Martensite Particles Research Articles

Influence of solid solution time on microstructure and precipitation strengthening of novel maraging steels

Effective subsequent heat treatment is crucial for achieving the desired microstructure and excellent mechanical properties in laser-deposited high-performance maraging steel. In this paper, we systematically investigate the synergistic relationship and tuning mechanism of different solution treatment times on the microstructure-property synergy of new maraging steels fabricated using laser direct energy deposition (LDED). To determine the optimal heat treatment process, solution treatment was conducted at 840 °C for varying durations, followed by aging at 530 °C for 2 h to induce precipitation strengthening. The results indicate that after 2 h of solution treatment, the alloy exhibits optimal ductility with an elongation of 7.90 % ± 0.15 %, attributed to the refinement of the martensitic matrix and precipitated phases, along with the formation of a small amount of residual austenite. When the solution treatment time is extended to 4 h, the alloy achieves its highest tensile strength, reaching 1958 ± 24 MPa. However, the elongation decreases to 7.31 % ± 0.12 % due to the coarsening of the martensite and secondary phase particles. After 6 h of solution treatment, significant coarsening and aggregation of the martensite and Fe2Mo intermetallic compounds markedly reduce the hardness, strength, and toughness of the alloy. By adjusting the solution treatment time, the size, morphology, and distribution of the martensitic matrix, Fe2Mo, and nanoscale precipitated phases play a critical role in the strengthening and fracture processes. Therefore, optimizing the precipitation behavior of the martensitic matrix and Fe2Mo intermetallic compounds through rational solution heat treatment is key to enhancing the mechanical properties of laser-deposited new maraging steels.

Quantitative characterization of local deformation-fracture behavior in ferrite-martensite dual-phase steels with different martensite distributions

Low-carbon dual-phase (DP) steels, composed of a soft ferrite phase and a hard martensite phase, are known as promising advanced high-strength steels (AHSSs) due to their high strength and good ductility at low fabrication cost. However, the deformation behavior of DP steels is still not fully understood because of their complex mixed-phase distribution and mechanical interactions between two phases during deformation. The present study quantitatively investigated the effect of martensite distribution on mechanical properties and local deformation-fracture behavior, using the digital image correlation (DIC) technique. Two types of DP structures were prepared: one with a chained martensite distribution (chained DP) and one with an isolated martensite distribution (isolated DP). The chained DP specimen exhibited a superior tensile property, achieving both high strength and large ductility compared to the isolated DP specimen. DIC strain analysis revealed that the chained DP structure showed relatively homogeneous deformation due to the greater contribution of martensite to plastic deformation. In contrast, the isolated DP specimen experienced significant strain localization in the soft ferrite grains. Despite high global strains, non-plastically deformed zones were observed in the central regions of large, isolated martensite particles. Notable differences in micro-void evolution were also observed between the two specimens. The chained DP specimen had a large number of randomly distributed micro-voids, ranging from 0.5 μm to 2 μm in size. In contrast, the isolated DP specimen contained fewer micro-voids, with some aligned at an angle of ∼65° to the tensile direction, potentially leading to early tensile fracture.

Three-Dimensional Micromechanical Modeling of Martensite Particle Size Effects on the Deformation Behavior of Dual-Phase Steels.

The objective of this study was to examine the influence of martensite particle size on the formation of stress and strain in microstructures of dual-phase steels. In order to achieve this objective, the 3D representative volume element (RVE) method was utilized. Particle size distributions were obtained from the microstructures of DP600 and DP1000 dual-phase steels as they actually exist. Virtual dual-phase steel microstructures were generated according to the above distribution and subsequent validation analyses were performed. In the subsequent phase, microstructures of varying martensite particle sizes (1 µm, 1.98 µm, 3 µm for DP600 and 1.15 µm, 2 µm, 3 µm for DP1000) were formed, and the effects of particle size on deformation behavior under tensile loads were determined. The findings indicated that an increase in martensite particle size resulted in a reduction in tensile strength, accompanied by an increase in deformation amount.

Fatigue and Impact Behavior of Friction Stir Processed Dual-Phase (DP600) Steel Sheets

This study investigates the impact of friction stir processing (FSP) on the deformation behavior of 1.1 mm-thick DP600 steel sheets under both static and dynamic loading scenarios, with a focus on the automotive applications of the material. During the process, the large plastic shear strains imposed by FSP resulted in a maximum temperature of 915 °C, leading to a morphological transformation of the martensite phase from well-dispersed fine particles into lath martensite and grain refinement of the ferrite phase. DP600 steel showed an almost two-fold increase in static strength parameters such as the hardness value, yield strength, and ultimate tensile strength. As-received and processed DP600 steel exhibited a plastic deformation behavior governed by strain hardening. However, uniform elongation and elongation to failure after FSP took lower values compared to those of the as-received counterpart. Following the improvement in the static strength of the steel, the fatigue strength of the steel increased from 360 MPa to 440 MPa after the FSP. The finite-life fatigue fracture surfaces of the as-received samples were characterized by the formation of fine bulges due to the variation in the crack propagation path in the vicinity of the martensite particles/clusters. After FSP, the transformation of the martensite particles into coarser lath martensite also transformed the fracture surface into a step-like morphology. The microstructural evolution after FSP caused a decrease in the absorbed impact energy and maximum striker reaction force from 239 J and 37.6 kN down to 183 J and 33.6 kN, respectively. However, the energy absorption capacity of the processed steel up to failure was higher than the absorbed energy value of the as-received steel at the same impact displacement. The simultaneous decrease in both impact energy and reaction force is attributed to the higher cracking tendency of the processed microstructure due to the lower volume fraction of the ferrite phase. The experimental results reported in this study mainly show that FSP is an easy-to-apply and functional solution to significantly improve the static and cyclic strength of DP600 steel. However, it is clear that the reduced total impact energy absorption capacity after FSP may be taken into account in design strategies.

Combining multiscale structure and TRIP effect to enhance room temperature tensile properties of 304 stainless steel by cryogenic cyclic plastic strengthening

In this paper, 304 austenitic stainless steel was strengthened by cryogenic cyclic plastic strengthening (CCPS) method to obtain a material that combined multiscale structure and transformation-induced plasticity (TRIP) effect. After strengthening, there were harder small α′-martensite particles limited to the dense slip bands skeleton in the austenite coarse grains, which led to dynamic strain partitioning during uniaxial tension tests at room temperature. In addition, martensitic transformation occurred steadily throughout the strain range of the tensile tests. At room temperature, the characteristics of cryogenic transformation were maintained and the saturation value of the transformation was increased. The strengthened material showed an excellent combination of high strength and ductility at room temperature.

Micromechanical Modeling of Strain‐Hardening Behavior in Dual‐Phase Steels

A mathematical model is developed to consider the impact of microstructural parameters, including the volume fraction and the average particle size of martensite, on the flow stress and strain‐hardening behavior of dual‐phase microstructure. In this regard, the micromechanical approach is applied for partitioning the stress and strain in ferrite and martensite. Martensite carbon content and geometrically necessary dislocations, generated from austenite‐to‐martensite transformation, and strain accommodation at the ferrite–martensite interface, are involved to modify the partitioned stress of martensite and ferrite, respectively. Having partitioned stress in each phase, the global stress is estimated as the function of steel chemical composition, ferrite grain size, martensite particle size, aspect ratio, and volume fraction. To evaluate the applicability of the proposed model, four dual‐phase steels containing 12, 25, 34, and 48% volume fractions of martensite are prepared from the intermediate quenching process, and then after the strain‐hardening stages are investigated. Comparing the experimental result and model output reveals that the presented model shows good predictive capabilities to identify strain‐hardening stages and estimate the inverse of the strain‐hardening exponent.

In-situ generation of high-strength AISI 1045 steel with SiO2 nano-precipitation by selective laser melting (SLM)

High-strength AISI 1045 steel with SiO2 nano-precipitated particles is prepared via selective laser melting (SLM). The microstructure and mechanical properties of the as-built samples were studied and characterized by Scanning electron microscope (SEM), X-Ray Diffraction (XRD), Electron backscatter diffraction (EBSD) and Transmission electron microscope (TEM). XRD analysis shows that the powder mainly contains α-Fe phase, while γ-Fe phase appears in the SLMed specimen. The peak intensity increases slightly with the increasing of the processing energy density. In addition, ultra-fine martensite and SiO2 nano-precipitated particles were found in the as-built samples. Moreover, excellent mechanical performances were achieved by SLM-AISI 1045 (Vickers hardness: 502 ± 25 HV0.1, yield strength: and 1250 MPa), which are 1.63 times and 3.29 times higher than those of traditional commercial AISI 1045 respectively. Finally, the strengthening mechanism of the SLM-AISI 1045 was analyzed and discussed.

Heterogenous structure and formation mechanism of white and brown etching layers 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 layers (WEL and BEL, respectively) on the rail raceway during service. Bainitic rail steels with an excellent strength–toughness balance are promising candidates for next generation rail steels. It is generally difficult to generate hard and brittle WEL/BEL in bainitic rails because of the low carbon content. This study reports, for the first time, the formation of a unique multilayer heterostructured WEL/BEL in a field-tested bainitic rail. The heterogenous structures of WEL/BEL were characterized using transmission electron microcopy, focused ion beam, energy-dispersive X-ray spectroscopy, electron energy loss spectroscopy, electron probe microanalysis, atom probe tomography, and nano-indentation techniques. Results demonstrate that WEL is mainly composed of fine-grained martensite or ferrite and retained austenite, whereas BEL is a composite structure containing nanocrystalline martensite or ferrite, retained austenite, cementite, and oxide or O-rich particles. The oxide or O-rich particles are primarily formed within BEL through local oxidation, resulting in secondary partitioning of the alloying elements between the oxidation product and its surrounding matrix. BEL is hard but brittle due to the formation of brittle twinned martensite, cementite, and oxide particles, whereas the adjacent WEL is relatively soft and ductile. Based on these findings, the heterostructured WEL/BEL formation mechanism is preliminarily discussed in terms of inhomogeneous grain refinement, localized oxidation, chemical alterations (including both chemical segregation in the initial microstructure and secondary partitioning of the alloying elements during local oxidation), and phase transformation.

Surface layer characterization of 2205 duplex stainless steel subjected to abrasive water jet descaling

Abrasive water jet (AWJ) is an environmentally friendly and acid-free descaling technology, that has the added benefit of strengthening the material surface. However, the effects of AWJ descaling on the surface layer of heterostructured (HS) materials and the application of deformable abrasives involving high cycle times for AWJ descaling have not been extensively reported. In this study, martensitic stainless steel particles with high cycle times were subjected to AWJ descaling to investigate the effect of AWJ on the surface layer characterization of 2205 duplex stainless steel (DSS). The properties of the 2205 DSS surface layer were characterized by the surface morphology, surface roughness, microstructure, and microhardness. When exposed to an AWJ intensity of 0.245 mmA, the surface of 2205 DSS exhibited the formation of extrusion craters and flat debris. Additionally, the surface roughness Ra was measured at 3.778 µm. The gradient hardening layer was formed on the surface of the AWJ specimen, and the depth of the hardening layer was 550 µm. The maximum microhardness was 395 HV, which is approximately 30% higher than that of the original specimen. Within the AWJ specimens, dislocation networks were observed in the ferrite, while the austenite exhibited dislocation cells, deformation twins, and strain-induced martensite. Furthermore, the experimental results indicate that plastic deformation was unevenly distributed between the two phases, with the majority of deformation occurring within the austenite phase. Owing to the requirement of deformation compatibility, several of geometrically necessary dislocations (GNDs) accumulated near the austenitic phase boundary to regulate the plastic strain gradient, and the hetero-deformation induced (HDI) stress generated by these dislocations strengthens the austenite. The AWJ refines the surface material structure; increases the density of dislocations, phase boundaries, and stacking faults; forms strain-induced martensite; and introduces a residual compressive stress field, which improves the material surface hardness.

Simultaneous enhancement of strength and ductility in a medium carbon low-alloy steel induced by secondary martensite and Cu-rich particles

This study aims to simultaneously improve the strength and ductility of a medium carbon low-alloy steel via Cu addition and the partitioning treatment. The obtained microstructure was heterogeneous that included temper martensite, retained austenite, M-A island/secondary martensite, bainitic ferrite, and nanosized Cu-rich particles. The partitioning temperature had significantly influence on the microstructure, leading to the distinct variations of mechanical properties. With the increase of partitioning temperature from 240 °C to 330 °C, yield strength decreased from 1470 MPa to 1110 MPa. However, it increased to 1210 MPa when the partitioning temperature further increased to 380 °C. In contrast, the ultimate tensile strength gradually decreased from 2300 MPa to 1440 MPa with the increase of partitioning temperature from 240 °C to 380 °C, while the ductility significantly increased from 9% to 26%. The concurrent increases of yield strength and ductility is attributed to the numerous nanosized Cu-rich particles formed in the temper martensite. In contrast, the increase of ultimate tensile strength was caused by the increasing secondary martensite which had high carbon concentration. By using the band contrast values obtained by EBSD mapping and Gaussian fitting, the distribution of temper martensite, secondary martensite, and bainitic ferrite were quantified. The results showed that the distribution of temper martensite and bainitic ferrite were correlated, which was apt to improve the coordinated deformation between the neighboring phases. In addition, the Cu-rich particles were found to preferentially nucleate at the edge of the strip carbide because the carbide edge acted as a channel for the rapid diffusion of Cu atoms. Surprisingly, the formation of numerous nanosized Cu-rich particles was always accompanied with the increasing ductility without the loss of strength, which provides a great potential for breaking through the trade-off between strength and ductility in steels.

Microscale Strain Localizations and Strain-Induced Martensitic Phase Transformation in Austenitic Steel 301LN at Different Strain Rates

Microscopic strain and strain-induced phase transformation during plastic deformation in metastable austenitic steel were investigated at different strain rates. Quasi in-situ tension tests were performed sequentially with well-defined elongation intervals at room temperature at strain rates of 10−3 s−1 and 10−1 s−1. The tests were monitored by high-resolution optical imaging with a microscopic lens at a resolution of 0.23 µm/pixel. The macroscopic temperature was also measured with an infrared (IR) camera. The microstructure-level strain localizations were observed on the surface of the etched specimens by means of microscale digital image correlation (µDIC). Additionally, the microstructure was characterized by electron backscatter diffraction (EBSD) at the same location before and after deformation. The results of the study indicated that microscopic strain localizations favored the formation of α′-martensite particles. At the lower strain rate, high local strain concentrations formed at several locations in the microstructure, correlating with the areas where the formation of large martensite islands occurred. Martensite particles of various sizes formed nearby each other at the lower strain rate, whereas at the higher strain rate, martensite islands remained small and isolated. Although the macroscopic increase in temperature at both the studied strain rates was very low, at the higher strain rate, local heating on the microscopic scale could take place at the newly nucleated martensite embryos. This inhibited the further growth of the martensite particles, and local strain distribution also remained more homogeneous than at the lower strain rate.

A micromechanical study on the correlation of the microstructure and failure mechanism of dual-phase steels under tension

In this study, the influence of the volume fraction of the martensite phase as well as the size of the martensite particles on the mechanism of particle fracture in dual-phase steel were examined. A combined continuum/dislocation based approach was used in order to model the average stress in the martensite particles. It was found that the model predictions are in accordance with the experimental results. For the same volume fraction of the martensite particles, the model predicts an increase of the internal stress and the average stress in the martensite particles with increasing the particles size. Since the fracture strength of the martensite depends on its volume fraction, the particle size has no effect on the mechanism of particle fracture. Increasing the volume fraction of the martensite particles results in the enhancement of the internal stress in the martensite particles. However, it has a slight influence on the average stress in the particles. Nevertheless, because of decreasing the fracture strength of martensite with increasing its volume fraction, this parameter has a main role in the occurrence of the particle fracture mechanism.

Deformation behavior and formability of friction stir processed DP600 steel

Abstract The effect of friction stir processing (FSP) on the formability of DP600 steel was experimentally investigated and the basic relationships between biaxial deformation behavior and FSP-induced evolutions in microstructural and mechanical properties were established. FSP formed a microstructure that consists of lath martensite with increased volume fraction compared to as-received (AR) microstructure that mainly consisted of well-distributed fine martensite particles in a ferrite matrix. Consequently, AR yield strength (301 MPa) and ultimate tensile strength (621 MPa) increased to about 811 and 1054 MPa, respectively. This strength enhancement achieved accompanied by adequate uniform elongation and elongation to failure values of 6.3 and 13.0%, respectively. Under biaxial loading conditions, good strain hardenability of the AR DP600 steel brought about a large membrane stretching regime leading to high punch force for biaxial flow. After FSP, both punch displacements within the membrane stretching regime decreased due to the increased volume fraction of lath martensite leading to higher cracking tendency. In result, cup depth and peak punch force of FSPed DP600 decreased from 8.7 mm and 33.2 kN to 7.1 mm and 28.1 kN, respectively. The obtained results simply indicate that FSP can be employed to enhance the strength of dual-phase steels with a reasonable level of formability.

Fatigue and tensile deformation behaviors of laser powder bed fused 304L austenitic stainless steel

Unique microstructure equilibrium state induced by the laser powder bed fusion (LPBF) process endows the deposited metallic parts with excellent mechanical properties. However, the microstructure evolution of LPBF metallic materials during fatigue deformation and its influence on fatigue properties remain still unclear. This paper systematically investigated the microstructural evolution and deformation behavior of LPBF 304L austenitic stainless steel (ASS) during tensile and fatigue testing. Experimental results indicated that LPBF 304L ASS exhibited superior tensile and fatigue properties compared with its annealed counterparts, which could be attributed to different deformation mechanisms. The superior tensile properties were commonly attributed to the regulation of cell walls on the dislocation flow. Many fine α′-martensite particles were distributed in the γ-austenite matrix grains of fatigue-fractured LPBF 304L ASS, indicating the occurrence of martensitic transformation during cyclic deformation. The cell walls with strong chemical misfits and high coherent internal stress might provide the non-parallel lattice shear forces for α′-martensite nucleation during cyclic deformation. The martensitic transformation could inhibit the cyclic softening and postpone the strain localization, thus contributing to the remarkable prolongation of the steady stage and the significant improvement of fatigue lifetime. Collectively, this study established the microstructural evolution role in regulating the tensile and fatigue deformation of LPBF 304L ASS.

Mechanical Properties of TC11 Titanium Alloy and Graphene Nanoplatelets/TC11 Composites Prepared by Selective Laser Melting.

Titanium matrix composites (TMCs) with excellent mechanical properties, reinforced by graphene, is deemed the lightweight and high strength structural materials. In this study, TC11 titanium alloy powder and graphene nanosheets (GNPs) were used as raw materials, and the composite powder with good uniformity and fluidity was obtained through non-interventional homogeneous mixing by a planetary mixer. The microstructure and mechanical properties of the GNPs-TC11 composites and TC11 alloy were compared. The results showed that the microstructure of TC11 and the composites was acicular martensite α’ phase under the process parameters of 280 W laser power, 1200 mm/s scanning speed, and 0.1 mm hatch spacing. The GNPs in addition, in the composites, reduced the acicular martensite particle size and expanded the proportion of low-angle grain boundaries. The tensile strength and percentage elongation after the fracture of the TC11 titanium alloy were 1265 MPa and 4.3%, respectively. Because of addition of the GNPs, the strength and percentage elongation after the fracture of the composite increased to 1384 MPa and 8.1%, respectively, at a GNPs mass content of 0.2%. The enhancement of mechanical properties can be attributed to grain refinement, dislocation strengthening, Orowan strengthening, and load transfer strengthening.

Corrosion and wear characteristics of Al-Zn based composites reinforced with martensitic stainless steel and silicon carbide particulates

The corrosion and wear characteristics of stir-casting processed Al-Zn based composites reinforced with (6, 8, and 10 wt%) martensitic stainless steel (SS), was studied. Potentiodynamic polarization based electrochemical method was used for the corrosion studies in 0.3 M H2SO4 solution; while dry sliding wear test conducted using a tribometer, was adopted for the wear assessment. The results indicate that the thermodynamic feasibility and kinetic susceptibility for corrosion in the acidic solution, reduced with increase in the wt% of martensitic stainless steel in the Al-Zn based composites. Also, the corrosion resistance of the Al-Zn based composite containing 10 wt% martensitic stainless steel was higher than that of the composite grade containing 10 wt% SiC. The improved corrosion resistance was linked to the chromium and nickel contents in the stainless steel, which aid the formation of stable passive films on the composite surface that mitigate corrosion attack substantially. Similarly, the wear resistance of the Al-Zn based composite improved with increase in wt.% of the martensitic stainless steel – with the Al-Zn composite reinforced with 10 wt% SS having the best wear resistance value of 0.09136 mm3/n/mm and the least worn track section value of 232373.8 µm3. The improved wear resistance was attributed to the presence of more martensitic stainless steel particles in the composites; however the wear mechanism was essentially by abrasion regardless of the composite composition.

Influence of microcracks on Poisson's ratio during plastic deformation of austenitic steel

We researched the influence of damage accumulation on the Poisson's ratio measured by echo-pulse acoustic method during plastic deformation of 12Kh18N10T steel. On the basis of the obtained experimental data we calculated the partial contributions to the change in the Poisson's ratio of damage accumulation and formation of the strain induced martensite phase. The characteristics of stable cracks forming near strain-induced martensite particles at small degrees of plastic strain have been analyzed by computer simulation. The theoretical dependence of the change in the Poisson's ratio due to crack formation during plastic deformation has been constructed. A good agreement between the experimental data and theoretical calculations has been obtained. Keywords: plastic deformation, austenitic steel, Poisson's ratio, martensite transformation, dislocation, mesodefect, configurational force method.

Fatigue of metastable austenitic steel: Micromechanics aspects

Microstructural changes accompanying fatigue of a metastable austenitic steel comprise two main factors: (a) microcracking and (b) nucleation and growth of the martensitic phase. Both factors produce changes in the effective elastic properties that can be monitored by high-precision acoustic techniques. In order to focus on monitoring microcracking, its effect on the elastic properties should be separated from the effect of the martensitic phase, since the latter has elastic constants that are different from the ones of the austenitic phase. To this end, the eddy current technique is used to monitor the volume fraction of martensite. The volume fraction parameter is generally insufficient for the prediction of the elastic properties since they depend not only on the volume fraction but, also, on the shapes of the martensitic particles – that are complex and not fully known. However, if the elastic contrast between the austenitic and martensitic phases is weak-to-moderate, the effect of shapes can be neglected, so that the volume fraction information is sufficient. This allows non-destructive monitoring of microcracking by a combination of the acoustic and the eddy current data.

Влияние микротрещин на коэффициент Пуассона при пластическом деформировании аустенитной стали

We researched the influence of damage accumulation on the Poisson's ratio measured by echo-pulse acoustic method during plastic deformation of 12Kh18N10T steel. On the basis of the obtained experimental data we calculated the partial contributions to the change in the Poisson's ratio of damage accumulation and separation of the strain induced martensite phase. The characteristics of stable cracks forming near strain martensite particles at small degrees of plastic strain have been analyzed by computer simulation. The theoretical dependence of the change in the Poisson's ratio due to crack formation during plastic deformation has been constructed. A good agreement between the experimental data and theoretical calculations has been obtained.

Microstructural evolution and mechanical properties of laser repaired 12Cr12Mo stainless steel

In this study, the laser repairing (LR) method is applied to 12Cr12Mo stainless steel, and the corresponding microstructural evolution, texture characteristic, and hardness variation are investigated. A 50/50 uniaxial tensile-test is conducted, and the mechanical failure mechanism is investigated using scanning electron microscope and transmission electron microscopy. The results show that the microstructure consists mainly of martensite and tiny M23C6 carbide particles in the deposited zone (DZ), while the substrate contains tempered martensite and coarse M23C6 carbide. The high laser-power generates a cubic texture in the DZ, while the substrate maintains a texture-free matrix. From the substrate to the DZ, the hardness increases sharply at the fusion line and then decreases gradually along the building direction. The tensile-test result indicates that the overall mechanical properties of LRed 12Cr12Mo stainless steel are comparable to wrought parts. Moreover, the microvoid-coalescence fracture indicates good plasticity of the LRed samples. Using this approach, we successfully repaired a 17th-stage stator cascade of a gas turbine, which indicates that LR can be used for the type of steel investigated in this study.