Isothermal Dilatometry Research Articles

High‐Precision Dilatometry for the Study of Precipitation Processes and Microalloying Effects in Lightweight Alloys ‐ A Specific Review

By dilatometry, the absolute change in the length of a sample is measured. As length, i.e., volume, is a state variable, its change can be monitored for long time ranges. This is the decisive advantage compared to the widely applied calorimetric methods which are based on a rate (change of heat) and where the maximum time range is determined by the resolution limit of the heat flow measurement. In contrast, if the measurement environment is kept at a constant temperature, dilatometric signals can be measured for very long time ranges down to very small changes with resolution in the nm range. The application of high‐stability isothermal dilatometry for studying precipitation phenomena in Al(Mg,Si)‐, Ti(V)‐, and Ti(Cr)‐alloys is summarized in the present review. Al(Mg,Si) turns out as a prime example where the various phases can unambiguously be distinguished by their characteristic length change features. For Ti(V), the influence of the oxygen impurity on the omega phase formation, and for Ti(Cr), the effect of Sn alloying on the omega phase formation can be revealed by dilatometry. The results on Al(Mg, Si) are compared with dilatometry on Al(Cu)‐alloys upon time‐linear heating, for which also the effect of microalloying with Au is studied.

Study on Transformation Mechanism and Kinetics of α’ Martensite in TC4 Alloy Isothermal Aging Process

The law of microstructure evolution and transformation mechanism of the α′ martensite decomposition during 400–600 °C were studied by the isothermal dilatometry. The transformation process of α′ martensite was quantitatively characterized based on Johnson–Mehl–Avrami (JMA) model, and the microstructure evolution under different aging processes was observed and compared on Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The results showed that α′ → α + β is the elemental diffusion transformation, the position and shape of the precipitate gradually change with the holding time and temperature. The decomposition rate of α′ martensite was positively correlated with the aging temperature. The whole transformation process was divided into two stages based on the value of the Avrami exponent n, and the corresponding average values of the transformation activation energies Q are 46.1 kJ/mol and 116.8 kJ/mol, respectively. The calculated model had good agreement with the experimental data, and the transformation curve of α′ martensite with time and temperature during the isothermal aging at 400–600 °C was drawn.

Microstructural evolution and sintering properties of palygorskite nanofibers

AbstractIn this study, the morphological evolution and sintering properties of the palygorskite nanofibers were studied along with the increase of temperature, using raw palygorskite as materials. The palygorskite powder was calcined at different temperatures in the range of 100°C‐1200°C, and the microstructural evolution of the palygorskite nanofibers was investigated by thermogravimetric and differential thermal analysis (TG‐DTA), X‐ray diffraction (XRD), scanning electron microscopy (SEM), and high‐resolution transmission electron microscope (HRTEM). Furthermore, the palygorskite powder was shaped to bars by dry pressing and sintered from 700°C to 1200°C. The properties of the sintered palygorskite were characterized by bending strength, mercury intrusion porosimeter (MIP), and stepwise isothermal dilatometry (SID). The results showed that the morphology of palygorskite nanofibers maintained unchanged till 1000°C. The palygorskite nanofibers molted to bind each other and formed a solid interwoven network structure at 1100°C. Correspondingly, it was shown from the sharply decrease of the sintered palygorskite porosity from 45.46% at 1000°C to 1.82% at 1100°C that the dense sintering of palygorskite started at 1100°C. With the sintering proceeding, some closed micropores fused each other to form bigger opening pores, resulting in a slight increase of porosity at 1200°C. However, the pore size distribution got more uniform and the density of the sintered body increased. So the bending strength of the sintered body reached the maximum of 176.67 Mpa and finally the main crystalline phases of the sintered sample changed to quartz, enstatite, and kyanite. The sintering activation energy of the palygorskite was measured by means of SID with a value of 906.46 kJ·mol−1.

Sintering kinetics of nanocrystalline Ce0.9Zr0.1O2 prepared by gel combustion method

The sintering process of nanometric Ce0.9Zr0.1O2 mixed oxides prepared by the amino acid–nitrate gel combustion method was analyzed by dilatometry using the constant heating rate (CHR) and stepwise isothermal dilatometry (SID) methods. The morphology of the mixed oxides was studied by scanning and transmission electron microscopies. The evolution of crystallite size with temperature was analyzed by time-resolved X-ray powder diffraction with synchrotron radiation. The sintering behavior resulted dependent on the morphology of the powders. Samples with nanometric crystallite size start to sinter even 500 K before than micrometric sample. We concluded that sintering process of nanometric Ce0.9Zr0.1O2 is controlled by grain-boundary diffusion for temperatures below 1373–1573 K, depending on the morphology of the sample. At high temperature, the grain-boundary and lattice diffusion become parallel controlling mechanism for sintering.

The Preparation and Properties of Porous Sepiolite Ceramics

In this paper, a new type of porous ceramics was prepared using the raw sepiolite mineral. The porous ceramics was shaped by the dry pressing method and sintered in the range of 700 ~ 1200 °C. The temperature-microstructure evolution and the properties of porous sepiolite ceramics were investigated by thermo gravimetric and differential thermal analyses (TG-DTA), X-ray diffraction (XRD), bending strength, compressive strength, scanning electron microscopy (SEM) and mercury intrusion porosimeter (MIP). The sintering kinetics of the porous ceramics from sepiolite was investigated by means of stepwise isothermal dilatometry (SID). The mechanical properties improved with the increasing sintering temperature, and the bending strength and compression strength reached a maximum of 52 MPa and 32 MPa respectively at 1200 °C. The porosity increased with the sintering temperature until 1100 °C attaining the value of 55.40% and then decreased to a value of 46.48% at 1200 °C. The main crystal phases of the porous ceramics were akermanite and diopside. At 1200 °C, the pores inside the ceramics basically follows a unimodal distribution, which was mainly located near 553 nm. The sintering activation energy of porous sepiolite ceramics was measured by step isothermal thermal expansion with a value of 791.42 kJ/mol in the range of 1000 °C to 1200 °C.

High-precision isothermal dilatometry as tool for quantitative analysis of precipitation kinetics: case study of dilute Al alloy

An in-depth case study of precipitation kinetics for alloys is presented utilizing recent progress in high-precision isothermal dilatometry by measuring relative length changes down to the range of 10^{-5} and covering large timescales exceeding 10^5 s. Using a dilute Al–Mg–Si alloy as model system, the different phases that form during isothermal heat treatment could quantitatively be analyzed both with respect to the absolute amount of precipitates and with respect to the underlying kinetics. Owing to the distinct length change features upon multi-step precipitation processes, the formation of the metastable beta ''- and beta '-phases can unambigously be detected and furthermore can specifically be distinguished and resolved. From the reaction rate analysis of the precipitation-induced relative length change, that was isothermally measured for temperatures between 170 ^circ hbox {C} and 260 ^circ hbox {C}, the evolution with time of the atomic fraction of both the beta ''- and the beta '-phase was determined. The results were also used to construct the isothermal time–temperature–precipitation diagrams which are important for technologically relevant processes.

Sintering Kinetics of Porous Ceramics from High-Alumina Fly Ash

ABSTRACTThe sintering kinetics of porous ceramics from high-alumina fly ash was studied by stepwise isothermal dilatometry associated with scanning electron microscopic observation, X-ray diffraction phase identification, and mechanical properties analysis. This work used Makipirtti-Meng method to analyze the date of stepwise isothermal shrinkage. The shrinkage data was fitted well to the rate equation which was analyzed by Makipirtti-Meng method. The apparent activation energy ΔE values obtained at 1250o-1400oC are 679.28 kJ.mol–1. The sintering temperature has a great influence on sintered products’ properties which include open porosity and flexural strength. The results were well supported by microstructure analysis of sintered samples.

Isothermal differential dilatometry based on X-ray analysis applied to stress relaxation in thin ion-beam-sputtered Pt films

Relaxation of stress and point defects in ion-beam-sputtered Pt films with a thickness of 20 and 40 nm during isothermal annealing was investigated. First, isothermal differential dilatometry measurements based on X-ray analysis were carried out between 130 and 400 °C. They show that the relaxation of compressive stress is associated with the formation of vacancies at the surface. From the measurements, an activation enthalpy of 0.14 eV was estimated for the stress relaxation process. In addition, self-diffusion experiments of Pt were carried out on the same type of films using stable 194Pt tracer. From secondary ion mass spectrometry on samples annealed for longer times, an activation enthalpy of 0.5 eV for Pt diffusion in grain boundaries was estimated. The influence of vacancy creation at the surface, vacancy transport, and the annihilation of non-equilibrium bulk interstitials and thermally created vacancies on stress relaxation is discussed.

Evaluating the Sintering Kinetics of Porous Sic Ceramics

Porous ceramics made by burning out a pore-former during the sintering process were investigated by stepwise isothermal dilatometry (SID), x-ray diffraction (XRD) phase identification, scanning electron microscopy (SEM), and mechanical-property analysis. SID data obtained by the Makipirtti-Meng method was analyzed and was found to fit well with data on shrinkage obtained by means of the rate equation proposed for use with this method. Kinetic parameters were evaluated and it was established that sintering takes place through two main mechanisms. These mechanisms are related to the generation and sintering of oxidation products and have activation energies of 55.4 and 325.4 kJ/mole. The sintering temperature greatly affects such properties of sintered specimens as open porosity and ultimate flexural strength. The results obtained in the investigation were well-supported by microstructural analysis of the sintered specimens.

Solid-state phase transformation kinetics in the near-equilibrium regime

Solid-state phase transformation kinetics in the near-equilibrium regime behaves differently from that in extremely non-equilibrium regime since their thermodynamic states are different. Incorporating temperature- and transformed fraction-dependent thermodynamic terms, a thermo-kinetic model is derived to describe the transformation kinetics in the near-equilibrium regime. The model predicts a sluggish stage in isothermally conducted transformation and a temperature-dependent stage in non-isothermally conducted transformation. Then, the kinetics of γ/α transformations in two binary substitutional Fe-based alloys (i.e. Fe-3.28 at.% Mn and Fe-1.73 at.% Co), measured by isothermal and non-isothermal dilatometry, is investigated by the newly proposed model. The model quantitatively describes the retarded isothermal kinetics in Fe-3.28 at.% Mn alloy and the abnormal non-isothermal kinetics in Fe-1.73 at.% Co alloy.

Sintering kinetic study of the nano-crystalline 3-mol% yttria-samaria codoped tetragonal zirconia polycrystal ceramics

Dilatometric shrinkage data was utilized to study the sintering kinetics of the in-house synthesized nano-crystalline 3-mol% yttria-samaria codoped tetragonal zirconia polycrystal (TZP) ceramics. The objective was to determine activation energy (Q) of sintering and the sintering mechanism (n) relevant to the initial stage of sintering. The product of activation energy and sintering parameter, i.e., “nQ” was calculated from the shrinkage data acquired from the constant rate of heating experiment. The apparent activation energy of sintering (Q) was calculated using modified-“Dorn” method. Modified Johnson’s equation was used to determine value of “n” using the activation energy obtained from the Dorn method. Stepwise isothermal dilatometry technique was utilized as an independent method to determine the “n” value. The activation energy of sintering was in the range of 400–525 kJ mol−1 and found to be dependent on the dopant concentration. The value of “n” was found to be ~0.33 for both 3 mol% yttria-doped (3Y-TZP) and yttria-samaria codoped (3(Y,S)-TZP) TZP, whereas for 3 mol% samaria-doped tetragonal zirconia (3S-TZP), the value of “n” was ~0.40. From the obtained “n” values, it may be concluded that grain boundary diffusion (GBD) was the dominant sintering mechanism in 3Y-TZP and 3(Y,S)-TZP, whereas an intermediate of GBD and volume diffusion influences the initial sintering stage in the 3S-TZP.

Kinetic Study of the Austempering Reactions in Ductile Irons

Kinetics of the reaction that occur during the austempering heat treatment in unalloyed and alloyed ductile irons with 1Cu-0.25Mo, 1Ni-0.25Mo, and 0.7Cu-1Ni-0.25Mo, was studied. The austenitization and austempering cycles were achieved by isothermal dilatometry in cylindrical samples of 2 mm in diameter and 12 mm in length. The specimens were austenitized at 870 °C for 120 min, followed by isothermal holding for 300 min at temperatures between 270 and 420 °C. Kinetic parameters such as the order of reaction “n” and the rate of reaction “k” were calculated using the Johnson-Mehl equation while the empirical activation energy was calculated by means of the Arrhenius equation. It was found that the values of “k” decreased with the addition of Cu, Ni, and Mo as well as with the reduction of the isothermal temperature. The activation energy changes with the austempering temperature, in the range 30,348-58,250 J/mol when the heat treatment was carried out between 370 and 420 °C and 10,336-26,683 J/mol when the temperature varied from 270 to 350 °C. The microstructures in samples austempered at 370 and 315 °C were observed by transmission electron microscopy. No carbides precipitation was observed on samples heat treated at 370 °C for less than 120 min, while at 315 °C carbides of hexagonal structure e(Fe2.4C) were found from the beginning of the transformation. The smallest value of activation energy and a slower kinetic transformation seem to be related with the presence of a carbide phase. Additionally, the time results obtained for transformation fractions of 0.05 and 0.95 by the dilatometry analysis were used to build the temperature-time-transformation diagrams for the irons.

Sintering kinetics study of mechanically alloyed nanocrystalline Mo–30 wt.% W

The paper details the results of sintering kinetics studies conducted on nanocrystalline Mo–30 wt.% W alloy powders prepared through mechanical alloying route. Both, constant rate of heating method as well as Stepwise Isothermal Dilatometry (SID) technique were used for studying the sintering kinetics. Measured step isothermal shrinkage data were analyzed by Mekipritti-Meng method. The shrinkage data was found to fit well with the rate equation proposed in this method and its validity was established for mechanically alloyed systems. Kinetic parameters were evaluated and sintering was found to occur through two major mechanisms operative successively, which are grain boundary diffusion and lattice diffusion with corresponding energies of activation as 230 kJ/mol and 480 kJ/mol, respectively. The results have been well supported by micro structural evaluation of specimens at different stages of sintering.

Advanced Multi-Metallic SOFC Anode Development by Mechanical Alloying Route

Anodes composed of Ni-YSZ (yttria-stabilised zirconia) cermets are the key material to allow direct biofuel feeding to Solid Oxide Fuel Cell (SOFC) devices due to its internal reforming capability. The main challenge among these materials is related to carbon deposition poisoning effect when C-bearing fuels are feed. The work deals with these issues by alloying Ni with some metals like Cu to conform a multi-metallic anode material. Mechanical alloying (MA) at shaker mills is chosen as the route to incorporate the metal and ceramic powders in the anode material, also leading to better sintering behaviour. A projected cermet material is conceived where a third metal can be added based on two criteria: low Cu solubility and similar formation enthalpy of hydrides regarding Ni. Refractory metals like Nb, W and Mo, seems to fulfil these characteristics, as well as Ag. The MA resulted powder morphology is highly homogeneous showing nanometric interpolated metal lamellae. The sintering behaviour is investigated by conventional dilatometry as well as by stepwise isothermal dilatometry (SID) quasi-isothermal method to determine the sintering kinetic parameters. Based on these tools, it is found the Cu additive promotes sintering to obtain a denser anode and therefore allowing lower process temperatures. The consolidation is achieved through the sintering by activated surface (SAS) method allied to liquid phase sintering process, where the third metal additive also has influenced. The final cermet can be obtained at one sole process step, dispensing pore-forming additives and reduction treatments. The sintered microstructure demonstrates the material is homogeneous and possesses suitable percolation networks and pore structure for SOFC anode applications.

Low‐Temperature Densification of Lanthanum Strontium Manganite (La1−xSrxMnO3+δ), x=0.0–0.20

Intermediate‐stage sintering of lanthanum strontium manganite (LSM, where Sr=0.00, 0.05, 0.10, 0.15, and 0.20) was shown in dilatometry studies to be accelerated when subjected to alternating flows of air and nitrogen. The extent of rate enhancement decreased with increased Sr content, and decreased with increased temperature, which coincides with diminished oxygen nonstoichiometry. Shrinkage rates were further shown to be sensitive to the difference in oxygen content in the alternating gas flows. Baseline air sintering rates were measured using stepwise isothermal dilatometry, from which kinetic parameters were calculated using the Makipirtti–Meng model. Activation energies for sintering in air were determined to be 255 ± 26, 258 ± 28, 308 ± 32, 373 ± 37, and 417 ± 41 kJ/mol for Sr=0.0, 0.05, 0.10, 0.15, and 0.20, respectively. A diffusion‐based model is proposed that is consistent with trends in accelerated shrinkage versus temperature. Transient cation vacancy gradients, which lead to higher cation mobility, were calculated from established oxygen diffusivities and oxygen nonstoichiometry as a function of temperature and time. A potential application of this approach is the processing of LSM‐based cathode‐side contact pastes in solid oxide fuel cells.

Sintering studies on Ni–Cu–YSZ SOFC anode cermet processed by mechanical alloying

New 40 vol%[(Cu)–Ni]–YSZ cermet materials processed by mechanical alloying (MA) of the row powders are prepared. The powder compacts are sintered in air, hydrogen and inert (argon) atmospheres at a dilatometer and tubular furnace up to 1,350 °C. Sintering by activated surface concept (SAS) can anticipate and enhance the densification in such powders. Stepwise isothermal dilatometry (SID) sintering kinetics study is performed allowing determining kinetic parameters for Ni–YSZ and Ni–Cu–YSZ pellets. Two-steps sintering processes is indicated while Cu-bearing material features the smallest activation energy for sintering. The allied MA–SAS method is a promising route to prepare SOFC fuel cell anode materials.

Sintering kinetics of submicron sized cobalt powder

The paper details the results of sintering kinetics studies on submicron sized fine cobalt metal powders prepared through oxalate decomposition route using both conventional sintering method as well as stepwise isothermal dilatometry (SID) technique. Powder preparation and processing parameters such as decomposition temperature, compaction pressure and sintering atmosphere, etc. were optimized to achieve highest sintered density with lowest activation energy through conventional sintering. Role of submicron sized powder and surface contamination layers on initial shrinkage was explained towards early shrinkage of cobalt compacts by dilatometric analysis. Measured step isothermal shrinkage data were analyzed by Makipirtti–Meng method. The shrinkage data were found to fit well with the rate equation proposed in this method and its validity was established for the metallic systems also. Kinetic parameters were evaluated and sintering was found to occur through three major mechanisms operative successively, which are grain boundary diffusion, lattice diffusion and plastic/viscous flow with the energy of activation as 135, 234 and 373 kJ/mol, respectively. The results were well supported by microstructural analysis of sintered specimens.

Formation and kinetics of porous cordierite from fly ash

Porous cordierite ceramics were prepared from a mixture of coal fly ash and basic magnesium carbonate at 1100–1350°C. Porosity, flexural strength and thermal expansion coefficient of the samples sintered at 1300°C were estimated to be 26%, 65 MPa and 4.21×10−6/°C, respectively. The kinetics of the formation progress was investigated by stepwise isothermal dilatometry (SID) accompanied with XRD, SEM and porosity measurement. It was found that the isothermal shrinkage data from SID could be well analyzed to get kinetic parameters according to the empirical rate equation developed by Makipirtti-Meng, dY/dt = nk(T)Y(1 − Y)(Y/1 − Y)n/1, where Y is the fractional shrinkage during the sintering process and n is a dimensionless component. The apparent activation energy ΔE values for 900–1000°C and 1050–1150°C were 1294 and 1778 kJ/mol, respectively.

Nickel-Zirconia cermet processing by mechanical alloying for solid oxide fuel cell anodes

This paper describes the development of a process based on high energy milling (or mechanical alloying—MA) of metallic Ni and YSZ at 40 vol% Ni composition for the preparation of solid oxide fuel cell anode material. The cermet powder is consolidated using the surface activated sintering (SAS) method. The cermet pellets possess microstructural characteristics that can potentially lead to higher electrocatalytic activity and fuel reforming capability. In addition to the development of a new processing method for this purpose, a further differential of this work is the addition of Cu in partial substitution of Ni as a means to prevent the formation of carbon on its surface and, hence, the anode’s degradation during service. The prepared powder samples are well dispersed and structured at the nanometric level, showing thin lamellar constituents. Suitable sintered pellets can be obtained from the powders with the required porosity and microstructure. The higher the energy delivered by MA the lower the initial sintering temperature. Activation energies are determined by stepwise isothermal dilatometry (SID) for Ni-YSZ and Ni/Cu-YSZ pellets, involving a 2-step sintering process. The Cu additive promotes sintering and leads to a refined microstructure.

Enhanced Shrinkage of Lanthanum Strontium Manganite (La0.90Sr0.10MnO3+δ) Resulting from Thermal and Oxygen Partial Pressure Cycling

Exposure of (La0.90Sr0.10)0.98MnO3+δ (LSM‐10) to repeated oxygen partial pressure cycles (air/10 ppm O2) resulted in enhanced densification rates, similar to behavior shown previously due to thermal cycling. Shrinkage rates in the temperature range 700°–1000°C were orders of magnitude higher than Makipirtti–Meng model estimations based on stepwise isothermal dilatometry results at a high temperature. A maximum in enhanced shrinkage due to oxygen partial pressure cycling occurred at 900°C. Shrinkage was the greatest when LSM‐10 bars that were first equilibrated in air were exposed to gas flows of lower oxygen fugacity than in the reverse direction. The former creates transient cation and oxygen vacancies well above the equilibrium concentration, resulting in enhanced mobility. These vacancies annihilate as Schottky equilibria are reestablished, whereas the latter condition does not lead to excess vacancy concentrations.