Lattice Parameter Of Austenite Research Articles

Microstructural evolution, coarsening behavior of vanadium carbide and mechanical properties in the simulated heat-affected zone of modified medium manganese steel

The microstructural evolution, coarsening behavior of vanadium carbide (VC) and the mechanical properties of the simulated heat-affected zone (HAZ) of modified medium manganese steel (MMMS) were investigated as a function of peak temperature (Tp). Cementite precipitated at the grain boundary at 650°C–850°C, and the formation of a carbide network was not observed even at a higher Tp. The nucleation of VC precipitates occurred at a Tp of 650°C, and with increasing Tp, they grew at a rate that is greater than the theoretical value calculated according to the volume diffusion-controlled mechanism. However, coarse VC particles disappeared when the Tp reached 1250°C, and VC particles less than 2.0nm precipitated during cooling. The lattice parameter of austenite in the simulated HAZ first decreased with VC precipitation, then increased with VC dissolution and finally decreased again at 1250°C due to VC regeneration. Meanwhile, the formation of the VC nanoparticles increased the microhardness and tensile strength of the simulated HAZ.

EFFECT OF Al AND Si ON THE FORMATION OF AUSTENITE IN INTERCRITICAL TEMPERATURE RANGE IN Cr – Ni – Mo-STEEL

Aluminum and silicon alloying of Cr – Ni – Mo steel has been investigated. The described steel is used in energetic and heavy machine building engineering. The main aim of the present work was to improve mechanical properties of the steel such as plasticity and impact strength. The high temperature X-ray analysis is applied for understanding the ratio of α- & γ-phases in various temperature ranges. The critical points were determined and it was shown that the aluminum addition expands the critical temperature region while the silicon addition only moves this region to the higher temperatures. The crystal lattice parameter of austenite is the smallest at 740 °С. Silicon addition makes this temperature higher up to 760 °С. The influence of aluminum is more intensive, the temperature is 780 °С.

Tailored thermal expansion alloys

Current approaches to tailoring the thermal expansion coefficient of materials or finding materials with negative thermal expansion rely on careful manipulation of either the material's composition and/or the complex fabrication of composites. Here, by contrast, we report a new principle that enables the precise control of macroscopic thermal expansion response of bulk materials via crystallographic texture manipulation and by taking advantage of anisotropic Coefficients of Thermal Expansion (CTE) in a large class of martensitically transforming materials. Through simple thermo-mechanical processing, it is possible to tailor the thermal expansion response of a single material––without manipulating its composition––over a wide range of positive and negative values. We demonstrate this principle by gradually tuning the macroscopic CTE in a model NiTiPd alloy between a positive (+14.90 × 10−6 K−1) and a negative (−3.06 × 10−6 K−1) value, simply by incrementally increasing tensile plastic deformation in the martensite phase. This surprising response is linked to the large positive, +51.33 × 10−6 K−1, and negative, −34.51 × 10−6 K−1, CTE anisotropy, at the lattice level, along the different crystal directions in martensite. Similar CTE anisotropy is also shown experimentally in CoNiGa and TiNb alloys. In a model TiNb alloy, giant macroscopic CTEs of +181 × 10−6 K−1 and −142 × 10−6 K−1 are measured. A connection between the CTE anisotropy and the martensitic transformation in these and other materials systems such as NiTi, pure uranium, and PbTiO3 is later made. It is shown that negative or positive thermal expansion crystallographic directions are connected to the crystallographic relationship between the austenite and martensite lattices, and can easily be predicted using the lattice parameters of austenite and martensite phases. Our current observations and analyses suggest that the tunability of the macroscopic CTE through thermo-mechanical processing is universal in materials––both ceramic and metals––that undergo martensitic transformations.

Influence of Al and Si on austenite formation in Cr–Ni–Mo steel in the intercritical temperature range

The effect of adding aluminum and silicon to 35XH1M2ΦA steel is investigated in order to analyze the influence of those elements on austenite formation. 35XH1M2ΦA steel, which is used in heavy industry and in power equipment, is selected for study because its mechanical properties—in particular, its plasticity and impact strength—require improvement. High-temperature structural analysis provides data regarding the ratio of α and γ phases in the steel. On the basis of the results, the critical points Ac1 and Ac3 are calculated. It is found that introducing aluminum in 35XH1M2ΦA steel expands the intercritical temperature range, whereas introducing silicon shifts the intercritical temperature range to higher temperatures. The lattice parameter of austenite is least at 740°C in 35XH1M2ΦA steel. Silicon and aluminum raise this temperature to 760 and 780°C, respectively.

Effect of non-isothermal deformation of austenite on ferrite transformation behavior studied by in-situ neutron diffraction

The microstructure evolution and phase transformation of high strength 22SiMn2TiB steel during non-isothermal deformation were investigated by using in situ time-of-flight (TOF) neutron diffraction technique. The results indicate that the deformation of austenite promotes pearlite and ferrite transformation while suppresses bainite transformation. Deformation texture forms in austenite and then it influences the evolution of transformation texture. Deformation of austenite brings the changes in lattice parameters of austenite caused by carbon partitioning and elastic strains during the transformation. Volume fraction of the retained austenite decreases with a decreased carbon content as deformation amount increases.

Tempering of a martensitic stainless steel: Investigation by in situ synchrotron X-ray diffraction

Tempering of a martensitic stainless steel is investigated by means of in situ X-ray diffraction using high-energy synchrotron radiation. The simultaneous evolutions of the fractions of martensite/ferrite, retained austenite, M23C6 and M2X (X=N,C) precipitates are determined during a continuous heating, and show that precipitation of M2X can be observed directly only above 650°C, concomitantly to the transformation of austenite into ferrite. However, a careful cross-analysis of the evolution of the lattice parameters of all phases shows the precipitation of metastable carbides/nitrides at low temperatures, followed by some stress relaxation. Between 500 and 650°C, the lattice parameters of M2X feature changes associated with the evolution of their composition, as supported by the analysis of additional isothermal agings, and consistently with the literature. Finally, the complex variations of the lattice parameters of austenite are analyzed thanks to a simple micromechanical model, supporting the previous conclusions and highlighting the subtle interplay between precipitation and stress relaxation.

The influence of silicon and aluminum alloying on the lattice parameter and stacking fault energy of austenitic steel

The influence of Si and Al on the lattice parameter and stacking fault energy (SFE) of austenitic steel was studied. X-ray diffraction indicated that 2.5 wt.% Al alloying increased the austenite lattice parameter by 1.41 × 10−3 nm, whereas 2.5 wt.% Si alloying had little effect. Partial dislocation separation measurements from weak-beam dark-field images indicated that 2.5 wt.% Al alloying increased the SFE by 12–17 mJ m−2 and 2.5 wt.% Si alloying decreased the SFE by 6–7 mJ m−2.

Investigation of Triaxial Stress State in Retained Austenite during Quenching of a Low Alloy Steel by <i>In Situ</i> X-Ray Diffraction

In situ XRD measurements were performed at ESRF, Grenoble, France (ID11) during quenching of a ball bearing steel AISI 52100 (100Cr6) with varying carbon content in solution. The evolution of austenite lattice parameter during cooling is nearly linear until Ms is reached and then, a divergent behavior can be observed. Assuming that the extrapolation of the linear range to room temperature gives the stress-free lattice spacing, an increasing compressive hydrostatic stress state is resulting. A strong effect of the carbon content was found. These results were confirmed by theoretical calculations based on data from the literature.

Microstructure of nitrided and nitrocarburized layers produced on a superaustenitic stainless steel

The expanded austenite γN can be produced in austenitic stainless steels by plasma nitriding, carburizing or nitrocarburizing at low temperatures. This metastable phase presents higher hardness and toughness if compared with traditional nitride layers whilst also maintaining the corrosion resistance. However, the application of plasmas composed by both nitrogen and carbon is technologically recent and the effect of such process on the microstructure and properties of the nitrocarburized layers is still under investigation. In this study, samples of UNS S31254 superaustenitic stainless steel were produced by plasma nitriding and nitrocarburizing at 400°C, 450°C and 500°C for 5h. The plasma treated samples were observed by optical and transmission electron microscopy and also analyzed by X-ray diffraction. The thickness of the layers increased with temperature and the nitrocarburized layers were thicker than nitrided at a given temperature. The presence of expanded austenite was confirmed by X-ray diffraction through its characteristic anomalous shift on the diffracted peaks related to the austenite. Nitride formation on samples produced at 400°C was only identified by transmission electron microscopy where fine rounded particles with 10–15nm size revealed reflections consistent with the CrN cubic chromium nitride. The estimated lattice parameter from the expanded austenite ranged from 0.38 to 0.41nm depending on the employed {hkl} reflection which was found to be 6–11% larger than the untreated austenite lattice parameter.

New observations on formation of thermally induced martensite in Fe–30%Ni–1%Pd alloy

Kinetical, morphological, crystallographical and thermal characteristics of thermally induced martensite in an Fe–30%Ni–1%Pd alloy has been studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and X-ray diffraction method. Kinetics of transformation was found to be as athermal. SEM and TEM observations and X-ray method revealed α′ (bcc) martensite formation in the austenite phase of alloy by thermal effect. The crystallographic orientation relationship between austenite and α′ (bcc) martensite was found to be having Kurdjumov–Sachs (K–S) type relationship. In addition, the lattice parameters of austenite and martensite phases were calculated from X-ray diffraction patterns.

Stability of expanded austenite, generated by ion carburizing and ion nitriding of AISI 316L SS, under high temperature and high energy pulsed ion beam irradiation

Expanded austenite can be generated on austenitic stainless steels either by ion carburizing or ion nitriding. In both cases the resulting fcc crystal structure, supersaturated with nitrogen or carbon, is strongly hardened with improved wear-resistance, while maintaining the original resistance to corrosion. In this work, we have studied the stability of expanded austenite, generated by ion nitriding and ion carburizing on AISI 316L SS with N and C, under: a—high temperature (225°C – 504°C), and b—under irradiation with high energy (30keV – 500keV), high fluence (~1015cm−2), short duration (~400ns) light (deuterium and helium) ion beams. It was found that expanded austenite is stable below 325°C. Between 325°C and 504°C expanded austenite lattice parameter presents gradual reduction with increasing temperature. We observed microstructural changes related only to the temperature treatment. We did not observe any microstructure change due to the duration of the heat treatment. Over 504°C, the lattice parameter returns to the material's austenite original parameter. On the other hand, when irradiated with pulsed ion beams, a gradual reduction of the lattice parameter corresponding to the expanded austenite with the number of pulses was observed. This behavior can be explained through the thermal shock induced on the surface by each beam, consisting in fast heating followed by fast cooling that induces the gradual exo-diffusion of N (or C). Nevertheless, after 20 ion pulses, a final lattice parameter slightly higher than the corresponding to the original austenite was found as stable limit. This residual expansion can be attributed to partial amorphization of the first few micrometers that induces stresses on the crystals of austenite which are closer to the surface layers.

In situ X-ray diffraction study of stacking fault formation in the near-surface region of transformation induced plasticity steels

Formation of stacking faults in the near-surface region of Cr–Mn–Ni steels that shows the effect of transformation induced plasticity (TRIP) was investigated by means of in situ X-ray diffraction experiments during a tensile deformation of the samples up to approximately 9%. The X-ray diffraction measurements revealed simultaneously the phase transformation of the TRIP steel during its plastic deformation and the changes in the interplanar spacing of the diffraction lines of austenite. As the interplanar spacing in austenite is influenced both by the residual elastic lattice strain and by the stacking fault density, the crystallographic anisotropy of the cubic lattice parameter was employed to distinguish these effects. Despite a partial correlation between the cubic invariant, which describes the crystallographic anisotropy of the stress-induced lattice deformation, and the orientation factor of the stacking faults in austenite, which describes the effect of the stacking faults on the orientation-dependent shift of the X-ray diffraction lines, the crystallographic anisotropy of the lattice parameter of austenite was proven to be capable of tracking the formation of microstructure defects and of distinguishing individual microstructure defects from each other. The X-ray diffraction experiments were complemented by transmission electron microscopy, which allowed the different observed anisotropies of the cubic lattice parameter to be assigned to specific microstructure defects.

Rigorous version of ınfinitesimal deformation approach to the crystallography of fcc ® bcc martensitic phase transformation observed in Fe-31 wt % Ni and zirconia alloys

Following the mathematical approach of Kelly (2003), rigorous version of infinitesimal deformation approach (ID) was applied to analyze fcc ® bcc martensitic phase transformation observed in Fe-31wt% Ni alloys considering the twinning shear system as the lattice invariant system (LIS). The expressions for crystallographical parameters associated with the martensitic phase transformation such as habit plane orientation, rotation matrix, total shape deformation matrix, orientation relationships between austenite and martensite phases, etc., have been obtained by using the rigorous version of infinitesimal deformation approach, taking into account only the information about the lattice parameters of the austenite and martensite phases. The habit plane orientation obtained from the infinitesimal deformation approach (ID), for example, for the value of volume fraction 0.39876 in Fe-31wt% Ni alloy, is found as (0, 0.58033, 0.81438) while the habit plane obtained from rigorous version of (ID) approach associated with the martensite transformation is (0.185064, 0.594242, 0.782705) which is an agreement with that calculated from the Wechsler-Lieberman-Read (W-L-R) theory. The difference between the two habit plane orientations is 10.80. Moreover, the calculations carried out for zirconia, in which there are small values of the principal distortions, in the present study certainly indicate that infinitesimal deformation approach (ID) gives almost the same results as (W-L-R) theory. For the other crystallographical parameters, such as, magnitude of total shape deformation, volume fraction, orientation relationship, the rigorous version of (ID) approach gives essentially the same crystallographical solutions as those calculated (W-L-R) theory. So, it is concluded that the rigorous version of (ID) approach in the habit plane orientation is better than the infinitesimal deformation approach (ID) for the alloys in question. Key words: Martensitic transformation, solid-solid phase transitions, crystallography of martensite phase transformations.

On measurement of carbon content in retained austenite in a nanostructured bainitic steel

In this study, the carbon content of retained austenite in a nanostructured bainitic steel was measured by atom probe tomography and compared with data derived from the austenite lattice parameter determined by X-ray diffraction. The results provide new evidence about the heterogeneous distribution of carbon in austenite, a fundamental issue controlling ductility in this type of microstructure.

In-Situ X-Ray Investigations and Computer Simulation during Continuous Heating of a Ball Bearing Steel

This article presents a comparison between transformation kinetics obtained by in-situ X-ray diffraction during continuous heating of a ball bearing steel AISI 52100 to austenitizing temperature and computer simulation. In order to have acquisition times short enough for good time resolution, X-ray diffraction experiments were done with special equipment such as a rotating anode producing intense X-ray radiation and an area detector. Computer simulations of austenitizing were performed with a Johnson–Mehl–Avrami equation. Besides transformation kinetics, the lattice parameters of ferrite and austenite were also studied. Lattice parameters measured by in-situ X-ray diffraction match calculations from models. Good agreement between phase contents obtained by simulation and by X-ray diffraction was obtained.

In situ X-ray phase analysis and computer simulation of carbide dissolution of ball bearing steel at different austenitizing temperatures

This paper presents the results of in situ X-ray diffraction experiments and computer simulation of AISI 52100 steel during soaking at different austenitizing temperatures. A study of carbide dissolution kinetics and austenite lattice parameter variations has been executed. In situ X-ray diffraction investigations were carried out with a rotating anode and an area detector to allow a good time resolution. For the computer simulation, a Johnson–Mehl–Avrami equation was used. Lattice parameters of austenite obtained by X-ray diffraction show good agreement with values calculated from a model based on neutron diffraction experiments. Carbide contents measured by X-ray diffraction are close to those obtained by simulation. But discrepancies are present which are essentially due to local heterogeneity of the chemical composition.

A study of the carbon distribution in retained austenite

Cold-rolled and annealed transformation-induced plasticity (TRIP) steels were overaged to modify the carbon concentrations (Cγ) in retained austenite. Experimental Cγ values were directly obtained by electron energy loss spectroscopy and compared with data derived from X-ray diffraction measurements of the austenite lattice parameter (aγ). In this way, we evaluated the different expressions available in the literature relating Cγ to aγ.

In‐situ Investigation during Tempering of a High Speed Steel with X‐ray Diffraction

AbstractThe combined use of a X‐ray diffractometer and a high temperature chamber allows in‐situ determination of microstructure during heat treatment. X‐ray diffractograms are recorded during tempering of high speed steels. The Rietveld method is used in connection with size‐strain analysis. Lattice parameters of austenite and martensite are utilized for estimating the change of carbon content that has massive influence on the secondary hardening. Nevertheless, it was shown that the heat treatment of deep‐frozen high speed steel cannot be deduced a priori from the applied tempering process under conventional quenching conditions. The differently conditioned microstructures are based on different phase amounts and microstrain‐situations.

Martensitic transformation of Ni 50Ti 45Ta 5 shape memory alloy

Ni 50Ti 45Ta 5 alloy is similar to equiatomic NiTi alloy with near equal transformation temperatures and having B2 ↔ B19′ one stage transformation. The main lattice twin is 〈0 1 1〉 type II twin. The lattice parameter of B2 austenite is a = 0.3025 nm, and the lattice parameters of B19′ martensite is a = 0.290 nm, b = 0.412 nm, c = 0.473 nm and β = 98.2°. The transformation temperature depression of Ni 50Ti 45Ta 5 alloy is larger than that of NiTi alloy under the same number of thermal cycles due to the presence of more particles. The martensite stabilization occurs due to the deformation. It is closely related to the variation of the elastic energy and the irreversible energy during deformation. The Ni 50Ti 45Ta 5 alloy has a higher martensite stabilization than NiTi alloy under the same degree of deformation due to the presence of a soft β-Ta phase.

A Dilatometry Study of the Austenitization and Cooling Behavior of Ductile Iron Meant for the Production of Austempered Ductile Iron (ADI)

An understanding of the kinetics of transformation during austenitization, cooling, and austempering of ductile iron is critical to achieving the desired microstructures and ultimately mechanical properties in austempered ductile iron (ADI). To this end, dilatometry experiments have been carried out to study the austenitization and cooling behavior of an unalloyed ductile iron. When a typical austenitization temperature of 900°C is used, unlike in steels, there is an initial expansion of the specimen, which levels off as the soaking time is increased. This occurs despite the fact that the temperature remains constant. This phenomenon, hitherto unreported, highlights the subtle differences between the austenitization of ductile irons and steels. The initial expansion is attributed to the increase in austenite lattice parameter, arising from the diffusion of carbon from the graphite nodules. The levelling off signals the saturation of austenite with carbon and can therefore be used as an indicator of the appropriate austenitization time. Studies of the cooling behavior of unalloyed ductile iron have also shown that the dilatometer can be used as a tool for determining the minimum cooling rates, which guarantee the formation of ausferrite during austempering. When ductile iron is appropriately heat-treated based on results from dilatometry studies, the mechanical properties obtained are typically superior and consistent.