Austenite Domains Research Articles

Annealing behavior of a 304L stainless steel processed by large strain cold and warm rolling

A 304L-type austenitic stainless steel was subjected to plate rolling at ambient temperature and at 573K to total strains of 3 and then annealed at temperatures of 873, 973 and 1073K. The structural changes during annealing were associated with the austenite reversal (for the cold rolled samples), recrystallization and grain growth, which depended significantly on the annealing temperature. The grain growth exponent of 4 and 5 was obtained after annealing at 973K/1073K for the cold and warm rolled samples, whereas very sluggish grain coarsening took place at 873K. The texture evolution during annealing in the austenite domain was characterized by gradual weakness of cold/warm rolled textures, although the main texture components such as Brass, {110}<112>, and S, {123}<634>, remained in the annealed samples irrespective of annealing mechanisms of microstructure evolution. The grain coarsening during annealing was accompanied by gradual softening. The yield strength of ultrafine grained steel processed by cold/warm rolling followed by annealing could be expressed by a Hall-Petch type relationship with σ0=160MPa and ky=470MPam0.5.

Modelling approach to support design of new high nitrogen austenitic steel for wind turbines components

The alloy design of a new high nitrogen austenitic steel for bearing used in wind turbine gearboxes has been approached. The alloy design was based on a modelling approach to the different aspects of manufacturing routes. The mechanical strength was predicted by means of empirical models. Nitrogen solubility in liquid steel was estimated on the basis of Sievert’s law and Thermo-Calc calculations. Scheil calculations and models developed by Centro Sviluppo Materiali SpA (CSM) were used for estimating nitrogen solubility. The stability of austenite has been studied by thermodynamic calculations. The results show that amounts of at least N = 0.5% are required to fall in austenite domain approaching the required mechanical strength. Vacuum induction melting + pressure electroslag remelting is required to manufacture alloys with N ⩾ 0.6%, but the addition of elements as Mn can potentially allow to increase N amounts achievable with only vacuum induction melting route processing.

Combining gradient structure and TRIP effect to produce austenite stainless steel with high strength and ductility

We report a design strategy to combine the benefits from both gradient structure and transformation-induced plasticity (TRIP). The resultant TRIP-gradient steel takes advantage of both mechanisms, allowing strain hardening to last to a larger plastic strain. 304 stainless steel sheets were treated by surface mechanical attrition to synthesize gradient structure with a central coarse-grained layer sandwiched between two grain-size gradient layers. The gradient layer is composed of submicron-sized parallelepiped austenite domains separated by intersecting ε-martensite plates, with increasing domain size along the depth. Significant microhardness heterogeneity exists not only macroscopically between the soft coarse-grained core and the hard gradient layers, but also microscopically between the austenite domain and ε-martensite walls. During tensile testing, the gradient structure causes strain partitioning, which evolves with applied strain, and lasts to large strains. The γ → α′ martensitic transformation is triggered successively with an increase of the applied strain and flow stress. Importantly, the gradient structure prolongs the TRIP effect to large plastic strains. As a result, the gradient structure in the 304 stainless steel provides a new route towards a good combination of high strength and ductility, via the co-operation of both the dynamic strain partitioning and TRIP effect.

Investigation of the relationships between mechanical properties and microstructure in a Fe-9%Cr ODS steel

Ferritic-martensitic Oxide Dispersion Strengthened (ODS) steels are potential materials for fuel pin cladding in Sodium Fast Reactor (SFR) and their optimisation is essential for future industrial applications. In this paper, a feasibility study concerning the generation of tensile specimens using a quenching dilatometer is presented. The ODS steel investigated contains 9%Cr and exhibits a phase transformation between ferrite and austenite around 870 °C. The purpose was to generate different microstructures and to evaluate their tensile properties. Specimens were machined from a cladding tube and underwent controlled heat treatments inside the dilatometer. The microstructures were observed using Electron Backscatter Diffraction (EBSD) and tensile tests were performed at room temperature and at 650 °C. Results show that a tempered martensitic structure is the optimum state for tensile loading at room temperature. At 650 °C, the strengthening mechanisms that are involved differ and the microstructures exhibit more similar yield strengths. It also appeared that decarburisation during heat treatment in the dilatometer induces a decrease in the mechanical properties and heterogeneities in the dual-phase microstructure. This has been addressed by proposing a treatment with a much shorter time in the austenitic domain. Thereafter, the relaxation of macroscopic residual stresses inside the tube during the heat treatment was evaluated. They appear to decrease linearly with increasing temperature and the phase transformation has a limited effect on the relaxation.

A Study of Free Recovery in a Fe – Mn – Si – Cr Shape Memory Alloy

Video recording of the free recovery of “hot shape” (typical for the austenitic domain) in shape-memory alloy Fe – 28% Mn – 6% Si – 5% Cr during heating of specimens with a “cold shape” typical for the martensitic domain is performed. Prior to each measurement the specimens are deformed by caliber bending at room temperature in martensitic condition. The thermomechanical training consists in 10 cycles of bending – heating – cooling. Displacements of the free ends of the specimens are plotted as a function of the temperature and the plots are used to determine the critical temperatures of the reverse martensitic transformation.

Smart Alloys for Automotive Bumpers

Bumper beam absorbs the accidental kinetic energy by deflection in low-speed impact and by deformation in high-speed impact. Based on the last years necessity of lighter materials and safer usage of vehicles we try to come with a new class of materials for bumper systems. Analyze of metallic materials is cheaper when the analyze take place on a computer avoiding the metallic loss or energy consume. We present few results obtained in Catia software about the behavior of some metallic materials under external solicitations in function of the mechanical properties of metallic elements, geometry of the element, restrains and solicitation points. Shape memory alloy are smart materials that can use the external mechanical energy damping to thermal energy in bumper applications. In martensite to austenite domain we observe an increase of damping mechanical capacity with possible applications in bumper systems.

Contribution on the influence of steel ladle processing (LF) upon the nitrogen removal rate

Nitrogen, which is present in the composition of steel either dissolved or as a gas, represents an element, which is generally unwanted, except for the cases when the aim is to obtain nitrides or to increase the austenitic domain in stainless steels. The paper shows the results obtained in increasing the nitrogen removal rate during the secondary treatment of steel meant for oil industry pipes, into a Ladle Furnace-type installation. The processed data allowed the determination of variation domains, respectively graphical and analytical correlations between the nitrogen removal rate and the parameters of the secondary treatment process (bubbling duration, steel temperature and argon pressure).

Plastic deformation of NiTi shape memory alloys

Dislocation slip in B2 NiTi is studied with atomistic simulations in conjunction with transmission electron microscopy (TEM). The atomistic simulations examine the generalized stacking fault energy (GSFE) curves for the {011}, {2¯11} and {001} planes. The slip directions considered are 〈100〉, 〈111〉 and 〈011〉. The results show the smallest energy barriers for the (011)[100] case, which is consistent with the experimental observations of dislocation slip reported in this study. To our knowledge, slip on the (011)[11¯1] system is illustrated for the first time in our TEM findings, and atomistic simulations confirm that this system has the second lowest energy barrier. Specimens that underwent thermal cycling and pseudoelasticity show dislocation slip primarily in the austenite domains while the bulk of martensite domains does not display dislocations. The results are discussed via calculation of the ideal slip nucleation stress levels for the five potential slip systems in austenite.

Corrosion behavior of a duplex stainless steel under cyclic loading: a scanning Kelvin probe force microscopy (SKPFM) based microscopic study

Corrosion fatigue behavior of the duplex stainless steel AISI 2205 was studied by the scanning Kelvin probe force microscopy (SKPFM) in the ambient temperature and pressure condition (25 °C, one standard atmospheric pressure). Surface-polished AISI 2205 samples were subject to a cyclic tensile stress while immersed in a 0.5 M NaCl solution. At specified time intervals, surface Volta potential of the samples was measured using the SKPFM in an oxygen and water regulated environment. The steel samples demonstrated a map of potentials with high contrasts between ferritic and austenitic grain domains, which was then linked to the actual corrosion potential (w.r.t. a saturated calomel electrode—SCE) based on a rigorous calibration procedure. The corrosion fatigue behavior of AISI 2205 was studied by comparing the SKPFM-measured Volta potentials of the same sample under applied cyclic strain and under no applied cyclic strain. It was found that AISI 2205 became more prone to corrosion when the applied tensile strain exceeds one percent (1 %).

Local characterization of austenite and ferrite phases in duplex stainless steel using MFM and nanoindentation

Abstract

Three-dimensional shear-strain patterns induced by high-pressure torsion and their impact on hardness evolution

The shear strain imposed on austenite/ferrite duplex stainless steel discs at different stages of high-pressure torsion (HPT) processing was imaged in plan-view and cross-section using optical microscopy and scanning electron microscopy. The effect of the shear strain was correlated to the hardness evolution of the discs. The shear-strain patterns are complex and are different on the top and bottom surfaces of the discs. A double-swirl pattern emerged on the top surface in the early stages of HPT. These two centres of the swirl moved towards the centre of the disc as the numbers of HPT revolutions was increased and ultimately the double-swirl evolved into a single-swirl. Less regular shear-strain patterns were observed on the bottom surfaces of the discs. Multiple ring-like patterns with mirror symmetry over the central axes of the discs were visible from cross-sectional observations. Nanoindentation testing on the two surfaces and a cross-section of HPT discs showed that the hardness is insensitive to specific shear-strain patterns, but is closely related to the widths of the austenite and ferrite phase domains. Late in the deformation process, the hardness in the interior of an HPT disc may be higher than at either of the disc surfaces because of the development of finer microstructural phase distributions.

Hot Deformation and Ductility Analysis of Continuous Cast C40 Steel by Means of Tensile and Compression Tests

The steel production from scrap using continuous cast technology has increased in last decades. Sometimes, steels processed via this route display poor ductility at high temperature. This feature is associated to cooling conditions and chemical composition, which in turns affect the segregation pattern and vary the transformation temperatures and the phase transformation kinetics. The material under study was a C40 steel with a dendrite solidification microstructure coming from an industrial continuous casting plant. The high temperature ductility was evaluated by means of tensile tests up to fracture at strain rate of 0.001 s-1 in a temperature range of 1100 to 710°C. The reduction in area at fracture as a function of temperature graphs show a clear reduction of the steel ductility in the intercritical region, but also after the pearlite transformation. Single deformation compression tests were also carried out on the steel in the austenitic temperature domain, 900 to 1100°C, and at strain rate of 0.001 to 1 s-1. A modification of the Garofalo hyperbolic sine equation has been employed to derive the peak and steady stresses of the flow curve. The work hardening, U, and dynamic recovery, Ω, parameters which describe the flow curve before dynamic recrystallization takes place and the k and t50 parameters, based on the JMAK model, to describe the recrystallization kinetics were also calculated for every test and expressed as a function of the Zener Hollomon parameter, Z.

Pearlite in hypoeutectoid iron–nitrogen binary alloys

In this study, hypoeutectoid Fe–N binary specimens have been prepared by gas nitriding pure iron in austenite domain at 840 °C. The slow cooling of these specimens led to the α-ferrite + γ′-Fe4N pearlitic microstructure which is similar to the pearlite in Fe–C binary system. This pearlitic microstructure has been characterized by electron microscopy. The crystal structure of the γ′-Fe4N nitride has been identified by electron microdiffraction and the Nishiyama–Wassermann (N–W) and near Kurdjumov–Sachs (K–S) orientation relationships have been found between the α-ferrite and the γ′-Fe4N.

Micro-thermomechanical constitutive model of transformation induced plasticity and its application on armour steel

Based on the crystallographic theory of martensitic transformation and internal variable constitutive theory, a micromechanical constitutive model of martensitic transformation induced plasticity was developed. Plastic strains of product and parent phases as well as the volume fraction of each martensitic variant were considered as internal variables describing the microstructure evolution. The plasticity flow both in austenite and martensitic variants domain is described by J 2 flow theory. The thermodynamic driving force acting on these internal variables was obtained through the determination of the intrinsic dissipation due to plastic flow and the growth of martensitic domains. The evolution laws of the internal variables are derived, furthermore macroscopic response due to the change of internal variables is obtained. Thermomechanical behavior of armour steel under uniaxial loading was tested which showed a good agreement with experimental results.

Finite element simulations of martensitic phase transitions and microstructures based on a strain softening model

A new approach for modeling multivariant martensitic phase transitions (PT) and martensitic microstructure (MM) in elastic materials is proposed. It is based on a thermomechanical model for PT that includes strain softening and the corresponding strain localization during PT. Mesh sensitivity in numerical simulations is avoided by using rate-dependent constitutive equations in the model. Due to strain softening, a microstructure comprised of pure martensitic and austenitic domains separated by narrow transition zones is obtained as the solution of the corresponding boundary value problem. In contrast to Landau–Ginzburg models, which are limited in practice to nanoscale specimens, this new phase field model is valid for scales greater than 100 nm and without upper bound. A finite element algorithm for the solution of elastic problems with multivariant martensitic PT is developed and implemented into the software ABAQUS. Simulated microstructures in elastic single crystals and polycrystals under uniaxial loading are in qualitative agreement with those observed experimentally.

Magnetocaloric effect and magnetization in a Ni–Mn–Ga Heusler alloy in the vicinity of magnetostructural transition

The magnetic and thermodynamic properties of a Ni2.19Mn0.81Ga alloy with coupled magnetic and structural (martensitic) phase transitions were studied experimentally and theoretically. The magnetocaloric effect was measured by a direct method in magnetic fields 0-26 kOe at temperatures close to the magnetostructural transition temperature. For theoretical description of the alloy properties near the magnetostructural transition a statistical model is suggested, that takes into account the coexistence of martensite and austenite domains in the vicinity of martensite transformation point.

Effect of competing hardening and softening mechanisms on the flow stress curve modeling of ultra-low carbon steel at high temperatures

An ultra-low carbon steel was deformed in torsion over a temperature range of 650–1100 °C and diagrams of the work-hardening rate vs. applied stress were drawn up for the hardening region of the flow stress curves. The work-hardening rate decreased continuously as the applied stress increased in the austenite domain, displaying an inflexion point near the peak stress while, in the ferrite region, where the work-hardening rate decreased linearly as the applied stress increased, no inflexion point was observed. These data were compared with one-parameter models, in which the total dislocation density was selected as the determining microstructural characteristic. The difference in the expected microstructural evolution is discussed and equations are given for the flow stress curve modeling for both phases.

Shape memory behavior of FeNiCoTi single and polycrystals

We present experimental and theoretical evidence of thermoelastic martensites in Fe29Ni18Co4Ti alloys. In this class of alloys, the high strength in the austenite domains limits the slip deformation as verified with transmission electron microscopy. The restriction of slip permits a higher degree of recoverability of the transformation. Using both single crystals with [123] orientation and polycrystals, the appearance of martensite plates upon deformation, and their reversion back to austenite upon heating (the shape memory effect), is revealed with in-situ optical microscopy. Theoretical results for the transformation strains and the detwinning of martensite are presented, which demonstrate convincingly the potential of these classes of alloys. Electrical resistance measurements identified the stress and temperature levels at the onset of forward and reverse transformations in isothermal deformation and thermal cycling experiments, respectively. The return of the electrical resistance to its reference value, upon austenite to martensite followed by martensite to austenite transformation, verified the recovery in the transformation strains measured in the experiments.

Strain–temperature behavior of NiTiCu shape memory single crystals

Single crystal specimens of NiTi10Cu alloys were subjected to temperature cycling conditions under constant tensile and compressive stresses and the transformation strains were monitored. The [111] orientation exhibited the highest experimental transformation strains (6.64%) in tension while the [001] provides the highest transformation strains in compression (5.34%). These transformation strain levels are significantly higher than previously reported values on NiTiCu alloys. The theoretical treatment includes both the calculation of the CVP (correspondent variant pair) formation strain incorporating the growth of monoclinic phase from the most favorably oriented orthorhombic variant, and the concomitant detwinning of the monoclinic martensite. The experimental transformation strain values are consistently below the theoretical levels due to two main reasons: the slip deformation in the austenite domains as confirmed with TEM studies, and the incomplete transformation resulting in a mixture of orthorhombic and monoclinic phases as determined from diffraction patterns. The experimental transformation strains are higher in tension compared to compression for most single crystal orientations due to two factors: the additional strain associated with the detwinning of the B19′phase in the final microstructure (such as in [111] case), and the partial completion of the second step of the transformation limiting the compression strains.

Cyclic deformation behavior of single crystal NiTi

Single crystals of NiTi (with 50.8 at.% Ni) were subjected to cyclic loading conditions at room temperature which is above the M s (martensite start) temperature of −30°C. The single crystals exhibited remarkable cyclic hardening under zero to compression strain control experiments. The stress range under strain control increased by as much as a factor of 3 in compression. The increase in stress range is primarily due to the increasing strain hardening modulus. In the tension case, loop shape changes occurred but the increase in stress range is rather small. The fatigue cycling was undertaken with a strain range of 3% which is far below the theoretical transformation strains levels exceeding 6%. The maximum stress levels reached in the experiments are below those that cause martensite slip. Therefore, the stress–strain response is governed by transformation from the austenite to the martensitic phases and the dislocation structure evolution in the austenite domains. Two single crystal orientations [148] and [112] were examined during the experiments with single and double CVP (correspondent variant pair) formations respectively. The strain hardening in compression cases is rather substantial with the stress range in the double CVP case surpassing the single CVP case. Two heat treatments were selected to produce coherent and incoherent precipitates in the microstructure respectively. The influence of the coherent precipitates on the stress–strain response is significant as they lower the transformation stress from austenite to martensite, and at the same time, they raise the flow stress of the austenite and martensite domains leading to higher saturation stresses in fatigue.