Martensite Particles Research Articles

Martensitic Transformation Effect on the Dislocation Emission from a Semi-infinite Crack Tip in Nanocomposites

A theoretical model is established to investigate the effect of martensitic transformation particle on the dislocation emission from a crack tip in ceramic-matrix nanocomposites. Using the model of dislocation-based strain nucleus and the Green’s function method, the expressions of complex potentials and stress fields are derived in closed form. The critical stress intensity factors for the first-lattice dislocation emission and the maximum number of emitted dislocations are well calculated. The effects of important parameters such as the size of transformation particle, the dislocation emission angle and the distance from the crack tip to the transformation particle on dislocation emission are discussed in detail. The results reveal that the transformation particle shows a significant shielding effect on the dislocation emission from the crack tip, and the shielding effect enhances with an increase in the size of transformation particle. On the other hand, the results also imply that the emission of edge dislocations is closely related with the dislocation emission angle, and there exists a probable angle $${\vert }{\theta }{\vert }\approx 74^{\circ }$$ making the dislocation emission easiest. Besides, the remarkable crack blunting induced by the dislocation emission is quite difficult for small grain size but easy for the growth of crack.

Spheroidization heat treatment and intercritical annealing of low carbon steel

Spheroidization annealing of low carbon steel and its effects on the microstructure and mechanical properties of dual phase (DP) steel were studied. It was revealed that the reduction in strength and hardness of the quenched martensitic microstructure was much more pronounced compared to the fully annealed ferritic-pearlitic banded microstructure with spheroidizing time. This was related to the confinement of spheroidized carbide particles to distinct bands in the latter, and the uniform dispersion of carbides and high-temperature tempering of martensite in the former. During intercritical annealing of the spheroidized microstructures, the tendency to obtain martensite particles as discrete islands was observed. This, in turn, resulted in an inferior strength-ductility balance compared to the DP steel obtained from the intercritical annealing of martensite, which negated the usefulness of the spheroidized microstructures as the initial microstructures for the processing of DP steels.

The Effect of Different Arc Currents on the Microstructure and Tribological Behaviors of Cu Particle Composite Coating Synthesized on GCr15 Steel by PTA Surface Alloying

In order to improve the tribological performance of the slanted guide pillar, Cu particle reinforced composite coatings were synthesized on the surface of GCr15 steel using the plasma transferred arc (PTA) alloying technique. A systematic experimental investigation was conducted to study the effects of PTA current on the microstructure and microhardness of alloyed coatings. In addition, tribological behaviors at room temperature (RT) and high temperature (HT) were investigated. The results indicate that at low PTA current (70A), due to the insufficient current, no Cu particles are dissolved in the alloyed coating and a Cu-rich layer is observed on the surface. With the increase in the PTA current, Cu particles are gradually dissolved into the alloyed layer and the microstructure of alloyed coating mainly consists of bamboo-like martensite, retained austenite, and dispersed Cu particles. The microhardness of the PTA samples is approximately four times that of the untreated sample. The tribological results exhibit that an abrasive wear at RT and slight abrasive wear with oxidation wear at HT are the dominant wear mechanisms of alloyed coatings. The PTA samples show far superior antifriction properties compared to the untreated and remolten samples at both RT and HT, which can be attributed to the formation of lubricating Cu films and the improvement in microhardness.

Effect of Niobium on the Dry Sliding Friction and Wear Properties of 100Cr6 Bearing Steel

In this work, the dry sliding friction and wear properties were studied by wear test for 100Cr6 bearing steel when Nb content was 0.018% and 0.040%. In addition, in order to explain the differences, the experimental samples were processed for spheroidizing annealed and the quenched-tempered microstructure and hardness was analyzed. The result indicated that their friction coefficient was decreased to 0.047 when Nb content was 0.018% and the worn surface is microcutting and spalling without plowing. When Nb content was 0.040%, the friction coefficient was decreased to 0.006 and maximum wear depth was the deepest owing to obvious cutting. In order to increase the properties of the friction and wearing, Nb content should be decreased. With Nb content increased, the properties of the friction and wearing are decreased. The incorporation of Nb into bearing steel promotes the formation of martensite and carbide particles, which results in the diversity of the wear behaviors, eventually.

Damage Mechanisms and Mechanical Properties of High-Strength Multiphase Steels.

The usage of high-strength steels for structural components and reinforcement parts is inevitable for modern car-body manufacture in reaching lightweight design as well as increasing passive safety. Depending on their microstructure these steels show differing damage mechanisms and various mechanical properties which cannot be classified comprehensively via classical uniaxial tensile testing. In this research, damage initiation, evolution and final material failure are characterized for commercially produced complex-phase (CP) and dual-phase (DP) steels in a strength range between 600 and 1000 MPa. Based on these investigations CP steels with their homogeneous microstructure are characterized as damage tolerant and hence less edge-crack sensitive than DP steels. As final fracture occurs after a combination of ductile damage evolution and local shear band localization in ferrite grains at a characteristic thickness strain, this strain measure is introduced as a new parameter for local formability. In terms of global formability DP steels display advantages because of their microstructural composition of soft ferrite matrix including hard martensite particles. Combining true uniform elongation as a measure for global formability with the true thickness strain at fracture for local formability the mechanical material response can be assessed on basis of uniaxial tensile testing incorporating all microstructural characteristics on a macroscopic scale. Based on these findings a new classification scheme for the recently developed high-strength multiphase steels with significantly better formability resulting of complex underlying microstructures is introduced. The scheme overcomes the steel designations using microstructural concepts, which provide no information about design and production properties.

An empirical non-proportional cyclic plasticity approach under multiaxial low-cycle fatigue loading

The non-proportional hardening is revealed to be a main reason to influence the fatigue life of materials which are sustained to multiaxial low-cycle fatigue loading. Microstructurally speaking, increase of stacking faults to the grain size, formation of micro-twins in deformation bands and/or nucleation of martensite particles under non-proportional loading can introduce the strain hardening effect. Nevertheless to quantify the correlation between the microstructural phenomena and the macrostructural characteristic of material is not yet a trivial task. After quantitative analysis of the correlation between microstructural development and periodic deformation behaviour, an empirical non-linear hardening approach based on Benallal's NP factor estimation, which reflects the relationship between microstructural condition and internal stress responses is presented in this paper. Our approach is verified to be effective by experimental observations under different loading paths.

Micromechanics of multiaxial plasticity of DP600: Experiments and microstructural deformation modeling

Multiaxial plasticity behavior of Dual Phase (DP) 600 steel was evaluated through macroscopic testing and predicted through both continuum and micromechanics simulations. The stress state dependent mechanical properties of the material were examined through multiaxial loading under five stress states. The proposed continuum plasticity model was found to accurately predict the multiaxial mechanical response of the material through macroscopic simulations. The microstructure of DP600 contains a relatively soft ferrite matrix and relatively hard martensite particles, suggesting a more complex deformation behavior at the microscale compared to single phase materials. To understand the deformation mechanisms, a representative volume element (RVE) model based on the observed microstructures was built, which was able to predict the macroscopic multiaxial plasticity behavior of the material from the microstructural level. The local deformation inhomogeneity and stress states were also examined through this RVE model. The simulation results showed that in the DP steel studied, closely situated martensite particles increased the local plastic strain and stress triaxiality of the ferrite between them, with no trend for Lode angle parameter.

Strengthening and toughening austenitic steel by introducing gradient martensite via cyclic forward/reverse torsion

Converting austenite to martensite is a very effective and low-cost strategy for steel strengthening, but it results in a significant loss of ductility. In this study, we propose a novel method which simultaneously strengthens and toughens austenitic steels by introducing a gradient of martensite phase. We find that a gradient of deformation-induced martensite (α′-M) particles, with a volume fraction increasing from core to surface can be obtained in cylindrical AISI 304 stainless steel (304 SS) rods by applying free-end-torsion (FET). We compared the microstructures and tensile properties of gradient-structured 304 SS prepared by unidirectional-torsion (UT) and cyclic forward/reverse torsion (CFRT). It appears that piled-up dislocations formed near the core region during FET processing play a key role in the subsequent tensile deformation, and control the strain-hardening ability of the FET treated samples. The gradient α′-M enhances the strength of the surface layer and improves the tensile properties of the FET treated samples as a whole. Compared to UT, CFRT is more effective in inducing martensitic transformation, and enhances the gradient distribution of the α′-M. These findings provide a pathway for developing high strength and good ductility steels and other alloyed metals via gradient distributed second phase particles.

Effects of martensite particle size and spatial distribution on work hardening behaviour of fine-grained dual-phase steel

ABSTRACTModel dual-phase steel microstructural variants having fine ferrite grain size and a range of martensite particle sizes and spatial distributions were produced by varying the starting microstructure prior to the intercritical annealing treatment. Superior tensile properties were obtained for the microstructural variant having the smallest (∼1 µm) uniform ferrite grain structure and a corresponding uniform distribution of small (∼0.5 µm) martensite particles. This microstructural variant also exhibited superior work hardening properties, as determined from a Crussard–Jaoul analysis and plots of instantaneous work hardening exponent vs. strain. The true work hardening rate had a positive dependence on at low strain ( < 2%) for all three microstructural variants, consistent with the geometrically necessary dislocation mechanism. At higher strains, Stage III work hardening is operative and the dislocation annihilation factor exhibited a positive dependence on .

Effect of unit size on thermal fatigue behavior of hot work steel repaired by a biomimetic laser remelting process

AISI H13 hot work steel with fatigue cracks was repaired by a biomimetic laser remelting (BLR) process in the form of lattice units with different sizes. Detailed microstructural studies and microhardness tests were carried out on the units. Studies revealed a mixed microstructure containing martensite, retained austenite and carbide particles with ultrafine grain size in units. BLR samples with defect-free units exhibited superior thermal fatigue resistance due to microstructure strengthening, and mechanisms of crack tip blunting and blocking. In addition, effects of unit size on thermal fatigue resistance of BLR samples were discussed.

Microstructure and wear property of Fe–Cr13–C hardfacing alloy reinforced by WC particles

Tungsten as the most effective carbide-forming element was added in the Fe–Cr13–C hardfacing alloy to precipitate WC particles. Optical microscope (OM), scanning electron microscope (SEM) and energy-dispersive spectrometer (EDS) were used to investigate the microstructures of the hardfacing alloy. The wear resistance was tested through a slurry rubber wheel abrasion test machine, and the wear behavior was also studied. The results indicate that the microstructures of the hardfacing alloy consist of lath martensite, residual austenite and WC particles. The wear resistance can be significantly improved through the addition of tungsten element being provided by the precipitation of WC particles. And the predominant wear mechanism was microcutting with shallow grooves and spalling.

Microstructural characteristics and densification behavior of high-Nb TiAl powder produced by plasma rotating electrode process

In the present research, the microstructure and phase composition of high-Nb TiAl powder, produced by plasma rotating electrode process (PREP) have been investigated in details. PREPed powder with a mean particle size larger than 150μm demonstrates a dendrite structure, and that smaller than 75μm indicates a martensitic structure. PREPed powder is mainly composed of a hexagonal a2 phase, while in fine powder and coarse powders minor β phase and γ phase are detected, respectively, which is related to solidification path and cooling rate of molten droplets in PREP process. Phase composition and microstructure of the high-Nb TiAl powder have intrinsic effects on the resultant microstructure of hot isostatic pressed (HIPed) billets. After HIP processing, most martensitic and dendritic particles transformed into near γ structure, while few coarse dendritic powder was prone to introduce defects, such as residual primary particle boundaries (PPBs) or coarse lamellae structure, which were harmful to the mechanical properties of HIPed billets. The residual primary particle boundaries (PPBs) or coarse lamellae structure obviously decreased the ultimate tensile strength at room and elevated temperature.

Micromechanical Finite Element Analysis of the Effects of Martensite Particle Size and Ferrite Grain Boundaries on the Overall Mechanical Behavior of Dual Phase Steel

This paper focuses on micromechanical finite element (FE) modeling of the effects of size and morphology (particularly elongation or aspect ratio (AR) along the loading direction) of martensite particles and the ferrite grains on the overall mechanical behavior of dual-phase (DP) steels. To capture the size-effect of the martensite particles and ferrite grains, the core and mantle approach is adapted in which a thin interphase of geometrically necessary dislocations (GNDs) is embedded at the martensite–ferrite boundaries. It is shown that as the martensite particles size decreases or their aspect ratio increases, both the strength and ductility of DP steel increase simultaneously. On the other hand, as the ferrite grain size decreases or its aspect ratio increases, the overall strength increases on the expense of the ductility. The conclusions from this study can be used in guiding the microstructural design of DP steels.

Effect of microstructural factors on void formation by ferrite/martensite interface decohesion in DP980 steel under uniaxial tension

The effect of microstructural factors on ferrite/martensite (F/M) interface decohesion in DP980 steel under uniaxial tension was investigated using ex-situ tensile testing and the crystal plasticity finite element method (CPFEM). Ex-situ tensile testing of a miniaturized specimen with a 100 μm-thick grooved gauge region revealed that the main ductile failure mechanism in DP980 steel was F/M interface decohesion. First, the effect that the crystallographic orientation of ferrite exerted on the ductile fracture by F/M interface decohesion was investigated using the representative volume elements (RVEs) of single-crystalline ferrite matrix containing a hard martensite particle. Then, the effects of martensite morphology and grain boundary alignment on the heterogeneity of strain-stress partitioning, in-grain orientation gradient for the ferrite matrix, and the corresponding void formation by F/M interface decohesion were investigated using three types of RVEs with a bi-crystalline ferrite matrix that contained one of the followings: (i) a circular martensite particle, (ii) an elliptical martensite particle, and (iii) two adjacent elliptical martensite particles at the F/F grain boundary. To capture the void formation by F/M interface decohesion, a cohesive zone model based on traction-separation law was introduced along the interfaces between the ferrite and martensite phases. The simulation revealed that the crystallographic orientation for ferrite, the martensite morphology, and the grain boundary alignment all significantly affected the heterogeneity of strain-stress partitioning, the in-grain orientation gradient for the ferrite matrix, and the void formation by F/M interface decohesion in DP980 steel under uniaxial tension.

Refinement of the magnetic composite model of type 304 stainless steel by considering misoriented ferromagnetic martensite particles

We improved a magnetic composite model that combines the Jiles−Atherton model and Eshelby’s equivalent inclusion method to consider misoriented martensite particles. The magnetic permeability of type 304 stainless steel were analyzed by using both experimental data on the orientation distribution of type 304 stainless steel specimens and the improved model. We found that the model is able to qualitatively explain the variation of permeability with the orientation angle and orientation distribution, an effect that depends on the direction of the excitation magnetic field.

Damage micromechanisms in dual-phase steel investigated with combined phase- and absorption-contrast tomography

The single-distance phase retrieval technique was applied to contrast-enhanced imaging of the dual-phase microstructure of a ferrite/martensite dual-phase with only 1.4% difference in density between the two phases. Each high-resolution absorption-contrast image was registered with a corresponding phase-contrast image, to analyse damage evolution behaviour. The loading step at which each microvoid was nucleated was identified by tracking the microvoid throughout tension, together with its nucleation site. Premature damage initiation was observed at a relatively early stage at various nucleation sites, such as the ferrite interior, martensitic interior and ferrite/martensite interfaces; however, the subsequent growth of such microvoids was relatively moderate. On the other hand, microvoids were also initiated later due to martensitic cracking after the maximum load was reached, and these microvoids subsequently exhibited rapid growth. The martensite cracking induced additional damage evolution mainly along nearby ferrite/martensite interfaces and intersections between the martensite and the ferrite grain boundary. It is notable that the microvoids originating from martensitic cracking exhibited characteristic shear-dominated growth under macroscopic tension, whereas those originating from the other nucleation sites exhibited traditional triaxiality-dominated growth. It was concluded that the ductile fracture was dominated by the substantial force driving the growth of microvoids located on morphologically characteristic martensitic particles.

Damage Initiation in Dual-Phase Steels: Influence of Crystallographic and Morphological Parameters

Typical microstructures of dual-phase (DP) steels consist of hard martensite particles dispersed within a ductile ferritic matrix. These microstructures possess a complex network of grain and interphase boundaries, which, together with the mechanical contrast of their phase composition, control micro-damage initiation mechanisms, induced by deformation. Accordingly, in this study we analyze the influence of individual microstructural features and interfaces on damage nucleation and progression in DP steels with respect to applied tensile strain. Prominent micro-damage mechanisms include cracking of martensite and damage initiation at interphase boundaries. Influence of martensite morphology is discussed based on a statistical analysis of the damage features as observed by electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) maps. Prior austenite grain boundaries (PAGbs) in martensite show a brittle behavior and are highly susceptible to crack propagation.

Micromechanical finite element analysis of the effects of martensite morphology on the overall mechanical behavior of dual phase steel

Strong and ductile dual-phase (DP) steels are an important class of advanced high strength steels that are commonly used by the automotive industry. Micromechanical computational modeling is imperative for guiding the material design of DP steels with simultaneous enhancements in strength and ductility. In this study, the effect of the morphology of the martensite hard phase on the overall stress-strain response of DP steels is studied. Through generating realistic virtual representative volume elements (RVEs) and adapting a microstructural-based approach using a finite deformation elastic-viscoplastic constitutive model, it was possible to conduct various parametric studies of the effects of martensite phase on the strength and ductility of DP steel. Morphological parameters of the martensite phase that are studied include (1) effect of aspect ratio (equiaxed versus elongated), (2) effects of the combination of various aspect ratio within an RVE, and (3) effect of interconnectivity between martensite particles and its thickness. It is shown that elongated martensite particles lead to a better overall performance than that of equiaxed martensite particles. It is found that by having the martensite phase interconnected, both strength and ductility increase. Furthermore, it is shown that the increase in the volume fraction of equiaxed martensite particles within microstructures of elongated particles reduces the strength and ductility simultaneously. Overall, it is concluded that the morphology of the martensite phase in DP steels impacts more the ductility than strength.

The effect of Nb on texture evolutions of the ultrafine-grained dual-phase steels fabricated by cold rolling and intercritical annealing

Dual phase (DP) steels with different amounts of Nb were produced by utilizing cold rolling and intercritical annealing of a ferrite-martensite duplex starting structure. The effects of Nb contents (0.00, 0.06, 0.12 and 0.18 wt%) on microstructure and texture evolutions were studied. Addition of Nb promoted formation of ultra-fine grained structures with the ferrite grain size of about ∼1 μm and uniformly-dispersed martensite particles. The results showed increasing the fraction of the high angle grain boundaries and formation of {111} fiber components in the microstructures with increasing Nb content. In addition, the texture power was decreased with the addition of Nb, resulting in random texture components.

Very high cycle bending fatigue behaviors of FV520B steel under fretting wear

Abstract