Martensite Particles Research Articles

Investigation of the Effect of Deformation on γ‐α Phase Transformation Kinetics in Hot‐Rolled Dual Phase Steel by Phase Field Approach

Dual phase steels, consisting of hard martensite particles in a ductile ferritic matrix, offer high strength and deformability at the same time. Additionally, they are cost effective by a dilute alloying concept. In industrial production, two manufacturing concepts have been implemented: intercritical annealing of cold rolled sheet, or hot rolling. The current work has investigated the effect of deformation on the γ-α phase transformation kinetics in the dual phase steel production using the hot rolling scheme. The pancaked austenite grains containing denser nucleation sites have a strong influence on the ferrite transformation kinetics. In addition, the multiplication of dislocations which results in the increase in elastic strain energy and dislocation core energy contributes to some acceleration in ferrite growth kinetics. A modelling approach for the γ-α phase transformation kinetics in dual phase steels has been developed employing the phase field theory. The nucleation behaviour, i.e. the number and size of nuclei developed after an elapsed time as well as their nucleation sites which were evaluated from microstructure analysis, and the increase in the driving force of grain growth were integrated into this model.

Simulation of orientation gradients in a ferrite matrix containing hard martensite under plane-strain flange deformation

The plastic deformation behavior of dual phase (DP) steel under plane-strain flange deformation during a cup deep-drawing process was simulated using a crystal plasticity finite element method (CPFEM). The representative volume elements of a simple geometry were used to capture the in-grain orientation gradients of a ferrite matrix containing a hard martensite particle. The divergence of the reorientation rate vector and the stability parameter were used to determine relatively stable, metastable and unstable orientations for the ferrite matrix. The simulation showed that both the formation of the in-grain orientation gradients and the inhomogeneity of the local average misorientation (LAM) distribution were enhanced by the hard martensite particle. The analysis showed that the orientation gradients for the ferrite phase within the DP were simultaneously influenced by both the initial orientation and the martensite. The relative contribution of the two factors was strongly dependent on the stability of the initial orientation under the plane-strain flange deformation.

Low cycle fatigue behavior and failure mechanism of a dual-phase steel

Low cycle fatigue behavior and failure mechanism of a dual-phase steel was investigated by using LCF tests, interrupted LCF tests, SEM, TEM and XRD peak broadening. LCF tests were performed at constant strain amplitudes of ±0.002, ±0.004, ±0.006 and ±0.1. Microscopic investigations were carried out on gauge surfaces of the interrupted LCF specimens at various stages of their fatigue life. It was observed that at high strain amplitudes damage was started at fractured martensite particles and passed through areas with high density of martensite. At low strain amplitudes damage was started at separated ferrite/martensite interface and passed through areas with low density of martensite. All specimens showed fatigue hardening during LCF tests. It was also observed that the rate of hardening was affected by strain amplitude. The results show that XRD peak broadening is sensitive to the strain amplitude and the stage of damage in the specimens.

Transmission Electron Microscopy and Nanoindentation Study of the Weld Zone Microstructure of Diode-Laser-Joined Automotive Transformation-Induced Plasticity Steel

We have used transmission electron microscopy (TEM) and nanoindentation to characterize the dominant phases present in the weld zone of a diode-laser-welded transformation-induced plasticity (TRIP) steel, examining the unaffected base metal as a baseline. The microstructure of the base metal consists predominantly of ferrite, retained austenite, martensite, and occasional large carbide particles. The dominant microstructure of the weld zone is of differently oriented packets having a bainitic morphology. The weld also contains allotriomorphic ferrite, idiomorphic ferrite, as well some twinned martensite that is surrounded by austenite. The TEM analysis of the bainitic-morphology packets indicates that they consist of a lath ferrite phase separated by an interlath carbon-enriched retained austenite. In most cases, the orientation relationship (OR) between the lath ferrite and the interlath retained austenite can be approximated as Nishiyama–Wasserman (N-W). We used site-specific nanoindentation to further characterize the packets and the allotriomorphic ferrite, confirming through the hardness values the conclusions reached by TEM. While martensite was regularly present in the base metal, it was only sparsely distributed within the weld zone, boding well for the weld’s mechanical properties.

234 Effect of Microstructure on Fatigue Crack Growth Behavior of Structural steel

The fatigue crack growth tests were performed on low carbon structural steel with five different microstructures, i.e. steel with pearlite particle (15 and 50μm grain size), with bainite particle (15 and 5μm grain size) and with martensite particle. The results showed that microstructure influence fatigue crack growth behavior in both threshold region and Paris region and the presence of hard phase enhances fatigue crack growth resistance. The results were discussed based on the crack path observation and crack closure behavior

Nucleation and growth of the α′ martensitic phase in Pu–Ga alloys

In a Pu–2.0 at.% Ga alloy, it is observed experimentally that the amount of the martensitic α′ product formed upon cooling the metastable δ phase below the martensite burst temperature ( M b) is a function of the holding temperature and holding time of a prior conditioning (“annealing”) treatment. Before subjecting a sample to a cooling and heating cycle to form and revert the α′ phase, it was first homogenized for 8 h at 375 °C to remove any microstructural memory of prior transformations. Subsequently, conditioning was carried out in a differential scanning calorimeter apparatus at temperatures in the range between −50 and 370 °C for periods of up to 70 h to determine the holding time and temperature that produced the largest volume fraction of α′ upon subsequent cooling. Using transformation peak areas (i.e., the heats of transformation) as a measure of the amount of α′ formed, the largest amount of α′ was obtained following holding at 25 °C for at least 6 h. Additional time at 25 °C, up to 70 h, did not increase the amount of subsequent α′ formation. At 25 °C, the Pu–2.0 at.% Ga alloy is below the eutectoid transformation temperature in the phase diagram and the expected equilibrium phases are α and Pu 3Ga, although a complete eutectoid decomposition of δ to these phases is expected to be extremely slow. It is proposed here that the influence of the conditioning treatment can be attributed to the activation of α-phase embryos in the matrix as a beginning step toward the eutectoid decomposition, and we discuss the effects of spontaneous self-irradiation accompanying the Pu radioactive decay on the activation process. Subsequently, upon cooling, certain embryos appear to be active as sites for the burst growth of martensitic α′ particles, and their amount, distribution, and potency appear to contribute to the total amount of martensitic product formed. A modeling approach based on classic nucleation theory is presented to describe the formation of α-phase embryos during conditioning. The reasons why the holding times during conditioning become eventually ineffective in promoting more α′ formation on cooling are discussed in terms of the differences in the potency of the embryos created in the δ matrix during conditioning and in terms of growth-impeding volume strains in the matrix resulting from an increasing number of martensite particles, thus opposing further growth. It is suggested that the disparate amounts of the α′ formation reported in the literature following various studies may be in part a consequence of the fact that conditioning times at ambient temperatures are inevitably involved in any handling of radioactive samples prior to testing.

The effect of microstructural evolution on hardening behavior of type 17-4PH stainless steel in long-term aging at 350 °C

The effect of microstructural evolution on hardening behavior of 17-4PH stainless steel in long-term aging at 350 °C was studied by X-ray diffraction and transmission electron microscopy. The results showed that there is the matrix of lath martensite and nanometer-sized particles of ε-Cu precipitated from the matrix after the alloy is solution treated and tempered. When the alloy was aged 350 °C for 9 months, α-α′ spinodal decomposition occurred along the grain boundaries and caused an increase in hardness which compensated for the weakening effect due to ripening of the ε-copper precipitates. Upon further aging to 12 months, the Cr-rich α′-phase and M 23C 6 precipitated, both of which strengthened the alloy considerably and led to enhanced hardening despite the continued softening by overaging of the ε-copper precipitates. With the aging time extended to 15 months, substantial reversed austenite transformed and precipitation of the intermetallic G-phase occurred near the ε-Cu precipitates in the matrix. The abundant amount of reversed austenite that transformed led to rapid softening.

Martensite–ferrite interface strength in a low-carbon bainitic steel

Abstract Damage by void nucleation at martensite particles in a low-carbon bainitic steel occurs at a true tensile strain of 0.90 ± 0.05. Several void-nucleation models are considered, and the strength of the martensite–ferrite interface is determined to be 2.4–2.5 GPa.

The tensile fracture behaviour of intercritically annealed and quenched + tempered ferritic ductile iron with dual matrix structure

The tensile fracture behaviour of oil quenched from intercritical annealing temperature (ICAT) range (α + γ) and then tempered ferritic ductile iron having dual matrix structure (proeutectoid ferrite + martensite) with different martensite volume fraction and its morphologies have been studied for a ductile ferritic cast iron containing 3.5 C wt%, Si wt% and 0.318 wt% Mn. In both fine and coarse dual matrix structures microvoids formed at the interface between the graphite nodules and the surrounding structure, martensite particles, inclusions and martensite/ferrite interfaces in the necked region. The martensite morphology had an influence in determining martensite cracking. Coarse and interconnected high C martensite distributed along the intercellular boundaries cracked easily. Martensite cracking was less frequent and the microvoids were smaller in the fine structure than the coarse ones. Microvoid coalescence was the dominant form of fracture in both structures. The specimens with fine structure had higher densities of voids. In these samples, voids initiated mostly by decohesion at the interface, and by some examples of fracture of martensite.

Burst Martensitic Transformations in a Pu-Ga Alloy

AbstractMartensitic transformations can occur via two modes: thermoelastic and burst. In thermoelastic martensites, deformation can be accommodated elastically and transformations occur smoothly with changes in temperature or stress. Burst martensitic transformations require both elastic and plastic deformation to accommodate strain; individual martensite particles form at the speed of sound, and the overall accumulation of martensite may increase in discrete, incremental steps. Here, we examine a unique martensitic transformation and reversion in a Pu-2.0 at% Ga alloy and show evidence that they proceed via the burst mode. Upon cooling from ambient conditions, the metastable delta phase partially transforms martensitically to the alpha-prime phase with a volume contraction of 20%. This large volume change suggests a burst transformation. Furthermore, using differential scanning calorimetry (DSC), we observed that the alpha-prime to delta reversion proceeds in discrete increments, which appear as sharp peaks in DSC data. The DSC data is compared to similar results obtained using dilatometry and resistometry. This incremental progression is believed to be the result of autocatalytic cascades of many alpha-prime particles reverting nearly-simultaneously to the delta phase. Finite-element modeling suggests that residual stresses in the regions of reverted alpha-prime particles may catalyze (or retard) additional transformation. These stresses could initiate cascades of alpha-prime particles that revert nearly-simultaneously. The cascades are likely quenched by stress and/or temperature changes resulting from the transformation itself. During the forward delta to alpha-prime transformation, burst events are not observed with the above techniques. The transformation, however, is still expected to proceed via the burst martensite mode because of the large volume changes required. Because alpha-prime must be nucleated in the delta matrix before it can grow as an individual burst, the transformation may not occur cooperatively. These individual bursts may be too small to be resolved by the above techniques, and the signal observed corresponds to a cumulative total of all the events.

Magnetic states and the crystal volume of Nd 2/3Ca 1/3MnO 3: analysis of phase transformations

The neutron diffraction and magnetic studies of the Nd 2/3Ca 1/3MnO 3 ceramic compound have been carried out aiming to reveal the signs of phase separation in the system. A well-expressed step in the inverse magnetic susceptibility of the system found at T ∼ 212 K was considered to be a result of the charge ordering. The elastic neutron scattering study upon heating has shown a diminishing to zero of intensities of the diffraction peaks, corresponding to the ferromagnetic B-type structure, antiferromagnetic DE-type and pseudo-CE-type structures at ∼70 and ∼80 K by the first order phase transitions, and at ∼130 K by the second order transition, respectively. We suppose that the set of the revealed magnetic structures relates to the set of structure regions formed due to the nucleation of the martensitic particles within the parent matrix below the charge ordering temperature.

The effect of intercritical heat treatment on the mechanical properties of AISI 3115 steel

This paper presents the effect of intercritical heat treatment (IHT) on the mechanical properties of AISI 3115 alloy steel. The steel was intercritically heat-treated at a temperature range of 730–830°C followed by water quenching. The properties of tensile strength, fatigue, hardness and microstructure were evaluated from the mechanical tests and metallographic analysis, respectively. The experimental results showed that tensile strength increases but impact strength decreases with increasing intercritical temperature, correspondingly with the increase in amount of martensite in the steel. It was found that the resulting microstructures are predominantly ferrite with martensite, which is known as ‘dual-phase steels’. This indicates that both the soft and ductile ferrite matrix and strong and tough martensite particles play an important role in determining the dual-phase properties, especially continuous yielding behaviour of the steel. The fractographic analysis results are also discussed in this paper.

Effect of Initial Microstructure on Superplasticity in Ultrafine Grained 18Cr-9Ni Stainless Steel

Ultrafine grained austenitic structure was obtained in 18Cr-9Ni stainless steel by thermomechanical treatment using reversion from deformation-induced martensite. The superplastic deformation behavior was investigated at 923 K for the steels containing various amounts of retained martensite particles in the initial structure before tensile testing. The retained martensite was effective for suppressing grain growth of austenite and necessary for the superplasticity although it was thermodinamically unstable phase and gradually decreased its volume fraction with superplastic deformation. Therefore, the superplastic elongation was strongly dependent on the initial volume fraction of the retained martensite. The total superplastic elongation was enlarged with increasing the initial amount of martensite, and the maximum elongation of about 270% was obtained when the volume fraction was controlled to be around 10 vol%. However, the increase in elongation was leveled off in the range above 15 vol% martensite. The effect of the retained martensite on the superplasticiy was discussed in connection with the changes in volume fraction of the martensite, austenite grain size and deformation mechanism.

Multiscale computational simulation of deformation behavior of TRIP steel with growth of martensitic particles in unit cell by asymptotic homogenization method

Due to the strain-induced martensitic transformation (SIMT), the strength, ductility and toughness of TRIP (transformation-induced plasticity) steel are enhanced. Such favorable mechanical properties of TRIP steel can be realized by the appropriate combination of the SIMT behavior and deformation behavior of both austenite and martensite. However, since the SIMT and deformation behavior of both phases depend strongly on the conditions of the working environment such as temperature, strain rate and stress system, the desired mechanical properties are obtained only in a fairly restricted working environment. On the other hand, martensitic particles of various sizes, shapes and orientations can be observed and are distributed spatially in the microstructure of TRIP steel, therefore it is possible that the mechanical properties of TRIP steel strongly manifest a dependence on the geometrical inhomogeneity and configuration of martensite. If the geometrical configuration and inhomogeneity of martensite can be controlled, we can expect to enhance the mechanical properties of TRIP steel. Here, in order to investigate the effects of the geometrical configuration and inhomogeneity of martensite in the austenitic matrix on the macroscopic deformation behavior of TRIP steel, the computational simulation of the deformation behavior of TRIP steels with the growth of an ellipsoidal martensitic particle embedded in the center of the austenitic unit cell, is performed using the homogenization technique.

Micromechanical modeling of the effect of particle size difference in dual phase steels

Dual phase (DP) steels having a microstructure consisting of martensite islands, referred to as particles, dispersed in a ferrite matrix have received a great deal of attention due to their useful combination of high strength, high work hardening rate and ductility, all of which are favorable properties for forming processes. The martensite particles display two distinct deformation mechanisms, depending on their size. Small particles are reported in the literature to undergo no measurable plastic deformation and thus can be described as rigid particles dispersed in a matrix of ferrite. On the other hand, large particles reportedly experience a small degree of plastic deformation, which has a significant influence on the mechanism of deformation of such materials. Although most micromechanical models assume a uniform particle size, a distribution of sizes in DP-steels is a more realistic assumption. In this work, a micromechanical model is developed to capture the effect of particle size differences on the mechanical behavior of DP-steels. It is shown that the difference becomes most significant when the ratio of the small to large particle size is approximately 1/2. At low volume fractions of martensite, the effect of a distribution of particle sizes is negligible, but at intermediate and high volume fractions of martensite the interaction due to the size difference becomes quite important. The model displays the intrinsic ability of capturing the steep rise in the strain-hardening rate observed in DP-steels. The model also successfully predicts the mechanisms involved in the deformation process in the DP-steels in agreement with experimental observations reported in the literature.

Assessment of Geometrical Orientation of Martensitic Particles in TRIP Steel by Fourier and Wavelet Transformation Image Analysis

Assessment of Geometrical Orientation of Martensitic Particles in TRIP Steel by Fourier and Wavelet Transformation Image Analysis

The effect of martensite volume fraction and particle size on the tensile properties of a surface-carburized AISI 8620 steel with a dual-phase core microstructure

This study is focused on the production of a dual-phase steel structure in the core of a surface-carburized AISI 8620 cementation steel and the effect of martensite volume fraction (MVF) and martensite particle size (MPS) on tensile properties. Experimental results showed that, compared with specimens with a fully martensitic microstructure in the core, those with a dual-phase microstructure in the core exhibited slightly lower tensile and yield strength but superior ductility without sacrificing surface hardness. In specimens with a dual-phase microstructure in the core, the tensile strength increased and ductility decreased with increasing MVF. Both the tensile strength and the ductility increased with decreasing MPS at constant MVF. The best combination of tensile strength and ductility was obtained with a fine MPS at a constant MVF of 25%.

The effect of martensite particle size on tensile fracture of surface-carburised AISI 8620 steel with dual phase core microstructure

In this study, the production of dual phase microstructure in the core of surface-carburised AISI 8620 cementation steel and the effect of martensite particle size at constant martensite volume fraction on tensile fracture have been investigated. The results showed that specimens with dual phase microstructure in the core exhibited slightly lower tensile and yield strength but superior ductility without sacrificing surface hardness than a specimen with nearly fully martensitic microstructure in the core produced by conventional heat treatment involving quenching from ∼900 °C. Tensile properties increased with decreasing martensite particle size. The effects of martensite particle size on tensile fracture has also been observed. In the core of the specimens with dual phase having fine and coarse microstructures, microvoids formed at martensite particles, inclusions and martensite-ferrite interfaces in the necked region. Martensite particle size had an influence in determining martensite cracking. In the specimen with coarse martensite particles, interconnected martensite distributed along ferrite grain boundaries cracked easily and the fracture mode was predominantly cleavage type. Martensite cracking was less frequent and the microvoids were smaller and microvoid density were higher in the specimen with fine martensite particles and the form of fracture was dimple depression type. In this specimen, voids initiated mostly by decohesion at the interface, and by some examples of fracture of martensite. Microvoid coalescence was the dominant form of fracture in both fine and coarse microstructures.. The specimen with nearly fully martensitic microstructure in the core produced by conventional heat treatment showed completely cleavage type fracture.

Numerical modelling of martensitic growth in an elastoplastic material

Abstract A finite-strain-continuum thermomechanical approach is applied to the problem of temperature-induced martensitic transformation in elastoplastic materials. The key issue is to determine the transformation-induced stress and plastic strain fields and their effect on the thermodynamics and kinetics of transformation. The problems of appearance and growth of a temperature-induced rectangular martensitic unit in an austenitic matrix in both bulk and near-free-surface environments are formulated, solved and analysed. Very complex and heterogeneous stress-strain fields in the austenite and martensite phases and their non-monotonic time variation are predicted. Plastic shear strain at some points can reach 60∼ and, after an elastic growth stage, change sign and vary by 40∼. The interface velocity as a function of temperature and interface position is calculated. After the appearance of a martensitic particle, transformation work, which is the only variable part of the interface driving force, decreases by a factor of two during lengthening of the particle by 10∼. The component of the athermal interfacial friction due to dislocation forest hardening increases significantly as well, leading to interface deceleration and growth arrest. A free surface does not significantly affect the driving force until its distance from the moving interface falls below 0.75 times the particle thickness. Then the transformation work increases while the accumulated plastic strain and associated dislocation forest hardening decrease sharply. If the interface is not arrested at this point, it then accelerates to the free surface where an asymmetric plastic zone leaves ‘tent-shaped’ surface relief. The motion of a small step at the martensitic plate interface is studied as well. In contrast with elastic growth, the thermodynamic conditions for lengthening and thickening are almost equivalent for plastic growth. The results obtained allowed us to address the following fundamental problems: firstly, the morphological transition from plate to lath martensite; secondly, the relation of thermally activated kinetics at the interface level to athermal kinetics at the macrolevel; thirdly, the concentration of dislocations and plastic strain in martensite rather than in austenite despite the much higher yield stress of martensite in comparison with austenite.

The effect of new ferrite content on the tensile fracture behaviour of dual phase steels

The effect of new ferrite present with different volume fractions and morphologies of martensite on microvoids formation and tensile fracture behaviour in dual phase steels has been studied for a steel containing 0.065% C, 1.58% Mn and 0.5% Ni. Fine and coarse dual phase microstructures were obtained from two different starting conditions. Martensite contents were kept constant at ∼18 and ∼25% and new ferrite content was varied by controlled cooling from intercritical annealing temperature of 740, 750 and 785°C. In both fine and coarse dual phase structures microvoids formed at martensite particles, inclusions and martensite-ferrite interfaces in the necked region. Martensite morphology had an influence in determining martensite cracking. Coarse and interconnected martensite distributed along ferrite grain boundaries cracked easily. Martensite cracking was less frequent and the microvoids were smaller in the fine structure than the coarse ones. Microvoid coalescence was the dominant form of fracture in both structures. The specimens with higher new ferrite contents had higher densities of voids. In these samples, voids initiated mostly by decohesion at the interface, and by some examples of fracture of martensite