Stability of retained austenite in martensitic high carbon steels:Part I: Thermal stability

Thermal stability of retained austenite in 1C-1.5Cr steels with two Si and Mn contents is studied. Time-resolved high resolution synchrotron X-ray radiation and dilatometry are employed. The threshold transformation temperatures, decomposition kinetics, associated transformation strain, as well as the influence of Si and Mn were investigated. The coefficients of linear thermal expansion for both the bulk materials and individual phases are also obtained. The results indicate that an increase in the Mn and Si contents show little influence on the onset of retained austenite decomposition, but result in more thermally stable austenite. The decomposition is accompanied by a simultaneous increase in ferrite content which causes an expansive strain in the order of 10 − 4 , and subsequent cementite development from 300 − 350 ° C which causes a contraction that helps to neutralise the expansive strain. During decomposition, a continuous increase in the carbon content of austenite, and a reduction in that of the tempered-martensite/ferrite phase was observed. This process continued at elevated temperatures until full decomposition was reached, which could take less than an hour at a heating rate of 0.05 ° C /s. Additionally, the observation of austenite peak splitting on samples with high Mn and Si contents suggests the existence of austenite of different stabilities in such matrix

Modelling martensitic transformation in titanium alloys:The influence of temperature and deformation

New theory is presented to describe the occurrence of plasticity-induced transitions in titanium alloys. The approach is able to predict the composition dependence of transformation induced plasticity (TRIP), superelasticity, as well as martensite formation upon quenching. Martensite formation in the absence of stress is considered as the result of a competition between elastic strain energy and chemical driving force. Assuming that the formation of martensite is the result of a thermally activated nucleation process followed by athermal growth, a nucleation parameter is postulated to describe the conditions under which martensite is formed upon quenching; the parameter accounts for the ratio between the available thermal energy and an energy barrier for nucleation, suggesting that ω phase is not the main factor controlling martensite inhibition. This nucleation parameter is able to describe, for the first time, martensite occurrence in 130 alloys from the literature, quantifying the martensite start temperature (Ms) reported for 49 alloys with great precision. An empirical parameter ([Fe]eq) is proposed and, when combined with the Ms prediction, it allows to define regions within which TRIP and superelasticity occur. By defining threshold values for the Ms, the [Fe]eq and the nucleation parameter, candidate alloys likely to display TRIP, superelasticity or martensitic transformation upon quenching can be identified. As a result, this method can be adopted to design alloys with tailored plasticity behaviour. © 2019 Acta Materialia Inc

Modelling solid solution hardening in high entropy alloys

Solid solution hardening (SSH) is one of the major contributions to the excellent mechanical properties displayed by high entropy alloys (HEAs). SSH is first analysed for binary systems in face-centred cubic and body-centred cubic alloys with different elemental additions in the temperature range 5-623 K. The prediction of the SSH has been possible by using Labush's approach for SSH modelling, where the necessary parameters have been incorporated without fitting to experimental data. Among these parameters, elastic misfit is shown to be prominent; experimental evidence suggests it has a dominant effect with respect to other misfit forms. Nevertheless, Labush's approach cannot be directly applied to model SSH in HEAs, since it is based on the misfit produced in the lattice of a solvent/reference atom, which does not exist in HEAs. Its extension to HEAs has been performed by using Mooren's approach for the computation of interatomic spacing in multicomponent alloys, allowing the creation of a model for elastic misfit in HEAs. This has led to a methodology for computing SSH effect in HEAs, where the results have successfully been compared with a collection of experimental data from the literature. The explanation of how different atoms can modify the yield strength can be formulated in terms of this approach

Modelling and Design of Magnesium and High Entropy Alloys Through Combining Statistical and Physical Models

Physical and statistical models are combined to describe and design magnesium and high entropy alloys. A principal component analysis is applied to merge material datasets, and it is shown that limits in properties can be envisaged. Extrapolation techniques can be employed to devise properties of non-existing alloys, such as specific heat capacity, melting point and Young’s modulus. These in turn can be input to physical models to predict, for example, yield strength and modulus of toughness. The tools described herein can readily be used for materials discovery, and are being implemented in the Accelerated Metallurgy project

Towards efficient microstructural design and hardness prediction of bearing steels — An integrated experimental and numerical study

The present work develops a numerical approach combining thermodynamic and kinetic simulations to investigate the austenitisation process on spheroidised bearing steel. The approach incorporates the dissolution of spheroidised cementite present prior to austenitisation and the influence of austenitisation temperature. It allows predictions including the chemical driving force of austenite formation, the evolution of phase constituents and their chemical compositions during austenitisation, as well as an assessment on the austenite stability upon quenching. The calculated results further allow to predict the hardness of the produced martensitic steels. The model predictions are validated against experimental data in two commercial bearing steels with six austenitisation processes. Good agreement between the experimental results and numerical predictions is obtained on the steel microstructure, austenite stability and material hardness. In addition, comparison of the two steels show that 100Cr6 requires to be austenitised at temperatures 10 °C higher than 100CrMnSi6-4, to achieve the same driving force for austenite formation, and 20 °C higher to achieve identical austenite stability upon quenching. The method can be adopted beyond bearing steels to design austenitisation processing schedules

Thermodynamic design of ultra-strong titanium alloys undergoing plasticity induced martensitic transformations

The strength of β titanium alloys can be increased up to 20 % via inducing martensitic transformations under plastic deformation, without compromising on other properties such as ductility. Tailoring such transformations for various alloy compositions and deformation temperatures may be performed with the aid of thermodynamic computations. A thermodynamic framework is presented to control Plasticity induced transformations in titanium alloys, and its application in developing novel alloys subjected to various deformation schemes is presented. The prospects of developing a new family of metastable β alloys displaying improved strength/ductility relationships via plasticity induced transformation effects is outlined

Unravelling the materials genome:Symmetry relationships in alloy properties

Metals and alloys have been indispensable for technological progress, but only a fraction of the possible ternary systems (combinations of three elements) is known. Statistical inference methods combined with physical models are presented to discover new systems of enhanced properties. It is demonstrated that properties originating from atomic-level interactions can be described employing a linear regression analysis, but properties incorporating microstructural and thermal history effects require a balance between physical and statistical modelling. In spite of this, there is a remarkable degree of symmetry among all properties, and by employing a principal components analysis it is shown that ten properties essential to engineering can be described well in a three dimensional space. This will aid in the discovery of novel alloying systems

Modelling dislocation assisted tempering during rolling contact fatigue in bearing steels

Rolling contact fatigue in bearing steels is manifested by dark-etching regions, which are attributed to deformation induced tempering. In order to quantitatively explain this phenomenon, a model is suggested for martensite tempering assisted by dislocation glide during rolling contact fatigue. In the model, dislocations transport carbon from the matrix to carbide particles, provided that the carbon is located at a certain distance range from the dislocation contributing to the tempering process. By calculating the amount of carbon in the matrix, the kinetics of carbide thickening and hardness reduction are computed. It is found that the dark-etching region kinetics can be controlled by both bearing operation conditions (temperature and deformation rate) and microstructure (type, size, and volume fraction of carbides). The model is validated against tested bearings, and its limitations are discussed

Modelling the steady state deformation stress under various deformation conditions using a single irreversible thermodynamics based formulation

A new unified description for the steady state deformation stress in single and polycrystalline metals and for various deformation conditions is presented. The new formulation for dislocation controlled deformation stems from the field of irreversible thermodynamics. The model applies to conditions of dynamic recovery as well as dynamic recrystallization and has been validated for constant strain rate and creep loading conditions. Unlike existing approaches, the new model captures transitions between deformation mechanisms within a single formulation. For conditions of dynamic recrystallization, the average dislocation density is found to be a function of the shear strain rate and a term combining the dislocation climb velocity and the grain boundary velocity