Dilatometric Data Research Articles

Application of Intercritical Austenitizing in a Commercial CMnSi Steel as a Tool for Optimizing a One‐Step Quenching and Partitioning Cycle Aiming to Achieve a Third‐Generation Advanced High‐Strength Steel

Advanced high‐strength steels (AHSS) have been extensively studied to develop its third generation from relatively low‐cost alloys, but seeking optimized mechanical properties. In this context, the effects of quenching heat treatments are evaluated in this work after intercritical austenitizing (IA) on microstructure, alloying elements partitioning, and Ms temperature for a commercial CMnSi alloy. Applying dilatometric data and microstructural characterization, different heat cycles are evaluated. The obtained experimental results are compared with those ones obtained by computer simulations at equilibrium conditions. Based on this characterization, an optimized one‐step quenching and partitioning (Q&P) cycle (after IA) is proposed and applied to tensile test specimens. Then, the specimen microstructure is characterized by scanning and transmission electron microscopy as well as by X‐ray diffraction. Using the proposed methodology, it is possible to produce a Q&P steel with a product of tensile strength and elongation of 29.5 GPa% and a 7% volumetric fraction of retained austenite. It is concluded that the precise knowledge about the IA effects on the phase transformations and microstructure evolution in a commercial CMnSi alloy has great potential to be applied in defining a one‐step Q&P heat treatment that favors a multiphase microstructure, which meets mechanical properties requirements for a third‐generation AHSS.

Phase transformation kinetics in a coarse-grain Ti17 alloy determined by laser ultrasonics and dilatometry

Different methods are used to determine the phase transformation titanium alloys. While ex-situ methods require a large number of samples and accurate quantification of the phases, in-situ methods can provide the kinetics of transformation continuously and with less samples. However, the results are based on the changes of physical properties, and the interpretation of the results in terms of phase transformation are not direct. This study compares two in-situ methods to measure the kinetics of phase transformation for a Ti17 alloy, namely: laser ultrasonics for metallurgy (LUMet) and dilatometry. While LUMet is coupled to a Gleeble® 3500 thermo-mechanical simulator with ohmic resistance heating, the dilatometer device uses induction heating. We performed two types of experiments by heating the specimens above the β-transus temperature (∼ 865 °C) and 1) and then fast cooled below the β-transus temperature and held at different temperatures, or 2) continuously cooled at different rates. The β→α + β phase transformation is inferred by the evolution of the longitudinal wave speed with the LUMet method and by the change in the length of the specimen by dilatometry. We obtained the information about the isothermal phase transformation using the normalized changes in length for the dilatometric data and in longitudinal ultrasonic velocity. In the investigated isothermal temperature range (580 – 700 °C), the β →α + β transformation is completed within one hour of holding with the fastest transformation rates at 580 °C. For the continuous cooling treatments, the small change in length measured by dilatometry makes it impractical to use the lever rule to accurately determine the relative transformation fraction. However, we could successfully apply the lever rule to the LUMet data. Furthermore, the derivatives of the change in length and ultrasonic velocity are used to estimate the transformation temperatures as a function of the cooling rates. Further, the in-situ measurements are supplemented with ex-situ characterization of the microconstituents after heat treatments using scanning electron microscopy.

Porous 8YSZ Ceramics Prepared with Alkali Halide Sacrificial Additives.

8 mol% Y2O3-stabilized ZrO2 (8YSZ) ceramics were prepared with KCl and LiF additions to obtain porous specimens with high skeletal density. Thermogravimetric and differential thermal analyses (TG/DTA) were carried out on 8YSZ and on 8YSZ mixed to 5 wt.% KCl or 5 wt.% LiF as sacrificial pore formers that were thermally removed during sintering. The melting and evaporation of the alkali halides were evaluated by differential thermal analysis. Dilatometric analysis was also carried out following the same TG/DTA temperature profile with results suggesting rearrangement of the 8YSZ particles during LiF and KCl melting. The dilatometric data of 8YSZ green pellets mixed to KCl or LiF exhibited an initial expansion up to the melting of the alkali halide, followed by shrinkage due to sintering evolution with grain growth and pore elimination. The time that the alkali halide molten phase was kept during sintering was found to be an important parameter for obtaining 8YSZ-sintered specimens with specific pore content; bulk density and open porosity could then be tuned by controlling the time the alkali halide remained liquid during sintering. Scanning electron microscopy images of the pellet fracture surfaces showed pores that contributed to increasing the electrical resistivity as evaluated by impedance spectroscopy analysis.

Production of Al2O3-SiC composites from micrometer α-Al2O3 powder obtained via sol-gel

Alumina (Al2O3) is an advanced ceramic material developed for different applications as refractories, precision tools, pacemaker, etc. Solid state sintering of alumina or matrix ceramic composites (CMCs) compacts starts from powders. Once method to produce high quality aluminum oxide powders is the sol-gel technique. Alumina begins as pseudo-crystallized aluminum hydroxide gel which is produced under moderate reaction conditions trough a colloidal suspension. In this work, Al2O3 powder was produced by precipitation of pseudoboehmite (PB) through sol-gel process. Subsequently, a mixture of Al2O3/SiC powders with 5 wt.% of SiC as reinforcement was produced. This mixture was used to manufacture green compacts by uniaxial pressing at 440 MPa. Afterward, some samples were applied a heat treatment (pre-sintered) at 1200°C for 6 h in air. Sintering was carried out in a vertical dilatometer Linseis L75 V up to 1500°C for 2h under argon atmosphere. Pseudobohemite, alumina powders and Al2O3/SiC composites were characterized through X-ray diffraction technique and Scanning Electron Microscopy (SEM). Dilatometric shrinkage data into densification curves obtained were analyzed. Images obtained with SEM showed a uniform Al2O3 powder morphology of submicron size, otherwise Al2O3/SiC composite images showed the interaction of the reinforcement particles on the ceramic matrix. Experimental results demonstrated the pre-sintering reduce the decomposition of SiC particles on the compact surface. This behavior was attributed to formation of SiO2 around the reinforcement particle due it act as protective barrier.

Comparative Insights into the Fundamental Steps Underlying Gelation of Plant and Algal Ionic Polysaccharides: Pectate and Alginate.

Pectate and alginate are among the most important biopolymers able to give rise to ionotropic gelation upon the addition of di- or multivalent counterions. The two ionic polysaccharides exhibit several common aspects of the gelation mechanism with calcium ions, the physiologically and commercially most relevant counterion type. The first one pertains to the role that specific Ca2+/polyion interactions play in the establishment of the ion-mediated chain/chain cross-links. Such interactions include both a specific affinity of the territorially condensed Ca2+ counterions for the polyuronate(s) and the formation of long-lasting chemical bonding (inner ion-sphere complex) of specific interchain sites accompanied by high conformational ordering. As to the first mechanism, it is dominated by the strong desolvation of the interacting ionic species, with concomitant positive variations in both enthalpy and entropy, the contribution of the latter prevailing over the former due to the favorable liberation of a very large number of water molecules of hydration. Both dilatometric and microcalorimetric data point to the higher affinity of Ca2+ for pectate than for alginate. The selective accumulation of calcium ions close to the polyanion(s) favors the onset of the second-chemical bonding-mode, which is associated with charge neutralization at the bonding site. This mode coincides with the largely accepted "egg-box" model for the calcium-mediated interchain junction of pectate and alginate. A new approach was devised for the calculation of the fraction of chemically bound divalent ions; it was based on the available circular dichroism data (further supported by scattering and viscosity results) and successfully tested by comparison with an independently determined fraction in the case of pectate. In detail, the strong bonding mode manifests in two sequential bonding modes. The first one (at low concentrations of added Ca2+ ions) entails a cross-link in which only one calcium ions is bracketed in a "twisted" egg-box between two chains; upon further counterion addition, a series of nearest-neighboring "perfect" egg-box structures develops. Both dilatometric and microcalorimetric changes associated with the latter chemical bonding modes are quantitatively larger for pectate than for alginate; clearly the latter polyuronate suffers from the relevant presence of the weakly calcium-binding mannuronic acid repeating units. Light-scattering experiments provided a clear-cut demonstration of the intermolecular bonding of calcium ions from the very beginning of the linker addition.

Sintering of Si3N4 with Li-exchanged zeolite additive

Abstract This paper deals with the densification and phase transformation of Si3N4 with additives of Li-exchanged zeolite during pressureless sintering at significantly reduced temperatures. Dilatometric shrinkage data show that the first liquid forms as low as 1080 °C. Upon sintering at 1500 °C the bulk density increases to more than 95% of the theoretical density without phase transformation from α-S3N4 to β-Si3N4, i. e. the phase transformation lags behind the densification process. Above 1500 °C the secondary phase is completely converted into a glass and the α-to-β transformation takes place. Under these conditions the grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. The results show that a very effective low-temperature sintering additive for silicon nitride can be obtained from Li-exchanged zeolite.

Enhanced mechanical and thermal properties of fly ash-based geopolymer composites by wollastonite reinforcement

The present study investigated the mechanical and thermal properties of geopolymer composite. The geopolymer composite was prepared by mixing fly ash and wollastonite with the alkaline activator, which was 6 M KOH:K2SiO3 in a mass ratio of 1:1 and a solid:liquid mass ratio of 3:2. The compressive strength at 28 days of geopolymer was 33.3 MPa and possessed the highest strength of 38.3 MPa when 30 wt% wollastonite was added. The flexural strength presented differently whereby it increased from 2.1 MPa to 6.8 MPa. It increased remarkably up to 200% with the addition of 50 wt% wollastonite. The geopolymer composites were exposed to high temperatures at 800℃ to 1100°C for 2 h. Cracks were reduced since 20 wt% wollastonite was added. A high percentage of wollastonite presented excellent thermal stability. The total weight loss of the geopolymer composite at temperatures of 30℃ to 1400°C was minimized. It decreased from 25% to 12% when 50 wt% wollastonite was added, and the dilatometric data resulted in a dimensional change of almost zero. The phase development of the geopolymer composites at high temperatures showed the crystallization of leucite, kalsilite, calcium silicate, calcium aluminium silicate, and calcium aluminium oxide, which were the high temperature stable phases. The results indicated that wollastonite reinforced fly ash-based geopolymer composites are promising for use in high temperature applications.

Dilatometric Analysis and Kinetics Research of Martensitic Transformation under a Temperature Gradient and Stress

Based on material constitutive models and the classic Koistinen–Marburger (KM) kinetics model, a new dilatometric analysis model was developed to extract the kinetics curve of martensitic transformation under a temperature gradient and stress from the measured dilatometric data and to determine the transformation parameters. The proposed dilatometric analysis model is generally for athermal martensitic transformation, relying only on the average atom volume of martensite and austenite. Furthermore, through theoretical calculations, the proposed model also provided a more accurate method for obtaining the martensite start temperature, which is different from the traditional method. According to the dilatometric analysis results for the martensitic transformation of a type of high-strength low-alloy steel, and the thermodynamic basis of martensitic transformation, a refined kinetics model was developed that successfully predicted the martensitic transformation kinetics curves under different stresses, taking into account the physical significance of the transformation parameter α and the driving force of stress for martensitic transformation.

Characterization of untransformed ferrite in 10Cr and 12Cr ODS steels

Two new ferrito-martensitic oxide dispersion strengthened (ODS) steels reinforced with (Y, Ti, O) nanoparticles were elaborated using a high-energy attritor. The milled powder was consolidated by hot extrusion at 1050 °C. The two types of ODS steels differ by chromium content, with 10 wt% Cr and 12 wt% Cr respectively. According to thermodynamic calculations, those grades are supposed to exhibit an austenitic transformation at high temperatures. X-ray diffraction (XRD) above austenitic temperature transformation reveals the presence of both ferrite and austenite phase. This unexpected ferrite phase is assumed to be untransformed low temperature ferrite. The α→γ phase transformation specific enthalpy is monitored by differential scanning calorimetry (DSC). The untransformed ferrite fraction is calculated using dilatometric data and confirmed by electron backscatter diffraction (EBSD) microstructural analysis. The quenched samples from the austenitic domain give an image of the high-temperature partitioning. EBSD maps reveal two distinct elementary microstructures, one martensitic inherited from austenite and the other corresponds to the untransformed ferrite. This untransformed ferrite keeps the crystallographic α-fiber conferred by hot-extrusion. The 10 Cr ODS has equiaxed untransformed ferrite areas. In contrast, the untransformed ferrite into 12 Cr ODS is distributed as elongated areas, parallel to the hot-extrusion direction. Moreover, electron probe micro analyzer (EPMA) mapping exhibits chromium content gradients, consistent with phase partitioning at high temperatures. Creep properties are evaluated at 650 °C for both grades. Small-angle X-rays scattering (SAXS) shows a similar size and distribution of the oxide particles in both grades.

Near zero thermal expansion in metal matrix composites based on intermediate valence systems: Al/SmB6

This work is focused on the fabrication and characterization of a new type of composite invar materials combining near-zero thermal expansion and functional properties that are important for applications. This is accomplished through the use of particles of SmB6, an intermediate valence system with negative thermal expansion, embedded in Al matrix. The composite based on SmB6-21 vol% was fabricated by hot pressing and was characterized by XRD, optical/electron microscopy, X-ray computed tomography and capacitive dilatometry. The study of thermal expansion revealed that the sample exhibits invar behaviour up to ~60 K with a zero value of the coefficient of thermal expansion near 45 K. In comparison to pure aluminum, the temperature range has increased by about 20 K. A quantitative analysis of dilatometric experimental data performed on the basis of widely used theoretical models showed that the thermal expansion of the Al/SmB6 composite was well reproduced within the Turner model.

Proposition of an Empirical Functional Equation to Predict the Kinetics of Austenite to Ferrite Transformation in a Continuous Cooled IF-Ti-Stabilized Steel

The kinetics of phase transformations in isothermal processes is well described by the classic JMAK model. However, it is known that most industrial facilities employ continuous cooling processes and, for this condition, the JMAK model is adapted but, sometimes, without success. Considering the importance to predict critical temperatures, the present work proposes an alternative empirical functional equation to describe the kinetics of steel phase transformation under continuous cooling, being useful when the JMAK use is not successful. In this study, the continuous cooling austenite to ferrite transformation was experimentally characterized by dilatometry for an IF-Ti-stabilized steel. The proposed equation was compared with the classic adapted JMAK model regarding to evaluate its efficacy to fit the dilatometric experimental data. Using the constants obtained by the both model fittings as input parameters, a computational simulation was performed to determine the CCT diagram of the IF-Ti steel. The proposed functional equation was efficient to predict the critical temperatures, the kinetics of austenite decomposition and the CCT diagram of the studied steel. The results predicted by the proposed model greatly met the experimental data measured by dilatometry.

Thermal expansion of vanadium in the temperature range of 98–2400 K

The experimental results of a study of the thermal expansion of polycrystalline vanadium in the temperature range of 98–1723 K are presented. The correlation of the dilatometric data and γ-ray attenuation technique was established. The temperature dependence of thermal properties up to 2400 K was obtained and reference table of recommended values was calculated.

3D Dilatometer Time-Series Analysis for a Better Understanding of the Dynamics of a Giant Slow-Moving Landslide

This paper presents a methodological approach to the time-series analysis of movement monitoring data of a large slow-moving landslide. It combines different methods of data manipulation to decrease the subjectivity of a researcher and provides a fully quantitative approach for analyzing large amounts of data. The methodology was applied to 3D dilatometric data acquired from the giant San Andrés Landslide on El Hierro in the Canary Islands in the period from October 2013 to April 2019. The landslide is a creeping volcanic flank collapse showing a decrease of speed of movement during the monitoring period. Despite the fact that clear and unambiguous geological interpretations cannot be made, the analysis is capable of showing correlations of the changes of the movement with increased seismicity and, to some point, with precipitation. We consider this methodology being the first step in automatizing and increasing the objectivity of analysis of slow-moving landslide monitoring data.

Design, fabrication and validation of an improved coil for induction dilatometry

Dilatometry is a versatile tool to investigate thermal expansion or phase transformations in various materials based on the length changes upon heating and cooling. Since most modern thermal processes tend to take place at high heating and cooling rates it can be demonstrated that a large longitudinal temperature gradient within dilatometric samples impedes the accurate investigation of physical mechanisms. In this study we investigate how an instrumental advancement of the induction heating system can be implemented in a dilatometric setup in order to mitigate the temperature gradient and its effect on the measured length change for a wide range of heating rates. For this purpose a numerical method for heating design optimization using a FEM simulation of the dilatometric experiment is developed and validated. The presented heating component design method in combination with additive manufacturing capabilities allows for a more accurate and flexible method of detecting dilatation in a dilatometric setup. Extreme temperature differences of 140∘C for a maximum heating rate of 800∘C/s could be reduced by 15-45% within the sample using new optimized instrumentation. This leads to a more profound and exact interpretation of dilatometric data for high heating rates and complex time temperature profiles.

Thermal Properties of Na2O-MgO-TiO2-Al2O3-B2O3-SiO2 Glasses and Prospects for Their Use for Sealing Solid Oxide Fuel Cells

A series of Na2O-MgO-TiO2-Al2O3-B2O3-SiO2 glasses promising as sealants for solid oxide fuel cells are prepared. The glassy state is confirmed by X-ray diffraction analysis, and the elemental composition is determined by inductively coupled plasma atomic emission spectroscopy. The coefficient of thermal expansion (CTE) is calculated from the dilatometric data; it varies within (69–77) × 10−7 K−1 and increases with the MgO to Al2O3 concentration ratio. The CTE calculation by the Appen method gives the results that are underestimated by 5–8% relative to the experimental data. The temperature dependences of the heat capacity of the glasses are determined by differential scanning calorimetry and calculated using the Kopp-Neumann rule. Comparison of the calculated values with the experimental data shows that the Kopp-Neumann rule is observed for the systems under consideration with relatively high accuracy. The sensitivity of different methods in determination of the glass transition point has been evaluated.

Correlation of Cooling Rate, Microstructure and Hardness of S34MnV Steel

A study of the hardenability, the microstructure and the phase transformations as a function of the cooling rate, of a marine crankshaft S34MnV steel has been performed using a Jominy end-quench test and dilatometry tests. The cooling rate has a distinct influence on the hardness and microstructure characteristics. As the cooling rate decreases, the hardness drops from HRC53 at the quenched end to HRC22 at the top end of the Jominy bar. The corresponding microstructures change from hard martensite to bainite, then soft pearlite and ferrite. The critical temperatures Ac1, 740 °C and Ac3, 830 °C were determined, and a continuous cooling transformation diagram was constructed from the dilatometric data. The important role of cooling rate on final hardness, phase transformation and microstructures are analyzed.

Investigation and two-stage modeling of sintering behavior of nano/micro-bimodal powders

Nano/micro-bimodal powders take advantages of both nano- and micro-powders. However, densification of the bimodal powder samples is hardly predictable with conventional sintering models due to their particular shrinkage behaviors. In this study, sintering behaviors of micro-, nano- and nano/micro-bimodal powders were investigated, and a two-stage master sintering curve model was developed. Micro-, nano- and three bimodal powder samples were prepared via powder injection molding process. The samples were sintered using a dilatometer to analyze the densification behavior, and microstructures of the samples with various sintering temperatures were observed. The results demonstrated the sintering mechanism of the bimodal and nano-powder samples could be divided into two stages. Based on observation, the two-stage master sintering curve model was suggested by introducing transition temperature, and sintering activation energy and material parameters were obtained for each region from the dilatometric data. The two-stage master sintering curve model was successfully developed with high accuracy. The model significantly reduced the maximum error of prediction density from 6 to 0.8%. The result indicated the two-stage master sintering curve model could describe the sintering behaviors of the bimodal and nano-powder samples.

Model of phase transformations in steels subject to heating-cooling thermal cycles in continuous annealing line

ABSTRACTThe need for fast mean field models describing phase transformations in varying temperatures was the motivation for the research. The aim was to propose an austenite-ferrite transformation model which can be successfully substituted for the JMAK model and eliminate its weak points. The model consists of a second order differential equation which describes the kinetics of both the austenite-ferrite and the reverse transformations. The main advantage of such an approach is the ability to perform calculations for varying temperature without using the additivity rule. The coefficients in the equations were determined by performing the inverse analysis for the dilatometric data. The inverse approach was transformed into an optimisation task, which was solved using the Particle Swarm Optimization (PSO) algorithm. The first part of the work is devoted to the description of the model and is completed with the calculation of the coefficients obtained for two DP steels. The second part consists of numerical tests and the validation of the model, which was performed by simulations of two industrial thermal cycles. The final part of the work is devoted to the optimisation of the thermal cycle for continuous annealing, in order to obtain the required phase composition by changing parameters of the thermal cycle. This part also contains the results of sensitivity analysis of final phase distribution.

Sintering kinetics of ZrO2 nanopowders modified by group IV elements

AbstractThe sintering behavior of tetragonal zirconia nanopowders modified by the group IV elements at the initial sintering stage was investigated. It was found that different additives SiO2, SnO2, and GeO2 have a significant influence on the densification kinetics of 3Y‐TZP nanopowders obtained by coprecipitation during sintering as it depends on the amount of additives (0‐5 wt%). The shrinkage of zirconia‐based specimens during the nonisothermal sintering was analyzed using the dilatometric data. The constant rate of heating technique was applied in order to determine the dominant mass transfer mechanism at the initial stage of sintering in modified zirconia nanopowders. It was found that there was a change in the mass transfer mechanism and diffusion activation energy in 3Y‐TZP as a result of the additives. The dominant sintering mechanism in 3Y‐TZP changed from the volume diffusion to the grain boundary diffusion due to the addition of SiO2 and SnO2 and the sintering activation energy increased in these cases. However, GeO2 additive activated the viscous flow mechanism in sintering process of 3Y‐TZP nanopowders which led to acceleration of the densification due to the decrease in the diffusion activation energy.

H-complexes in the “4-n-alkoxybenzoic acid: 4-pyridyl 4′-n-alkoxybenzoate” system. IR spectroscopy and quantum chemical calculations

IR spectra of individual 4-n-dodecyloxybenzoic acid (A) and 4-pyridyl 4′-n-dodecyloxybenzoate (B) compounds as well as IR spectra of 2A:1B and 1A:1B systems were recorded. For the assignment of the experimental vibrational spectra a series of quantum chemical calculations of DFT(B97-D)/6-311++G** level was carried out. Hydrogen-bonded complexes of types A⋯A, A⋯B, as well as various trimers that can be formed at different component ratios in system A-B were simulated. The geometric structure of these complexes was optimized and the vibrational frequencies were calculated. The conclusions on the molecular organization of system A-B for different ratios of the components A and B were based on the interpretation of IR spectra and the analysis of calculated thermodynamic characteristics of self-assembly processes. Thus, it is determined that the system A consists of cyclic A⋯Acycl dimers; in 1A:1B system the H-complexes of A⋯B type are formed. In the 2A:1B system in the process of self-assembly, instead of the complexes A⋯A⋯B and A⋯B⋯A of stoichiometric composition, the complexes A⋯B and A⋯Acycl are formed in the ratio 2:1. These results are confirmed by the dilatometric method data.