Criterion for the action of applied stress in the martensitic transformation

Residual Stresses in Ribbed Reinforcing Bars

Ribbed reinforcing bars (rebars) are used for the reinforcement of concrete structures. In service, they are often subjected to cyclic loading. In general, the fatigue performance of rebars may be influenced by residual stresses originating from the manufacturing process. Knowledge about residual stresses in rebars and their origin, however, is sparse. So far, residual stress measurements are limited to individual stress components, viz., to the non-ribbed part of the rebar surface. At critical points of the rebar surface, where most of the fatigue cracks originate, i.e., the foot radius regions of transverse ribs, the residual stress state has not yet been investigated experimentally. To extend the knowledge about residual stresses in rebars within the scope of this work, residual stress measurements were carried out on a rebar specimen with a diameter of 28 mm made out of the rebar steel grade B500B. In addition, numerical simulations of the TempCoreTM process were carried out. The results of the experimental investigations show tensile residual stresses in the core and the transition zone of the examined rebar specimen. Low compressive residual stresses are measured at the non-ribbed part of the rebar surface, while high compressive residual stresses are present at the tip of the transverse ribs. The results of the numerical investigations are in reasonable accordance with the experimental results. Furthermore, the numerical results indicate moderate tensile stresses occurring on the rebar surface in the rib foot radius regions of the transverse ribs. High stress gradients directly beneath the rebar surface, which are reported in the literature and which are most likely related to a thin decarburized surface layer, could be reproduced qualitatively with the numerical model developed.

A Phase Field Study of the Influence of External Loading on the Dynamics of Martensitic Phase Transformation

An elastoplastic phase field model was employed for simulations to investigate the influence of external loading on the martensitic phase transformation kinetics in steel. The phase field model incorporates external loading and plastic deformation. During the simulation process, the authenticity of the phase field model is ensured by introducing the relevant physical parameters and comparing them with experimental data. During the calculations, loads of various magnitudes and loading conditions were considered. An analysis and discussion were conducted concerning the volume fraction and phase transition temperature during the phase transformation process. The simulation results prominently illustrate the preferential orientation of variants under different loading conditions. This model can be applied to the qualitative phase transition evolution of Fe-Ni alloys, and the crystallographic parameters adhere to the volume expansion effect. It is concluded that uniaxial loading promotes martensitic phase transformation, while triaxial compressive loading inhibits it. From a dynamic perspective, it is demonstrated that external uniaxial loading accelerates the kinetics of martensitic phase transformation, with uniaxial compression being more effective in accelerating the phase transformation process than uniaxial tension. When compared to experimental data, the simulation results provide evidence that under the influence of external loading, the martensitic phase transformation is significantly influenced by the applied load, with the impact of external loading being more significant than that of plastic effects.

Mechanical Behavior of Multi-Phase Steels Comprising Retained Austenite

The retained austenite (RA) in advanced high-strength steel (AHSS) grades, such as dual-phase (DP) steels, plays an important role on their formability. Thanks to the transformation-induced plasticity (TRIP) effect that occurs during the mechanically induced transformation of RA into martensite, additional ductility is obtained. Martensite has a higher flow stress than austenite; hence, the transformation results in an apparent hardening, which is beneficial for the stability of deformation. The stability of RA at a given temperature strongly depends on its carbon content, which, in AHSS, is not uniform but distributed. The aim of this study is to build a model that predicts the transformation as well as TRIP in a DP steel grade with RA. A physics-based kinetic model is presented that captures the transformation of retained austenite based on the thermodynamic driving force of the applied stress. A direct analytical estimate of transformation plasticity is provided, which is consistent with the kinetic model. Transformation kinetics is incorporated in a self-consistent, mean-field homogenization-based constitutive model. Finally, an indication of the effect of transformation of retained austenite on formability is given.

Dilatometric Analysis and Kinetics Research of Martensitic Transformation under a Temperature Gradient and Stress

Based on material constitutive models and the classic Koistinen–Marburger (KM) kinetics model, a new dilatometric analysis model was developed to extract the kinetics curve of martensitic transformation under a temperature gradient and stress from the measured dilatometric data and to determine the transformation parameters. The proposed dilatometric analysis model is generally for athermal martensitic transformation, relying only on the average atom volume of martensite and austenite. Furthermore, through theoretical calculations, the proposed model also provided a more accurate method for obtaining the martensite start temperature, which is different from the traditional method. According to the dilatometric analysis results for the martensitic transformation of a type of high-strength low-alloy steel, and the thermodynamic basis of martensitic transformation, a refined kinetics model was developed that successfully predicted the martensitic transformation kinetics curves under different stresses, taking into account the physical significance of the transformation parameter α and the driving force of stress for martensitic transformation.

Hydrogenation treatment under several gigapascals assists diffusionless transformation in a face-centered cubic steel

The use of hydrogen in iron and steel has the potential to improve mechanical properties via altering the phase stability and dislocation behavior. When hydrogen is introduced under several gigapascals, a stoichiometric composition of hydrogen can be introduced for steel compositions. In this study, a face-centered cubic (fcc) stainless steel was hydrogenated under several gigapascals. When the steel was not hydrogenated, the microstructure after depressurization was an fcc with a hexagonal close-packed (hcp) structure. In contrast, the hydrogenation treatment resulted in a fine lath body-centered cubic (bcc) structure arising from diffusionless transformation. In particular, the bcc phase formed through the following transformation sequence: fcc → hcp → dhcp (double hexagonal close-packed phase) → bcc. That is, the use of hydrogenation treatment realized fine microstructure evolution through a new type of diffusionless transformation sequence, which is expected to be used in future alloy design strategies for developing high-strength steels.

Model of local hydrogen permeability in stainless steel with two coexisting structures

The dynamics of hydrogen in metals with mixed grain structure is not well understood at a microscopic scale. One of the biggest issues facing the hydrogen economy is “hydrogen embrittlement” of metal induced by hydrogen entering and diffusing into the material. Hydrogen diffusion in metallic materials is difficult to grasp owing to the non-uniform compositions and structures of metal. Here a time-resolved “operando hydrogen microscope” was used to interpret local diffusion behaviour of hydrogen in the microstructure of a stainless steel with austenite and martensite structures. The martensite/austenite ratios differed in each local region of the sample. The path of hydrogen permeation was inferred from the time evolution of hydrogen permeation in several regions. We proposed a model of hydrogen diffusion in a dual-structure material and verified the validity of the model by simulations that took into account the transfer of hydrogen at the interfaces.

Future Trends on Displacive Stress and Strain Induced Transformations in Steels

Displacive stress and strain induced transformations are those transformations that occur when the formation of martensite or bainitic ferrite is promoted by the application of stress or strain. These transformations have been shown to be one of the mechanisms by which the mechanical properties of a microstructure can be improved, as they lead to a better ductility and strength by the transformation induced plasticity effect. This review aims to summarize the fundamental knowledge about them, both in fully austenitic or in multiphase structures, pointing out the issues that—according to the authors’ opinion—need further research. Knowing the mechanisms that govern the stress and strain induced transformation could enable to optimize the thermomechanical treatments and improve the final microstructure properties.

Study of diffusionless and diffusional transformations using in situ cooling and heating techniques in a scanning electron microscope

In situ electron microscopy can be an effective tool to investigate the underlying science of many transformation mechanisms in materials science. Useful utilization of these experimentations will provide greater insight into many of the existing theories, as microstructural changes can be visualized in real time under some applied constraints. In this study, we have investigated two basic phase transformation phenomena: diffusionless and diffusional mechanisms with the help of in situ cooling and heating techniques in scanning electron microscope (SEM). In situ cooling experiments have been carried out on secondary hardening ultra-high-strength steels to understand the diffusionless transformation of austenite to martensite. Nucleation and growth of the martensites have been observed with cooling in different steps to -194°C. Details of the formation of different variants of martensites in steel were studied with the help of orientation imaging microscopy. Diffusional transformations were studied in terms of oxidation of pure copper in SEM using in situ heating technique. Different heating cycles were adopted for different samples by in situ heating to a maximum temperature of 950°C for the oxidation study. Nucleation of copper oxides and subsequent growth of the copper oxides at different temperatures were studied systematically. Raman spectroscopy and orientation imaging were done to confirm the formation of oxides and their orientations. The thermal cycling phenomenon was replicated inside SEM with heating and cooling and it has been demonstrated how the nature of copper and its oxides changes with the thermal cycle. This article is part of a discussion meeting issue 'Dynamic in situ microscopy relating structure and function'.

In Situ Determination of Phase Stress States in an Unstable Medium Manganese Duplex Steel Studied by High-Energy X-ray Diffraction

Duplex medium Mn steels are high-potential advanced high-strength steels (AHSS) for automotive construction. Their excellent forming properties stem from the specific stress partitioning between their constituting phases during deformation, namely the ferritic matrix, unstable retained austenite, and strain-induced fresh martensite. The stability of the retained austenite and the 3D stress tensors of each phase are determined simultaneously in this work by in situ high energy X-ray diffraction on synchrotron beamline during a tensile test. The role of internal stresses inherited from the manufacturing stage are highlighted for the first time as well as new insights to understand the origin of the serrations shown by these alloys.

On the Factors Governing Austenite Stability: Intrinsic versus Extrinsic

In this review, we separate the different governing factors on austenite stability into intrinsic and extrinsic factors, depending on the domain defined by austenite grain boundaries. The different measuring techniques on the effectiveness of the governing factors in affecting the austenite stability are discussed. On the basis of the austenite stability, a new alloy design strategy that involves the competition between the intrinsic and extrinsic factors to control the transformation-induced plasticity (TRIP) effect to realize the stronger the more ductile steel is proposed. The present review may provide new insights into the development of novel thermal-mechanical processing to advance the mechanical properties of steels for industrial applications.

Predicting the available work from deformation-induced α′′ martensite formation in metastable β Ti alloys

Deformation-induced α′′ martensite formation is essential to the mechanical properties of a variety of metastable β Ti alloys by extending elasticity or contributing to work-hardening during plastic deformation. Nevertheless, to date, a comprehensive analysis of the effect of β texture and applied stress state on the martensitic transformation to α′′ is still lacking. The present study therefore provides a detailed analysis of the work which is made available from the shape strain of the martensitic transformation under a variety of in-plane stress states and as a function of β crystal orientation. The available work was found to strongly depend on the applied stress state and the parent grain orientation. The shape strain of the martensitic transformation was obtained from applying the phenomenological theory of martensite crystallography. In cases where this theory was not applicable, an approximation of the shape strain by the Bain strain was found to provide a good approximation of the available work. Analysis of three different metastable β Ti alloys showed no strong effect of the alloy composition on the available work. Martensite formation from typical cold- and warm-rolling β texture components under different stress states is discussed. Cases are highlighted to show how the cold- and warm-rolling β textures can be tailored to hinder martensite formation upon subsequent industrial forming operations.

Microstructure and Mechanical Properties of an Austenitic CrMnNiMoN Spring Steel Strip with a Reduced Ni Content

The article presents the mechanical properties of the austenitic stainless steel X5CrMnNiMoN16-4-4 after deformation by cold rolling and subsequent short-term tempering (deformation and partitioning (D&P) treatment). Tensile strengths of 1700–900 MPa and beyond were achieved both after work hardening and in the D&P-treated strip. The initial state of austenite in terms of grain size and pre-strengthening, as well as the selected cold rolling temperature significantly influenced the deformation-induced formation of α’ martensite and thus the flow and hardening behavior of the steel. The usage of two different rolling temperature regimes showed that the strength properties in the cold strip can be specifically adjusted. Lower deformation-induced martensite fractions enabled a larger thickness reduction of the strip without increasing the rolling force, while high deformation-induced martensite fractions led to strong hardening at low deformation levels. The D&P-treatment permits the strength of the cold-rolled strip with a predominantly austenitic microstructure to be increased to the required level. The total elongation of such a D&P strip was well over 2%. The D&P treatment of the spring steel strip is a cost-effective alternative to conventional tempering treatment.

TRIP Steels: A Multiscale Computational Simulation and Experimental Study of Heat Treatment and Mechanical Behavior

A multiscale investigation of the microstructure and the mechanical behavior of TRIP steels is presented. A multi-phase field model is employed to predict the microstructure of a low-alloy TRIP700 steel during a two-stage heat treatment. The resulting stability of retained austenite is examined through the Msσ temperature. The phase field results are experimentally validated and implemented into a model for the kinetics of retained austenite during strain-induced transformation. The kinetics model is calibrated by using experimental data for the evolution of the martensite volume fraction in uniaxial tension. The transformation kinetics model is used together with homogenization methods for non-linear composites to develop a constitutive model for the mechanical behavior of the TRIP steel. A methodology for the numerical integration of the constitutive equations is developed and the model is implemented in a general-purpose finite element program (ABAQUS). Necking of a bar in uniaxial tension is simulated and “forming limit diagrams” (FLDs) for sheets made of TRIP steels are calculated. The models developed provide an integrated simulation toolkit for the computer-assisted design of TRIP steels and can be used to translate mechanical property requirements into optimised microstructural characteristics and to identify the appropriate processing routes.

Electron backscatter diffraction study of variant selection during ordering phase transformation in L10-type red gold alloy

A shape-memory effect is known to appear in red gold alloys with compositions close to Au–Cu. The aim of this paper is to study by electron backscatter diffraction (EBSD) the variant selection in the A1 → L10 transformation occurring under stress, in bending conditions. The L10 domains are successfully identified by this technique despite the c/a ratio being close to unity. The orientation relationship between the cubic and tetragonal phases is determined by a careful analysis of the EBSD data. The distortion of the lattice for each variant is then modelled and calculated from the experimental orientations. The mechanical work associated with the transformation is computed from the lattice distortion by neglecting the obliquity. Finally, the distribution of this mechanical work is compared with the case of a uniform distribution of all variants, in order to evaluate the extent of variant selection. The maximal work criterion, often used for martensitic transformations, enabled quantification of the variant selection phenomenon.

Deformation Behavior of a Double Soaked Medium Manganese Steel with Varied Martensite Strength

The effects of athermal martensite on yielding behavior and strain partitioning during deformation is explored using in situ neutron diffraction for a 0.14C–7.14Mn medium manganese steel. Utilizing a novel heat treatment, termed double soaking, samples with similar microstructural composition and varied athermal martensite strength and microstructural characteristics, which composed the bulk of the matrix phase, were characterized. It was found that the addition of either as-quenched or tempered athermal martensite led to an improvement in mechanical properties as compared to a ferrite plus austenite medium manganese steel, although the yielding and work hardening behavior were highly dependent upon the martensite characteristics. Specifically, athermal martensite was found to promote continuous yielding and improve the work hardening rate during deformation. The results of this study are particularly relevant when considering the effect of post-processing thermal heat treatments, such as tempering or elevated temperature service environments, on the mechanical properties of medium manganese steels containing athermal martensite.

Variant Selection in Fe-20Ni-1.8C under Bending

Variant selection is commonly observed in martensitic steels when a stress is applied to the material during transformation. Classically, the selection phenomenon is modelled considering the work of the shape strain in the applied stress field. This shape strain is generally calculated by using the Phenomenological Theory of the Martensite Crystallography (PTMC). In the present study, we studied the martensitic transformation occurring in a Fe-20wt%Ni-1.8wt%C alloy transformed while loaded in four-point bending. A significant variant selection is observed, but surprisingly its nature cannot be explained by the classical approach. A crystallography-based empirical model which accounts for the experimental results is proposed instead.

Deformation mechanisms and microplasticity of austenitic TRIP/TWIP steel under flyer plate impact

Abstract. The focus of this study is on the deformation mechanisms of high-alloy cast austenitic TRIP/TWIP steel with the nominal composition Fe-16Cr-6Mn-6Ni. Due to its chemical composition, the material exhibits a low stacking-fault energy of 17.5 mJ/m2 which facilitates the formation of the deformation-induced γ (fcc) → ε (hep) → α’ (bcc) transformation. Consequently, the steel exhibits a tensile strength of 800 MPa with fracture elongation of 55 % under quasi-static loading. The experiments presented demonstrate the response of this steel to flyer-plate impact (FPI) at room temperature using two different test setups. In the first setup, laser interferometry measurements of the sample free surface were used for determination of the dynamic mechanical properties (Hugoniot elastic limit / HEL. spall strength) after impact with aluminium plates accelerated up to 650 m/s. In the second setup, an experimental shock testing device developed at the Freiberg High-Pressure Research Centre was used for impacting large cylindrical samples without the occurrence of spallation. Subsequently, microstructural investigations were carried out by scanning electron microscopy (SEM) and transmission election microscopy (TEM) in combination with diffraction techniques and magnetic martensite measurements. Their results facilitate the representation of a complete image of deformation mechanisms during shock wave loading.

Stress-induced reverse martensitic transformation in a Ti-51Ni (at%) alloy aged under uniaxial stress

Ti-51Ni (at%) alloys including coherent precipitates of Ti₃Ni₄ exhibits thermally-induced B2-R transformation. If the Ti₃Ni₄ is formed under tensile stress, it orientates preferentially so that its habit plane becomes perpendicular to the tensile axis. In such specimens, stress-induced reverse R-B2 transformation is reported to occur. In the present study, the stress-induced reverse R-B2 transformation behavior was studied by infrared camera and in situ X-ray analysis. The infrared camera observation revealed that the temperature of the specimen decreases homogeneously by the application of tensile stress within the resolution of the camera. The in situ X-ray analysis revealed that stress-induced reverse R-B2 transformation and rearrangement of variants of the R-phase occurs simultaneously in the specimen.

Design for Fe-high Mn alloy with an improved combination of strength and ductility

Recently, Fe-Mn twinning-induced plasticity steels with an austenite phase have been the course of great interest due to their excellent combination of tensile strength and ductility, which carbon steels have never been able to attain. Nevertheless, twinning-induced plasticity steels also exhibit a trade-off between strength and ductility, a longstanding dilemma for physical metallurgists, when fabricated based on the two alloy design parameters of stacking fault energy and grain size. Therefore, we investigated the tensile properties of three Fe-Mn austenitic steels with similar stacking fault energy and grain size, but different carbon concentrations. Surprisingly, when carbon concentration increased, both strength and ductility significantly improved. This indicates that the addition of carbon resulted in a proportionality between strength and ductility, instead of a trade-off between those characteristics. This new design parameter, C concentration, should be considered as a design parameter to endow Fe-Mn twinning-induced plasticity steel with a better combination of strength and ductility.

Investigation of Parent Austenite Grains from Martensite Structure Using EBSD in a Wear Resistant Steel

Crystallographic reconstruction of parent austenite grain boundaries from the martensitic microstructure in a wear resistant steel was carried out using electron backscattered diffraction (EBSD). The present study mainly aims to investigate the parent austenite grains from the martensitic structure in an as-rolled (reference) steel sample and samples obtained by quenching at different cooling rates with corresponding dilatometry. Subsequently, this study is to correlate the nearest cooling rate by the dilatometer which yields a similar orientation relationship and substructure as the reference sample. The Kurdjumov-Sachs orientation relationship was used to reconstruct the parent austenite grain boundaries from the martensite boundaries in both reference and dilatometric samples using EBSD crystallographic data. The parent austenite grain boundaries were successfully evaluated from the EBSD data and the corresponding grain sizes were measured. The parent austenite grain boundaries of the reference sample match the sample quenched at 100 °C/s (CR100). Also the martensite substructures and crystallographic textures are similar in these two samples. The results from hardness measurements show that the reference sample exhibits higher hardness than the CR100 sample due to the presence of carbides in the reference sample.

Hydrogen diffusion and the percolation of austenite in nanostructured bainitic steel

The diffusion of hydrogen in austenite is slower than in ferrite. Experiments have been conducted to study the behaviour of hydrogen in a nanostructured steel sample consisting of a mixture of thin plates of bainitic ferrite and intervening films of retained austenite, with the latter phase present in a quantity larger than the percolation threshold, i.e. it has three-dimensional connectivity. The structure was then heat treated to control the fraction of austenite, and hence to study the role of hydrogen when the austenite decomposes below the value required to sustain percolation. The experiments have involved both thermal desorption analysis and permeation, and when combined with theoretical analysis, indicate a significant influence of percolating austenite in hindering the passage of hydrogen into the steel during hydrogen charging, and its permeation through the composite nanostructure. The effect is not as large as might be expected from a simple comparison of independent data on the diffusivities of hydrogen in the two lattices, because the effective diffusivity in ferrite is found to be much smaller than in the defect-free ferrite, owing to trapping effects. The morphology of the austenite is demonstrated to play a role by comparing with a sample containing a larger volume fraction of austenite but present as isolated grains which are ineffective to the permeation of hydrogen.

Influence of Temperature on Phase Transformation and Deformation Mechanisms of Cast CrMnNi-TRIP/TWIP Steel

At different temperatures ranging from ‑60°C to 200°C a cast CrMnNi-TRIP steel was deformed by uniaxial tension. The resulting microstructure was investigated using XRD, EBSD and LOM. The correlation of the phase transformation with the deformation temperature was examined. Depending on temperature, a transition in the deformation mechanisms was observed. Starting with the generation of deformation bands, accompanied by martensitic phase transformation, followed by twinning, the deformation mechanism turned to conventional dislocation glide with raising temperature. Between -60°C and 20°C the TRIP (TRansformation Induced Plasticity)-effect is the dominating deformation mechanism, whereas between 20°C and 200°C the TWIP (Twinning induced plasticity) effect is observed. The geometrical arrangement of martensite within the microstructure is considered within this study. The amount of α'-martensite is mainly responsible for the hardening rate and the resulting mechanical properties.

Statistical Aspects of Martensitic Nucleation

Analysis of the Cech-Turnbull small-particle martensitic transformation experiments in terms of heterogeneous nucleation theory defines an exponential nucleation-site potency distribution. Sensitive acoustic-emission detection of martensitic nucleation events shows that the same form of distribution describes the behavior of bulk polycrystals, and the influence of heat treatment on the distribution amplitude can be identified in this way. On the assumption of a random distribution of pre-existing nucleation-site orientations, the effect of applied stress on the effective potency distribution has been calculated, thus accounting for an observed nonlinear stress dependence of the transformation kinetics. The model also predicts a transformation “yield locus” for multiaxial stress.

Nanostructured bainite

An alloy system based on iron is described in which it has been possible to create a high density of interfaces by heat treatment alone. The resulting structure consists of a mixture of slender platelets of bainitic ferrite, just 20–40 nm in thickness, embedded in a matrix of carbon-enriched austenite. The rate at which this structure evolves is slow by conventional standards, but this permits components to be made which are large in all three dimensions, with uniform properties throughout. The fundamental mechanisms behind this novel nanostructured steel are reviewed, along with the factors determining its strength, ductility and fracture toughness. It is argued that, although reasonable toughness can be achieved in the context of strength levels exceeding 2000 MPa, the impact toughness remains poor and that it may not be possible to improve this particular parameter.

Crystallographic texture of stress-affected bainite

A method is presented for calculating both the macroscopic strains and the crystallographic bias which develop when a polycrystalline sample of austenitic steel is transformed into bainite or martensite under the influence of an applied stress or a system of stresses. Any texture present in the austenite prior to transformation is taken into account, as is the detailed crystallography of the transformation. Comparisons with experimental data are encouraging. A strong correlation has been observed between the proportion of the driving force for transformation that is attributed to stress and the extent of variant selection.

Phase Transformations in Pearlitic Steels Induced by Severe Plastic Deformation

The paper presents an overview of a number of unusual phase transformations which take place in pearlitic steels in conditions of the severe deformation, i.e. combination of high pressure and strong shear strain. Strain-induced cementite dissolution is a well-documented phenomenon, which occurs during cold plastic deformation of pearlitic steels. Recently new results which can shed additional light on the mechanisms of this process were obtained thanks to 3DAP and HRTEM investigations of pearlitic steel deformed by high pressure torsion (HPT). It was shown that the process of cementite decomposition starts by carbon depletion from the carbides, which indicates that the deviation of cementite’s chemical composition from the stoichiometric is the main reason for thermodynamic destabilisation of cementite during plastic deformation. Important results were obtained regarding the distribution of released carbon atoms in ferrite. It was experimentally confirmed that carbon segregates to the dislocations and grain boundaries of nanocrystalline ferrite. Another unusual phase transformation taking place in nanocrystalline pearlitic steel during room temperature HPT is a stress induced α→γ transformation, which never occurs during conventional deformation of coarse grained iron and carbon steels. It was concluded that this occurred due to a reverse martensitic transformation. The atomistic mechanism and the thermodynamics of the transformation, as well as issues related to the stability of the reverted austenite will be discussed.