Martensite Grains Research Articles

Segregation engineering enables nanoscale martensite to austenite phase transformation at grain boundaries: A pathway to ductile martensite

In an Fe–9at.% Mn maraging alloy annealed at 450°C reversed allotriomorphic austenite nanolayers appear on former Mn decorated lath martensite boundaries. The austenite films are 5–15nm thick and form soft layers among the hard martensite crystals. We document the nanoscale segregation and associated martensite to austenite transformation mechanism using transmission electron microscopy and atom probe tomography. The phenomena are discussed in terms of the adsorption isotherm (interface segregation) in conjunction with classical heterogeneous nucleation theory (phase transformation) and a phase field model that predicts the kinetics of phase transformation at segregation decorated grain boundaries. The analysis shows that strong interface segregation of austenite stabilizing elements (here Mn) and the release of elastic stresses from the host martensite can generally promote phase transformation at martensite grain boundaries. The phenomenon enables the design of ductile and tough martensite.

Origin of retarded martensitic transformation in Heusler Ni–Mn–Sn melt-spun ribbons

Experimental evidences have shown that the martensitic transformations in Heusler Ni–Mn–X (X = Ga, Sn, In, Sb) melt-spun ribbons occur at lower temperatures as compared to the bulk alloys. The objective of this present study is to find out its origin based on martensitic Ni50Mn41Sn9 ribbons at room temperature. In the cross sectional view of the ribbon, novel highly-ordered columnar microstructure is observed. It is proposed that large internal stress was produced due to the refined highly-oriented martensitic columnar grains, which produced refined martensite plates in different orientations. The refined microstructure and dense martensite plates in different orientations cause the retardation in martensitic transformation. Meanwhile, the martensitic crystal structure evolves from seven-layered monoclinic in the bulk as-cast alloy to four-layered orthorhombic in the melt-spun ribbons with the decrease of the transition temperature.

Microstructure Modelling of Dual-Phase Steel Using SEM Micrographs and Voronoi Polycrystal Models

The microstructure of dual-phase (DP) steels is composed of a matrix of ferrite reinforced by islands of martensite and the common interphase boundaries. To study the mechanical behavior of DP steels, steel with 45% ferrite and 55% martensite was fabricated and tested in the laboratory. Two types of finite element models were then created based on SEM images. The first model directly created the grains and boundaries from the SEM images, while the second model used a Voronoi type algorithm to construct geometries which are statistically similar to the SEM images. The models consider the measured morphology of ferrite, martensite, and their phase boundaries. The Gurson damage model was then used for the ferrite and boundary regions. The obtained results correctly predicted the failure mechanisms in a DP material. The results indicate that the deformation is localized due to microstructural inhomogeneities and the nucleation of voids in the boundaries between the ferrite and martensite grains. The good correlation between the numerical and experimental observations from SEM micrographs shows the efficiency of the proposed models in predicting the failure mechanism of DP steels.

Metallurgical analysis and fatigue resistance of WaveOne and ProTaper Nickel–Titanium instruments

The aim of the study was to evaluate cyclic fatigue resistance of two NiTi instruments and to analyse their surface, fractographic and matrix morphology under ESEM/EDS and optical microscopy. WaveOne Primary and ProTaper Universal F2 brand new instruments were subjected to fatigue testing in an artificial canal with 5.0 mm radius and 60° angle of curvature. Seventy-two instruments were divided into three groups (n = 24), according to the selected kinematics: WaveOne using reciprocation (A); ProTaper using reciprocation (B) or rotation (C). Time to fracture was recorded. Data were analysed with ANOVA and Tukey test. ESEM/EDS analysis was conducted on new files to examine surface characteristics and on fractured fragments to identify the fractographic features. Metallographic analysis was performed with optical microscope on new instruments to evaluate alloy properties. Significant differences were found with Group A, which was statistically more resistant to cyclic fatigue (P < 0.05) than the other groups. Surface analysis of new instruments showed both in WaveOne and ProTaper files the presence of deep milling marks. ESEM fractographic analysis of WaveOne showed multiple crack origins with an area of fatigue propagation wider than ProTaper instruments, in which a single crack origin could be detected. EDS analysis confirmed the equiatomic NiTi composition. Metallographic analysis under optical microscope revealed in WaveOne instruments the presence of nano-crystalline martensitic grains embedded in austenite matrix, presence which could not be found in ProTaper files. WaveOne NiTi files revealed higher resistance to fatigue stress, suggesting extended working time in clinical applications.

Martensite Formation and Recrystallization Behavior in 17Mn0.06C2Si3Al1Ni TRIP/TWIP Steel after Hot and Cold Rolling

This work evaluates the evolution of the microstructure and its influence on the mechanical behavior of steel containing 17% Mn, 0.06% C, 2% Si, 3% Al, and 1% Ni after hot rolling at 1070°C, cold rolling with 44% reduction, and annealing at 700°C for different time periods. The resultant athermal, strain-induced martensite and austenite grains were analyzed by optical and scanning electron microscopy (SEM). The volume fractions of the g, e, and α’ phases of martensite were confirmed by X-ray diffraction, dilatometry, and SEM-electron backscatter diffraction (EBSD) techniques. It was found that cold reduction results in the formation of more a’ martensite. The Vickers microhardness values were higher for the cold-rolled condition and lower for recrystallized samples, as expected. However, this reduction is counterbalanced by the formation of athermal e and a’ martensite during the cooling process. The sizes of the recrystallized grains change exponentially during their growth and remain within 1–3 mm. The yield and tensile strength of the hot-rolled steel reach values close to 250 and 800 MPa, respectively, with a total elongation of 40%, which demonstrates the high work-hardening rate of the steel.

Numerical investigation and experimental validation of a plasticity model for sheet steel forming

This paper investigates a recently developed elasto-plastic constitutive model. For this purpose, the model was implemented in a commercial finite element code and was used to simulate the cross-die deep drawing test. Deep drawing experiments and numerical simulations were conducted for five interstitial-free steels and seven dual-phase (DP) steels, each of them having a different thickness and strength. The main interest of the adopted model is a very efficient parameter identification procedure, due to the physical background of the model and the physical significance of some of its parameters and state variables. Indeed, the dislocation density, grain size and martensite volume fraction explicitly enter the model's formulation, although the overall approach is macroscopic. For the DP steels, only the chemical composition and the average grain sizes were measured for the martensite and ferrite grains, as well as the martensite volume fraction. The mild steels required three additional tensile tests along three directions, in order to describe the plastic anisotropy. Information concerning the transient mechanical behavior after strain-path changes (reverse and orthogonal) was not collected for each material, but for only one material of each family of steels (IF, DP), based on previous works available in the literature. This minimalistic experimental base was used to feed the numerical simulations for the twelve materials that were confronted to deep drawing experiments in terms of thickness distributions. The results suggested that the accuracy of the numerical simulations is very satisfactory in spite of the scarce experimental input data. Additional investigations indicated that the modeling of the transient behavior due to strain-path changes may have a significant impact on the simulation results, and that the adopted approach provides a simple and efficient alternative in this regard.

Fusion bonding of solidified TiC–TiB2 ceramic to Ti–6Al–4V alloy achieved by combustion synthesis in high-gravity field

By taking combustion synthesis in high-gravity field to prepare the solidified TiC–TiB2 ceramic, a novel and available strategy for achieving the joint of near-full-density fine-grained TiC–TiB2 ceramic to Ti–6Al–4V alloy in continuously-graded composition was discussed. The presence of microstructure transformation rather than a clear interface in the joint was considered a result of the achievement of the joint in multilevel and multiscale, which characterized by the size and the distribution of TiB phases. Fusion bonding and atomic interdiffusion between liquid TiC–TiB2 and liquid Ti in thermal vacuum circumstances were considered to initiate peritectic reaction of TiB2(s)+Ti(l)→TiB(s), direct growth of TiB solids from liquid Ti and eutectic reaction of TiB solids and liquid Ti at the final stage of joint solidification, successively, so that consumption of TiB2 solids and the decrease of TiB crystals in size and volume both occurred, and the joint of the ceramic to Ti alloy was achieved in multilevel and multiscale. The joint in continuously graded composition and microstructure represents not only a transitional change of Vickers hardness but also the high shear strength of 450±25MPa between the solidified ceramic and Ti alloy, and shear fracture usually happen at the solidified area of HAZ (heat-affected zone) due to the presence of the coarsened α′-Ti martensitic grains and shrinkage cavities.

Surface Modification of Pure Titanium by High Current Pulsed Electron Beam

Polycrystalline pure titanium was irradiated by high-current pulsed electron beam (HCPEB). The microstructure changes and material strength were investigated by using microhardness tester, optical microscope, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) technique. The experimental results indicate that many craters are inevitably formed on the irradiated surface. The eruption of the craters makes the material surface cleaned, which can improve the corrosion resistance of materials. Furthermore, martensitic structure, ultra-fine grains and high-density dislocations are formed on the irradiated surface, which increase the hardness of the treated samples. The microhardness of 20-pulsed sample reaches 286Hv, which is 71% higher than the initial sample. Martensitic transformation, grain refinement and dislocation strengthening induced by HCPEB treatment are the dominating mechanism for the improvements of material strength. It is suggested that HCPEB technique is becoming an effective approach to surface modification for pure titanium and titanium alloy.

Nanoscale austenite reversion through partitioning, segregation and kinetic freezing: Example of a ductile 2 GPa Fe–Cr–C steel

Austenite reversion during tempering of a Fe–13.6 Cr–0.44 C (wt.%) martensite results in an ultra-high-strength ferritic stainless steel with excellent ductility. The austenite reversion mechanism is coupled to the kinetic freezing of carbon during low-temperature partitioning at the interfaces between martensite and retained austenite and to carbon segregation at martensite–martensite grain boundaries. An advantage of austenite reversion is its scalability, i.e. changing tempering time and temperature tailors the desired strength–ductility profiles (e.g. tempering at 400°C for 1min produces a 2GPa ultimate tensile strength (UTS) and 14% elongation while 30min at 400°C results in a UTS of ∼1.75GPa with an elongation of 23%). The austenite reversion process, carbide precipitation and carbon segregation have been characterized by X-ray diffraction, electron back-scatter diffraction, transmission electron microscopy and atom probe tomography in order to develop the structure–property relationships that control the material’s strength and ductility.

Phase field simulation of the carbon redistribution during the quenching and partitioning process in a low-carbon steel

Phase-field modelling is used to simulate the quenching and partitioning process in a low-carbon transformation-induced plasticity (TRIP) steel, in order to understand the carbon redistribution in the microstructure during the heat treatment. The simulations show that, depending on local characteristics of the microstructure, including phase distributions and carbon-concentration gradients, different features in the carbon evolution during the partitioning step occur that are physically and practically relevant, but are not accessible for experimental observation. The overall carbon partitioning from martensite to austenite occurs not only by direct diffusion from martensite to austenite, but also through the bulk ferrite grains. The simulations also show interface migration driven by the free-energy difference between austenite and martensite, which affects the fractions of phases and the dimensions of the austenite grains. The carbon content of individual austenite, martensite and ferrite grains as well as average values are analysed, showing that the carbon concentration within the austenite grains is strongly inhomogeneous at short partitioning times, which contributes to a variable mechanical stability of individual austenite grains, affecting the occurrence of TRIP.

Microstructural Evolution of Surface Layer of TWIP Steel Deformed by Mechanical Attrition Treatment

A nanocrystalline layer was synthesized on the surface of TWIP steel samples by surface mechanical attrition treatment (SMAT) under varying durations. Microhardness variation was examined along the depth of the deformation layer. Microstructural characteristics of the surface at the TWIP steel SMATed for 90 min were observed and analyzed by optical microscope, X-ray diffraction, transmission and high-resolution electron microscope. The results show that the orientation of austenite grains weakens, and α−martensite transformation occurs during SMAT. During the process of SMAT, the deformation twins generate and divide the austenite grains firstly; then α−martensite transformation occurs beside and between the twin bundles; after that the martensite and austenite grains rotate to accommodate deformation, and the orientations of martensite and between martensite and residual austenite increase; lastly the randomly oriented and uniform-sized nanocrystalline layers are formed under continuous deformation.

Individual mechanical properties of ferrite and martensite in Fe–0.16mass% C–1.0mass% Si–1.5mass% Mn steel

Abstract The dual-phase steel generally contains a ferrite matrix and an appropriate amount of martensitic phase. Strength and deformability of the dual-phase steel are related to a difference in mechanical property between ferrite and martensite. Reliable experimental information of the above relationship is, however, rather limited. In order to examine individual mechanical properties of ferrite and martensite in the dual-phase steel, the nanoindentation technique was applied focusing on the microstructure. The chemical composition of the steel was 0.16 mass% C, 1.0 mass% Si, 1.5 mass% Mn, 0.01 mass% P, 0.003 mass% S and balance Fe. The sheet specimens were isothermally annealed at 1048 K for 600 s, water-quenched and tempered in a temperature range between 473 and 923 K for 300 s, to obtain a dual-phase steel with 65 vol.% of ferrite. Tensile tests of the heat-treated specimens were carried out. Nanoindentation experiments were performed with the peak load of 1000 μN, using a cube corner type as an indenter. Although almost all the peak loads in this study are the same, indentation sizes in ferrite grains are larger than those in martensitic grains. This implies ferrite has lower deformation resistance than martensite in the dual-phase steel. The nanohardness for ferrite and martensite is 2.8 GPa and 7.2 GPa in the specimen tempered at 473 K, respectively. The nanohardness for martensite decreases with increasing the tempering temperature, which is similar to the temper softening of the tensile strength. On the other hand, the yield stress is almost constant with tempering temperatures below 473 K and significantly increases in the temperature range between 473 and 623 K. These results mean that the mixing rule cannot be applied to the yield strength, but it is applicable to tensile strength.

Bi-modal microstructure in a powder metallurgical ferritic steel

Ferritic steel with a nominal composition of Fe–14Cr–3W–0.42Ti–0.32Y was prepared by mixing gas-atomized prealloyed powder and mechanically alloyed powder. The microstructure is much different from other ferritic steels with the same composition and prepared via only mechanically alloyed powder. A bi-modal structure, which consists of pure ferritic grains and martensitic grains, was obtained after hot forging and air cooling. A phase transformation of αbcc → γfcc → α'bcc was also discovered in microstructural observation. The bi-modal microstructure shows a good combination of high strength and high ductility.

Thermal stability of bulk nanocrystalline Ti–10V–2Fe–3Al alloy

Abstract Thermal stability of the nanocrystalline Ti–10V–2Fe–3Al alloy produced by cold-forging and corresponding hardness variations were investigated. When the annealing temperature was lower than 400 °C, the primary features were the decline of lattice defect density and the increase of lattice ordering level in the vicinity of grain boundaries, named as grain boundary relaxation. The nanocrystalline grains grew slowly. In comparison, when the annealing temperature was higher than 400 °C, the decomposition of the deformation-induced α″ martensites and the subsequent rapid grain growth were observed. Vickers hardness measurement shows an initial hardening followed by a softening with the increase of annealing temperature. The hardening peak appeared at 400 °C. The critical grain growth temperature, i.e., Tcgg, in the nanocrytalline Ti–10V–2Fe–3Al alloy, was slightly lower than those in other nanocrystalline alloys. That could be attributed to the martensitic decomposition-induced grain growth in this alloy.

Insitu neutron diffraction study of micromechanical interaction and phase transformation in dual phase NiTi alloy during tensile loading

It is well known that the shape memory effect of NiTi alloy is closely related to the micro-structural characteristics. Neutron diffraction method can used to explore the changes of the phase transformation, lattice strain and twining reorientation of bulk NiTi alloy during deformation caused by the applied stress. In this paper, combining the four types of deformation characteristics in the macro stress-strain curves of dual phase NiTi alloy and using in-situ neutron diffraction measurement, the micromechanical interactions and phase transformation are determined. The volume fraction of the initial austenite before deformation is about 22%. The contrast transformation, which is corresponding to the lattice strain rapid decreasing of (110)B2 and increasing of (002)B19', reveals that the stress-induced transformation from austenite to martensite phase appears with the volume fraction of austenite decreasing rapidly and 011 II type twinning increases at the low strain hardening stage. At the same time, the initial martensite grains change their orientation to a favorable direction and the new {201} type martensite twinnings induced with the increase of applied stress cannot recover after unloading. At the high strain hardening stage, the twinning deformation is considered to be the main mechanism from the observing of the changes in the full width at half maximum (FWHM). Meanwhile, the slipping caused by dislocation is the main deformation mechanism corresponding to the obvious increas of the FWHM at the statured stage of the strain hardening.

ANALYSIS OF THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF DUAL PHASE STEEL UNDER THE EFFECTS OF DIFFERENT BRAZING RATES

ABSTRACTEnvironmental, concerns regarding reducing CO2 emissions and the drive of having better fuel economy have already enthused the car manufacturer to use the weight materials having better mechanical properties. Automotive industry has shown a great interest in Dual Phase steels due to the possibility of reducing weight of vehicles and increasing the passenger safety at a very competitive cost. Automotive applications unavoidably entail welding and joining in the manufacturing process and the fatigue resistance of welded joints due to the integrity and safety requirements. The variation of welding parameters (voltage, current and speed of welding) affects weld performance, mechanical, and metallurgical properties.The CMT (Cold Metal Transfer) braze welding is a relatively new technology that partially decouples the arc electrical transients from the filler wire feed rate. It allows reducing the heat required for welding and permits higher joining speeds.The aim of this work is to study the interfacial microstructures and intermetallic compounds produced by cold metal transfer welding of two plates of galvanized DP600 dual phase steel with CuSi3 as filler metal. The study was performed by applying a CMT braze welding with three different joining speeds. The welded microstructures and microhardness were determined and related to the welding process conditions.A small HAZ, constituted by martensite, bainite and coarse ferrite grains, has been highlighted. Furthermore, an intermetallic Fe-Si-Cu compound layer formed at the interface between steel and filler metal. The joining speed sways the size of ZTA since the heat input Q affects the phase transformation in the weld and heat affected zoneThis parameter also affects the thickness of the compound layer and the size of precipitates in the filler metal, likewise the mechanical characteristics. The fracture starts at the interface steel-copper where intermetallic compounds formed.

Measurements of Residual Stresses in Cold-Rolled 304 Stainless Steel Plates Using X-Ray Diffraction with Rietveld Refinement Method

The determination of the residual stresses using X-ray powder diffraction in a series of cold-rolled 304 stainless steel plates, deforming 0, 34, 84, 152, 158, 175 and 196 % reduction in thickness has been carried out. The diffraction data were analyzed using the Rietveld structure refinement method. The analysis shows that for all specimens, the martensite particles are closely in compression and the austenite matrix is in tension. Both the martensite and austenite, for a sample reducing 34% in thickness (containing of about 1% martensite phase) the average lattice strains are anisotropic and decrease approximately exponential with an increase in the corresponding percent reduction (essentially phase content). It is shown that this feature can be qualitatively understood by taking into consideration the thermal expansion mismatch between the martensite and austenite grains. Also, for all cold-rolled stainless steel specimens, the diffraction peaks are broader than the unrolled one (instrumental resolution), indicating that the strains in these specimens are inhomogeneous. From an analysis of the refined peak shape parameters, the average root-mean square strain, which describes the distribution of the inhomogeneous strain field, was predicted. The average residual stresses in cold-rolled 304 stainless steel plates showed a combination effect of hydrostatic stresses of the martensite particles and the austenite matrix. Received: 14 March 2008; Revised: 13 October 2008; Accepted: 6 November 2008

Microstructural Analysis of Type 304 Metal Sleeve

In order to form thin metal sleeve with the thickness of 0.03 mm, type 304 austenitic steel sheet was deeply drawn to a cup and spinning method applied to its body. The sleeve shows high strength with a dual-phase microstructure of fine austenite and transformed martensite. Pancaked austenite and martensite grains were highly elongated along RD (drawing direction) in the layer structure, and their grain width was about 100 nm. Dynamically recovered austenite grains were highly aligned from {101} to {101}. The strain-induced martensite grains mainly showed two components of {001} and {111}. Recover and recrystallization of the sleeve appeared at the temperature from 873 K to 1073 K. Annealed at 1073 K the austenite grains were mostly recrystallized with intensifying {101}, and the martensite grains were also reverse-transformed to austenite.

Metallurgical Characterization of M-Wire Nickel-Titanium Shape Memory Alloy Used for Endodontic Rotary Instruments during Low-cycle Fatigue

Introduction Rotary instruments made of a new nickel-titanium (NiTi) alloy (M-Wire) have shown improved cyclic fatigue resistance and mechanical properties compared with those made of conventional superelastic NiTi wires. The objective of this study was to characterize microstructural changes of M-Wire throughout the cyclic fatigue process under controlled strain amplitude. Methods The average fatigue life was calculated from 30 M-Wire samples that were subjected to a strain-controlled (∼4%) rotating bend fatigue test at room temperature and rotational speed of 300 rpm. Microstructural evolution of M-Wire has been investigated by different metallurgical characterization techniques, including differential scanning calorimetry, Vickers microhardness, and transmission electron microscopy at 4 different stages (as-received state, 30%, 60%, and 90% of average fatigue life). Results During rotating bend fatigue test, no statistically significant difference ( P > .05) was found on austenite finish temperatures between as-received M-Wire and fatigued samples. However, significant differences ( P < .05) were observed on Vickers microhardness for samples with 60% and 90% fatigue life compared with as-received and 30% fatigue life. Coincidentally, substantial growth of martensite grains and martensite twins was observed in microstructure under transmission electron microscopy after 60% fatigue life. Conclusions The results of the present study suggested that endodontic instruments manufactured with M-Wire are expected to have higher strength and wear resistance than similar instruments made of conventional superelastic NiTi wires because of its unique nano-crystalline martensitic microstructure.

Characterization of Gas Metal Arc Welded Hot Rolled DP600 Steel

AbstractDual‐phase (DP) steels are suitable candidates for automotive applications due to their high strength and ductility. These advanced mechanical properties result from the special microstructure of the DP steel with 5∼20% martensite phase in a soft ferrite matrix. However, during welding, which is an important process in automotive industry, this special microstructure is destroyed. In this research the characterization of Gas Metal Arc (GMA) welded joining zones was performed by optical microscopy and hardness mapping. Tensile tests were also performed keeping the welded portion in the gauge length. Scanning Electron Microscopy (SEM) was used for the fracture investigation. From the characterization and tensile tests, the soften zones were found, which are caused by the tempered martensite and larger ferrite grain size than that in base metal. Furthermore, GMA welding make a large Heat Affected Zone (HAZ).