Martensite Grains Research Articles

In-situ EBSD study of deformation behaviour of 600 MPa grade dual phase steel during uniaxial tensile tests

This paper studied the plastic deformation behaviour of DP600 steel subjected to uniaxial tension, by means of in-situ EBSD technique. It provides experimental evidences and detailed insight into the microstructural aspects of plastic deformation. A phase identification method based on the band slope map of EBSD was adopted to differentiate martensite from ferrite. The results show that the plastic strain localization lies mainly in the ferrite grains, fracture could usually start in ferrite grains close to hard martensite grains. With the increase of strain, average misorientation angle decreased while the fraction of LAGBs increased. Average Taylor factor for the whole microstructure became higher at high strains due to work hardening process, and plastic deformation results in soft regions with zonal distribution parallel to the loading direction. In the undeformed state, the texture orientation (111)[01¯1] and (111)[11¯0] are the major components of the γ-fibre while (223)[11¯0] and (221)[11¯0] are the main components of α-fibre. The intensity of the α-fibre slightly decreased, while the intensity of the γ-fibre increased with increasing strain. Plastic deformation occurred in some grains which were subdivided into different regions due to the activation of different slip systems. The tensile axis orientation of the grain rotated gradually to the line link <100>−<101>, and lattice rotation within one single grain differs from regions to regions.

Microstructure, Mechanical Properties, and Constitutive Models for Ti–6Al–4V Alloy Fabricated by Selective Laser Melting (SLM)

The mechanical performances and microstructure of Ti–6Al–4V built by selective laser melting were evaluated by optical microscopy, transmission electron microscopy, and room temperature tensile testing, and compared with the wrought and as-cast material. The flow behavior of the as-produced Ti–6Al–4V at temperatures varying from 700–900 °C at an interval of 50 °C and strain rates ranging from 10−2–101 s−1 was experimentally acquired. According to the experimental measurement, the Johnson–Cook, modified Arrhenius model, and artificial neural network were constructed. A comparative investigation on the predictability of established models was performed. The as-produced microstructure is made up of non-equilibrium martensite and columnar grains, leading to higher strength and lower ductility with respect to the conventional material. In room temperature tensile tests, the SLMed Ti–6Al–4V shows the characteristics of continuous yielding and unobvious work-hardening. The flow stress rapidly reaches the peak, and the softening rate depends on the strain rates and deformed temperatures in hot compression. The Johnson–Cook model could well predict the flow stress during quasi-static tensile deformation, but the model constants might vary with the process conditions. For dynamic compression, the artificial neural network exhibits higher accuracy to fit the flow stress of SLMed Ti–6Al–4V, and higher error to predict the conditions out of the model data, compared to the modified Arrhenius model involving the compensation of strain rate and strain.

Effect of fusion and friction stir welding techniques on the microstructure, crystallographic texture and mechanical properties of mild steel

The present investigation aims at studying the effect of different welding techniques on the microstructure and mechanical properties of the welds. Weld zone (WZ) in the metal inert gas (MIG) welding, contains higher martensite fraction compared to arc and tungsten inert gas (TIG) welding. The micro-hardness of the WZ was found to be maximum after MIG welding (220 ± 14 HV) in comparison to arc welding (190 ± 12 HV), TIG welding (142 ± 10 HV) and friction stir welding (FSW) (202 ± 10 HV) as well as the base metal (BM) (108 ± 14 HV). The joints fabricated via MIG welding exhibited higher tensile strength (371 ± 10 MPa) as compared to other welding techniques (arc and FSW). The ductility of the parent metal was 42 ± 2% whereas the ductility of the MIG and arc joints were lower (13 ± 2% and 17 ± 3% respectively) due to the presence of martensite during fusion welding. The FSW joint shows higher yield and tensile strengths (356 ± 8 MPa) along with higher elongation (22 ± 4%) compared to fusion-welded joints due to the presence of fine, equiaxed recrystallized grains in the WZ. Fracture occurs at the HAZ/base metal interface, where the hardness is lower compared to the WZ. Compressive and tensile residual stress is obtained in the WZ upon fusion and FSW, respectively. In this paper, analytical tools are used to calculate the orientation of prior austenite (γ) in the MIG weld joint. The texture of high temperature γ phase is simulated for the different welding techniques. The occurrence of the Kurdjumov–Sachs (KS) orientation relationship (OR) is determined from the martensite grains in MIG weld. The texture investigation showed the occurrence of texture memory effect of ferrite in the HAZ after MIG welding.

Microstructure characterization and mechanical properties of laser additive manufactured oxide dispersion strengthened Fe-9Cr alloy

Oxide dispersion strengthened (ODS) alloys are attractive material candidates for advanced fission and fusion reactor application owing to their excellent radiation tolerance and high temperature creep strength but traditional manufacturing process encounters some difficulties in producing complex components. An ODS Fe-9Cr alloy with nominal composition of Fe-9Cr-1.5W-0.3Ti-0.3Y (wt%) were manufactured by laser engineered net shaping (LENS) method with different laser power. Equiaxed martensitic grains together with micro-pores were observed in the as-deposited alloys. With the increase of laser power, the density and size of pores decrease, the grain size of the alloys increases while the number density of nanoscale oxides decreases due to the formation of thicker Y-Ti-O-enriched and Al-O-enriched slag layer in the block surface. Hot isostatic pressing (HIP) processing of the as-deposited ODS alloy with low laser power reduces significantly the density of and the size of micro-pores, refines the grain size, and precipitates higher density of nanoscale oxides (Y2TiO5 and Y2Ti2O7), which improves significantly the tensile properties that is comparable at room temperature and is better at 873 K than ODS EUROFER steel produced by conventional powder metallurgy.

Influence of weld parameters on weld regimes and vaporization rate in electron beam welding of Ti6Al4V alloy

Electron beam welding (EBW) is the most preferable and efficient joining technique to weld titanium alloys, since it provides a vacuum atmosphere and eliminates the use of shielding gas during the welding process. The present work is focused on understanding the electron beam weld parameters which controls the weld limits, microstructural and mechanical properties and vaporization rate of metal elements in the Ti6Al4V alloy. The influence of power density on characteristics of electron beam welded Ti6Al4V alloy at constant linear energy conditions is examined. The welding experiments were performed to evaluate the weld geometry, weld regimes, microstructural properties and microhardness characteristics using fundamental parameters such as power density and interaction energy per unit volume (IE). Moreover, the vaporization heat transfer model of the EBW process is developed by employing Fourier’s law of heat conduction to estimate the vaporization rate of Ti6Al4V alloying elements. The rate of vaporization of major alloying elements of Ti6Al4V is analysed with respect to power density. Based upon the results obtained, it is witnessed that the weld bead geometry, weld limits and grain size are controlled by power density for constant linear energy conditions. Also, the solidified microstructure in the weld zone at different power density significantly effects the final strength of the weld joint. The conduction and keyhole weld limits are identified for power density values below 0.31 W cm−2 and above 2.12 W cm−2, respectively. The transition mode welding in the Ti6Al4V alloy is identified by a nominal increase in aspect ratio for a wide range of power density and IE values. The results also established that the size of β grain in the fusion zone reduces with increase in power density at constant linear energy condition. Also, the average α1 martensitic grain size in the weld zone is determined to be higher in keyhole welds (128 µm) relative to conduction welds (48 µm) on account of higher solidification rate. In effect, the solidified martensitic phases influence the microhardness distribution proportionately across the weld sample of Ti6Al4V alloy. Vaporization of metal elements of the Ti6Al4V alloy is induced in few milliseconds and evaporation rate of ‘Al’ element is least followed by ‘Ti’ and ‘V’ elements. It is also observed that by reducing the power density from 2.54 to 2.12 W cm−2, keyhole mode welding can be acquired and vaporization rate of ‘Al’ is reduced by 20.5%.

Transmission of Plasticity Through Grain Boundaries in a Metastable Austenitic Stainless Steel

Austenitic metastable stainless steels have outstanding mechanical properties. Their mechanical behavior comes from the combination of different deformation mechanisms, including phase transformation. The present work aims to investigate the main deformation mechanisms through the grain boundary under monotonic and cyclic tests at the micro- and sub-micrometric length scales by using the nanoindentation technique. Within this context, this topic is relevant as damage evolution at grain boundaries is controlled by slip transfer, and the slip band-grain boundary intersections are preferred crack nucleation sites. Furthermore, in the case of metastable stainless steels, the interaction between martensitic phase and grain boundaries may have important consequences.

High Tensile Ductility and Strength in Dual-phase Bimodal Steel through Stationary Friction Stir Processing

The combination of high strength and good ductility are very desirable for advanced structural and functional applications. However, measures to enhance strength typically lead to ductility reduction due to their inverse correlation, nano-grained structures for an instance. Bi-modal grain structure is promising in this regard, but its realization is limited by multiple complex processing steps. Here, we demonstrate a facile single-step processing route for the development of bimodal grain structure in austenitic stainless steel, SS316L. The bimodal structure comprised of fine martensite grains (<500 nm) sandwiched between coarse austenite grains (~10 µm). The dual-phase bimodal structure demonstrated higher yield strength (~620 MPa) compared to ultra-fine grain structure (~450 MPa) concurrent with high uniform tensile ductility (~35%). These exceptional properties are attributed to unique dual-phase, bimodal grain structure which delayed the onset of plastic instability resulting in higher strength as well as larger uniform elongation and work-hardening rate. Our approach may be easily extended to a wide range of material systems to engineer superior performance.

Impact of second phase morphology and orientation on the plastic behavior of dual-phase steels

Martensite volume fraction, composition and grain size are the known primary factors controlling the mechanical behavior of ferrite-martensite dual-phase steels. Recently, excellent performances of dual-phase steels with a fibrous microstructure have been reported. However, the precise role of martensite morphology and orientation has not been thoroughly elucidated yet. This study develops a two-scale micromechanical modeling strategy in order to investigate the effect of particle morphology and orientation on the elastoplastic behavior of dual-phase steels. Finite element simulations are carried out on 3D periodic unit cells, each having a given orientation and volume fraction of spheroidal particles. The overall response is obtained by averaging the response of grains with different orientations, thus bypassing the need for costly full-field simulations on representative volume elements of realistic microstructures. A detailed parameter study systematically investigates the effect of particle morphology and orientation at grain level and at grain assembly level. While particle morphology and orientation effects lead to significant differences at grain level in terms of strain hardening behavior and back-stress development, the impact of the phase morphology at the homogenized multigrain level is almost negligible up to the onset of necking. However, the mechanical fields at the micro-scale are considerably influenced by both particle morphology and orientation, and are expected to largely impact the damage behavior through, among others, generating large grain-to-grain heterogeneities.

Inconsistent effects of austempering time within transformation stasis on monotonic and cyclic deformation behaviors of an ultrahigh silicon carbide-free nanobainite steel

Given the transformation stasis after the incomplete transformation of carbide-free bainite, this paper aimed to study the effects of austempering time within transformation stasis on the bainite microstructure, monotonic deformation behavior, and especially cyclic deformation behavior using an ultrahigh silicon (2.59 wt%) steel. With increasing austempering time, the dislocation density of bainitic ferrite decreased, carbon content of retained austenite slight increased, and carbon distribution in retained austenite blocks gradually homogenized. The best combination of strength and ductility was obtained through a longer austempering time within transformation stasis. But the longer austempering time, indeed, did not result in longer fatigue life. The opposite trend could be explained by the fact that the primary factors affecting these mechanical properties were different. The lower density of mobile dislocation pre-existed in the starting microstructure of the samples austempered for a longer time was primarily responsible for its lower cyclic hardenability. Moreover, one retained austenite block with completely homogeneous carbon distribution was only transformed to one martensite grain, which increased the cyclic softening and degrade the fatigue life. By contrast, the deformation-induced martensite transformation from the retained austenite with higher mechanical stability in the samples austempered for a longer time enhanced strain hardenability at higher monotonic tensile strains and well delayed the necking, thus improving the combination of strength and ductility.

Tailoring the Mechanical Properties of Laser Cladding-Deposited Ferrous Alloys with a Mixture of 410L Alloy and Fe⁻Cr⁻B⁻Si⁻Mo Alloy Powders.

The effects of different ratios of 410L alloy and Fe–Cr–B–Si–Mo alloy powders on the microstructure and mechanical properties of laser cladding-deposited ferrous alloys were investigated. The experimental results revealed that the 410L alloy had good strength and excellent ductility due to its microstructure consisting of large elongated ferrite dendrites surrounded by a small number of martensite grains, while the Fe–Cr–B–Si–Mo alloy had high strength and poor ductility because of its eutectic microstructure composed of ferrite and Fe2B/Cr2B. As the concentration of Fe–Cr–B–Si–Mo alloy powder added to the 410L alloy powder increased, the ferrite grains became finer and the volume fraction of the eutectic increased, which eventually improved the strength and reduced the plasticity. Then, 410L + 12.5% Fe–Cr–B–Si–Mo alloy powder was successfully deposited onto AISI 1060 steel substrate via laser cladding deposition, and the mechanical properties met those of the substrate, which verified that tailoring the mechanical properties of the laser cladding-deposited alloys with a mixture of 410L and Fe–Cr–B–Si–Mo alloy powders for steel repairing applications is a feasible solution.

Improvement of densification and microstructure of ASTM A131 EH36 steel samples additively manufactured via selective laser melting with varying laser scanning speed and hatch spacing

ASTM A131 (EH36 grade) steel samples with a high density (>99.5%) were additively manufactured through a selected laser melting (SLM) process. The additive manufacturing (AM) process parameters were optimized through tuning scanning speed from 100 to 300 mm/s and hatch spacing from 0.08 to 0.13 mm with a layer thickness of about 50 µm using a laser power of 175 W under a chess board scanning strategy. By varying heat input mainly via lowering scanning speed, a near full density of the samples could be achieved. The processing at lower scanning speed was broadened to a larger window of hatch spacing, which improved the density of the printed samples above 99%. The microstructure of the samples under different scan speeds was subsequently studied and compared. Cellular-dendritic grains with a diameter of around 1 µm and acicular grains of up to 2–3 µm were observed at different locations. A large plate-like martensite structure was found in coarse columnar grains (up to 20 µm). With lower scanning speed, the grains were coarser and more like a cellular structure, while with higher scanning speed the grains were finer and more like a cellular-dendritic structure. However, different morphologies of martensite grains were found mainly in cellular, columnar and lath shapes, which probably resulted from different transformation mechanisms and were controlled by cooling rate. Electron back scattered diffraction analysis showed that the prior elongated columnar grains with the martensitic plate-like structure delineated the columnar grain boundaries. The micro-hardness and tensile properties of the selected samples were correlated to their microstructures. The resultant mechanical properties were attributed to a combined effect of martensitic structure and possible precipitation, solid solution and dislocation strengthening for the EH36 steel samples built via SLM. The fractured surfaces, examined with scanning electron microscopy, showed mostly a dimple type of failure but with a low elongation of up to about 6%. The microsegregation behavior of the samples was believed to accompany this rapid solidification process and the degree of element escaping from the lattice was attributed to the cooling rate.

Optimization of Quenching Process Parameters of 15CrNiMo Steel Used for Roller Bit Bits

The microstructure and mechanics of 15CrNiMo steel with different quenching temperatures (800°C, 825°C, 850°C, 875°C, 900°C) and different quenching holding time (15min, 25min, 35min) were studied for steel 15CrNiMo for roller bit. The relationship between the properties of different quenching parameters and the mechanical properties such as microstructure, impact toughness and hardness was analyzed. The results show that the microstructure of the annealed precursor is mainly ferrite and pearlite. With the increase of quenching temperature, the ferrite is gradually reduced, the lath martensite is gradually increased, and the martensite grains tend to grow. The strength increases first and then decreases, and the plasticity increases slightly. The quenching and holding time has no obvious effect on the test steel, and the austenitization of the core is the best time. The best quenching temperature is 850°C, the best holding time is 25 min, (tempering temperature is 220°C, tempering holding time is 50 min) and the mechanical properties are optimal.

Mapping of local lattice parameter ratios by projective Kikuchi pattern matching

We describe a lattice-based crystallographic approximation for the analysis of distorted crystal structures via Electron Backscatter Diffraction (EBSD) in the scanning electron microscope. EBSD patterns are closely linked to local lattice parameter ratios via Kikuchi bands that indicate geometrical lattice plane projections. Based on the transformation properties of points and lines in the real projective plane, we can obtain continuous estimations of the local lattice distortion based on projectively transformed Kikuchi diffraction simulations for a reference structure. By quantitative image matching to a projective transformation model of the lattice distortion in the full solid angle of possible scattering directions, we enforce a crystallographically consistent approximation in the fitting procedure of distorted simulations to the experimentally observed diffraction patterns. As an application example, we map the locally varying tetragonality in martensite grains of steel.

Microstructural and mechanical evaluation of a dissimilar joining between SAE 1020 and AISI 304 steel obtained via ultra-high-frequency-pulsed GTAW

Two welding process parameters (the amplitude and frequency of the pulsed current) were varied in order to investigate their influence on the microstructure of the weld region and the mechanical properties of the joints. This investigation and the subsequent discussion were made based on the results of metallographies, tensile tests, and Vickers microhardness tests. The introduction of pulsed current at ultra-high frequencies resulted in a significant reduction in the size of the heat-affected zone (HAZ) for both materials: up to 54% for carbon steel and up to 73% for stainless steel. An increase in the grain size in the carbon steel HAZ was also noted. There are indications that the pulsed current at ultra-high frequencies has accelerated the atomic diffusion process of alloying elements from the fusion zone (FZ) to the carbon steel HAZ. Martensitic grains were observed in the FZ and the microhardness test verified this finding, once the microhardness values measured were around 400 HV0.3. The tensile strength was around 450 MPa and the rupture occurred on the carbon steel side, for all samples.

Microstructure characteristics of 12Cr ferritic/martensitic steels with various yttrium additions

12Cr ferritic/martensitic steels with 0, 0.1 wt%, 0.2 wt% and 0.3 wt% theoretical yttrium (Y) additions were fabricated by vacuum inducting melting and casting method. Solubilities of Y in the 12Cr steels are 0.027, 0.078 and 0.17 for 12Cr-0.1Y, 12Cr-0.2Y and 12Cr-0.3Y, respectively. Phase transformations and microstructure characteristics under different heat-treatment schedules were investigated. The starting temperature of ferrite-to-austenite transformation Ac1 are maintained about 850 °C, but the finishing temperature of ferrite-to-austenite transformation Ac3 are about 950, 970, 980 and 1000 °C for 12Cr-0Y, 12Cr-0.1Y, 12Cr-0.2Y and 12Cr-0.3Y, respectively, which indicates that Ac3 increases gradually with the addition of Y. Martensite accompanied with a few δ-ferrite is the dominant structure in all the steels. The amount of δ-ferrite shows a strong dependence with the Y content and austenitizing temperature. Area fraction of δ-ferrite increases with the content of Y, which is the ferrite favouring element. The minimum amount of δ-ferrite are achieved at 950 °C for 12Cr-0Y, 12Cr-0.1Y, 12Cr-0.2Y and 1000 °C for 12Cr-0.3Y. Besides, more carbides precipitate along the martensite laths and grain boundaries in the Y-bearing steel due to the redistribution of carbon between austenite and ferrite resulting from the ferrite favouring element of Y.

Study on the wear resistance of laser cladding iron-base alloy by heat treatment

The friction and wear properties of laser cladding iron-base alloy by heat treatment were investigated in order to improve the durability of scraper conveyors by the wear test of the mechanical properties test. It mainly includes microstructure observation, hardness test, wear rate and appearance. The results show the crystalline grains of the heat treatment scheme are smaller, austenite and lath martensite grains can be observed clearly, which are helpful in improving the hardness. Meanwhile, gravimetric wear loss and friction coefficient are reduced. The wear situation of the heat treatment generates smooth surfaces, which can better prove their wear resistance is better.

Micromechanical modeling of the effect of phase distribution topology on the plastic behavior of dual-phase steels

In this paper, a topology optimization based numerical method is proposed to investigate the micromechanical plastic behavior of dual-phase (DP) steels. Representative volume elements (RVEs) are constructed using the topology optimization based artificial microstructures. Micromechanical behavior under various loading conditions are predicted for the RVEs to investigate the plasticity, strain localization and strengthening mechanism affected by the microstructural characteristics of DP steels. Plastic strain patterns including shear band are found during the deformation. Due to the twisting movement of martensite grains, the direction of the strain localization bands in the shear loading case is 0° or 90° to the loading direction, while it is 45° in the tensile and compressive cases. Moreover, the effective flow behavior of the material under shear loading is lower than those found in tensile and compressive cases. The influence of various microstructural features, such as, martensite fraction, distribution of each phase, on the effective flow properties and the local strain partitioning has also been identified. Both of the effective flow properties and strain localization exhibit the tendency to be strengthened with the increase of martensite phase fraction. Furthermore, the RVE with more uniform martensite distribution leads to the decrease of effective flow properties and strain localization. Longer martensite-ferrite interface results from the clustering of martensite, which increases the strain localization effect during the plastic deformation.

Grain refinement and strengthening of austenitic stainless steels during large strain cold rolling

ABSTRACTThe microstructure/texture evolution and strengthening of 316 L-type and 304 L-type austenitic stainless steels during cold rolling were studied. The cold rolling was accompanied by the deformation twinning and micro-shear banding followed by the strain-induced martensitic transformation, leading to nanocrystalline microstructures consisting of flattened austenite and martensite grains. The fraction of ultrafine grains can be expressed by a modified Johnson-Mehl-Avrami-Kolmogorov equation, while inverse exponential function holds as a first approximation between the mean grain size (austenite or martensite) and the total strain. The deformation austenite was characterised by the texture components of Brass, {011}<211>, Goss, {011}<100>, and S, {123}<634>, whereas the deformation martensite exhibited a strong {223}<110> texture component along with remarkable γ-fibre, <111>∥ND, with a maximum at {111}<211>. The grain refinement during cold rolling led to substantial strengthening, which could be expressed by a summation of the austenite and martensite strengthening contributions.

Effects of austempering temperature on bainitic microstructure and mechanical properties of a high-C high-Si steel

High tensile strength of ~ 1550 MPa, uniform elongation of ~ 30%, and impact energy of ~ 80 J were achieved in a high-C high-Si bainitic steel, by austempering around the nose-tip temperature of the time–temperature–transformation (TTT) diagram. This bainitic microstructure revealed multiple lengths of nanobainite plates and various sizes and shapes of retained austenite. Its bainite transformation during austempering and microstructure evolution during tensile deformation were discussed in depth. Results demonstrated that large blocks of retained austenite (with inhomogeneous carbon distribution) transformed partly and gradually to multiple martensite grains as strain increases, due to the strong effect of carbon on mechanical stability of retained austenite. Ductile long nanobainite plates (~ 0.13 at% C) suppressed the propagation of microcracks, because of stress relief effect ahead of the current microcracks.

TENSILE AND HIGH CYCLE FATIGUE PROPERTIES OF ANNEALED TI6AL4V (ELI) SPECIMENS PRODUCED BY DIRECT METAL LASER SINTERING

The tensile and high cycle fatigue (HCF) properties of high temperature annealed (HTA) direct metal laser sintered (DMLS) Ti6Al4V extra low interstitial (ELI) machined and polished specimens were investigated. The HTA heat treatment of the specimens resulted in the nucleation and growth of the alpha and beta grains from the acicular ?′ martensite grains, improving their elongation to failure. The specimens were micro-CT scanned in an attempt to relate the pores in the specimens to their fatigue properties. The micro-CT pore information from suspected crack initiation pores on the surfaces of eventual fracture was used to calculate the stress intensity factors, which correlated well with the decreasing cycles to failure of the fatigue test specimens for all three build directions. Three representative specimens were analysed, and the ‘killer pore’ was identified in each micro-CT scan and fractograph, all of which were proximal to the surface of the specimen.