Austenite Martensite Research Articles

Giant magnetoelectric effect of Ni–Mn-Ga/piezopolymer composites tailored by a martensitic transformation

Polymer-based magnetoelectric (ME) composites, composed of magnetostrictive and piezoelectric phases, in addition to exhibiting an effective coupling between the magnetic and electric orders of the matter, also offer important advantages for the Internet-of-Things, digitalization, and 4.0 revolution environments: light weight, flexibility, wearability, environmental friendliness, printability, and biocompatibility. Nevertheless, their successful implementation on applications such as sensors, actuators, energy harvesters, biomedical devices and spintronics strongly depends on the increase of its ME voltage response.This work explores, both experimentally and theoretically, the magnetostrictive Ni–Mn-Ga's martensite ‒ austenite phase transformation to shift/tailor/design the ME resonance peak exhibited by Ni–Mn-Ga/P(VDF-TrFE) piezoelectric composites. In the austenite phase, the determined peak value of the ME coefficient of 18.1 V cm−1 Oe−1 was much higher than the value of 6.05 V cm−1 Oe−1 obtained for the martensite phase, whereas the estimated theoretically value of magnetoelastic constant in the austenite is much smaller. The ways to further increase the magnetically induced ME response are also outlined.

In Situ Observation of the Grain Growth Behavior and Martensitic Transformation of Supercooled Austenite in NM500 Wear-Resistant Steel at Different Quenching Temperatures.

In situ observations of the austenite grain growth and martensite transformations in developed NM500 wear-resistant steel were conducted via confocal laser scanning high-temperature microscopy. The results indicated that the size of the austenite grains increased with the quenching temperature (37.41 μm at 860 °C → 119.46 μm at 1160 °C) and austenite grains coarsened at ~3 min at a higher quenching temperature of 1160 °C. Furthermore, a large amount of finely dispersed (Fe, Cr, Mn)3C particles redissolved and broke apart at 1160 °C, resulting in many large and visible carbonitrides. The transformation kinetics of martensite were accelerated at a higher quenching temperature (13 s at 860 °C → 2.25 s at 1160 °C). In addition, selective prenucleation dominated, which divided untransformed austenite into several regions and resulted in larger-sized fresh martensite. Martensite can not only nucleate at the parent austenite grain boundaries, but also nucleate in the preformed lath martensite and twins. Moreover, the martensitic laths presented as parallel laths (0~2°) based on the preformed laths or were distributed in triangles, parallelograms, or hexagons with angles of 60° or 120°.

Evidence for austenite to non-modulated martensite transformation crystallography and variant organization in Ni-Mn-Ga-Co ferromagnetic shape memory alloys

Knowledge of transformation crystallography and variant organization of product phase was a prerequisite for tuning microstructure and enhancing functionalities in materials with displacive structure-transformation. However, the transformation orientation relationship (OR) between parent austenite and non-modulated (NM) martensite in traditional NiMnGa ferromagnetic shape memory alloy was not yet well-determined via direct experimental results, thus leading to ambiguity in understanding the self-accommodated configuration. In this work, by generating a microstructure with coexisting austenite and NM martensite through minor Co substitution and proper heat treatment, the transformation OR was unambiguously determined to be the N-W relation with {111}A//{101}NM and <2¯11>A//<101¯>NM on the basis of accurate EBSD orientation measurements. Analysis on deformation gradient matrix constructed from the N-W OR revealed that, the internal nano-twined structure allowed a maximum profit for eliminating the overall latticed deformation caused by transformation, overweighing the “sandwich” micro-variant pair structure. Such strain accommodation mechanisms consequently contributed to the resultant self-accommodated hierarchically twinned structure of martensite. Moreover, it was found that the prior austenite grain boundaries (PAGBs) provided dominated nucleation sites for NM martensite variants, where the preferential variant selection at PAGBs mainly depended on the inclination angle between the PAGB and matching close-packed direction (<2¯11>A//<101¯>NM) of variants. The present study provided comprehensive information on transformation crystallography and displacive characters of austenite to NM martensite transformation, which was useful for the efforts in property optimization and theoretical simulation.

Austenite Formation and Manganese Partitioning during Double Soaking of an Ultralow Carbon Medium‐Manganese Steel

Double soaking (DS) is a thermal processing route intended to produce austenite–martensite microstructures in steels containing austenite‐stabilizing additions and consists of intercritical annealing (primary soaking), followed by heating and brief isothermal holding at an increased temperature (secondary soaking), and quenching. Herein, experimental dilatometry during DS of a medium‐manganese (Mn) steel with nominally 7 wt% Mn and an ultralow residual carbon concentration, in combination with phase‐field simulations of austenite formation during secondary soaking, is presented. The feasibility of maintaining heterogeneous Mn distributions during DS is demonstrated and insight is provided on the effects of the secondary soaking temperature and prior Mn distribution on the ferrite‐to‐austenite phase transformation during the secondary soaking portion of the DS treatment.

A micromechanical constitutive model of high-temperature shape memory alloys

In this work, a new micromechanical constitutive model is constructed for high-temperature shape memory alloys. Different from the classical constitutive models, the microstructural patterns, heterogeneities of stress and strain fields at the microscopic scale are addressed. In the proposed model, three different inelastic deformation mechanisms, i.e., reversible martensitic transformation, irreversible transformation-induced plasticity, viscoplasticity, as well as their complex interactions, are considered. In addition to the well-known austenite and martensite phases, a new austenite-martensite interface phase is introduced to address the high local stress induced by the martensitic transformation. The modified Mori-Tanaka's homogenization scheme and Hill's interfacial operator are employed to determine the non-uniform stress fields of the three phases. To satisfy the thermodynamic compatibility of the proposed model, the driving forces and evolution equations of the three inelastic deformation mechanisms are derived based on the constructed Helmholtz free energy and the fundamental laws of irreversible thermodynamics. Inheritances of the plastic deformation induced by moving austenite-martensite (A-M) interface during repeated martensitic transformation and its reverse are incorporated. Temperature evolution caused by internal heat production and heat exchange with the ambient medium is also addressed. To validate the capability of the new constructed model, the predicted results of creep behavior and stress-assisted two-way shape memory effect of NiTiPd high-temperature shape memory alloys are compared with the experimental ones. The proposed model provides a theoretical tool for simultaneously analyzing the deformation behaviors at the macroscopic scale and underlying mechanisms at the microscopic scale.

Influence of the Inherited Structure Induced by Al and Si Alloying on Microstructure Evolution and Mechanical Properties of 100Cr6 Steels

The fatigue lifetime of high‐strength 100Cr6 steels can be improved by an increased content of retained austenite that can be induced by Al and Si alloying. The related deformation‐induced retention of the austenite–martensite transformation during cyclic loading increases their local strain hardening capacity. However, for those 100Cr6 steels containing retained austenite, sufficient dimensional stability must be ensured. In this study, two standard 100Cr6 steels alloyed either with 1.5 wt% Al or 1.5 wt% Si (to diminish carbide formation and accordingly promote austenite retention) are laboratory melted and processed to adjust a microstructure of bainite, retained austenite, and carbides. The segregation simulation in the as‐cast condition and the corresponding microstructures in the forging and heat‐treating conditions are investigated. The inheritance of chemical heterogeneity leads to structural heterogeneity on both the nano‐ (nm) and micro (μm) scales. This heterogeneity is much more pronounced in the Al‐alloyed steel, which can be attributed to inheritance from the as‐cast state. While the results of the quasistatic tensile tests are comparable for both alloys, the cyclic load increase tests indicate a higher fatigue strength of the Si‐alloyed steel, which can be explained with the more homogenous microstructure and the finer distribution of the retained austenite.

Carbon Migration Behavior during Rolling Contact in Tempered Martensite and Retained Austenite of Carburized SAE4320 Steel

The changes in the state of carbon in tempered martensite and retained austenite in carburized SAE4320 steel under the rolling contact fatigue (RCF) were investigated using atom probe tomography (APT). In the tempered martensite, the carbons in solid solution and in carbon cluster were readily transferred to the preexisting metastable (ε) carbide due to rolling contact, resulting in a localized change from tempered martensite to ferrite accompanied by the growth of carbides. This supports the recently proposed dislocation assisted carbon migration theory. On the other hand, retained austenite with uniformly distributed enriched solute carbon was partially transformed into the very fine deformation-induced martensite due to rolling contact. Furthermore, carbon seemed to be partitioned into retained austenite from the deformation-induced martensite during further rolling contact cycles. This is a new insight into the characteristics of deformation-induced martensite and retained austenite generated by rolling contact. The present study provides a plausible explanation to the phenomenon that the deformation-induced martensitic transformation improves the RCF life.

Abnormal Trend of Ferrite Hardening in a Medium-Si Ferrite-Martensite Dual Phase Steel

In this paper, the effects of carbon, Si, Cr and Mn partitioning on ferrite hardening were studied in detail using a medium Si low alloy grade of 35CHGSA steel under ferrite-martensite/ferrite-pearlite dual-phase (DP) condition. The experimental results illustrated that an abnormal trend of ferrite hardening had occurred with the progress of ferrite formation. At first, the ferrite microhardness decreased with increasing volume fraction of ferrite, thereby reaching the minimum value for a moderate ferrite formation, and then it surprisingly increased with subsequent increase in ferrite volume fraction. Beside a considerable influence of martensitic phase transformation induced residual compressive stresses within ferrite, these results were further rationalized in respect of the extent of carbon, Si, Cr and Mn partitioning between ferrite and prior austenite (martensite) microphases leading to the solid solution hardening effects of these elements on ferrite.

Damage Evolution and Microstructural Fracture Mechanisms Related to Volume Fraction and Martensite Distribution on Dual‐Phase Steels

The scope of this work is the analysis of the damage evolution and microstructural fracture mechanisms of dual‐phase steels through quasi‐static uniaxial and cyclic tensile tests, considering the influence of the volume fraction and distribution of martensite. The samples are intercritically annealed at different temperatures to obtain three different martensite volume fractions (MVF). Damage evolution is evaluated as a function of stiffness loss. The steels show lower ductility and strength under cyclic loading conditions. The work hardening rate is affected by MVF in different stages of the deformation. A higher MVF is linked to a prompter damage evolution rate. The primary void nucleation mechanisms are ferrite‐martensite and ferrite–ferrite decohesion. Martensite fracture can be activated depending on MVF and martensite distribution along the ferrite grain boundary. The dislocation density on grain boundaries is high due to the austenite–martensite transformation during the quenching process and subsequent plastic deformations. Dislocation density is related to stored strain energy, stress concentration, and the observed fracture mechanisms, particularly near martensite grains. Nanoindentation is conducted to evaluate the ferrite and martensite hardness, which depends on its carbon content. It is observed that the martensite/ferrite hardness ratio affects the uniform elongation of the material.

Influence of Martensite–Austenite Constituents and Precipitates on Reheat Cracking Susceptibility of Coarse-Grained Heat-Affected Zone in T23 and Modified T23 Steels

Influence of Martensite–Austenite Constituents and Precipitates on Reheat Cracking Susceptibility of Coarse-Grained Heat-Affected Zone in T23 and Modified T23 Steels

Study on the improving effect of Nb-V microalloying on the hydrogen induced delayed fracture property of 22MnB5 press hardened steel

Press-hardened steel (PHS) is widely used in manufacturing automobile parts and is sensitive to hydrogen embrittlement (HE). A novel 0.04Nb-0.04 V (wt%) PHS (22MnB5NbV) was developed based on 22MnB5. The constant-strain bending test shows the duration of no-cracking for 22MnB5NbV in 0.5 mol/L HCl is > 300 h even though the pre-bending stress is 1500–1600 MPa, and that for 22MnB5 is 12 h. The prior austenite grain size and martensite lath width in 22MnB5NbV decrease from 13.4 to 8.2 μm and from 0.35 to 0.15 μm due to Nb-V microalloying, respectively, resulting in the hydrogen trapped in reversible hydrogen traps increases by 2.09 times according to thermal desorption spectrometry analysis. The leaching experiment presents that 88% Nb and 25% V exist in the form of precipitates rather than in solution in 22MnB5NbV. The nano-precipitates (< 10 nm) are mainly (Nb, Ti, V)C, in which Nb-bearing precipitates account for the largest proportion and supply the heterogeneous nucleation sites for V-bearing precipitates, this is confirmed by atom probe tomography (APT). APT confirms that the hydrogen is mainly trapped at the interface of Fe matrix/carbide. The dispersed nano-carbide and refined microstructure resulting from the Nb-V microalloying are the key factors improving the HE of 22MnB5NbV.

Influence of the thermal cycle on microstructure formation during direct laser deposition of bainite-martensitic steel

The article presents the formation of structure and mechanical properties of bainite-martensitic steel in the process of exposure to a laser source. By varying the inter-pass temperature, the thermal cycles take place in a different temperature range, at which the formation of martensite and metastable austenite occurs. The formation of residual metastable austenite has a significant impact on the mechanical properties and performance of the product. During continuous growth, the process of phase transitions takes place in the high-temperature region. In the high-temperature region and slow cooling, bainite ferrite forms. The ferritic structure leads to brittle fracture at negative temperatures, which is unacceptable in cold-resistant steels. When specimens are deposition with a pause, the formation of the structure occurs in the low-temperature region, forming mainly bainite-martensitic structures. The resulting properties are quite high, but anisotropy is also preserved.

Investigating the effect of nanovoid inelastic surface stress and the austenite–martensite interface inelastic stress on the martensitic growth at the nanovoid surface

Investigating the effect of nanovoid inelastic surface stress and the austenite–martensite interface inelastic stress on the martensitic growth at the nanovoid surface

Microstructures and properties of a novel 115 mm thick 08Cr9W3Co3VNbCuBN heat-resistant steel tube joints welded by shielded metal arc welding

A novel heat-resistant steel tube, 08Cr9W3Co3VNbCuBN (G115), is the primary choice for crucial components of ultra-supercritical (USC) thermal power plants. In this paper, the 115 mm thick G115 steel tube was welded successfully by shielded metal arc welding (SMAW) and gas tungsten arc welding (GTAW), and the microstructures and mechanical characteristics of joint were studied. The results indicated that the microstructure of the weld zone (WZ) was mostly ferrite and martensite, and the content of ferrite at the weld root was 19.9% lower than that at the weld center. However, at the weld toot the grain size was reduced by about 20%, and the content of M23C6 was increased by 30.9%. Previous austenite grain boundaries (PAGBs) and martensite occurred in the heat-affected zone (HAZ) because to varying heat input, and a tiny quantity of fine Cu-rich phase precipitated from the root of HAZ. The hardness distribution of joint from the base metal (BM) to WZ demonstrated a low to high phenomena, with the average hardness of the BM being 235 HV and the average hardness of WZ being enhanced by 1.16 times. The fine secondary phases precipitated and diffused in the WZ, therefore reinforcing the martensite and improving its mechanical properties. The average impact absorption energy of the weld zone was 43.89 J, and the average impact energy of the HAZ was 54.57 J, both of which were sufficient to fulfill the actual standard (>31 J). The fracture morphology revealed a complex fracture mechanism with dimples and cleavages on the surface of joint, and the toughness was positive. The causes of the decrease in hardness and the rise in toughness might be the Cu-rich phase precipitated from the root of HAZ. Furthermore, the corrosion resistance of the welded joint (WZ: Icorr = 10.542 μA cm−2, HAZ: Icorr = 11.241 μA cm−2) was lower than that of the BM (Icorr = 9.4844 μA cm−2).

Embrittlement mechanism of 460 MPa-Grade nuclear power steel deposited after heat treatment

The effects of post-welded heat treatment on microstructure evolution and embrittlement mechanism of 460 MPa-grade nuclear power steel deposited metal were studied using optical microscope, scanning electron microscope, transmission electron microscope and electron backscattered diffraction (EBSD). The results showed that there was no significant change in the type of deposited metal microstructure before and after heat treatment, except for some acicular ferrite turning into block ferrite. Further studies showed that the dispersive martensite-austenite (M-A) constituent did not worsen the toughness of the deposited metal. However, after heat treatment, the M-A constituent decomposed into ferrite and carbides, where the carbides were precipitated on the grain boundary, resulting in grain boundary embrittlement. The precipitated carbides also increased the dislocation tangle on the grain boundary, which was prone to causing stress concentration and induced crack initiation, resulting in the cleavage fracture. The analysis of the orientation characteristics of the impact fracture profile via EBSD showed that the ferrite laths were coarsened and the high-angle grain boundaries were reduced after heat treatment. Meanwhile, the plastic crack propagation was terminated earlier resulting in the early initiation of cleavage fracture. Moreover, the crack propagation path was smooth and secondary cracks formed during the propagation process. Finally, after heat treatment the inclusions easily fell off and formed micro-voids, which provided favorable channels for crack propagation and accelerated the deterioration of impact toughness. All of these were factors for the significant decline in the toughness of the heat-treated deposited metal.

Cleavage fracture micromechanisms in simulated heat affected zones of S690 high strength steels

High strength steels are widely used for structural applications, where a combination of excellent strength and ductile-to-brittle transition (DBT) properties are required. However, such a combination of high strength and toughness can be deteriorated in the heat affected zone (HAZ) after welding. This work aims to develop a relationship between microstructure and cleavage fracture in the most brittle areas of welded S690 high strength structures: coarse-grained and intercritically reheated coarse-grained HAZ (CGHAZ and ICCGHAZ). Gleeble thermal simulations were performed to generate three microstructures: CGHAZ and ICCGHAZ at 750 and 800 °C intercritical peak temperatures. Their microstructures were characterised, and the tensile and fracture properties were investigated at − 40 °C, where cleavage is dominant. Results show that despite the larger area fraction of martensite-austenite (M-A) constituents in ICCGHAZ 750 °C, the CGHAZ is the zone with the lowest fracture toughness. Although M-A constituents are responsible for triggering fracture, their small size (less than 1 μm) results in local stress that is insufficient for fracture. Crack propagation is found to be the crucial fracture step. Consequently, the harder auto-tempered matrix of CGHAZ leads to the lowest fracture toughness. The main crack propagates transgranularly, along {100} and {110} planes, and neither the necklace structure at prior austenite grain boundaries of ICCGHAZs nor M-A constituents are observed as preferential sites for crack growth. The fracture profile shows that prior austenite grain boundaries and other high-angle grain boundaries (e.g., packet and block) with different neighbouring Bain axes can effectively divert the cleavage crack. Moreover, M − A constituents with internal sub-structures, which have high kernel average misorientation and high-angle boundaries, are observed to deflect and arrest the secondary cracks. As a result, multiple pop-ins in load-displacement curves during bending tests are observed for the investigated HAZs.

Promotion of thermomechanical processing of 2-GPa low-alloyed ultrahigh-strength steel and physically based modelling of the deformation behaviour

A low-alloyed ultrahigh-strength steel comprising CrNiMoWMnV was designed based on thermodynamic calculations and by controlling the microalloying elements to promote various strengthening mechanisms upon processing. The hot deformation behaviour and mechanism were correlated with the processing parameters, that is, strain rate and temperature. The fine features of the deformed microstructures were analysed using electron backscatter diffraction (EBSD) and MATLAB software, combined with the MTEX texture and crystallographic analysis toolbox. The flow stress behaviour at high temperatures was modelled using the dislocation density-based Bergström's model, which could be applied up to the peak strain. However, the diffusional transformation (i.e. recrystallisation)-based Kolmogorov–Johnson–Mehl–Avrami model has been applied to fit the flow stress over a wide deformation strain. The effective grain size (EGS) of martensite and prior austenite grain size (PAGS) were correlated with the deformation temperature and strain rate. Because the PAGS was significantly refined from 16 μm in the initial microstructure to 6 μm after processing at 850 °C/0.01 s−1, the corresponding martensite EGSs were 1.38 and 1.01 μm, respectively. Therefore, these fine-controlled characteristics of the processed microstructures at high temperatures help to enhance the mechanical properties, such as the strength and toughness, of the designed ultrahigh-strength steel.

Microstructural Heterogeneity and Mechanical Properties of a Welded Joint of an Austenitic Stainless Steel

This research presents the microstructural and mechanical evolution throughout the welded seam of an austenitic stainless steel (ASS) tube. It was found that the main hardness decrement occurred in the fusion zone (FZ), followed by the heat-affected zone (HAZ) and the base material (BM). Optical microscopy indicated a dendritic structure in FZ and heterogeneous austenitic grain size from the HAZ towards the BM, ranging from 100 µm to 10 µm. The welding process generated an intense texture around the FZ and the HAZ, while the BM still showed an extrusion-like texture. In terms of mechanical behavior, the largest austenite grain size in the FZ led to the lowest strength and ductility of all zones due to the earliest strain localization manifested by heterogeneous strain distribution. However, the strain localization in all zones appeared after 0.4 true strain, indicating an overall good ductility of the seam. These high values were related to two microstructure characteristics: (1) the 10% δ-ferrite after solidification in the FZ favored by the Creq/Nieq=1.67 relationship that delayed the crack propagation along the austenite grains and (2) the heterogeneous microstructure made up of soft austenite and hard martensite in the HAZ and BM producing multiple strain concentrations. Kernel Average Misorientation (KAM) maps obtained by Electron Back-Scattering Diffraction (EBSD) allowed observing higher internal misorientations in the FZ than in the HAZ due to interconnected walls between the δ-ferrite grains. However, the largest KAM values were observed in the BM between γ-austenite and the deformation-induced α’-martensite phases. X-ray diffraction revealed that the residual stresses in the cross-section of the welded seam were compression-type and then switched to tension-type in the outer surface.

Effect of microstructure on the mechanical properties and corrosion resistance of a welded joint of 620-grade marine steel

The relationship between the microstructure and the mechanical and corrosion properties of a welded joint of 620-grade marine steel was studied using metallographic microscopy, scanning electron microscopy, an energy dispersive spectrometer, transmission electron microscopy, and microhardness and tensile tests. The results showed that the strength and hardness of the weld center area (WMmid) were higher than those of the inner and outer welding surface region (WMin and WMout) because the volume fraction of the martensite-austenite (MA) constituents (21.6%) was higher than that in WMin and WMout (18.0% and 14.3%, respectively). There were numerous MnO-Al2O3-SiO2-TiO2-type inclusions located at the bottom of dimples in the fracture surface; however, the MA constituents took precedence over this kind of inclusion in inducing pitting corrosion. In contrast, pitting corrosion can be initiated by Al2O3-MgO-CaO-CaS inclusions in the heat-affected zone (HAZ) and base metal (BM). The corrosion resistance of the welded joint was in the order of weld metal &amp;gt; HAZ &amp;gt; BM. The WMmid with smaller dendrite spacing and a larger size of MA constituents had better corrosion resistance compared with the WMin and WMout. The corrosion resistance of the HAZ decreased in the sequence of coarse grain HAZ, fine grain HAZ, and intercritical HAZ.

Study of Tensile Deformation and Damage Law in Undermatching X80 Pipeline Steel Welded Joints

This study used a digital imaging technique (DIC) to obtain the strain distribution at various locations in undermatching X80 pipe girth-weld joints under uniaxial tensile loading. In addition, the microstructure characteristics and deformation patterns in different regions were analyzed by scanning electron microscopy (SEM). The results showed that there was strain heterogeneity between the various regions of the welded joint. Strain concentration existed only in the 12.8 mm base metal heat-affected zone (HAZ) and only in the elastic deformation stage. There was strain concentration in the weld metal (WM) and both sides of the HAZ close to the near-fracture stage, and the maximum deformation was in the WM. When εM = 12.2%, the KC was 6.27 and the KF was 1.73, and the KF was 113% and 152% of the KC and the KG, respectively. The large number of slip strips generated indicated serious damage in the WM near the fracture stage. In the elastic deformation stage, the strain concentration of the N1 HAZ was caused by the softened ferrite. The maximum deformation of the WM near the fracture stage was caused by the large grain size and the non-uniform martensite–austenite (M–A) islands, which may also lead to better local toughness of the cover weld and further affect the fracture mechanism of the welded joint.