of the material to the observed changes in the volume phase fraction. Moreover, the austenite transformation behaviour has been studied at the length scale of individual grains, where an in-depth characterization of four selected grains has been performed including grain volume, local carbon concentration and grain orientation. For the first time, a high resolution far-field detector was used to study the initial and evolving structure of individual austenite grains during uniaxial tensile deformation of the sample. The sub-grain size in austenite is found not to change significantly during the deformation. The final transformation to martensite occurred in either one or two loading steps.Radiation Science and TechnologyApplied Science

Taking advantage of multiple identities to reduce defensiveness to personally threatening health messages

A host of studies have shown that self-relevant health messages may result in increased defensiveness and rejection of protective recommendations. Drawing on research showing that multiple identities offer psychological resources to deal with identity threats, we sought to examine whether the salience of an alternative identity before people are exposed to a personally relevant health message may buffer the threat and reduce defensive responses. Two studies were conducted on samples of daily smokers asked to read an antismoking message before completing a range of measures of defensiveness. Half of the participants had an alternative identity made salient beforehand (vs. no salience condition). Consistent with our hypotheses, Study 1 (N = 90) showed that this manipulation significantly reduced defensiveness to the message. Study 2 (N = 95) additionally showed that such effects only occurred when the alternative identity overlapped highly with the threatened identity. The theoretical implications of these findings are discussed

Time-dependent synchrotron X-ray diffraction on the austenite decomposition kinetics in SAE 52100 bearing steel at elevated temperatures under tensile stress

We have studied the decomposition kinetics of the metastable austenite phase present in quenched-and-tempered SAE 52100 steel by in situ high-energy synchrotron X-ray diffraction experiments at elevated temperatures of 200-235 °C under a constant tensile stress. We have observed a continuous decomposition of austenite into ferrite and cementite. The decomposition kinetics is controlled by the long-range diffusion of carbon atoms into the austenite ahead of the moving austenite/ferrite interface. The presence of a tensile stress of 295 MPa favours the carbon diffusion in the remaining austenite, so that the activation energy for the overall process decreases from 138-148 to 82-104 kJ mol-1. Before the austenite starts to decompose, a significant amount of carbon atoms partition from the surrounding martensite phase into the metastable austenite grains. This carbon partitioning takes place simultaneously with the carbide precipitation due to the over-tempering of the martensite phase. As the austenite decomposition proceeds gradually at a constant temperature and stress, the elastic strain in the remaining austenite grains continuously decreases. Consequently, the remaining austenite grains act as a reinforcement of the ferritic matrix at longer isothermal holding times. The texture evolution in the constituent phases reflects both significant grain rotations and crystal orientation relationships between the parent austenite phase and the newly formed ferritic grains.</p

Time-dependent synchrotron X-ray diffraction on the austenite decomposition kinetics in SAE 52100 bearing steel at elevated temperatures under tensile stress

We have studied the decomposition kinetics of the metastable austenite phase present in quenched-and-tempered SAE 52100 steel by in situ high-energy synchrotron X-ray diffraction experiments at elevated temperatures of 200-235 °C under a constant tensile stress. We have observed a continuous decomposition of austenite into ferrite and cementite. The decomposition kinetics is controlled by the long-range diffusion of carbon atoms into the austenite ahead of the moving austenite/ferrite interface. The presence of a tensile stress of 295 MPa favours the carbon diffusion in the remaining austenite, so that the activation energy for the overall process decreases from 138-148 to 82-104 kJ mol-1. Before the austenite starts to decompose, a significant amount of carbon atoms partition from the surrounding martensite phase into the metastable austenite grains. This carbon partitioning takes place simultaneously with the carbide precipitation due to the over-tempering of the martensite phase. As the austenite decomposition proceeds gradually at a constant temperature and stress, the elastic strain in the remaining austenite grains continuously decreases. Consequently, the remaining austenite grains act as a reinforcement of the ferritic matrix at longer isothermal holding times. The texture evolution in the constituent phases reflects both significant grain rotations and crystal orientation relationships between the parent austenite phase and the newly formed ferritic grains.</p