Lattice Parameter Of Austenite Research Articles

Lattice Parameter of Austenite in Silicon Cast Irons

Upon solidification, graphitic cast irons undergo a volume change whose amplitude depends on two opposite terms, the contraction associated with austenite formation and the expansion due to graphite crystallization. During cooling after solidification, further precipitation of graphite occurs that continuously changes the physical properties of the material and possibly affects the eutectoid transformation that transforms the matrix from austenitic to ferritic or ferritic-pearlitic. This work intended to study the density of graphitic cast irons at high temperature, i.e., in the temperature range where the matrix is austenitic. High-temperature laboratory X-rays have been carried out on several alloys containing various carbon and silicon contents to characterize the austenite mean lattice parameter. By complementing these results with literature data, a statistical analysis was carried out that expresses the austenite mean lattice parameter as a function of temperature and composition, evidencing the high uncertainty related to the austenite carbon content. Finally, one of the investigated alloys was submitted to a simultaneous dilatometry and X-ray analysis in a synchrotron from room temperature to 1050 °C. The data are used to discuss the austenite lattice parameter prediction and the possibility of density prediction.

Lattice parameters of austenite and martensite during transformation for Fe–18Ni alloy investigated through in-situ neutron diffraction

Martensitic transformation is accompanied by the generation of microscale and macroscale internal stresses during cooling below the martensitic transformation start temperature. These internal stresses have been determined through X-ray or neutron diffraction, but the reported results are not consistent, probably because the measured lattice parameter is influenced not only by the internal stress but also by several factors, including solute elements and crystal defects. Therefore, in-situ neutron diffraction combined with dilatometry measurements during martensitic transformation and subsequent cyclic tempering were performed for an Fe–18Ni alloy. The phase strains calculated by lattice parameter variations show that a hydrostatic compressive strain in austenite and a tensile strain in martensite arose as the martensitic transformation progressed during continuous cooling or isothermal holding. However, the phase stresses of austenite and martensite estimated from these strains failed to hold stress balance law when dense crystal defects involved in the processes. After these crystal defects were removed by appropriate tempering, the stress balance law held well. Meanwhile, the phase stresses of austenite and martensite were changed to opposite, revealing their true identity. Furthermore, the lattice parameter of quenched full martensite and the specimen diameter were both decreased by tempering, demonstrating that there is a direct correlation among lattice parameter, specimen volume and dislocation density. As a result, various crystal defects in austenite and martensite, introduced by plastic accommodation, were suggested to affect their lattice parameters and then their phase stresses.

Evolution of austenite lattice parameter during isothermal transformation in a 0.4 C low alloyed steel

The evolution of internal stress during displacive transformation is a topic of continuous debate. Neutron diffraction was used to study the isothermal bainite transformation in a 0.4 C low alloyed steel from 773 to 623 K to provide a clearer basis for discussion regarding the change in the austenite lattice parameter. According to diffraction profile analysis, fresh bainite possesses a body-centered tetragonal structure, and its c/a ratio decreases rapidly over time. The austenite lattice parameter increases or decreases depending on whether the transformation temperature is above or below the nose of the Time-Temperature-Transformation (TTT) curve. This isothermal transformation behavior can be divided into two categories: above and below the nose of TTT curve, which correspond to the upper and lower bainites, respectively. The internal stresses caused by the transformation strains are relaxed by dislocation motion and vacancy formation. The yielded dislocations and vacancies not only affect the broadening of both austenite and bainitic ferrite diffraction peaks but also the lattice parameter. The first-principles calculations demonstrate that the austenite lattice parameter decreases as the vacancy density increases, which may account for the experimental observation in lower bainite.

A first-principle assisted framework for designing high elastocaloric Ni–Mn-based magnetic shape memory alloy

• An efficient first-principle-based computational method to predict ΔV/V 0 is proposed • The substitution of Ga for In is beneficial to increase ΔV/V 0 and ∆S vib • The substitution of Cu for Mn is beneficial to reduce ∆M and ∆S mag • A large transformation entropy change is obtained of the proposed alloy • The studied alloy exhibits excellent elastocaloric properties at room temperature A large adiabatic temperature change (∆ T ad ) is a prerequisite for the application of elastocaloric refrigeration. Theoretically, a large volume change ratio ( ∆V/V 0 ) during martensitic transformation is favorable to enhance ∆ T ad . However, the design or prediction of ∆V/V 0 in experiments is a complex task because the structure of martensite changes simultaneously when the lattice parameter of austenite is tuned by modifying chemical composition. So far, the solid strategy to tailor ∆V/V 0 is still urgently desirable. In this work, a first-principles-based method was proposed to estimate ΔV/V 0 for Ni–Mn-based alloys. With this method, the substitution of Ga for In is found to be an effective method to increase the value of ΔV/V 0 for Ni–Mn–In alloys. Combined with the strategies of reducing the negative contribution of magnetic entropy change (via the substitution of Cu for Mn) and introducing strong crystallographic texture (through directional solidification), an outstanding elastocaloric prototype alloy of Ni 50 (Mn 28.5 Cu 4.5 )(In 14 Ga 3 ) was fabricated experimentally. At room temperature, a huge ∆ T ad of -19 K and a large specific adiabatic temperature change of 67.8 K/GPa are obtained. The proposed first-principle-assisted framework opens up the possibility of efficiently tailoring ∆V/V 0 to promote the design of advanced elastocaloric refrigerants.

Microstructure Formation and Carbon Partitioning with Austenite Decomposition during Isothermal Heating Process in Fe-Si-Mn-C Steel Monitored by In Situ Time-of-Flight Neutron Diffraction

Retained austenite is a key feature used to realize the transformation-induced plasticity in bainitic high strength steels. In this study, the authors focused on the formation of metastable austenite in Fe-0.61C-1.9Si-0.98Mn (mass%) during isothermal heating processes using in situ neutron diffraction techniques. Quantitative discussion of carbon partitioning processes is enabled by applying an in situ phase fraction analysis considering crystallographic textures, in addition to the carbon concentration estimation based on the lattice parameter of austenite. The carbon partitioning behavior is inhomogeneous, resulting in a bimodal carbon concentration distribution in austenite. The carbon enriched, high carbon austenite is stable during isothermal heating at 673 K and is retained even after cooling to room temperature. The remainder is low carbon austenite, which is gradually consumed by bainite transformation. Above 723 K, the high carbon austenite also decomposes to ferrite and cementite due to the fast diffusion of Si. Conversely, below 623 K, cementite is stabilized even without the diffusion of Si. These cementite formation mechanisms prevent the formation and retention of high carbon austenite. The inhomogeneous carbon distribution and cementite formation must be carefully considered to precisely predict the microstructure formation in Si-added bainitic steels.

Neutron diffraction study of the low-temperature ageing of martensite in dual-phase steel

Abstract Neutron diffraction measurements were used to obtain a better understanding of the ageing behaviour in dual-phase ferrite + martensite steels. Therefore, the influence of different ageing treatments on the tempering of low-alloyed 0.4 and 0.7% C FeMnCrMo-martensite and 0.1% C FeMnCrMo dual-phase steels was investigated. On tempering at 170 and 350 °C, the sequential precipitation of g- and h-carbides, as well as the loss of tetragonality was observed in the martensite. The decrease in austenite lattice parameter after tempering is due to stress relaxation in the material. These observations offer an acceptable explanation for the large yield stress increase after ageing at 170 °C.

Effect of the as-built microstructure on the martensite to austenite transformation in a 18Ni maraging steel after laser-based powder bed fusion

During laser-based powder bed fusion, the non-equilibrium solidification conditions promote local elemental segregation, leading to a characteristic microstructure composed of cellular walls. These walls can display either low carbon BCC martensite or FCC retained austenite crystal structures, thus affecting the subsequent isochronal or isothermal martensite to austenite phase transformation mechanisms. In the present study, the effect of the non-homogeneous as-built microstructure on the martensite-to-austenite reversion phenomena was studied for a 18Ni maraging steel fabricated by laser-based powder bed fusion. In-situ synchrotron X-ray diffraction was used to retrieve the austenite volume fraction and lattice parameter evolution during the physical simulation of continuous heating cycles to the austenitic field; and during isothermal tempering cycles throughout the inter-critical tempered martensite + austenite (α’ + γ) field. The as-built microstructure resulted in the expansion of the inter-critical α’ + γ field during very slow heating rates. This was associated to the synergic effects of compositional segregations (anticipating reversion) and pre-existing retained austenite (delaying solubilization). During conventional inter-critical tempering, the as-built microstructure did not fundamentally alter the austenite reversion kinetics, resulting in similar high temperature microstructures at the end of the isothermal stage relative to the solution treated state.

Carbide-Free Bainite Transformations Above and Below Martensite Start Temperature Investigated by In-Situ High-Energy X-Ray Diffraction

Carbide-free bainitic advanced high-strength steels (AHSS) exhibits multiphase microstructures made of fine ferrite and retained austenite. The carbon partitions between these constituting phases during the bainitic transformation were studied above and below the martensite start temperature (MS) by in-situ high-energy X-ray diffraction experiments. The time-resolved analysis of the phase fraction, austenite lattice parameter, and ferrite tetragonality offer the possibility of establishing phase-scaled consistent and precise carbon balances. In the studied steel, the carbon content in ferrite ranges between 0.1 and 0.15 wt.% after isothermal treatments, meaning that the available carbon for austenite stabilization is reduced. The stasis conditions are close to the T0′ criterion below 400°C but are not reached at higher temperatures. Our multi-step experiments also prove that the transformation below MS is very sensitive to the carbon content in austenite and auto-catalytic nucleation processes inherited from prior transformations (martensite and bainite at higher temperature). The bainitic transformation thus does not follow additivity rules.

Carbon content evolution in austenite during austenitization studied by in situ synchrotron X-ray diffraction of a hypoeutectoid steel

Using in situ high energy X-ray diffraction study of austenite formation in hypoeutectoid steel with three different initial microstructures (ferrite-pearlite, tempered martensite and bainite), the lattice parameters of ferrite, cementite and austenite are examined on heating at 0.25, 10 and 100 °C/s.The lattice parameters of ferrite, cementite and austenite do not vary linearly with the temperature, especially, in the temperature range where the austenitization takes place. For the austenite, it is suggested that the deviation from the linearity is mainly associated to the carbon content variation. Using Dyson and Holmes equation, the carbon content in austenite is evaluated for any moment of the austenite formation for each initial microstructure and all heating rates.For the ferrite-pearlite microstructure heated at 0.25 °C/s, the carbon content in austenite after complete cementite dissolution corresponds to that of pearlite. Moreover, a rapid decrease in carbon content in the austenite is observed during the first stage of the austenitization (simultaneous dissolution of ferrite and cementite) followed by a slow further decrease during the transformation of the remaining ferrite. The obtained results are discussed using thermodynamic calculations.

Influence of carbon concentration and magnetic transition on the austenite lattice parameter of 30Mn-C steel

In this study, the effects of carbon concentration and magnetic transition on the austenite lattice parameter of Fe-30Mn-xC alloys were assessed. A linear relationship was obtained between the carbon content and the lattice parameter determined by neutron diffraction and neutron Bragg-edge transmission imaging for austenitic steels bearing 0.007–1.20 mass% C. The results obtained through both methods showed good agreement. In addition, carbon partitioning was evaluated in artificially piled-up discs with different carbon contents in order to develop a new technique for advanced multi-phase steels. The analysis of asymmetry and broadening of the diffraction line profile revealed that the carbon partitioning could be determined. In addition, the Néel temperature was determined by in situ neutron diffraction upon heating and cooling with magnetic diffraction peaks and the change in the lattice parameter with temperature being monitored. The temperature dependence of the austenite lattice parameter was found to become smaller below the Néel temperature. In conclusion, the results presented herein expand the current knowledge on the properties of high Mn austenite.

Real Time Observation of Martensite Transformation for a 0.4C Low Alloyed Steel by Neutron Diffraction

A high-intensity and high-resolution neutron diffractometer with a thermomechanically controlled processing simulator was employed in-situ to investigate martensite transformation behavior with and without ausforming for a medium carbon low-alloy steel. Serial neutron diffraction profiles have revealed that the transformation behavior could be successfully monitored during fast cooling and during the ausforming process. The hkl diffraction intensity of austenite increased at the beginning of transformation due to the primary extinction effect. The fresh martensite exhibits a body-centered tetragonal structure when it forms immediately below the martensite start (Ms) temperature, and its c/a ratio decreases rapidly as time elapses. The lattice parameter and the full width at half maximum of austenite peaks significantly decreases and increases upon martensite transformation, respectively. Ausforming slightly increases the Ms temperature, the amount of retained austenite at RT and the austenite lattice parameter. The dislocation densities in martensite and the retained austenite estimated using the convolutional multiple whole profile fitting method increase by ausforming.

ALUMINOBAROTHERMIC SYNTHESIS OF HIGH-NITROGEN STEEL

High-nitrogen austenitic steels are promising materials, combining high strength, plasticity and corrosion resistance properties. However, to produce high-nitrogen steel by conventional metallurgical methods under high nitrogen pressure, powerful and complex metallurgical equipment is required. From energy-saving viewpoint, an alternative and simpler method for producing high-nitrogen steels can be aluminothermy (reduction of metal oxides by metallic aluminum) under nitrogen pressure. Thermodynamic modeling of aluminothermic reactions in a nitrogen atmosphere was carried out by the authors. Aluminothermy under nitrogen pressure was used to produce high-nitrogen nickel-free Cr – N and Cr – Mn – N stainless steels with a nitrogen content of about 1 %. Microstructure (X-ray diffraction, metallography and transmission electron microscopy techniques) and mechanical properties were examined. Thermodynamic analysis has shown that the aluminothermic reduction reactions do not go to the end. The most important parameter of the synthesis is the ratio of Al and oxygen in the charge, the correct choice of which provides a compromise between completeness of oxides reduction, content of aluminum and oxygen in steel (the degree of deoxidation), and its contamination with aluminum nitride. Cr – N steel ingots in the cast state had the structure of nitrogen perlite (ferrite-nitride mixture), and Cr – Mn – N steel – ferrite-austenite structure with attributes of austenite discontinuous decomposition with Cr2 N precipitations. Quenching resulted in complete austenization of both steels. The compliance of the austenite lattice parameter obtained from the diffractograms for quenched Cr – Mn – N steel with the parameter predicted from the known concentration dependence for Cr – Mn – N austenitic steels indicated that all alloying elements (including nitrogen) were dissolved in austenite during aging at quenching temperature and fixed in the solid solution by quenching. Study of the mechanical properties of quenched Cr – Mn – N steel has shown a combination of high strength and ductility. It is concluded that by the aluminothermic method a high-nitrogen steel can be obtained, which, by mechanical properties, is not inferior to industrial steel – analog manufacted by electroslag remelting under nitrogen pressure.

Synthesis of High-Nitrogen Steel by SHS Reduction with Aluminum

High-nitrogen austenitic steel is characterized by high strength, plasticity, and corrosion resistance. However, its production by traditional methods (under high nitrogen pressure) calls for energy-intensive and complex equipment. In terms of energy conservation, a simpler alternative is the reduction of metal oxides by means of aluminum under nitrogen pressure. The present work is devoted to thermodynamic modeling of such reactions. High-nitrogen nickel-free stainless steels (of Cr–N and Cr–Mn–N type) with around 1% nitrogen are produced by the proposed method. The structure of the steel samples is investigated by X‑ray diffraction, metallography, and transmission electron microscopy; their mechanical properties are also determined. Thermodynamic analysis shows that the reduction processes are incomplete. In synthesis, the most important parameter is the ratio of the aluminum and oxygen concentrations in the batch. Correct selection of this ratio entails a compromise between the completeness of reduction of the oxides, the content of aluminum and oxygen in the steel (the degree of reduction), and its contamination with aluminum nitride. Cast ingots of Cr–N steel are characterized by the structure of nitrogen-containing pearlite (ferrite–nitride mixture), while ingots of Cr–Mn–N steel are characterized by ferrite–austenite structure, with traces of discontinuous austenite decomposition, accompanied by the precipitation of the nitride Cr2N. Quenching leads to complete austenitization of both types of steel. The agreement of the austenite lattice parameter obtained from the diffraction patterns for quenched Cr–Mn–N steel and the value expected from the concentration dependence for Cr–Mn–N steel indicates that all the alloying elements (including nitrogen) dissolve in austenite as a result of holding at the quenching temperature and are fixed in the solid solution by quenching. Tests of the mechanical properties shows that quenched Cr–Mn–N steel is characterized by both high strength and high plasticity. It may be concluded that reduction of oxides by means of aluminum under nitrogen pressure permits the production of high-nitrogen steel whose mechanical properties match those of corresponding steels produced by electroslag remelting under nitrogen pressure.

Cementite evolution in medium manganese twinning-induced plasticity steels

The precipitation and dissolution of cementite in austenitic medium Mn high C steels capable of deformation-induced twinning were studied based on the associated length changes. In continuous dilatometry heating cycles, the net contraction associated with the precipitation of cementite resulted in a decrease in the apparent coefficient of thermal expansion. The expansion associated with the reverse process of cementite dissolution in austenite, on the other hand, increased the apparent coefficient of thermal expansion. The contraction associated with the cementite precipitation was found by in-situ X-ray diffraction measurements to originate from the decrease in the lattice parameter of austenite. The latter contraction more than offset the expansion expected based on the pure consideration of austenite replacement by cementite. A model was developed to determine the evolution of cementite fraction based on the dilatometry data and the corresponding X-ray diffraction results during continuous heating cycles. The introduced dilatometry method confirmed the effect of aluminum in retarding the cementite precipitation in medium Mn high C steels.

In-Situ Synchrotron X-ray Diffraction Studies on Effects of Plastic and Elastic Loading on bcc Phase Transformations of a 3rd Generation 1\xa0GPa Advanced High Strength Steel

In this paper, we describe the effects of mechanical loading on bcc-to-bcc phase transformations of an Advanced High Strength Steel during cooling. In-situ synchrotron diffraction was employed to measure time–temperature–load diffraction patterns. Calculations were made of the volume fractions of the phases, the transformation kinetics, and the austenite lattice parameter during cooling and simultaneous loading. In addition, volume fractions and lattice parameters of retained austenite at room temperature under different loading conditions were obtained. The results show that applying a load during cooling of the fcc phase significantly increases the volume fraction of a bcc phase before the start of the martensitic transformation. The kinetics of phase transformations were affected by the applied loads. The volume fraction and lattice parameter of retained austenite at room temperature vary in different samples and the highest retained austenite and the largest lattice parameter were obtained in the sample subjected to the highest load.

Abnormal expansion due to pearlite-to-austenite transformation in high aluminium-added steels

Addition of alloying elements usually moves eutectoid point to left whilst aluminium enhances eutectoid carbon content. The carbon expands lattice parameters of austenite. High aluminium addition in steels led to volume expansion at pearlite-to-austenite transformation since specific volume of austenite is larger than that of mother phases, which is proposed to determine the volume expansion or contraction. It has been proven by calculations based on thermodynamics as well as accurate experimental observations of dilatometry study.

Heat treatment response of TiC-reinforced steel matrix composite

A particulate TiC-reinforced SKD11 steel matrix composite is fabricated by using a pressure infiltration casting, achieving a homogeneous distribution of the particles with 60 vol%. The retained austenite fraction in the composite matrix is approximately 19% after quenching from the austenitization temperature of 1010 °C, which is larger than 13% in as-quenched condition of unreinforced SKD11. A combined analysis on the austenite lattice parameter using XRD profiles and first-principle calculation suggests the increase of carbon content in the steel matrix possibly by partial dissolution of TiC during casting. The change of carbon content and prior austenite grain size reasonably accounts for the increase of retained austenite fraction in the composite matrix. In the austenitizing temperatures ranging from 950 °C to 1040 °C, the retained austenite fraction in the composite matrix in as-quenched condition increases more rapidly than that of unreinforced SKD11 with the increase of austenitization temperature, while the hardness of the composite is less sensitive to the austenitization temperature. This suggests that it is advantageous to conduct the austenitization at a temperature below 1010 °C, which is typical practice of austenitization of the unreinforced SKD11, because the retention of austenite is effectively suppressed while minimizing the loss of hardness.

Self-stabilization of untransformed austenite by hydrostatic pressure via martensitic transformation

For improving the understanding of austenite stability in steel, hydrostatic pressure in untransformed austenite that is generated via martensitic transformation was evaluated from macro- and micro-viewpoints, and its effect on austenite stability was investigated in a Fe-27%Ni austenitic alloy. X-ray diffractometry revealed that the lattice parameter of untransformed austenite is continuously decreased via martensitic transformation only when martensite becomes the dominant phase in the microstructure. This suggests that the untransformed austenite is isotropically compressed by the surrounding martensite grains, i.e., hydrostatic pressure is generated in untransformed austenite dynamically at a later stage of martensitic transformation. On the other hand, microscopic strain mapping using the electron backscatter diffraction technique indicated that a finer untransformed austenite grain has a higher hydrostatic pressure, while a high density of dislocations is also introduced in untransformed austenite near the austenite/martensite interface because of lattice-invariant shear characterized by non-thermoelastic martensitic transformation. Furthermore, it was experimentally demonstrated that the hydrostatic pressure stabilizes the untransformed austenite; however, the austenite stabilization effect alone is not large enough to fully explain a large gap between martensite start and finish temperatures in steel.

Influence of Martensite Fraction on Tensile Properties of Quenched and Partitioned (Q&P) Martensitic Stainless Steels

By means of a simplified quenching and partitioning (Q&P) processing with quench temperature equal to room temperature, austenite fractions of 56, 43, and 24 vol% are stabilized in the microstructures of Q&P‐processed Fe–Cr–C, Fe–Cr–C–Co, and Fe–Cr–C–N stainless steels, respectively. The mechanical stability of austenite is quantified by tensile tests at temperatures between 20 and 200 °C. In agreement with the Q&P concept, the stability of austenite is found to be highest for the steel with the highest martensite fraction. The carbon enrichment of austenite in a specimen of the Fe–Cr–C steel processed to have 82 vol% martensite is estimated by X‐ray diffraction measurement of the austenite lattice parameter. The results indicate that only about 12% of the carbon contained in the martensite has partitioned into the austenite. The formation of tempering carbides is one of the mechanisms behind the significant deviation of the carbon enrichments from the full partitioning assumption.

Structure and Mechanical Properties of Nitrogen Austenitic Steel after Ultrasonic Forging

Electron microscopy and X-ray diffraction have been used to investigate a nitrogen 07Kh17AG18 steel with an austenitic structure after the surface deformation treatment—ultrasonic forging. During ultrasonic forging, an austenitic structure transforms into a new structure with an elevated concentration of deformation-induced stacking faults, a lot of deformation microtwins, e-martensite crystals. The austenite lattice parameter is found to be decreased in the surface layer. After ultrasonic forging, nitrided steel exhibits enhanced strength properties with retained high plasticity.