Dilatometry Studies Research Articles

Wetting phenomena for liquid Cu and Cu alloys on Fe-base substrate

Liquid phase sintering is used for the consolidation of powder based materials to enhance densification by the activation of diffusion rates and sintering kinetics. However, an inappropriate interaction between the liquid and the solid powdered skeleton can promote excessive swelling instead of densification, and the purpose of the liquid phase process would not be fulfilled. In the Powder Metallurgy (PM) industry, Cu is among the main alloying elements used in the production of low alloyed sintered steels. Cu promotes the formation of transient liquid below the common sintering temperatures (1120°C). Although it favors rapid homogenization and can be used to effectively transport other alloying elements, it causes a significant swelling effect upon melting that needs to be controlled in order to maintain a proper dimensional stability. The dimensional expansion associated to Cu melting and its distribution in a Fe-base system has been mainly treated in the literature on the basis of dilatometry studies. A systematic study of the influence of different sintering parameters has never been included in that discussion and the information in the literature about the wetting behavior of liquid Cu or Cu alloys is scarce. This work presents an insight into Cu wetting behavior on plain Fe and steel substrates – which is compared to Cu-base a Ni-base liquid formers – using sessile drop experiments. The sensitivity to wetting conditions is addressed by modifying atmosphere conditions (either H2 containing atmosphere or Ar is used) and interfacial interactions between the liquid and the solid (each liquid former presents a dissolutive condition in contact with Fe, the Cu-based liquid dissolves up to 1.5wt.% Fe while the Ni-based around 35wt.%)

Tailoring master alloys for liquid phase sintering: Effect of introducing oxidation-sensitive elements

The use of low melting point master alloy (MA) powders contributes beneficially to sintering by increasing the distribution rate of alloying elements, enhancing homogenisation and sometimes also promoting densification. However, working with liquid phases poses important challenges like maintaining a proper dimensional control and minimising the effect of secondary porosity on the final performance of the steel. In this work, three different MA systems are compared: a low dissolutive Cu-based MA, and two systems with a higher degree of iron dissolution but different content in oxidation-sensitive elements. The combination of wetting experiments, step sintering tests and dilatometry studies show how the evolution of the microstructure, dimensional stability and overall densification are strongly affected by the characteristics of the liquid MA and in particular by its ability to dissolve the iron base particles, and by the amount of oxidation-sensitive elements present in the composition of the MA powder.

Microstructural development and effect of different Cu/C contents on dimensional changes during sintering of PM steels

Powder blends within the Fe–Cu–C system are extensively used by the Powder Metallurgy industry. Based on the optimisation of mechanical properties the purpose of this work is to further understand the dimensional changes produced in the powder compact during sintering. Green compacts containing copper and carbon within ranges 0.5–3.5%Cu and 0.3–0.9%C were die pressed and sintered under different conditions varying processing parameters, including compaction pressure, temperature and time. Better understanding of the effect of different Cu/C combinations on dimensional changes was approached using the results obtained from dilatometry studies combined with interrupted sintering experiments. For selected specimens the effect of elemental Ni additions, at 1 and 4%, on dimensional changes was studied along with a detailed description of microstructural development. It was clearly observed that swelling due to copper melting and grain boundary diffusion is diminished as graphite additions were increased. The impact of Cu–C–Ni additions on hardness was also evaluated.

Finite element analysis of the residual thermal stresses on functionally gradated dental restorations

The aim of this work was to study, using the finite element method (FEM), the distribution of thermal residual stresses arising in metal–ceramic dental restorations after cooling from the processing temperature. Three different interface configurations were studied: with conventional sharp transition; one with a 50% metal–50% ceramic interlayer; and one with a compositionally functionally gradated material (FGM) interlayer. The FE analysis was performed based on experimental data obtained from Dynamic Mechanical Analysis (DMA) and Dilatometry (DIL) studies of the monolithic materials and metal/ceramic composites.Results have shown significant benefits of using the 50% metal–50% ceramic interlayer and the FGM interlayer over the conventional sharp transition interface configuration in reduction of the thermal residual stress and improvement of stress profiles. Maximum stresses magnitudes were reduced by 10% for the crowns with 50% metal–50% ceramic interlayer and by 20% with FGM interlayer. The reduction in stress magnitude and smoothness of the stress distribution profile due to the gradated architectures might explain the improved behavior of these novel dental restorative systems relative to the conventional one, demonstrated by in-vitro studies already reported in literature.

Melting and Microstructure Analysis of β-Ti Alloy Ti–5Al–5Mo–5V–1Cr–1Fe With and Without Boron

β-Titanium alloys form one of the most versatile classes of structural materials due to their high specific strength, good hardenability, crack propagation resistance and substantial ductility. β-Titanium alloy with a composition (in wt%) of Ti–5Al–5Mo–5V–1Cr–1Fe was processed by double vacuum arc remelting route. In the present work, the effect of boron addition (up to 0.12 wt%) on the as-cast microstructure and β-transus (Tβ) of the alloy was studied using characterization tools like optical microscopy, electron back scattered diffraction, scanning electron microscopy, differential scanning calorimetry (DSC) and dilatometry. It was observed that boron addition has resulted in refinement of the as-cast microstructure due to precipitation of titanium boride whiskers along the grain boundaries. The DSC and dilatometry studies on the as-cast alloy revealed significant effect of boron addition on thermal stability of the alloy.

Self- and Solute Diffusion, Interdiffusion and Thermal Vacancies in the System Iron-Aluminium

Starting from fundamental aspects of thermal vacancies and solid-state self-and solute diffusion, this paper reviews procedures for tracer-and interdiffusion studies and of the major techniques for vacancy studies by dilatometry and positron annihilation in metals. Equilibrium vacancy and diffusion studies performed on pure iron and aluminium are mentioned at first. We also comment some peculiarities of solute diffusion in aluminium. Positron annihilation and differential dilatometry studies for Fe-Al alloys with various compositions are summarized and new experimental studies by the authors are reported for vacancy migration in Fe61Al39. All these studies indicate a relatively high fraction of thermal vacancies with relatively low mobility in this type of iron-aluminides as compared to pure metals. Tracer diffusion of iron and of several substitutional solutes such as Co, Ni, Cr, Mn, Zn, and In in Fe-Al from the Münster laboratory are summarized. The diffusion studies of Fe-Al cover various alloy composition between Fe3Al and FeAl and several structures such as A2, B2 and D03. Interdiffusion coefficients obtained from diffusion couples between Fe-Al alloys are discussed together with Fe tracer diffusion data. The Darken-Manning equation is used to deduce Al diffusivities therefrom. The latter are hardly accessible to radiotracer experiments due to a lack of a suitable Al tracer. Diffusion of Al is slightly faster than diffusion of Fe indicating diffusion mechanisms with coupled jumps of Fe and Al atoms.

Superior sinterability of nano-crystalline gadolinium doped ceria powders synthesized by co-precipitation method

Reduced sintering temperature of doped ceria can greatly simplify the fabrication process of solid oxide fuel cells (SOFCs) by utilizing the co-firing of all cell components with a single step. In the present study, nano-crystalline gadolinium doped ceria (GDC) powders of high sinterability at lower sintering temperature has been synthesized by co-precipitation at room temperature. The successful synthesis of nano-crystalline GDC was confirmed by XRD, TEM and Raman spectroscopy analysis. Dilatometry studies showed that GDC prepared by this method can be fully densified (97% relative density) at a sintering temperature of 950 °C which is fairly lower than ever before. It has also been found that the sintered samples have a higher ionic conductivity of 1.64 × 10 −2 S cm −1 at 600 °C which is suitable for the intermediate temperature SOFC application.

Residual strength properties of sodium silicate alkali activated slag paste exposed to elevated temperatures

The residual compressive strength behavior of alkali activated slag paste (AASP) after temperature exposures up to 1,200°C was investigated. Strength loss of approximately 60% occurred between 100 and 200°C and a further strength loss in the order of 30% at 800°C. Total loss of strength occurred at 1,200°C. Thermogravimetric studies (TGA/DTG) verified AASP contained no Ca(OH)2 which governs the chemical mechanism of strength loss for ordinary Portland cement (OPC) and blended slag cement pastes. However, the TGA results showed that AASP had a higher water loss than the other binders between 100 and 200°C and higher thermal shrinkage as indicated by the dilatometry studies. The high thermal shrinkage led to a differential thermal shrinkage gradient within the AASP and induced micro stresses and cracking which was more prominent for larger samples. Differential thermal shrinkage caused by the higher thermal shrinkage of the AAS material was concluded as the mechanism which gives lower residual strength in AASP compared to OPCP.

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.

Residual compressive behavior of alkali‐activated concrete exposed to elevated temperatures

AbstractThis paper reports the effect of elevated temperature exposures, up to 1200°C , on the residual compressive strengths of alkali‐activated slag concrete (AASC) activated by sodium silicate and hydrated lime; such temperatures can occur in a fire. The strength performance of AASC in the temperature range of 400–800°C was similar to ordinary Portland cement concrete and blended slag cement concrete, despite the finding that the AASC did not contain Ca(OH)2 , which contributes to the strength deterioration at elevated temperatures for Ordinary Portland Cement and blended slag cement concretes. Dilatometry studies showed that the alkali‐activated slag (AAS) paste had significantly higher thermal shrinkage than the other pastes while the basalt aggregate gradually expanded. This led to a higher thermal incompatibility between the AAS paste and aggregate compared with the other concretes. This is likely to be the governing factor behind the strength loss of AASC at elevated temperatures. Copyright © 2008 John Wiley & Sons, Ltd.

Reversible atomic processes as basic mechanisms of the glass transition

Reversible formation and disappearance of vacant spaces (vacancy-type defects) in bulk Zr57Cu15.4Ni12.6Nb5Al10 glass are directly evidenced by high-resolution, time-differential dilatometry studies. The vacancy kinetics are strongly temperature-dependent, with an effective migration enthalpy of HVM=3.34 eV. This may explain the strong temperature dependence of glass properties such as viscosity. The results presented here are of general importance for understanding amorphous condensed matter and biomaterials and for the technical development of amorphous steels.

Phase Transitions in p-(Phenyl Benzylidene)-p1-Alkylaniline Compounds: A Dilatometric Study

Dilatometric studies are carried out on liquid crystals (LCs), viz., p-(phenyl benzylidene)-p1- alkylanilines (PBnA for n = 7 - 10, 12, 14 and 16). LC PBnA compounds are found to exhibit four different types of phase variants. The lower homologues (for n = 3 - 6) exhibit an NBE trivariant, intermediate homologues (with n = 7 - 9) exhibit an NABE tetravariant, higher homologues (for n = 10 - 12) exhibit an ABE tetravariant, and long higher homologues (for n = 14 and 16) exhibit an AB bivariant phase sequence. Dilatometry studies in PBnA compounds infer the first order nature for IN (for n = 7, 8 and 9), IA (for n = 10, 12, 14 and 16), NA (for n = 8 and 9) and AB (for n = 14 and 16) phase transitions. Phase transitions involving the growth of the 3D smectic-B phase in PBnAs are found to be tuned by squeezing of orientational disorder. A comparative study of phase transitions exhibited by PBnA compounds is presented along with the data in other Schiff base compounds.

Synthesis, Characterization, and Dilatometric Studies on N-(p-n-Alkoxybenzylidene)-p-n-pentyloxyanilines Compounds

Synthesis, textural characterization, differential scanning calorimetry, and dilatometry studies are carried out on a new homologous series of N-(p-n-alkoxy benzylidene)-p-n-pentyloxy anilines (nO.O5) where n = 1–18. The lower homologues of this series (n = 1–5) and a higher homologue (n = 16) exhibit monotropic Liquid Crystal (LC) phase. The other members of the series show enantiotropic LC phases, while 18O.O5 is a nonliquid crystal. The dilatometric studies on 7O.O5, 10O.O5, 13O.O5 and 16O.O5 reveal the first-order nature of Isotropic to Nematic Phase (IN), Isotropic to Smectic A Phase (IA), Nematic to Smectic C Phase (NC), Smectic A to Smectic B Phase (AB), Smectic C to Smectic B Phase (CB), and Smectic A to Krystal Phase (AK) transitions. The pretransition effects are estimated from dilatometry results. The results are compared with the available data on nO.m compounds.

Mixed Conductivity of Ce- and Nb-Substituted (La, Sr)FeO<sub>3-δ</sub>

Due to relatively high oxygen permeability, lanthanum-strontium ferrite phases are of interest as ceramic membrane materials for the partial oxidation of natural gas. This work was focused on the study of perovskite-type ferrites co-doped with Sr2+ and Ce4+ or Nb5+, with particular emphasis on the ionic transport and thermodynamic stability limits at low oxygen chemical potentials. Dense membranes of La0.5-2xCexSr0.5+xFeO3-δ (x = 0 - 0.2) and La0.5-2ySr0.5+2yFe1-yNbyO3-δ (y = 0 - 0.1) were characterized employing X-ray diffraction (XRD), scanning electron microscopy (SEM), dilatometry, oxygen permeation and faradaic efficiency studies, and the measurements of total conductivity and Seebeck coefficient in the oxygen partial pressure range from 10-20 to 0.5 atm. The incorporation of Ce4+ or Nb5+ was found to decrease thermal expansion and electronic transport parameters, whereas the ionic conductivity behavior is complex, indicating the relevance of redox interactions of the variable-valence cations and the concentration of mobile oxygen vacancies.

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.

Temperature evolution of crystal field interactions across the Jahn-Teller transition in aLa7/8Sr1/8MnO3single crystal

We present electron-spin-resonance (ESR), x-ray-diffraction, and dilatometry studies in a ${\mathrm{La}}_{7/8}{\mathrm{Sr}}_{1/8}{\mathrm{MnO}}_{3}$ single crystal. ESR experiments were performed in the paramagnetic regime ${(T}_{C}$ $\ensuremath{\approx}180\mathrm{K}),$ from 220 K to 570 K at a microwave frequency of 9.4 GHz. We measured the ESR linewidth as a function of temperature with the static magnetic field $\mathbf{H}$ parallel to the crystallographic directions [100] and [001], referred to as the orthorhombic (Pbnm) axes. Detailed angular variations at constant temperature for $\mathbf{H}$ rotating in the (001), (010) and $(1\ifmmode\bar\else\textasciimacron\fi{}10)$ planes were also performed. The temperature dependence of the linewidth for fixed directions clearly reflects the changes in the symmetry of the crystal. At the Jahn-Teller (JT) transition, a very noticeable decrease of the linewidth is observed. The angular dependence of the linewidth reveals an anisotropy that is temperature dependent. The amplitude of the variations decreases with T and almost disappears above ${T}_{\mathrm{JT}}\ensuremath{\approx}280\mathrm{K}.$ However, there is a small remnant anisotropy present up to 400 K. We have carefully investigated the temperature dependence of this anisotropy across the Jahn-Teller transition. We showed that it is related with the evolution of the crystal field interactions. The symmetry of the ESR spectra is explained in terms of a spin Hamiltonian that includes isotropic Heisenberg superexchange, single-ion-crystal field, and Dzyaloshinsky-Moriya antisymmetric exchange. The experimental angular variation of the ESR linewidth and its temperature dependence were fitted using the calculated expression for the linewidth as obtained from a perturbative Kubo-Tomita approach. From the ESR data we have obtained a ratio $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{E}/\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{D}=\ensuremath{-}0.4(1)$ for the spin Hamiltonian coefficients associated with the orthorhombic $(\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{E})$ and tetragonal $(\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{D})$ Jahn-Teller distortions of the ${\mathrm{MnO}}_{6}$ octahedra. The temperature dependence of the parameters of the model is presented and the correlation with the dilatometry results are discussed.

Promotion of deactivated sintering by dry‐particle coating

AbstractDry‐particle coating was used to examine the feasibility of increasing the sintering temperatures of different particulate materials (hosts) by applying a discrete coating of a highly refractory material (guest). Increasing the sintering temperature reduces the sintering rate of the material, and hence the phenomenon is called “deactivated sintering.” Several host particles, such as glass beads, PMMA, γ‐alumina, and an alumina–silica composite, were coated with submicron‐size silicon carbide in three different dry‐particle coating devices: magnetically assisted impaction coating, mechanofusion, and hybridizer. The coated products were characterized using several techniques, including particle‐size analysis, SEM, EDX, BET, optical microscopy, and dilatometry studies to compare the coating performance of each device. By coating the host particles with silicon carbide, it was possible to appreciably increase the sintering temperatures of all four of the particulate host materials. Simple phenomenological sintering models based on volume diffusion for crystalline materials and viscous flow for amorphous materials were combined with dilatometry data to explain the phenomenon of deactivated sintering. For γ‐alumina, there was a significant reduction in the effective diffusivity due to the addition of a surface coating of SiC, whereas for SiC‐coated glass beads and PMMA, an increase in the effective viscosity was postulated as the cause of deactivated sintering.

Heating microscopy study of the sintering behaviour of glass powder compacts in the binary system SiO 2–TiO 2

Glass powders in the binary glass-forming SiO 2–TiO 2 system were synthesized by very slow hydrolysis of metal alkoxides using sol–gel process. The sintering behaviour of calcined glass powders was investigated using heating microscopy. Both the densification and crystallisation stages were studied. It was found that the sintering temperature, T sin, and the crystallisation temperature, T cry, decrease by increasing TiO 2 content, in agreement with previous results from dilatometry studies. Slightly but consistent higher T sin values were measured using heating microscopy in comparison with dilatometer data. The results confirm that heating microscopy is a useful, non-invasive technique to study sintering and crystallisation phenomena in glass powder compacts, presenting some advantages over dilatometry, especially the absence of a compressive load acting on the sample during the experiment.

Atomic vacancies in quasicrystals—a study by positron annihilation spectroscopy

For studying atomic vacancies in quasicrystals the specific technique of positron annihilation spectroscopy has been employed. In quasicrystals and approximants of the Al–Pd–Mn and the Zn–Mg–Ho systems structural vacancies in the concentration range of 10 −3 have been observed. We perform the first in-situ measurements of the positron lifetime at high temperatures, where the formation of thermal vacancies in i-Al 70.2Pd 21.3Mn 8.5 can be observed directly. The annealing behavior of quenched-in vacancies, which coincide with the data from earlier time differential dilatometry studies after fast temperature changes, gives access to the vacancy migration properties and kinetics in quasicrystals. From the coincident measurement of the Doppler broadening of the positron-electron annihilation photon line at ambient temperatures, we deduce that the vacancies observed in i-Al 70.2Pd 21.3Mn 8.5 are predominantly located in an Al rich environment. Since high-temperature processes such as diffusion, plasticity and structural phase transformations, are strongly influenced by thermal vacancies and their kinetics, the present experiments can provide information on these processes.

Atomic defects in quasicrystals: an approach with positron annihilation spectroscopy and time-differential dilatometry

Atomic defects have been studied in Al–Pd–Mn and Zn–Mg–Ho icosahedral quasicrystals by means of positron annihilation spectroscopy and time-differential dilatometry. In addition, the first in situ positron lifetime measurements on Al–Pd–Mn quasicrystals at high temperatures are performed. In the time-differential dilatometry studies, after fast cooling at high temperatures an anomalous time-dependent isothermal expansion of the specimen was observed. If this length change is attributed to atomic defects the defect formation and the migration can be studied.Positron lifetimes could be derived for icosahedral Al70.2Pd21.3Mn8.5 and icosahedral Zn61Mg28Ho11 with 214 and 212ps, respectively. These lifetimes are ascribed to structural vacancies in these quasicrystalline systems, which are supported, by electron irradiation and long-term annealing experiments.