Martensitic Plasticity Research Articles

Damage accumulation and its effect during thermal- and stress-induced cycling martensite transformation of laser powder bed fused (LPBF) NiTi alloy

Nitinol (NiTi) alloys produced via laser powder bed fusion (LPBF) exhibit promising potential across diverse applications, yet their suitability for devices necessitating repetitive actuation confronts uncertainties regarding microstructural evolution and functional fatigue behavior during cyclic operation. Our investigation unveils that damage accumulation during thermal cycling stems from geometrically necessary dislocations formed through finite-scale incoherent interface motion, contrasting earlier hypotheses attributing LPBF-induced dislocation activity as the primary contributor. Damage accumulation during mechanical cycling is characterized by notable multiplication of cyclic dislocations via dislocation twinning interactions, rather than severe martensitic plasticity. These newborn cycling dislocations inherit martensite's crystallographic characteristics, exerting a distinct influence mechanism on martensite transformation and deformation behavior, diverging from conventional studies on function fatigue in traditional NiTi alloys. This study sheds light on the distinctive damage accumulation traits observed during thermal/mechanical cycling of LPBF-fabricated NiTi alloys, offering valuable insights for probing their functional fatigue mechanisms.

A quasi-2D integrated experimental–numerical approach to high-fidelity mechanical analysis of metallic microstructures

Integrated experimental–numerical testing on bulk metal alloys with fine, complex microstructures is known to be highly challenging, since measurements are restricted to the sample surface, thereby failing to capture the effects of the 3D subsurface microstructure. Consequently, a quantitative comparison of deformation fields between experiments and simulations is hardly possible. To overcome this, we propose a novel ‘quasi-2D’ integrated experimental–numerical testing methodology that hinges on the fabrication of μm-thin specimens with practically through-thickness microstructures over large regions of >100 μm. The specimens are fully characterized from both surfaces and tested in-situ to retrieve microstructure-resolved deformation fields. Simultaneously, the full microstructure is discretized in 3D and simulated. This allows for a detailed, one-to-one quantitative comparison of deformation fields between experiments and simulations, with negligible uncertainty in the subsurface microstructure. Consequently, a degree of agreement between experiments and simulations is attained which we believe to be unprecedented at this scale. We demonstrate the capabilities of the framework on polycrystalline ferritic steel and dual-phase ferritic–martensitic steel specimens. At the mesoscale, the methodology enables quantitative comparisons of the interaction between multiple grains, while, at the microscale, it enables advancement of numerical models by direct confrontation with detailed experimental observations. Specifically, it is revealed that the individual slip system activity maps, identified with SSLIP, near a grain boundary can only be reasonably predicted by enhancing the adopted crystal plasticity simulations with a discrete slip plane model. Additionally, the experimentally observed strong anisotropic plasticity of martensite can only be captured with a substructure-enriched crystal plasticity model.

An integrated experimental-numerical study of martensite/ferrite interface damage initiation in dual-phase steels

Martensite/ferrite (M/F) interface damage is relevant to failure of many dual-phase (DP) steels, but the underlying microscale mechanisms remain unclear. Through an integrated experimental-numerical study, this work examines the recent hypothesis that (lath) martensite substructure boundary sliding triggers and dominates M/F interface damage initiation accompanied by apparent martensite plasticity. The mesoscale morphology and prior austenite grain reconstruction are used as modelling inputs. A multi-scale framework is adopted to predict the interface damage initiation. The M/F interface damage initiation sites predicted by the model based on a sliding-triggered interface damage mechanism adequately agree with those identified from in-situ experiments, confirming the key role of substructure boundary sliding. Moreover, the M/F interface damage initiation strongly correlates with a low M/F strain partitioning rather than the commonly accepted strong M/F strain partitioning. This fundamental understanding is instrumental for the future optimization of DP steel microstructures.

On the critical role of martensite hardening behavior in the paradox of local and global ductility in dual-phase steels

The ductility of sheet metal is typically limited by either localized necking or by damage and fracture. A recent ductility classification refers to these failure modes respectively as “global” and “local” formability. The forming limit curve (FLC) and the uniaxial tensile test assess global formability, whereas the fracture forming limit (FFL), the true thickness fracture strain, hole expansion ratios (HER), etc. are indicators of local formability. Experimental hole expansion data in the literature for different dual-phase (DP) steel grades of similar strength and composition presents a paradox: grades which are found to be ductile in a tensile test and/or FLC show a low ductility in hole expansion, whereas other grades with a low ductility in conventional tests perform surprisingly well at cut edges. In this work, an in-depth systematic statistical analysis of idealized artificial two-phase microstructures is carried out to unravel the underlying mechanisms of the observed paradoxical trends. This is done by scaling the hardness of martensite and its volume fraction to generate virtual DP steels of the same strength but different strain hardening and mechanical phase contrast (hardness difference). The proposed micromechanical model adequately reproduces the experimentally observed trends. The results show that in DP steels, a higher global (necking-driven) ductility is obtained upon delaying martensite plasticity by increasing the mechanical contrast of the two phases. Consequently, the stress–strain distributions becomes more heterogeneous, resulting in a lower fracture limit of one of the phases or interfaces in shear loading, thereby, reducing the local ductility. Global ductility is improved by higher mechanical phase contrast and lower martensite volume fraction, whereas local ductility is improved by low phase contrast and higher martensite volume fraction. The hardening behavior of martensite is the key to avoiding the above trade-off between local and global ductility. It is shown that if the strain hardening capacity of martensite in the later stages of deformation (in high strains) can be increased, this would result in removing the observed paradoxical trends of local and global ductility in DP steels.

A multi-scale model combining martensitic transformations with multi-phase crystallographic slip

Mechanically induced martensitic transformations are an essential mechanism influencing the constitutive behaviour of many advanced steel grades, such as austenitic stainless steels and Transformation Induced Plasticity (TRIP) alloys. In this work, a single-crystal constitutive model is developed to predict their complex microstructural behaviour during mechanical loading. The model is based on the work of Vieira de Carvalho et al. [16] and crucially proposes crystallographic slip in the martensitic phase after transformation from retained austenite. Austenite and martensite plasticity are incorporated following a standard finite strain single crystal plasticity approach and are fully coupled with the martensite formation kinematics, enabling the simultaneous evolution of the three mechanisms. The numerical treatment of the coupled constitutive equations is described in detail. In particular, the solution of the monolithic system of equations with the Newton-Raphson methodology, including the exact linearisation of both the local and global problems. An algorithmic strategy for handling the martensite volume fraction constraints is proposed for return-mappings in the restarted step. The model is validated using macroscopic experimental results for an austenitic stainless steel where an efficient calibration procedure for the micro-constitutive parameters is devised using derivative-free optimisation methods.

Influence of Microstructure on Tensile Properties and Fatigue Crack Propagation Behavior for Lath Martensitic Steel

This paper addresses the role of multilevel microstructures on the fatigue crack propagation behavior and the tensile properties of lath martensite with different substructure sizes. Microstructure characterization of the alloy was carried out by transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron back-scattering diffraction (EBSD), and optical microscopy (OM). Based on the classic Hall–Petch relationship, the results of tensile tests showed that martensitic block is the effective control unit of yield strength. Furthermore, the plasticity of lath martensite is not sensitive to grain size. The tensile deformation mechanisms were also discussed. Fatigue crack propagation tests revealed that the coarse grain has a higher crack propagation threshold and lower crack propagation rate than the fine grain in lath martensitic steel. The change in the plasticity zone ahead of the crack tip leads to the transitional behavior of the fatigue crack propagation rate. When plasticity zone sizes are equal to the block size, the fatigue crack propagation reverts to a stable propagation stage. Finally, an empirical model was established to predict the fatigue crack propagation rate of the stable propagation stage based on the tensile properties of the lath martensitic steel.

The effect of pre-deformation loading on the cyclic hysteresis behavior of nanocrystalline NiTi alloy：A molecular dynamic study

In this work, the effect of pre-deformation loading on the cyclic hysteresis behavior of nanocrystalline NiTi alloy is investigated by the molecular dynamic. The nanocrystalline NiTi is subjected to a large monotonic pre-deformation loading firstly to generate the irrecoverable deformation on the initial model. After this pre-deformation treatment, the other cyclic loading is exerted on the nanocrystalline NiTi to estimate the effect of pre-deformation on cyclic hysteresis behavior. The simulation results show that the plateau length of phase transition and the hysteresis loop area decreases with the increase of pre-deformation loading. When the nanocrystalline NiTi model undergoes a pre-deformation of martensite plasticity, the hysteresis curve transforms from sigmoidal-shaped to shuttle-shaped, and the MD simulation also reveals that the hysteresis behavior is gradually dominated by the dislocation motion instead of the phase transition. The corresponding micro mechanism of this degeneration of super-elasticity could be explained by the fact that the dislocations brought by the pre-deformation hinder the reverse phase transition, which results in the accumulation of residual martensite phase and disordered structure. Furthermore, the models with different grain sizes and temperatures are also considered in the MD simulation. The simulation results proved that the dislocation motion is enhanced by the temperature increase, which brings a higher hysteresis energy under a large pre-deformation loading. For the grain size effect, the reduction of the grain size leads to a higher dislocation density, which results in a more obvious phase transition degeneration phenomenon for the model with a smaller grain size.

Phase-field theory based finite element analysis for ratchetting behavior of medium-manganese steel

A phase field model for martensitic transformation considering crystal plasticity was developed to investigate the microscopic mechanism of the transformation-induced plasticity in response to ratchetting of medium-manganese steel with a ferritic matrix containing retained austenite. This model introduced various inelastic deformation mechanisms, i.e., martensitic transformation, the austenite, ferrite and martensite plasticity caused by dislocation slip. Combining phase field model with finite element method, the ratchetting behavior of medium-manganese steel under asymmetric load was simulated by COMSOL software, and the martensitic transformation and its effect on ratchetting was discussed. The simulated results show that the ratchetting strains evolutions of the medium-manganese steel are well described by the developed phase field model. Ratchetting strain increases with increasing stress amplitude, mean stress and stress ratio. The martensite content increases with increasing stress amplitude and decreases with increasing mean stress and stress ratio.When the plastic strain exceeds a certain value, the plastic strain induced by martensitic transformation will increase the ratchetting strain of medium-manganese steel, which indicates that the critical importance of the martensitic transformation for the overall ratchetting behavior of medium-manganese steel. The plastic deformation of medium- manganese steel is mainly composed of dislocation slip of austenite and ferrite, where the former is the main part. Because of martensitic transformation, the non-uniformity of micro strain field in medium-manganese steel is aggravated, and the equivalent plastic strain increases.

Martensite plasticity and damage competition in dual-phase steel: A micromechanical experimental–numerical study

Martensite damage in Dual-Phase (DP) steel has been studied extensively, yet, the exact deformation mechanisms that trigger or inhibit damage initiation remain mostly unexplored. Whereas generally assumed to be hard and brittle, lath martensite in fact deforms in a highly anisotropic manner, showing large strains under favorable habit plane orientations, which is attributed both to the lath morphology and to so-called ’substructure boundary sliding’. Yet, the correlation (or interplay) between plasticity and damage in lath martensite has not received much attention. Therefore, we raise the question whether these soft martensite plasticity mechanisms can delay or even inhibit damage initiation. We analyze several ’damage-sensitive’ martensite notches, i.e. thin contractions of two martensite islands, by combining several state-of-the-art experimental and analysis methods. Deformations are tracked in-situ at the nanoscale, aligned to detailed microstructure maps, and categorized, for each martensite variant, into habit plane or out-of-habit-plane slip. In these experiments, strong plasticity (>70%) is observed in martensite notches, enabled by slip along a favorably oriented habit plane, whereas damaged notches have unfavorably oriented habit planes, showing limited pre-damage strains (<10%), carried by out-of-habit-plane slip. Additionally, one-to-one experimentally based Crystal Plasticity (CP) simulations are performed in parallel, employing a recently introduced Enriched CP approach which models a soft plasticity mechanism on the variants’ habit plane. The Enriched CP simulations show considerably lower hydrostatic stresses in non-damaged and plastically deforming notches, thereby revealing that the soft habit plane mechanism is key for introducing the high plastic anisotropy that can lead to the inhibition of martensite damage in highly strained martensite notches. Finally, recommendations for improved damage inhibiting steels are proposed.

Superelasticity degradation of NiTi shape memory alloy in wide ranges of temperature and loading level: Experimental observation and micromechanical constitutive model

In this work, cyclic tension-unloading tests with different peak strains (10%-18%) in the temperature range from 313 K to 393 K are performed to investigate the effects of temperature and loading level on the cyclic deformation behavior of NiTi shape memory alloy (SMA) wires. Experimental results demonstrate that the superelasticity degradation occurs during the cyclic deformation process, and such phenomenon aggravates with the rise of ambient temperature and loading level, which can be attributed to the complex interactions among the martensite transformation (MT), austenite plasticity (AP), martensite plasticity (MP) and the transformation-induced plasticity (TRIP). Then, a micromechanical cyclic constitutive model is proposed based on the framework of irreversible thermodynamics and Eshelby's inclusion theory. The martensite (M) phase is treated as mobile inclusions with alterable eigenstrain embedded in the austenite (A) phase matrix. The volume fractions of the M-phase, A-phase, and A-M interface-phase are introduced. The non-uniform stress fields in the three phases are estimated by employing the Mori-Tanaka's homogenization scheme and interfacial operator. The driving forces of the four types of the inelastic deformation mechanisms, MT, AP, MP, and TRIP are derived based on the proposed new Helmholtz free energy, instantaneous growth hypothesis of M domains, and the energy dissipation inequality. The inheritances of the plastic deformation induced by the movement of A-M interfaces during the repeated MT and its reverse are incorporated. Finally, to validate the predictive capability of the proposed model, the predicted results for the superelasticity degradation of NiTi SMA with various peak strains at different ambient temperatures are compared with the experimental ones. Moreover, the dominant plastic deformation mechanisms and the effects of AP, MP, TRIP on the cyclic deformation of NiTi SMA are discussed.

Phase field study on the microscopic mechanism of the cyclic degradation of shape memory effect in nano-polycrystalline NiTi shape memory alloys

A three-dimensional non-isothermal and crystal plasticity based phase field model was proposed to simulate the cyclic deformation of nano-polycrystalline NiTi shape memory alloys (SMAs) reflecting their one-way shape memory effect (OWSME) and stress-assisted two-way effect (SATWSME), and then the microscopic mechanism of the cyclic degradation of shape memory effect (SME) was systematically revealed. The proposed model was numerically implemented in a two-dimensional plane strain nano-polycrystalline NiTi SMA system, where each grain was divided into a grain-interior (GI) phase and grain-boundary (GB) phase, and the GI phase was set to be embedded in the GB phase. For the GI phase, a crystal plasticity based phase field model was newly established by considering four inelastic deformation mechanisms, i.e., martensitic transformation (MT), martensitic reorientation (MR), austenitic plasticity caused by dislocation slipping and martensitic plasticity caused by dislocation slipping and deformation twinning, simultaneously; for the GB phase, a visco-plastic model was used to describe its plastic deformation. The simulated results show that: during the cyclic deformation of nano-polycrystalline NiTi SMAs involving the OWSME or SATWSME, the local internal stress caused by the MR or MT can lead to the accumulation of plastic deformation, which further increases the fraction of the martensite variants with favorable orientations by inducing a specific internal stress field after cooling and then results in the accumulation of residual martensite phase after heating; the accumulation of the irrecoverable strain mainly from the martensitic twinning and the decrease of recoverable strain cause the cyclic degradation of OWSME, which is further dependent on the loading rate and loading level; the SATWSME of the SMAs can exhibit a cyclic strengthening during the cyclic loading with a relatively low constant stress, while a cyclic degradation with a relatively high stress, which results from the competition between the transformation strain and the irrecoverable strain mainly caused by the austenitic plasticity that increases gradually during the cyclic deformation involving the SATWSME. These new findings are very helpful to clarify the physical nature of the cyclic degradation of SME and then alleviate the functional fatigue of NiTi SMAs by modulating the material design.

Phase field study on the microscopic mechanism of grain size dependent cyclic degradation of super-elasticity and shape memory effect in nano-polycrystalline NiTi alloys

A non-isothermal and crystal plasticity based phase field model was established by incorporating various inelastic deformation mechanisms, i.e., martensite transformation, martensite reorientation, the austenitic plasticity caused by dislocation slipping, and the martensitic plasticity caused by dislocation slipping and deformation twinning, in which the deformation twinning mechanism was newly considered. The grain size (GS) effect of martensite transformation and reorientation was characterized by introducing a grain boundary energy, and the GS dependent resistances of dislocation slipping and deformation twinning were considered in the phase field simulation for the first time to describe the GS dependence of plastic deformation. Through a series of phase field simulations on the inelastic cyclic deformation of two-dimensional nano-polycrystalline NiTi shape memory alloy (SMA) systems, the microscopic mechanism of the GS dependent cyclic degradation of the super-elasticity (SE), one-way shape memory effect (OWSME) and stress-assisted two-way shape memory effect (SATWSME) of such alloy systems was newly clarified. The simulated results show that with decreasing the GS, the cyclic degradations of the SE and SATWSME are both mitigated gradually, while that of OWSME is aggravated first and then mitigated; when reducing the GS to 10 nm, almost no functional degradation occurs. Further analysis shows that the GS dependence of the cyclic degradations of SE and SATWSME can be attributed to that: as reducing the GS, the plastic deformation resistances gradually increase due to the Hall-Patch effect, and the GS effect of martensite transformation and reorientation reduces the deformation mismatch introduced in the polycrystalline system and leads to the decrease of local internal stress level (i.e., the driving force of plastic deformation decreases) (Factor I) during the cyclic deformation, therefore, the generation and accumulation of plastic deformation are inhibited. However, for the cyclic degradation of OWSME, besides Factor I, since the reorientation-induced plasticity is mainly concentrated on the grain boundaries and their nearby grain interiors, the increase of grain boundary proportion results in the concentration and accumulation of plastic deformation at more locations during the cyclic deformation (Factor II). The competition between Factors I and II makes the cyclic degradation of OWSME show a non-monotonic variation with the decrease of the GS .

Plasticity and structure evolution of ferrite and martensite in DP 1180 during tension and cyclic bending under tension to large strains

This paper describes the main results obtained from a detailed experimental investigation into the dislocation and twinned structures that develop in dual phase (DP) steel sheets. In particular, the morphology and defect-structures present in DP 1180 steel having 55% ferrite and 45% martensite, were examined using transmission electron microscopy (TEM). Microstructure of the as-received DP 1180 consists of a twin-free martensite phase in the form of lath and island morphologies and a ferrite phase exhibiting a relatively low dislocation content (3.41 × 108 cm−2). The steel was deformed in simple tension (ST) to fracture and in continuous-bending-under-tension (CBT) to six cycles. As the CBT test facilitates stretching of the sheet considerably beyond the point of necking, strength increases and substantially exceeds that achieved in ST at fracture. As a consequence of plastic deformation, nano-twins are found in the island-shaped martensite, while the lath-shaped martensite remains twin-free. Additionally, some martensite regions develop a needle-like morphology after CBT. As dislocation density in the ferrite matrix increases with plastic strain, dislocation structures and accumulations of dislocations near the ferrite/martensite interface develop in the ferrite. Unlike in the as-received and ST-deformed DP 1180 samples, high density of dislocations (1.47 × 109 cm−2) and dislocation tangles are observed in ferrite regions of the CBT processed sample. We propose that the primary mechanisms enabling the achievement of high strength, while maintaining residual ductility upon CBT are intense plastic slip in the ferrite and profuse nano-twinning in the martensite regions.

Simultaneous improvement in strength and ductility in low carbon steel via intercritical annealing and quenching treatments

Abstract Hot-rolled low carbon steel sheets offer huge potential to be used in automotive body structures. Hot rolled steel sheets were subjected to intercritical annealing and quenching treatment to obtain high strength and ductility. Optical, scanning electron microscopy and X-ray diffraction were carried out to understand the underlying mechanisms. Microstructure and mechanical property correlation were established. High strength was achieved by increasing hard martensitic phase volume fraction in the soft ferrite matrix. Ductility was imparted by the ferrite matrix and martensite plasticity. Simultaneous improvements in strength and ductility were obtained by optimizing the ferrite-martensite microstructure during intercritical annealing and quenching.

Dislocation densities in a low-carbon steel during martensite transformation determined by in situ high energy X-Ray diffraction

The evolution of the dislocation densities in martensite and in austenite during the quench of a low-carbon (0.215 wt% C) steel is investigated in situ by the mean of a High Energy X-Ray Diffraction experiment on a synchrotron beamline. The line configuration offers an excellent time resolution well adapted to the studied martensitic transformation kinetics. The mean density of dislocations in martensite increases as the transformation proceeds confirming that dislocations are not homogeneously distributed between the laths in agreement with some recent post-mortem observations. The resulting spatial distribution of dislocations and the associated strain-hardening support the views assuming that lath martensite is a heterogeneous microstructure and behaves as a “multiphase” aggregate. In austenite, the increase in dislocation densities is even more significant meaning that austenite in martensite is also a hard phase, contradicting some recent theories attributing to films of retained austenite a major role in the plasticity of martensite.

The strengthening and toughening mechanism of dual martensite in quenching-partitioning steels

Based on the quenching-partitioning (Q&P) process, the mechanism underlying the improved plasticity of martensite under different quenching conditions during the deformation of ultra-high strength Q&P steels was explored. The microstructures of Q&P steels consisted of two kinds of martensite that were different in carbon contents and crystal orientations and retained austenite. During deformation, the coordination between the two kinds of martensite phases largely contributed to enhancing the strength and toughness of Q&P steels.

On the mechanical heterogeneity in dual phase steel grades: Activation of slip systems and deformation of martensite in DP800

We used micropillar compression experiments to study the plasticity of ferrite and martensite of two commercial dual phase steel grades (DP800). The activation of all three slip plane families, namely {110}, {112}, {123}, was observed in single crystalline ferrite pillars. They exhibit a comparable mean critical resolved shear stress (CRSS) of 147 ± 6, 143 ± 9, 146 ± 4 MPa for 3 µm pillars and are predominantly following Schmid´s law. A distinct size effect occurs when comparing the CRSS of 2 µm and 3 µm pillars. Martensite islands show uniform deformation and exhibit high compressive yield strength up to nearly 3 GPa. In most cases martensite pillars deform in an isotropic fashion without distinct slip traces. Despite the identical ultimate tensile stress of two steel grades their ferrite CRSS and martensite strength are largely different. It is found that the softer ferrite results in a lower macroscopic yield strength and a higher elongation to failure during macroscopic tensile testing. The results suggest that an increased local strain hardening capability suppresses global damage. The data provided here can serve as input parameter for crystal plasticity modeling.

Phase field simulation on the cyclic degeneration of one-way shape memory effect of NiTi shape memory alloy single crystal

In the framework of thermodynamics and based on the Ginzburg-Landau's theory and crystal plasticity, a three-dimensional phase field model was constructed to predict the cyclic degeneration of the one-way shape memory effect (OWSME) of NiTi shape memory alloy (SMA) single crystal. Two inelastic deformation mechanisms, i.e., austenite plasticity and martensite plasticity were newly introduced into the proposed model in order to describe the martensite transformation- and reorientation-induced plasticity and the stress-strain response of NiTi SMA single crystal more reasonably. From the phase field simulations, it is found that: during the thermo-mechanical cyclic deformation of NiTi SMA single crystal involving the OWSME, the Mode-I martensite reorientation (i.e., achieved by the migration of twinned interfaces) is gradually converted to the Mode-II one (i.e., achieved by the nucleation and growth of martensite variants with favorable orientations) due to the influence of plastic deformation; the reverse transformation is restrained by the plastic deformation, leading to the accumulation of residual martensite phase; moreover, the twinned martensite phase is gradually transited into the re-oriented one due to the interaction among the plastic deformation, residual martensite phase and temperature-induced martensite transformation; the irreversible plastic deformation and the accumulation of residual martensite phase lead to the decrease of recoverable strain, eventually resulting in the predicted cyclic degeneration of the OWSME of NiTi SMA single crystal.

Lath martensite plasticity enabled by apparent sliding of substructure boundaries

Lath martensite is widely present in advanced high strength steels as the key strengthening phase. Unexpectedly high ductility of lath martensite has been reported in the literature in both single-phase and multi-phase steels, however, without systematic identification of the underlying plasticity mechanisms. In this study, first, well-defined micro-tensile tests are carried out on fully martensitic steel with a clean large substructure and a variety of substructure boundary orientations with respect to the loading direction. Two deformation mechanisms of lath martensite were identified, namely, intra-lath crystallographic slip and apparent substructure boundary sliding, that compete with each other to carry the overall plasticity. The condition under which these two mechanisms are active has been clarified. It is found that boundary sliding is more easily activated than intra-lath crystallographic slip. In addition, for dual phase steel, as an example of multi-phase steels, the probability for sliding of lath martensite boundaries was estimated by boundary orientation characterization and micro-tensile tests. The results suggest that the apparent boundary sliding is also important for lath-martensite-containing multi-phase steels, which would explain prior reports in the literature of unexpectedly high local strains in the martensite regions.

Deformation behavior and microstructural evolution in ultra-high-strength dual-phase (UHS-DP1000) steel with different strain rates

The dynamic tensile behavior and deformation mechanism of ultra-high-strength dual-phase (UHS-DP1000) steel were investigated over a wide range of strain rates from 10−4 to 103 s−1. As the strain rate increases, the transition strain decreases from 2.73 to 1.92, and the martensite plastic deformation starts earlier. At strain rate of 10−4–0.5 s−1, the inhomogeneous plastic deformation ability increases because the dislocation density in the ferrite matrix increases. This leads to a decrease in uniform elongation and an increase in fracture elongation. When the strain rate increases from 0.5 to 500 s−1, the amount of mobile dislocation increases, which is the main reason for the enhancing uniform elongation and fracture elongation. Meanwhile, because the dislocation motion resistance rapidly increases, the yield strength and ultimate tensile strength also increase. When the strain rate is higher than 500 s−1, the hardening behavior caused by the dislocation motion resistance has not been offset by softening due to the mobile dislocation and adiabatic heating. The voids at the early stage of deformation could not uniformly form and grow, and thus the homogeneous plastic deformation ability decreases.