Austenite Films Research Articles

Novel Mn and Si alloyed complex phase steel for automotive applications

Automotive industries require materials with higher strength, plasticity and crashworthiness, aiming for 3rd generation of advanced high strength steels (AHSS) with medium Mn, Si and minor alloying elements. In this investigation, 3rd generation AHSS were synthesized using a vacuum arc melting furnace with manganese (Mn) and silicon (Si) as major alloying elements as well as minor additions such as Cr, Al, Ni, etc. The steels thus developed were characterized using FE-SEM, XRD, microhardness tester, universal testing machine (UTM) and 3-dimensional Atom Probe Tomography (APT). The alloy steels after casting and homogenization were subjected to hot rolling at 1100 °C, with rolling exit temperature 900 ± 50 °C. The FE-SEM micrographs in SE mode revealed a complex phase microstructure with martensite, ferrite, bainitic ferrite and retained austenite in the specimens obtained after rolling and air cooling. The microhardness of the developed alloys is found in the range of 395–502 VHN in the hot-rolled and air cooled condition of the specimen. The tensile strength of alloys was measured to be in the range of 1412–1614 MPa with elongation of 12.12 %–18.92 %. The analysis of fracture surfaces after tensile tests for developed alloys revealed that Alloy 1 (Fe-4Mn-1.5Si) had dimples indicating ductile fracture while Alloy 2 (Fe-6Mn-1.5Si) has a mixture of dimples and facets, and Alloy 3 (Fe-8Mn-1.5Si) has lower dimples but larger facets, confirming quasi-ductile fracture with a lower ductility limit of 12.12 %. Atom Probe Tomography was performed to study the complex phase structure at nanoscale through re-distribution of carbon and other alloying elements. 3D APT revealed the presence of very fine retained austenite film of thickness ∼4–5 nm and carbon content of 6–8 at.%. This complex phase microstructure obtained after hot rolling and normalizing is made of very fine bainitic ferrite with film type retained austenite providing TRIP effect, in addition martensite and ferrite resulting in high strength.

Microstructure and property evolution during tempering of a medium carbon bainitic steel

Tempering a medium carbon carbide-free bainite steel led to two distinct microstructures and properties below and above the bainitic start (Bs) temperature. Austenite decomposition during tempering, depended on carbon content, tempering temperatures and time, determining retained austenite (RA) stability. Up to 400 °C, most RA remained intact, but a significant reduction occurred above the Bs temperature. Film austenite primarily turned into carbides within the bainitic–ferrite matrix and form lines of carbide particles, while blocky austenite decomposed into troostite at 500 °C and sorbite at 600 °C. During tempering, on the other hand, bainitic ferrite lost tetragonality losing its carbon and resulting in the growth of bainitic–ferrite plates through grain boundary migration and misorientation annihilation. Above Bs, drastic microstructural changes caused significant alterations in tensile properties.

Effect of ageing temperature on the microstructural evolution and mechanical properties in M2C and NiAl co-precipitation secondary hardening ultrahigh-strength steel

A novel cost-effective secondary hardening ultrahigh-strength steel, strengthened by the co-precipitation of M2C and NiAl, was developed. The present study systematically investigated the co-precipitation evolution, austenite reversion, and mechanisms of strengthening and toughening at different ageing temperatures. As the ageing temperature increased, the quenched microstructure underwent various transformations, including precipitation and dissolution of cementite, nucleation and coarsening of M2C and NiAl nanoparticles, as well as reversion of austenite. At ageing temperatures of 427 °C and 454 °C, a small amount of cementite was present in the martensite matrix, and M2C and NiAl precipitates started to form, resulting in low strength and poor toughness. At 482 °C, the peak-ageing stage was reached, achieving a maximum yield strength of 1875 ± 4 MPa and an ultimate tensile strength of 2185 ± 3 MPa. This strength is primarily attributed to shear strengthening of NiAl particles and Orowan strengthening of M2C carbides. Additionally, the complete dissolution of cementite at this stage improved toughness. As the ageing temperature increased to 510 °C and 538 °C, the strength decreased due to coarsening of M2C, while the toughness improved as a result of film austenite reversion and enhanced dislocations bypassing around the M2C carbides.

Tailoring friction stir welding and postheat treatment of high-carbon steels to obtain nanostructured bainite

Conventional welding processes destroy the unique microstructural characteristics in nanobainitic steels. A novel method, i.e. tailored friction stir welding (FSW) and postheat treatment, was employed to overcome this problem. The postprocessing austempering was carried out on friction stir welded high-carbon steel to obtain nanobainite, and obtained microstructural characteristics were compared to that achieved by simple isothermal bainite transformation. Bainitic sheaves and austenite microblocks could be achieved in both production procedures. However, bainitic ferrites and austenite films within the bainitic sheaves were thinner, and austenite microblocks were finer when implementing the post-processing heat treatment after FSW. FSW enhanced the hardness of the primary steel from 385 HV to above 800 HV, which was maintained at about 520 HV even after post-heat treatment, higher than that of after implementing the isothermal bainite transformation (490 HV). This achievement can be related to FSW that reduced the mean diameter of austenite blocks, the thickness of bainitic ferrite, and the thickness of austenite films from 3820, 138, and 122 nm to 2200, 92, and 78 nm, respectively. Moreover, the microstructure and hardness of various microstructural zones of friction stir welded sample before and after heat-treatment were analyzed and discussed in detail. The outcome of this study can be applied in academic and industrial areas to propose a cost-effective method to weld nanobainitic steels.

Exploring the Influence of the Deposition Parameters on the Properties of NiTi Shape Memory Alloy Films with High Nickel Content

NiTi shape memory alloy films were prepared by magnetron sputtering using a compound NiTi target and varying deposition parameters, such as power density, pressure, and deposition time. To promote crystallization, the films were heat treated at a temperature of 400 °C for 1 h. For the characterization, scanning electron microscopy, energy dispersive X-ray spectroscopy, atomic force microscopy, synchrotron X-ray diffraction, and nanoindentation techniques were used on both as-deposited and heat-treated films. Apart from the morphology and hardness of the as-deposited films that depend on the deposition pressure, the power applied to the target and the deposition pressure did not seem to significantly influence the characteristics of the NiTi films studied. After heat treatment, austenitic (B2) crystalline superelastic films with exceptionally high nickel content (~60 at.%) and vein-line cross-section morphology were produced. The crystallization of the films resulted in an increase in hardness, Young’s modulus, and elastic recovery.

Sliding wear behaviour of austempered ductile iron, boron steel and AISI 1045 steel of similar hardness: effect of microstructure, yield strength, and strain hardening

ABSTRACT A study on dry sliding wear investigated four alloys: fully austenitized ADI (F-ADI), intercritically austenitized austempered ductile iron (I-ADI), hardened and tempered Boron steel (B-Steel), and hardened and tempered AISI 1045 steel (EN-Steel). Despite their similar hardness levels, these alloys differ in composition and microstructure. Testing was conducted using a Ball-On-Disc setup (ASTM G99-05) with a chrome steel ball, maintaining a constant sliding speed of 0.6 m/s and applying loads between 20 and 80 N. This study aimed to understand the influence of yield strength (σy ), strain hardening coefficient (n), and microstructure on sliding wear under uniform hardness conditions. The findings indicated a strong connection between sliding wear resistance, strain hardening coefficient, toughness, and yield strength. Wear mechanisms varied slightly among materials; F-ADI and I-ADI exhibited deformed layers with increased hardness and oxide formation, while B-Steel and EN-Steel showed micro-cutting and micro-ploughing. F-ADI's high-carbon austenite film and I-ADI's proeutectoid ferrite contributed to improved wear performance. The hardness of the deformation layer on FADI and I-ADI's worn surfaces highlighted the benefits of strain hardening in dry sliding wear.

Constrained incipient phase transformation in Ni-Mn-Ga films: A small-scale design challenge

Ni-Mn-Ga shape-memory alloys are promising candidates for large strain actuation and magnetocaloric cooling devices. In view of potential small-scale applications, we probe here nanomechanically the stress-induced austenite–martensite transition in single crystalline austenitic thin films as a function of temperature. In 0.5 µm thin films, a marked incipient phase transformation to martensite is observed during nanoindentation, leaving behind pockets of residual martensite after unloading. These nanomechanical instabilities occur irrespective of deformation rate and temperature, are Weibull distributed, and reveal large spatial variations in transformation stress. In contrast, at a larger film thickness of 2 μm fully reversible transformations occur, and mechanical loading remains entirely smooth. Ab-initio simulations demonstrate how an in-plane constraint can considerably increase the martensitic transformation stress, explaining the thickness-dependent nanomechanical behavior. These findings for a shape-memory Heusler alloy give insights into how reduced dimensions and constraints can lead to unexpectedly large transformation stresses that need to be considered in small-scale actuation design.

Hydrogen Embrittlement Behavior of a Commercial QP980 Steel

The hydrogen embrittlement (HE) behavior of a commercial QP980 steel is studied in this work. The HE susceptibility results indicate that QP980 suffers from a severe HE, and the fracture mode transforms from ductile dimpling to brittle quasi-cleavage under the attack of hydrogen. The EBSD results show that strain-induced martensite transformation can rarely occur at a strain close to the HE fracture strain, which is mainly attributed to the high mechanical stability of austenite. The TKD-KAM analysis results indicate that hydrogen-induced strain localization in martensite can be mitigated by the hydrogen-trapping effect of surrounding austenite, while it is most pronounced in martensite adjacent to ferrite. Correspondingly, HE cracking is considered to initiate in martensite adjacent to ferrite under the synergistic action of HELP and HEDE mechanisms, and then cracks can propagate through ferrite or along phase interfaces. Our findings suggest that to further improve the HE resistance of QP steel with stable austenite, it is necessary to consider introducing effective hydrogen-trapping sites (such as carbides, film austenite) into martensite, which is deemed to be beneficial for increasing the resistance against hydrogen-induced cracking initiation in martensite.

Microstructural evolution and ultra-high impact toughness of austempered lamellar bainitic steel far below Ms temperature

The present study investigated the effects of the austempering process far below Ms (martensite start temperature) on the microstructural evolution and impact toughness of bainitic steel with ultrahigh impact toughness and compared the results with the tempered martensitic steel. The results revealed that the lamellar bainitic steel composed of bainitic ferrite laths and retained austenite films is obtained by austempering far below Ms temperature. The obtained steel exhibited the Charpy V-notch impact absorbed energy of up to 131 ± 6 J, which was 2.7 times higher than that of the tempered martensitic steel (49 ± 1 J). The ultra-high impact toughness of bainitic steel was attributed to the deflection of the crack propagation direction by the refined bainitic ferrite laths and the passivation of the crack tip by the retained austenite films. Furthermore, the proportion of V2, V3, and V5 variants increased and formed HAGBs (high-angle grain boundaries) with the V1 variant in the austempered bainitic steel B-32. These HAGBs inhibited crack propagation effectively, thereby significantly improving the impact toughness of the austempered bainitic steel.

Re-Austenitisation of Thin Ferrite Films in C–Mn Steels during Thermal Rebound at Continuously Cast Slab Corner Surfaces

The influence of primary cooling and rebound temperature at C–Mn slab corner surfaces during continuous casting on ferrite film transformation and AlN precipitation was investigated. Laboratory simulations included primary cooling to minimum temperature, Tmin, rebounding to various maximum temperatures, Tmax, followed by secondary cooling. The negative effect of a low Tmin on hot ductility could not be readily reversed, even at relatively high temperatures. Quantitative metallography was employed to study the evolution of the microstructure during rebounding and secondary cooling. Following primary cooling to temperatures just above the Ar3, thin films of allotriomorphic ferrite formed on the austenite grain boundaries. These films did not completely transform to austenite during the rebound at 3 °C/s up to temperatures as high as 1130 °C and persisted during slow secondary cooling up to the simulated straightening operation. Whilst dilatometry did not indicate the presence of ferrite after high rebound temperatures, metallography provided clear evidence of its existence, albeit in very small quantities. Coincident with the ferrite at these high temperatures was the predicted (TC-PRISMA) grain boundary precipitation of AlN in bcc iron during the rebound from a Tmin of 730 °C. Importantly no thin ferrite films were observed, and AlN precipitation was not predicted to occur when Tmin was restricted to 830 °C. Cooling below this temperature promotes austenite grain boundary ferrite films and AlN precipitation, which both increase the risk of corner cracking in C–Mn steels.

Carbon segregation and cementite precipitation at grain boundaries in quenched and tempered lath martensite

Tempering is widely applied to make carbon atoms beneficially rearrange in high strength steel microstructures after quenching; though the nano-scale interaction of carbon atoms with crystallographic defects is hard to experimentally observe. To improve, we investigate the redistribution of carbon atoms along martensite grain boundaries in a quenched and tempered low carbon steel. We observe the tempering-induced microstructural evolution by in-situ heating in a transmission electron microscope (TEM) and by compositional analysis through atom probe tomography (APT). Probe volumes for APT originate from a single martensite packet but in different tempering conditions, which is achieved via a sequential lift-out with in-between tempering treatments. The complementary use of TEM and APT provides crystallographic as well as chemical information on carbon segregation and subsequent carbide precipitation at martensite grain boundaries. The results show that the amount of carbon segregation to martensite grain boundaries is influenced by the boundary type, e.g. low-angle lath or high-angle block boundaries. Also, the growth behavior of cementite precipitates from grain boundary nucleation sites into neighboring martensite grains differs at low- and high-angle grain boundaries. This is due to the crystallographic constraints arising from the semi-coherent orientation relationship between cementite and adjacent martensite. We also show that slower quenching stabilizes thin retained austenite films between martensite grains because of enhanced carbon segregation during cooling. Finally, we demonstrate the effect of carbon redistribution along martensite grain boundaries on the mechanical properties. Here, we compare micro-scale Vickers hardness results from boundary-containing probe volumes to nanoindentation results from pure bulk martensite (boundary-free) probe volumes.

Effect of ultra-fast heating on microstructure and mechanical properties of cold-rolled low-carbon low-alloy Q&P steels with different austenitizing temperature

We performed ultra-fast heating (150 °C/s) quenching and partitioning (Q&P) processes to a cold-rolled low-carbon low-alloy steel. Adjusting the annealing temperature, we obtained two types of ferrite-containing Q&P steels with different matrix structure, i.e., martensitic matrix and ferritic matrix. The effect of ultra-fast heating on microstructure evolution and mechanical properties was analyzed, compared with the samples treated with conventional heating rate (10 °C/s). We also investigated the strain partition and fracture behaviors, basing on in-situ tensile tests under SEM. Results show that ultra-fast heating can significantly increase the A c1 and A c3 temperatures, and extend the temperature span of intercritical zone. It can also delay the complete recrystallization of deformed ferrite and promote the austenite nucleation, which leads to the structure refinement of the sample with martensitic matrix, improving both the strength and ductility. The increase of austenite nucleation rate and the incomplete recrystallization of deformed ferrite jointly promote the austenite transformation, which results in a distinct increase of martensite islands fraction in the sample with ferritic matrix, leading to its obvious increase of tensile strength and decrease of ductility. Strain partition is an important factor affecting the TRIP effect in ferrite-containing Q&P steels. For the sample with a martensitic matrix, retained austenite mainly possesses a film-like morphology and exists between martensite laths. The relatively high mechanical stability of retained austenite films and low local strain in martensitic regions result in the weak TRIP effect. The retained austenite in the sample with a ferritic matrix mainly distributes among ferrite grains and exhibits a blocky morphology. The relatively low mechanical stability of blocky retained austenite and high local strain in ferrite jointly enhance the TRIP effect. • Ultra-fast heating delays ferrite recrystallization. • Ultra-fast heating restrains martensite tempering. • Ultra-fast heating results in grain refinement of martensitic matrix. • Strain partition is a crucial factor controlling TRIP effect. • Main ductility origin is compatible deformation of matrix structures.

Comparison of Novel Low-Carbon Martensitic Steel to Maraging Steel in Low-Cycle Fatigue Behavior

The study systematically compares the low-cycle fatigue (LCF) behaviors of novel martensitic steel (22MnSi2CrMoNi) and maraging steel (00Ni18Co9Mo4Ti). Results show that the two types of tested steel have a similar cyclic deformation behavior. The cyclic softening resistance of 22MnSi2CrMoNi steel is slightly inferior to that of 00Ni18Co9Mo4Ti steel at a low total strain amplitude. However, the gap gradually disappears with the increase of the total strain amplitude. At the same plastic strain amplitude, the LCF lifetime of 22MnSi2CrMoNi steel is higher than that of 00Ni18Co9Mo4Ti steel. The retained austenite film between martensite lath and the existence of precipitated phase in matrix can effectively improve the fatigue lifetime of the two types of tested steel.

Rationalization of brittleness and anisotropic mechanical properties of H13 steel fabricated by selective laser melting

This work revealed the causes of embrittlement and anisotropy in the mechanical properties of H13 steel fabricated by selective laser melting (SLM). It was found that the low mechanical stability of the carbon-enriched retained γ (austenite) films amongst martensite blocks transferred to high carbon twin martensite through stress-induced martensitic transformation upon plastic deformation, leading to a high susceptibility to cracking. Moreover, owing to the directional solidification nature in SLM, the majority of these retained γ films was aligned with the building direction (i.e. longitudinal direction). This significantly impaired the ductility of the steel when the load was perpendicular to these γ films (i.e. transverse loading condition).

Nano Scaled Checkerboards: A Long Range Ordering in NiCoMnAl Magnetic Shape Memory Alloy Thin Films with Martensitic Intercalations

Magnetic shape memory Heusler alloys, such as NiCoMnAl, are considered as promising candidates for magnetocaloric cooling applications. Grown in thin film systems of adjacent layers with austenite and martensite crystal structures of almost equal thicknesses, a long-range ordering phenomenon in the shape of a 3D checkerboard pattern occurs in NiCoMnAl samples. The crystallographic origin of the pattern is proven by transmission electron microscopy (TEM) techniques. The darker fields of the arrangement consist of martensite nuclei superposed with austenite, while the purely austenite regions appear bright in TEM cross sections. The nucleation process is presumably triggered by inhomogeneous local elastic stray fields of primary martensitic nuclei in the austenite matrix and limited by the thicknesses of the martensite and austenite thin films.

The investigation on Johnson-Cook model and dynamic mechanical behaviors of ultra-high strength steel M54

Ultra-high strength steel M54 has been considered as promising candidates for structural applications in spacecraft and aircraft, because of its excellent static and dynamic characteristics. Numerical simulation is widely accepted as an effective way to help understand the dynamic response to structures under impact loadings. In this study, the Johnson-Cook (J-C) constitutive parameters and failure parameters used in ANSYS were accurately determined by fitting static and dynamic experimental data. Based on the obtained experimental results, the good combination of ultra-high strength and good elongation under quasi-static condition was ascribed to its special chemical composition, the microstructure of ultrafine lath martensite with high dislocation density, ultra-high martensite content, uniform distribution of nano-sized M2C carbides and the existence of reverted austenite films. The dynamic compressive properties at high strain rates were investigated systematically by split Hopkinson pressure bar (SHPB) system. A remarkable strain rate strengthening effect came into evident on flow stress of the steel. The dynamic strengthening mechanisms were considered as fine-grain strengthening, substructure strengthening, and dislocation strengthening. Also, adiabatic shear behaviors of the steel under dynamic compressive tests were investigated. The results demonstrated that the strain rate was a key basis to judge stability of the M54 steel under impact loading. Recrystallization mechanism was responsible for the appearance of fine equiaxed grains within adiabatic shear bands (ASBs). The evolution of the ASBs was simultaneously revealed. The obtained experimental results and established J-C constitutive model could enhance the scientific understanding of dynamic mechanical behaviors of the M54 steel and provide reliable constitutive parameters for better guiding relevant engineering applications.

Effect of nano-bainite microstructure and residual stress on friction properties of M50 bearing steel

By means of the measurement of the residual stress and friction properties, the observation of the microstructure by the SEM, the X-ray diffraction (XRD) was used to phase analysis and the nano-bainite structure was determined by transmission electron microscopy (TEM). The effect of nano-bainite microstructure and residual stress on friction properties of M50 bearing steel was analyzed after austempered at 200 °C for 2 h, 16 h and 32 h. Results show that nano-bainite structure is composed of thin film austenite and bainite ferrite, the mean thickness of bainite ferrite lath is about 90 ± 10 nm. The contents of nano-bainite are 15.772%, 33.951% and 50.332% respectively after austempered for 2 h, 16 h and 32 h. The hardness increases from 2 to 16 h, which is related to the precipitation of secondary carbides in bainite. And the hardness decreases after 16 h due to the fact that the bainite content is the largest. The fluctuation range of the residual stress of the sample austempered for 32 h + tempering is the smallest and the distribution is the most uniform. Wear resistance of the sample gradually increases with the extension of austempering time which is mainly related to hardness, residual stress state and thermal stability.

Retained austenite-aided cyclic plasticity of the quenched 9Ni steel

This study focuses on the low cycle fatigue mechanism of the quenched 9Ni martensitic steel containing retained austenite films at martensite laths. HRTEM and EBSD suggested that extrusions developed along Low Angle Boundaries or along High Angle Boundaries at low and high strain respectively. Short cracks grew along LAB at low strain while they used any kind of boundary as well as crossed the martensite laths at high strain. Retained austenite allowed a flowing of matter along interfaces by acting as a lubricant. Consequently, the Coffin-Manson relation exhibited a bilinearity with a change in curve slope at Δεt = 1%.

Computational analysis of austenite film thickness and C-redistribution in carbide-free bainite

In this work, a methodology for the computational analysis of some essential microstructural features of a bainitic microstructure is developed. The focus lies in the accurate prediction of the ferritic subunit size, the thickness of the residual austenite films, their corresponding C-enrichment and the accompanying stabilization of the residual austenite. Basis of the approach is the T 0-temperature concept in combination with the numerical simulation of C-diffusion profiles utilizing the cell diffusion module of the thermokinetic software package MatCalc. This methodology gives the opportunity to predict the C-distribution under consideration of consecutively forming subunits, which is necessary to estimate the C-content of austenite films. The simulations also take into account the effect of C trapping at the dislocations formed inside the ferritic platelets due to plastic deformation and its influence on the chemical potentials. Good agreement is achieved between measured and predicted retained austenite layer thickness and the C-enrichment of the layers accompanying the C redistribution process.

On the austenite stability of cryogenic Ni steels: microstructural effects: a review

Austenite stability is essentially important in improving the cryogenic toughness of cryogenic Ni steels and guiding the development of Ni-saving cryogenic steels. The austenite stability in the cryogenic Ni steels is influenced by many microstructure features, making it a complicated issue which is lack of a systematic discussion. In this article, the microstructural effects on the thermal and mechanical stability of austenite in the cryogenic Ni steels are reviewed and discussed. The thermal stability of austenite (TSA) will be enhanced by the enrichment of austenite-stabilizing elements in the austenite which decreases the martensite-start (Ms) temperature. The grain refinement enhances the TSA by synergistically increasing the nonchemical driving force for the martensite transformation and the concentrations of austenite-stabilizing elements in the austenite. The excessive increase in the volume fraction of austenite weakens the TSA by decreasing the concentrations of austenite-stabilizing elements in the austenite. The film austenite is usually thermally more stable than the block austenite owing to its higher concentrations of austenite-stabilizing elements. The mechanical stability of austenite (MSA) is also influenced by the concentrations of austenite-stabilizing elements which affect the Ms temperature. The reports on the effect of grain size of austenite on the MSA are inconsistent. Both negligible and important effects of the grain size of austenite on the MSA are analyzed. The grain orientation of austenite affects the MSA via changing the Schmid factor and the additional driving force for the martensite transformation. The orientation which yields a larger value of Schmid factor would exhibit a lower MSA. The MSA is affected by the matrix or the neighboring phase due to the stress and strain partitioning among austenite and other constituent phases. The dislocation multiplication could weaken the MSA by assisting the nucleation and growth of martensite embryo and enhance the MSA by hindering the motion of embryo/austenite interfaces when dislocation density is sufficiently large. Austenite with a combination of a high TSA and a moderate or high MSA is considered to be effective strategies to enhance cryogenic toughness of the cryogenic Ni steels.