Martensite Particles Research Articles

The microstructure and properties of Mo alloying layer after surface alloying treatment induced by continuous scanning electron beam process

Surface alloying by scanning electron beams can improve the microstructure and mechanical properties of steel. In this study. Four electron energy densities were selected during the alloying process: E1 = 1.5J/cm2, E2 = 2.9J/cm2, E3 = 4.5J/cm2 and E4 = 5.9J/cm2. The obtained results show that the sample surface is composed of alloying zone and heat-affected zone. The microstructure of the alloy zone is concealed acicular martensite and molybdenum carbide particles. The microhardness of this area is 1250HV. The sample treated with an energy density of 5.9J/cm2 has the least amount of wear. After alloying treatment, the microhardness and wear amount of the scanned samples are significantly improved.

Effects of strain rate on strain-induced martensite nucleation and growth in 301LN metastable austenitic steel

The effects of strain rate on strain-induced α′-martensite nucleation and growth were analyzed in this work. Tension tests were performed at room temperature at strain rates of 2×10−4 s−1 and 0.5 s−1 using small polished specimens that fit inside a scanning electron microscope. The specimens were deformed incrementally, and microstructural evolution was tracked carefully at a specific location on the specimen surface. This approach allows the analysis not only of the spatial but also of the temporal evolution of the α′-martensite. Optical microscopy images and electron backscatter diffraction (EBSD) measurements were taken for each plastic deformation increment. The size and number of α′-martensite particles were evaluated from the EBSD images, whereas local microlevel strains were obtained using Digital Image Correlation (DIC). According to the results, the number of nucleation sites for α′-martensite does not seem to be affected much by the strain rate. However, there is a notable strain rate effect on how the transformation proceeds in the neighborhood of freshly formed α′-martensite particles. At a low strain rate, repeated nucleation and coalescence leads to the notable growth of α′-martensite particles, whereas at a high strain rate, once nucleated α′-martensite particles remain as small isolated islands that do not markedly grow with further plastic strain. This phenomenon can be attributed to local microstructure-level heating caused by plastic deformation and exothermic phase transformation. This reduces the local growth rate of the α′-martensite particles in the vicinity of the above-mentioned islands, thus leading to a lower bulk transformation rate at higher strain rates.

Large magnetostrains of Ni-Mn-Ga/silicone composite containing system of oriented 5M and 7M martensitic particles

In view of the exponential rate of advancement in robotics technology, a composite material, which is promising for high-speed and large-strain actuation and sensing applications, has been developed in this study. Single-crystalline Ni49.9Mn28.5Ga21.6 (SC Ni-Mn-Ga) ferromagnetic shape memory alloy particles were fabricated from the polycrystalline Ni-Mn-Ga alloy by a well-controlled mechanical crushing. X-ray diffractions revealed a mixture of 5M- and 7M-martensitic phases in the SC Ni-Mn-Ga particles. A composite consisting of the silicone rubber and 20 vol.% of the crystallographically and spatially oriented SC Ni-Mn-Ga particles was fabricated by curing under a magnetic field and the chain-oriented structure was confirmed by a micro-computed tomography. This composite, which exhibited a magnetic field induced rubber-like behavior with a magnetostrain as high as 4.0% despite reduced filling factor, was attributed to the co-existence of the 5M- and 7M-martensitic phases.

Nanoscale precipitation and ultrafine retained austenite induced high strength-ductility combination in a newly designed low carbon Cu-bearing medium-Mn steel

A novel low carbon Cu-bearing medium-Mn steel was designed and subjected to intercritical annealing (IA) and tempering (IAT). The yield strength and uniform elongation significantly increased from 659 MPa and 10% to 911 MPa and 20% by tempering. The microstructure of sample IAT was comprised of ferrite, ultrafine retained austenite, tempered martensite, and hierarchical Cu particles. Large size precipitates (9.7 ± 3.1 nm) were formed during intercritical annealing, while fine particles (1.85 ± 0.36 nm) were formed during tempering. Hierarchical Cu particles increased yield strength of ferrite by ~267 MPa, which compensated the strength loss induced by intercritical annealing and tempering. The carbon and alloying elements were further partitioned to austenite from martensite during tempering, which increased austenite stability. As a consequence, TRIP effect occurred over a wide strain regime, which contributed to a superior ductility. Before tempering, yielding of retained austenite was caused by martensite transformation at a low stress because of poor stability, which was avoided by the enhanced stability of retained austenite after tempering. As a result, the yield strength of austenite, and thereby the yield strength of the steel was increased.

Effects of Strain Rate on the Strain Induced Martensite Nucleation and Growth in 301LN Metastable Austenitic Steel

The effects of strain rate on the strain induced α’-martensite nucleation and growth were analysed in this work. Tension tests were performed at room temperature at strain rates of 2×10-4 s-1 and 0.5 s -1 using small polished specimens, which fit inside a scanning electron microscope. The specimens were deformed incrementally, and microstructural evolution was tracked carefully at the same location. This approach allows analysing not only the spatial but also the temporal evolution of the α’-martensite. Optical microscopy images and electron backscatter diffraction (EBSD) measurements were taken for each plastic deformation increment. The size and number of α’-martensite particles were evaluated from the EBSD images, whereas the local microlevel plastic strains were obtained with Digital Image Correlation (DIC). According to the results, the number of nucleation sites for α’-martensite does not seem to be affected much by the strain rate. However, there is a notable strain rate effect on how the transformation proceeds in the neighbourhood of freshly formed α’-martensite particles. At low strain rate, repeated nucleation and coalescence leads to notable growth of the α’-martensite particles, whereas at high strain rate, once nucleated α’-martensite particles remain as small isolated islands which do not markedly grow with further plastic strain. This phenomenon can be attributed to local microstructure level heating caused by plastic deformation and the exothermic phase transformation. This reduces the local growth rate of the α’-martensite particles in the vicinity of the above-mentioned islands, thus leading to a lower bulk transformation rate at higher strain rates.

Computational microstructural model of ordinary state-based Peridynamic theory for damage mechanisms, void nucleation, and propagation in DP600 steel

In this study, the evolution of microscopic damage mechanisms in DP600 was investigated by developing an ordinary state-based two-dimensional Peridynamic model. The real microstructure was extracted from scanning electron microscope (SEM) images and employed to predict the deformation and damage behavior of dual-phase (DP) steels under tension static loading. Isotropic hardening plasticity and nonlinear failure criterion were considered for both ferrite and martensite phases. The Peridynamic (PD) modeling procedure is improved to reduce simulation time and increase applied displacement steps. For improving the prediction of damage initiation, a new method was proposed for the properties of the martensite/ferrite interface. At four applied strains (no-damage, damage initiation, partly damage propagation, and coalescence of damages), deformation and damage patterns were investigated. The cumulative plastic stretch of interactions and the number of damaged interactions were obtained for every applied strain. These parameters were evaluated by the energy rate of acoustic emission signals. Void formation due to fracture of martensite particles and decohesion of the interface were predicted and validated by available experimental results and previous studies. Damage initiation was predicted in thin martensite grains, interfaces of martensite/ferrite, and sandwiched ferrite regions between martensite particles. In thin martensite phases, the crack was initiated at boundaries, and then it crossed through martensite phases. PD model successfully predicted crack propagation and branching that matched with the experimental data.

Effects of titanium content on the impact wear properties of high-strength low-alloy steels

In this study, the mechanical properties, wear behavior, and microstructures of high-strength low-alloy steels with titanium contents of 0, 0.04, 0.1, and 2 wt% were systematically investigated. Steel microstructures were examined via scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The wear properties of the steel samples were evaluated by measuring their weight losses, while the wear mechanism was elucidated via scanning electron microscopy. The worn steel surfaces exhibited very rough topographies with well-defined craters and grooves, which were produced by high-angle and low-angle impacts, respectively. The wear resistance of the steels strongly depended on their microhardness and impact toughness. Compared with the other steel specimens, the steel with 0.04 wt% titanium, which contained martensite and lower bainite particles, possessed superior mechanical properties, i.e., a hardness of 745 HV and an impact toughness of 97 J/cm3. Meanwhile, the steel with 2.0 wt% titanium exhibited the lowest abrasive resistance, impact toughness, hardness, tensile strength, and yield strength among all the specimens. These results indicate that excessive titanium content promotes the precipitation of large particles in the steel matrix, which degrades its mechanical properties.

Effects of aging time on the microstructure and mechanical properties of laser-cladded 18Ni300 maraging steel

In this work, 18Ni300 maraging steel was successfully fabricated, for the first time, by a laser cladding technique under atmospheric condition. The effects of different aging times (1, 3, 6, and 9 h) at 500 °C on the microstructure and mechanical properties of the as-cladded 18Ni300 maraging steel were carefully characterized and analyzed. The microhardness and tensile strength increase with increasing aging time up to 6 h, and subsequently, decrease with the time extending to 9 h. On the contrary, the elongation is shown a reverse trend. The original 18Ni300 maraging steel exhibits cellular microstructure with an average grain size of 2 µm, composed of martensite and nano-Ti2N particles. After aging treatment, Ni-rich nano-precipitates (Ni3(Mo,Ti) and Ni(Mo,Ti)) together with reverted austenite were formed and promoted with the extension of aging time. The optimal comprehensive performance of the 18Ni300 maraging steel can be obtained by aging 3 h at 500°C, with microhardness of 509 HV0.2, ultimate tensile strength of 1686 MPa, and elongation of 11.5%, respectively. The microstructural mechanisms accounting for the property changes are discussed in detail.

H-phase precipitation and its effects on martensitic transformation in NiTi-Hf high-temperature shape memory alloys

Precipitate microstructure in the B2 parent phase is known to have profound impacts on the properties of NiTi-based high temperature shape memory alloys (HTSMAs), including the martensitic transformation (MT) start temperature Ms, temperature- and stress-hysteresis, work output, dimensional stability and functional fatigue resistance. In order to understand the underlying mechanisms and hence to optimize aging heat treatments to achieve desired properties, we systematically investigate both the mechanical and chemical effects associated with nanoscale coherent precipitates on the behavior of MT. Using NiTi-Hf HTSMA as an example, we first study the equilibrium shape and stress and strain fields of an H-phase precipitate as a function of its size. We then determine quantitatively the elastic interaction energy between a precipitate and a nucleating martensitic particle consisting of either a single variant or multiple self-accommodating variants. In the meantime, we calculate the variation of concentration field around an H-phase precipitate during its growth. Finally, we quantify and compare the effects of the spatially inhomogeneous stress and concentration fields around an H-phase precipitate on Ms. The results indicate that the former is the dominant factor for long aging times while latter is the dominant factor for short aging times. Since the model predicts Ms as a function of aging temperature and time, it can aid the design of aging heat treatment schedule to achieve desired Ms.

A Novel Mechanism of the Formation of White and Brown Etching Layer in Bainitic Rail Steel

The failures of conventional pearlitic rail steels are strongly affected by the formation of hard and brittle White and Brown Etching Layer (WEL and BEL) on the rail raceway during service. The bainitic rail steels with excellent strength-toughness balance are promising candidates for the next generation of rail steels. It is generally stated that the hard and brittle WEL/BEL is difficult to generate in the bainitic rails because of low carbon content. However, we find a distinct multilayer WEL/BEL composite structure on the worn surface/subsurface of the bainitic rail after in-field testing. The multi-scale characterizations from the microscopic to the atomic scale reveal that the WEL and BEL exhibit different microstructural, chemical, and mechanical features. The oxide particles preferentially form in the BEL rather than the WEL. The local oxidation generates the Mn, Si, and Cr depletion but C enrichment in the BEL matrix. Consequently, the BEL exhibits hard but brittle features due to the formation of brittle twinning martensite, cementite, and oxide particles. In contrast, the WEL is relatively soft and ductile. Under plastic deformation and temperature rise upon wheel-rail contact, the heterogeneous microstructure and chemical micro-segregation in parent material lead to the inhomogeneous microstructural refinement, oxidation, and localized embrittlement, which is responsible for the formation of multilayer WEL/BEL structure in bainitic rail steels.

H-phase Precipitation and its Effects on Martensitic Transformation in NiTi-Hf High-Temperature Shape Memory Alloys

Precipitate microstructure in the B2 parent phase is known to have profound impacts on the properties of NiTi-based high temperature shape memory alloys (HTSMAs), including the martensitic transformation (MT) start temperature Ms, temperature- and stress-hysteresis, work output, dimensional stability and functional fatigue resistance. In order to understand the underlying mechanisms and hence to optimize aging heat treatments to achieve desired properties, we systematically investigate both the mechanical and chemical effects associated with nanoscale coherent precipitates on the behavior of MT. Using NiTi-Hf HTSMA as an example, we first study the equilibrium shape and stress and strain fields of an H-phase precipitate as a function of its size. We then determine quantitatively the elastic interaction energy between a precipitate and a nucleating martensitic particle consisting of either a single variant or multiple self-accommodating variants. In the meantime, we calculate the variation of concentration field around an H-phase precipitate during its growth. Finally, we quantify and compare the effects of the spatially inhomogeneous stress and concentration fields around an H-phase precipitate on Ms. The results indicate that the former is the dominant factor for long aging times while latter is the dominant factor for short aging times. Since the model predicts Ms as a function of aging temperature and time, it can aid the design of aging heat treatment schedule to achieve desired Ms.

Enhanced corrosion resistance of cold-sprayed and shot-peened aluminum coatings on LA43M magnesium alloy

A facile and environment-friendly strategy is proposed to prepare dense, thin and antioxidant aluminum coatings on LA43M magnesium alloy via cold spraying and subsequent shot peening technologies. High-energy martensitic stainless steel particles are used to tamp the cold-sprayed coatings, which resulted in complete plastic deformation of Al particles and elimination of pores. The samples with shot-peened Al coating showed excellent corrosion resistance in electrochemical tests. Moreover, long-term salt spray and immersion test revealed that the shot-peened Al coating can effectively protect LA43M alloy from corrosion. Shot-peened Al coating offered promising and long term applications time in the harsh environment.

The Effect of Welding Parameters on Joining Dissimilar Low Carbon Steel and 3CR12 Ferritic Stainless Steel by GTAW with ER308L Filler Metal

The effect of welding parameters on GTAW (gas tungsten arc welding) between low carbon steel and 3CR12 ferritic stainless steel with ER308L filler metal was investigated with regard to microstructure and mechanical properties. The welded metal presented good configuration. The microstructure of weldment was found to be of widmanstatten austenite, acicular ferrite and martensite. Martensite and Cr-rich carbide precipitated particles formed under certain conditions in weldment produced results of both better hardness values and tensile strength. The results indicated that influence of these welding parameters have significant effects on microstructure and mechanical properties in welded metal.

Influence of phase and interface properties on the stress state dependent fracture initiation behavior in DP steels through computational modeling

In dual phase (DP) steels, fracture can initiate within the ferrite phase, the martensite phase, or at the interfaces between these phases with the dominant fracture initiation mechanism expected to depend on a number of factors, including the phase and interface properties as well as the applied stress state. The present study aims to identify the links among fracture initiation behavior, phase/interface properties, and stress state in DP steels through finite element analysis using representative volume element (RVE) simulations. An idealized RVE model containing a circular martensite particle was loaded under five different stress states. The RVE model incorporated a ductile fracture criterion for ferrite, a brittle fracture criterion for martensite, and a cohesive zone model (CZM) for the ferrite/martensite interface. A parametric study was performed to determine the relative influence of fracture properties of each constituent and stress state on the failure initiation behavior, and to identify the conditions under which the fracture initiation behavior was stress state dependent.

Evaluation of magnetic properties in ferromagnetic martensite particle using type 304 stainless steel wire

In this study, the magnetic properties of a single martensite particle were investigated using type 304 stainless wire, which can reduce the number of crystal grains per unit area. First, the magnetization curve of wire specimens with different martensite fractions was measured by a SQUID magnetic flux meter. Then, the coercivity and susceptibility parameters were evaluated from the magnetization curve and factors contributing to these parameters were discussed. It was found that the coercivity values along the long and short axes of wire specimens with a diameter of 0.4 mm increased and subsequently decreased with an increase in the martensite fraction. Further, the susceptibility values of the same specimen along the long axis increased and along the short axis decreased with increasing martensite fractions. The results indicate that the coercivity and susceptibility of a martensite particle are affected by the size of variant clusters and the shape anisotropy of the martensite particle.

Plastic Behavior of Ferrite–Pearlite, Ferrite–Bainite and Ferrite–Martensite Steels: Experiments and Micromechanical Modelling

In this work, low carbon low alloy steel specimens were subjected to suitable heat treatment schedules to develop ferrite–pearlite (FP), ferrite–bainite (FB) and ferrite–martensite (FM) microstructures with nearly equal volume fraction of hard second phase or phase mixture. The role of pearlite, bainite and martensite on mechanical properties and flow behaviour were investigated through experiments and finite element simulations considering representative volume elements (RVE) based on real microstructures. For micromechanical simulation, dislocation based model was implemented to formulate the flow behaviour of individual phases. The optimum RVE size was identified for accurate estimation of stress–strain characteristics of all three duplex microstructures. Both experimental and simulation results established that FM structure exhibited superior strength and FP structure demonstrated better elongation while FB structure yielded moderate strength and ductility. The von Mises stress and plastic strain distribution of the individual phase was predicted at different stages of deformation and subsequent statistical analyses indicated that hard phases experienced maximum stress whereas, maximum straining occurred in soft ferrite phase for all three structures. Micromechanical simulation further revealed that strain accumulation occurred at the F–P and F–B interfaces while the same was observed within the martensite particles apart from the F–M interfaces for FM. These observations were further substantiated through the identification of void and crack initiation sites via subsurface examinations of failed tensile specimens.

Crystal plasticity based predictions of mechanical properties from heterogeneous steel microstructures

An accurate understanding of the influence of the microstructure on mechanical properties as well as how the material behaves in forming tests is vital for product development and process improvement. The heterogeneity of the microstructure is very different for the various families of steel grades. As a consequence spatial stress and strain distributions on the micro scale will also differ a lot in these materials during forming operations. In the important family of Dual Phase (DP) steel grades. the combination of ductile ferrite phase and hard martensite particles enables an excellent hardening behaviour. However these micro constituents also result in a strong locally heterogeneous behaviour caused by the high material property contrast. To enable an accurate prediction of the materials response in forming processes advanced microstructure models as well as fast crystal plasticity code is essential. Within Tata Steel both are available. A Multi-Level Voronoi microstructure generator, capable of generating very complex Representative Volume Elements, is directly connected to the DAMASK code of the Max Planck Institüt für Eisenforschung. This crystal plasticity software is based on an ultrafast Fourier Spectral Solver. Crystal plasticity simulations with different loading cases (e.g. uni-axial, plane strain, simple shear) study various effects of microstructure heterogeneity on stress and strain distributions as well as on global hardening behaviour. Outcome is that not only the volume fraction but also the spatial distribution of the martensite particles has a large impact on local stresses and strains as well as on average hardening behaviour. Another important heterogeneity parameter is the carbon content, which influences strongly the hardness and hardenability of the martensite particles. An attempt is made to incorporate also this effect in crystal plasticity simulations.

Effect of Different Heat Treatment Methods on Microstructure and Properties of 40Cr Steel

In this paper, the properties of 40Cr steel were studied by heat treatment. Different kinds of temperature quenching combined with tempering at different temperatures were used to study the influence of different heat treatment methods on the microstructure and properties of the samples, and find the optimal solution. The 40Cr steel samples were subjected to 850 °C oil quenching, respectively, and then tempered at 240 °C and 440 °C respectively, and then compared with the oil quenched parts which were not tempered. It can be known that the hardness of the quenched workpiece at 40 °C at the same quenching temperature of 850 °C is higher than that of the medium temperature tempering at 440 °C. When 40Cr steel is tempered at low temperature, the obtained structure is tempered martensite, and at the medium temperature tempering, the tempered tortite is obtained, the tempered martensite particles and the layered tempering hullite are fine. The hardened steel is organized from tempered martensite to tempered tortoise as the tempering temperature increases. 850 °C quenching + 240 °C tempering is the best solution.

The role of microstructure on edge cracks in dual phase and quench and partitioning steels subject to severe cold rolling

Rolling edge cracks are characterized in a wide range of steels. Four different crack morphologies are identified. The propensity to cracking has an inverse relationship to the martensite content in dual phase steels, related to the interconnectivity of the martensite particles within the microstructure. An additional effect due to grain refinement is seen in dual phase steels which have the same martensite content but a factor of three difference in ferrite grain size. Quench and partitioning steels exhibit considerably lower resistance to edge cracking than dual phase steels due to the specific characteristics of the martensite in each steel.

Experimental and 3D Micromechanical Analysis of Stress–Strain Behavior and Damage Initiation in Dual-Phase Steels

In the recent decade, dual-phase steel has been used extensively in the industry due to its high strength and formability. In this paper, deformation pattern and mechanical behavior of DP steel are predicted using the 2D and 3D representative volume element (RVE) and finite elements method. Also, the obtained results were compared with the experimental SEM images. The experiments are performed in three stages to obtain the stress–strain curve and failure pattern in the specimens with the different state of stress distribution at the test sections. In the 3D state of stress, the state of applied stress to the RVE is obtained from macro-modeling of the test specimens and this model was analyzed using the finite element method. Furthermore, the effects of volume fraction and mesh size in 3D simulation on the stress–strain behavior are investigated. Metallography and SEM images are also used to access the failure mechanism at the microscale. It is shown that the martensite particles remain almost in their original form and their deformations are very small for the specimens with the thickness-to-width ratio of 0.8 (3D state of stress). This phenomenon is completely different from those obtained for smaller thickness-to-width ratio that the martensite particles were pulled and its length was increased. On the other hand, SEM images show that voids are located in the phase boundaries. According to the experimental and numerical results, although the main factor in the formation and growth of 2D voids is shear stress, in the 3D state of stress, the voids grow in spherical form due to the existence of large hydrostatic stresses.