Individual Austenite Research Articles

Deformation-induced austenite grain rotation and transformation in TRIP-assisted steel

Uniaxial straining experiments were performed on a rolled and annealed Si-alloyed TRIP (transformation-induced plasticity) steel sheet in order to assess the role of its microstructure on the mechanical stability of austenite grains with respect to martensitic transformation. The transformation behavior of individual metastable austenite grains was studied both at the surface and inside the bulk of the material using electron back-scattered diffraction (EBSD) and X-ray diffraction (XRD) by deforming the samples to different strain levels up to about 20%. A comparison of the XRD and EBSD results revealed that the retained austenite grains at the surface have a stronger tendency to transform than the austenite grains in the bulk of the material. The deformation-induced changes of individual austenite grains before and after straining were monitored with EBSD. Three different types of austenite grains can be distinguished that have different transformation behaviors: austenite grains at the grain boundaries between ferrite grains, twinned austenite grains, and embedded austenite grains that are completely surrounded by a single ferrite grain. It was found that twinned austenite grains and the austenite grains present at the grain boundaries between larger ferrite grains typically transform first, i.e. are less stable, in contrast to austenite grains that are completely embedded in a larger ferrite grain. In the latter case, straining leads to rotations of the harder austenite grain within the softer ferrite matrix before the austenite transforms into martensite. The analysis suggests that austenite grain rotation behavior is also a significant factor contributing to enhancement of the ductility.

Conglomerate Stabilization Curve Design Method for Shape Memory Alloy Wire Actuators With Cyclic Shakedown

The high energy density actuation potential of shape memory alloy (SMA) wire is tempered by conservative design guidelines set to mitigate complex factors such as functional fatigue (shakedown). In addition to stroke loss, shakedown causes practical problems of interface position drift between the system and the SMA wire under higher stress levels if the wire does not undergo a pre-installation shakedown procedure. Constraining actuation strain eliminates interface position drift and has been reported to reduce shakedown as well as increase fatigue life. One approach to limit actuation strain is using a mechanical strain limiter, which sets a fixed Martensite strain position—useful for the development of in-device shakedown procedures, which eliminates time-consuming pre-installation shakedown procedures. This paper presents a novel conglomerate stabilization curve design method for SMA wire actuators, which accounts for shakedown with and without the use of mechanical strain limiters to enable higher stress designs to maximize actuator performance. Shakedown experimental data including the effect of strain limiters along with stroke and work density contours form the basis for this new design method. For each independent mechanical strain limiter, the maximum of the individual postshakedown Austenite curves at a range of applied stress are combined into a conglomerate stabilization design curve. These curves over a set of mechanical strain limiters including the zero set provide steady-state performance prediction for SMA actuation, effectively decoupling the shakedown material performance from design variables that affect the shakedown. The use and benefits of the conglomerate stabilization curve design method are demonstrated with a common constant force actuator design example, which was validated in hardware on a heavy duty latch device. This new design method, which accounts for shakedown, supports design of SMA actuators at higher stresses with more economical use of material/power and enables the utilization of strain limiters for cost-saving in-device shakedown procedures.

Observation of changing crystal orientations during grain coarsening

Understanding the underlying mechanisms of grain coarsening is important in controlling the properties of metals, which strongly depend on the microstructure that forms during the production process or during use at high temperature. Grain coarsening of austenite at 1273 K in a binary Fe–2 wt.% Mn alloy was studied using synchrotron radiation. Evolution of the volume, average crystallographic orientation and mosaicity of more than 2000 individual austenite grains was tracked during annealing. It was found that an approximately linear relationship exists between grain size and mosaicity, which means that orientation gradients are present in the grains. The orientation gradients remain constant during coarsening and consequently the character of grain boundaries changes during coarsening, affecting the coarsening rate. Furthermore, changes in the average orientation of grains during coarsening were observed. The changes could be understood by taking the observed orientation gradients and anisotropic movement of grain boundaries into account. Five basic modes of grain coarsening were deduced from the measurements, which include: anisotropic (I) and isotropic (II) growth (or shrinkage); movement of grain boundaries resulting in no change in volume but a change in shape (III); movement of grain boundaries resulting in no change in volume and mosaicity, but a change in crystallographic orientation (IV); no movement of grain boundaries (V).

Grain Scale Analysis of Variant Selection during the Gamma-Epsilon-Alpha' Phase Transformation in Austenitic Steels

Austenitic steels can exhibit a complex transformation sequence during deformation. Indeed, the austenitic phase transforms first into bands of ε (HCP) martensite. This transformation is then followed by the formation of α’ (BCC) martensite. In this study, the crystallography of the transformation together with the occurrence of variant selection is studied at the scale of individual austenite grains. About ten prior austenite grains deformed at different strain levels in uniaxial tension were analysed by means of EBSD techniques. One of the classical approaches to predict the variant selection phenomenon is based on the calculation of the interaction energy between the macroscopic stress and the shape deformation associated with the formation of the product phase. The formation of the α’ variants was observed to lead to a very strong variant selection that cannot be fully explained by energetic criterion. It is suggested that the crystallography of the transformation sequence can account for the unexpected variants.

Texture evolution and fcc/hcp transformation in Fe–Mn–Si–Cr alloys by tensile deformation

In this study, evolution characteristics of the texture and microstructure of Fe–Mn–Si–Cr shape memory alloys (SMAs) upon tensile deformation were investigated. Electron backscatter diffraction analysis revealed that the tensile deformation in the rolling direction (RD) of sample sheets resulted in the evolution of microstructure containing stress-induced ɛ martensite and texture components belonging to 〈1 1 1〉 and 〈1 0 0〉//RD fibers. Quantitative measurement of ɛ martensite formation in individual austenite grains at the strain levels of 4% and 5% revealed that among the 〈1 0 0〉, 〈1 0 1〉, and 〈1 1 1〉 grains which are oriented along the tensile direction, 〈1 0 1〉 oriented grain showed the highest ɛ martensite formation rate. In the recrystallized state, the volume fraction of 〈1 0 1〉 texture component in 20% tensile deformed samples was slightly higher in the transverse direction (TD) than in the RD. Because of that preferred orientation recovery strain was slightly higher in the TD sample than in the RD sample.

Investigation of strain-induced martensitic transformation in metastable austenite using nanoindentation

Strain-induced martensitic transformation of metastable austenite was investigated by nanoindentation of individual austenite grains in multi-phase steel. A cross-section prepared through one of these indented regions using focused ion beam milling was examined by transmission electron microscopy. The presence of martensite underneath the indent indicates that the pop-ins observed on the load-displacement curve during nanoindentation correspond to the onset of strain-induced martensitic transformation. The pop-ins can be understood as resulting from the selection of a favorable martensite variant during nanoindentation.

Quantitative Analysis of Variant Selection and Orientation Relationships during the γ-to-α Phase Transformation in Hot-Rolled TRIP Steels

The orientation relationships that apply to the fcc (γ) – bcc (α) phase transformation in high-performance hot-rolled TRIP-aided steels were characterised by EBSD techniques. A statistical treatment of the experimental data allows the mean orientation relationship to be determined. This mean orientation relationship was compared to the models commonly proposed in the literature and confronted qualitatively to the predictions of the phenomenological theory of martensite crystallography (PTMC). The variant selection phenomenon was also characterized quantitatively at the level of individual austenite grains. The reconstruction of the EBSD maps evidences that bainite grows by packets in which the bainite laths share a common {111}γ plane in the austenite. This growth mechanism is not influenced by the prior hot deformation of the austenite. The hot deformation has a critical influence on the number of packets that forms. The analysis of the crystallographic features of the bainite packets reveals that all possible variants are formed in a packet, though in different proportions.

404 低合金鋼/Ni基合金溶接部の熱処理による組織変化(OS4-(1)オーガナイズドセッション《材料と組織》)

Characteristic microstructure was formed in a low-alloy steel / Ni-base alloy welded interface region by solidification and thermal history. At welding, molten Ni-base alloy solidified epitaxially from low-alloy austenite grains in the beginning. However, the crystal growth often stopped at the place where concentrations of alloy elements changed rapidly. As a result, characteristic grain boundaries parallel to the interface were generated. During cooling after welding, a low-alloy steel region within one individual austenite grain transformed to martensite partially. Hence, the KS relationship was confirmed in such a grain, and the quenched martensite was tempered by the following heating.

Variant selection during the γ-to-α b phase transformation in hot-rolled bainitic TRIP-aided steels

The variant selection phenomenon during the austenite to bainite phase transformation in hot-rolled TRIP-aided steels was quantitatively characterized at the level of individual austenite grains. The reconstruction of the electron backscatter diffraction maps provided evidence that bainite grows by packets of laths sharing a common {1 1 1}γ plane in the austenite. The affect of hot deformation is to reduce the number of packets that form. It is suggested that slip activity is important in understanding this effect.

The effect of aluminium and phosphorus on the stability of individual austenite grains in TRIP steels

We have performed in situ synchrotron X-ray diffraction experiments to assess the influence of aluminium and phosphorus on the austenite stability in low-alloyed transformation-induced plasticity steels during the high-temperature bainitic holding and the subsequent martensitic transformation during cooling to temperatures between room temperature and 100 K. Although the addition of aluminium increases the chemical driving force for the formation of bainitic ferrite plates significantly, the phosphorus exerts a larger influence on the bainitic transformation kinetics. Consequently, the addition of phosphorus leads to a higher degree of carbon enrichment and a narrower grain volume distribution of the metastable austenite. The stability of the individual austenite grains with respect to their martensitic transformation depends on both the local carbon content and the grain volume for austenite grains smaller than 20 μm 3. The presence of aluminium and phosphorus further stabilizes the austenite grains.

Modelling of the plastic flow of trip-aided multiphase steel based on an incremental mean-field approach

An incremental mean-field model is developed for the prediction of transformation induced plasticity (TRIP) in multiphase steel. The partitioning of strain between softer and harder constituents is computed based on an elastic-plastic Mori–Tanaka approach that accounts for the progressive transformation of austenite into martensite. The latter transformation is predicted using an energy-balance criterion that is formulated at the level of individual austenite grains. The model has been tested against experimental data. Macroscopic stress-strain curves and rate of martensite formation have been measured on sheet samples subjected to various loading modes: uniaxial tension, simple shear, and (in-plane) uniaxial compression. These experiments were performed at 20 °C and the uniaxial tensile test was repeated at −30 °C. The mean-field model produces fair predictions of the macroscopic hardening resulting from TRIP on the condition that a sufficient proportion of the load is carried by the very hard martensite inclusions. Such prediction implies that one accounts for the stress heterogeneity across the ferrite-based matrix. At the same time, the model reproduces the elastic lattice strains and the plastic elongation which are measured within the phases by neutron diffraction and by image correlation in a scanning electron microscope, respectively. The model can be used in finite element simulations of forming processes which is illustrated in a study of necking of a cylindrical bar under uniaxial tension.

Characterization of individual retained austenite grains and their stability in low-alloyed TRIP steels

In situ three-dimensional (3-D) X-ray diffraction experiments have been performed at a synchrotron source on low-alloyed multiphase TRIP steels containing 0.25 wt.% Si and 0.44 wt.% Al and produced with different bainitic holding times, in order to assess the influence of the bainitic transformation on the thermal stability of individual austenite grains with respect to their martensitic transformation. A detailed characterization of the austenite grain volume distribution at room temperature was performed as a function of the prior bainitic holding time. In addition, the martensitic transformation behaviour of individual metastable grains was studied in situ during cooling to a temperature of 100 K. Both the carbon content and the grain volume play a key role in the stability of the austenite grains below 15 μm 3, while the carbon content exerts the dominant effect in the stability of the bigger grains. Measurements also suggest that the tetragonality of the thermally formed martensite is suppressed.

Growth of Individual Austenite Grains Measured with 3DXRD Microscopy

Studying austenitisation in steel, so far, was either limited to observations at the surface of the material or to the determination of the average grain growth behavior in the bulk. The development of the three-dimensional X-ray diffraction (3DXRD) microscope at beam line ID11 of the European Synchrotron Radiation Facility in Grenoble, France, made it possible to study the transformation kinetics in-situ and at the level of individual grains in the bulk of the material. Unique in-situ observations of austenite growth kinetics during continuous heating experiments were made for two commercial low-alloy steels (C22 and C35). The observed growth behavior of individual austenite grains gives a valuable contribution to understanding the phase transformations on heating, i.e. austenite formation from ferrite and pearlite.

Evolution of crack-tip transformation zones in superelastic Nitinol subjected to in situ fatigue: A fracture mechanics and synchrotron X-ray microdiffraction analysis

The ultrahigh spatial resolution (∼1 μm 2) of synchrotron X-ray microdiffraction is combined with fracture mechanics techniques to directly measure in situ three-dimensional strains, phases and crystallographic alignment ahead of a growing fatigue crack (100 cycles in situ) in superelastic Nitinol. The results provide some surprising insights into the growth of cracks in phase-transforming material at the microscale. Specifically, despite a macroscopic superelastic strain recovery of 6–8% associated with the phase transformation, individual austenite grains experience local strains of less than 1.5%. This observation indicates that it is the localized process of the accommodation of the transformation and subsequent loading of the martensite that provide the main source of the large recoverable strains. Furthermore, the plastic region ahead of the crack is composed of deformed martensite. This micromechanical transformation process is dependent upon the material texture, and directly influences the transformation zone size/shape as well as the crack path.

Elastic strain evolution and ε-martensite formation in individual austenite grains during in situ loading of a metastable stainless steel

The (hcp) ε-martensite formation and the elastic strain evolution of individual (fcc) austenite grains in metastable austenitic stainless steel AISI 301 has been investigated during in situ tensile loading up to 5% applied strain. The experiment was conducted using high-energy X-rays and the 3DXRD technique, enabling studies of individual grains embedded in the bulk of the steel. Out of the 47 probed austenite grains, one could be coupled with the formation of ε-martensite, using the reported orientation relationship between the two phases. The formation of ε-martensite occurred in the austenite grain with the highest Schmid factor for the active {111}<12¯1> slip system.

Grain Scale Assessment of Variant Selection in a Thermomechanically Processed TRIP Steel

Orientations of both the α and γ phases in a multi-phase commercial steel were measured by means of electron backscatter diffraction (EBSD) techniques. Using the average orientation of each austenite grain as the reference frame, the orientation relationships between the two lattices were compared with the common orientation relationships (i.e. the Kurdjumov-Sachs and Nishiyama-Wassermann) in Rodrigues-Frank space. The occurrence of variant selection in individual austenite grains was examined using a recent dislocation-based model. This model considers the role of the slip systems that were active during prior deformation, as well as those of in-plane reactions, cross-slip and the partial dislocations that are linked to specific variants. It also unites the competing K-S and N-W relationships through the dissociation of perfect dislocations. Reasonably good agreement was observed between the predictions and the observations. Possible explanations for some of the discrepancies are also presented.

Martensitic transformation of individual grains in low-alloyed TRIP steels

We have performed in situ synchrotron X-ray diffraction experiments on low-alloyed multiphase TRIP steels during cooling, to monitor the martensitic transformation of individual austenite grains within the bulk material. Direct experimental evidence is presented that the stability of the austenite grains is controlled not only by the local carbon level but also by the grain size. This new quantitative information on the martensitic transformation in complex microstructures is of great importance for the design of martensite-based metallic materials.

Evolution of Residual Strains in Metastable Austenitic Stainless Steels and the Accompanying Strain Induced Martensitic Transformation

The deformation behavior of metastable austenitic stainless steel AISI 301, suffering different initial cold rolling reduction, has been investigated during uniaxial tensile loading. In situ highenergy x-ray diffraction was employed to characterize the residual strain evolution and the strain induced martensitic transformation. Moreover, the 3DXRD technique was employed to characterize the deformation behavior of individual austenite grains during elastic and early plastic deformation. The cold rolling reduction was found to induce compressive residual strains in the austenite along rolling direction and balancing tensile residual strains in the ά-martensite. The opposite residual strain state was found in the transverse direction. The residual strain states of five individual austenite grains in the bulk of a sample suffering 2% cold rolling reduction was found to be divergent. The difference among the grains, considering both the residual strains and the evolution of these, could not be solely explained by elastic and plastic anisotropy. The strain states of the five austenite grains are also a consequence of the local neighborhood.

Effect of block size on the strength of lath martensite in low carbon steels

The microstructure and the strength of the lath martensite in Fe–0.2C and Fe–0.2C–2Mn alloys were analyzed as a function of the prior austenite grain size. The size of martensite packets formed within individual austenite grains was controlled by the austenite grain size but not affected by the Mn addition. However, the further subdivision of packets into blocks differed significantly in the two alloys, and at a given austenite grain size a smaller block size was observed in the Mn containing alloy. The yield strength of the two alloys was related to the packet size and the block size, respectively, and the results suggested that the block size is the key structural parameter when analyzing the strength–structure relationship of lath martensite in low carbon steels.

Variant Selection in Mechanically-induced Martensitic Transformation of Metastable Austenitic Steel

During martensite transformation, parent austenite usually has an orientation relationship with newly transformed martensite and thereby a crystallographic texture of the austenite has a great influence on a texture development in the inherited martensite. For a given orientation relationship, there are several equivalent orientations of the inherited phase, which is called variant. Table 1 shows the 24 variants for Kurdjumov–Sachs (K–S) orientation relationship which is usually observed in carbon steels. In idealized case, all variants can appear in an austenite grain with an equal probability during the transformation. However, it has been reported that some variants preferentially appeared during the transformation. In other words, some variants possibly have greater relative probability to be selected. This phenomenon, which is called variant selection, is known to have a significant effect on the texture development in the inherited phase. The slip system of parent phase, the grain boundary orientation and the existence of stress have been known to affect the variant selection during the transformation and the development of transformation texture. As for the slip activity, the criterion of variant selection was imposed in terms of slip distribution and was applied to diffusion-controlled transformation. Ray et al. reviewed the various kinds of transformation texture that is normally encountered in austenite-to-ferrite transformation in steel. As for the bainitic transformation in steel, the variant selection had been investigated by an experimental observation. Recently, present authors suggested that the probability for a nucleation site to really act during displacive transformation could be derived for each variant as a function of the mechanical interaction energy between externally applied stress and lattice deformation based on the Kurdjumov–Sachs (K–S) relationship. In present study, the variant selection in mechanically induced martensitic transformation of metastable austenite is investigated with respect to the interaction between external stress and lattice deformation of the transformation. The orientations of parent austenite and newly transformed martensite are measured for tensile and compressive deformation using electron back-scattered diffraction (EBSD). For an individual austenite grain, the orientation of 24 K–S variants are evaluated and compared with measured orientation of martensite. The interaction energy between externally applied stress and lattice deformation is calculated for each 24 K–S variant and the probability of variant selection is assessed. The assessed probability is compared with the experimental results.