Differential Dilatometry Research Articles

Fabrication of diamond particles reinforced Al-matrix composites by hot-press sintering

Al-matrix composites reinforced by 50vol.% diamond particles under a 60MPa sintering pressure were fabricated by a vacuum hot-pressing method. The composite obtained a relative density of 96.5%. The coefficient of thermal expansion (CTE) and the thermal conductivity (TC) of the diamond/Al composites were measured by the laser flash method and differential dilatometry, respectively. Results showed that diamond/Al composites have high TC and low CTE with high sintering pressure (60MPa) and high volume fractions of diamond particles (50vol.%). The TC of the 50vol.% diamond/Al composite was 321W/mK, which is 112W/mK higher than that of pure Al (209W/mK). At temperatures ranging from 298K to 573K, the composite obtained low CTEs in the range of 13.2×10−6/K to 8.3×10−6/K, which satisfied the CTE of electronic packaging materials. The CTE values obtained in the experiment were approximately equal to the CTE calculated by the Kerner model [17]. The effects of volume fractions of diamond particles and sintering pressure on the density, TC, and CTE of diamond/Al composites were investigated.

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.

Kinetics of Martensite Formation in Substitutional Fe-Al Alloys: Dilatometric Analysis

High-resolution differential dilatometry was employed to study the kinetics of the martensite formation upon isochronal cooling/quenching of substitutional Fe-(0.5, 0.7, and 1.0) at. pct Al alloys at fast cooling/quenching rates in the range of 17 K (17 °C) through 100 K (100 °C) s−1, with an emphasis on the as-yet unexpected influence of cooling/quenching rate. The martensite transformation initiated at nearly the same temperature (i.e., the \( M_{\text{S}} \) temperature) in the ferrite-phase region for all cooling/quenching rates applied, which indicates athermal nucleation: the chemical driving force governs the initiation of the nucleation of the martensite plates. Variation of the cooling/quenching rates revealed two principal kinetic features: both the temperature ranges passed during transformation and the grain size of the product martensite increase with the increase of cooling/quenching rates. A modular phase-transformation model, incorporating a classic partitioning analysis for nucleation and anisotropic growth for impingement, has been employed to extract the velocity of the migrating martensite/austenite interface from the dilatometric data. The thus obtained velocity of the martensite/austenite interface as function of temperature indicates a thermally activated growth governed by relatively lower activation energy, as determined by evaluation of the martensite-formation-rate maximum as function of cooling/quenching rate.

Phase transition studies in symmetric dimeric liquid crystals

As a part of the systematic studies on symmetric liquid crystal dimer homologous series, α,ω-bis-(4-n-alkylaniline benzylidene-4′-oxy) alkanes, (referred to as m.OnO.m with m = 3, 4, and 5; and n = 8, 9, and 10), we present in this article the nature of phase transitions across isotropic–nematic and nematic–smectic-A (N–SmA) phases exhibited by the just mentioned compounds. The methods employed are differential scanning calorimetry and dilatometry. The compounds studied were 3.O8O.3, 4.O8O.4, and 5.O8O.5; and 3.O9.03, 5.O9O.5, 3.O10O.3, 4.O10O.4, and 5.O10O.5. Different from the case of their corresponding monomers, all these compounds exhibit a nematic phase only with the exception of 5.O8O.5 which exhibits a SA phase in addition to the nematic phase. The phase transitions viz., isotropic–nematic transitions studied in all these compounds were confirmed to be of first-order nature, whereas the N–SmA transition exhibited by the compound 5.O8O.5 only was found to be of second-order nature. We also report in this article the calculated density jumps, thermal expansion coefficient maxima, and pressure dependence of transition temperatures which are analyzed in the light of the available literature data.

The Kinetics of Austenization Process of T91 Ferritic Heat-Resistant Steel

By fitting the calculated transformed fraction according to developed phase-transformation model to the experimental data obtained by differential dilatometry, the kinetic characteristics of the austenitization process in T91 steels have been investigated. According to the kinetic parameters fitted, we recognize that the nucleation and growth of austenite grain are mainly controlled by the diffusion of carbon in ferritic and austenite respectively. In addition, by increasing the diffusion active energy of carbon in austenite, carbides hinder the motion of interface and thus refine austenite grain.

Kinetics of isochronal austenization in modified high Cr ferritic heat-resistant steel

Employment of high Cr ferritic steels as a main structural material is considered as a way to achieve economical competitiveness of main steam pipe and nuclear reactors in power plants. Differential dilatometry and microstructure observation were employed to investigate the isochronal austenitic transformation of the modified high Cr ferritic steel. The kinetics of the isochronal austenitic transformation were described by a phase-transformation model involving site saturation (pre-existing nuclei), diffusion-controlled growth, and incorporating an impingement correction. The experimental results and kinetic analysis indicate that an increase of the heating rate promotes the diffusion-controlled austenitic transformation. The dissolving degree of precipitates during the austenization process affects the activation energy for diffusion and the undissolved precipitates lead to an increase of the onset temperature of the subsequent martensite transformation upon cooling.

Kinetics of the Austenite-Ferrite Transformation with and without Applied Stress

The formation of ferrite (α) from austenite (γ) and vice versa, upon thermo-mechanical processing of steels, are phase transformations of great technological importance. Often these transformations occur in the presence of externally or internally imposed stress. This paper provides an overview of recent research on the quantitative analysis of the transformation kinetics of the γ®a and a®g transformations subjected to uniaxial compressive stress below the yield stress of g and a, based on the application of the high-resolution differential dilatometry and the modular model of transformation kinetics. The application of uniaxially compressive stresses leads to antagonistic effects on the transformation kinetics: the stress applied upon the γ®a transformation prompts the transformation, while it obstructs the a®g transformation. These results can be quantitatively discussed in terms of chemical driving forces and transformation-induced deformation energies.

The Kinetics of the Austenite-Ferrite Phase Transformation of Fe-4Cr Alloy during Continuous Cooling

The massive transformation, austenite (γ) → ferrite (α), in Fe-4Cr alloy subjected to cooling from the high-temperature austenite down to room temperature at cooling rates of 75, 150, 330 K min-1 were experimentally investigated by means of high-resolution differential dilatometry. It is found that the onset transformation temperature and the overall transformation time decrease with increasing the cooling rate applied. Furthermore, by fitting the calculated transformed fraction according to developed phase-transformation model to the experimental data obtained by differential dilatometry, the kinetic parameters are obtained which show the activation energy for growth and the activation energy for nucleation are considerably different, this may be due to considerably different interfacial structure with the matrix for minute embryos and for the much larger growing particles.

The Possibility of New Ordering Reaction in Cu-50 mass% Au Alloy

Among the large number of superlattices, there is a special group which may be classified as long period superlattices, the copper-gold system provides a classical example of such structure. Despite that historically, the copper-gold alloys containing about 25 atom percent gold were among the first investigated alloys, it still represents a subject of numerous studies. In the present work, we study all the possible phase transitions in the binary intermetallic alloy Cu3Au using differential scanning calorimetry (DSC) and differential dilatometry. A new important anomaly due certainly to a new ordered reaction has been detected at low temperature range by these two methods. To confirm this new reaction, we have used other techniques as high resolution transmission electron microscopy (HRTEM), electrical resistivity and microhardness measurements. The kinetics parameters as activation energy Eact and Avrami exponent n have also been calculated respectively by Starink and John Mehl Avrami Kolmogorov (JMAK) methods.

Analysis of decomposition processes of ausferrite in copper–nickel austempered ductile iron

Analysis of thermal decomposition processes of the ausferrite obtained during simulation of austempering heat treatment was performed on austempered ductile iron (ADI). The analysis method consisting in investigation of inverse phase transformations was applied. The material with specified phase composition and well-defined thermophysical properties, both resulting from the conducted heat treatment cycle, was heated under controlled conditions and the thermal effects—enthalpy change and volume change were recorded by means of differential scanning calorimetry (DSC) and differential dilatometry. The process of ausferrite decomposition in the range of 100–800 °C was discussed; the identification scheme and temperature sequence of phase transformations accompanying the ausferrite decomposition were established. The elaborated decomposition scheme allows selection of the ADI heat treatment and its optimization by means of the non-isothermal thermal analysis methods.

The coupling between first-order martensitic transformation and second-order antiferromagnetic transition in Mn-rich γ-MnFe alloy

The fcc → fct martensitic transformation of a Mn-rich γ-MnFe alloy with a (011) martensite twin was investigated using transmission electron microscopy, dynamic mechanical analysis, electrical resistance measurements, differential scanning calorimetry and dilatometry. Experimental results show that the start temperature (M s) of the martensitic transformation and the temperature (T N) of the antiferromagnetic transition in Mn85.5Fe9.5Cu5 alloys are close and almost coincide. Direct and reverse martensitic transformations, normally a first-order transformation, exhibit features similar to those of a second-order continuous transition, which is attributed to coupling between the first-order martensitic transformation and the second-order antiferromagnetic transition.

Hydrogen Pick up in Zircaloy-4: Effects in the Dimensional Stability of Structural Components under Nuclear Reactor Operating Conditions

In the present work, the expansion coefficient due to hydrogen incorporation was measured for the axial direction of a Zircaloy-4 cooling channel, similar to that installed in the Atucha I PHWR, Argentina, trying to simulate the nuclear power reactor operating conditions. As a first step, the solubility curve of hydrogen in Zircloy-4 was determined by two techniques: differential scanning calorimetry and differential dilatometry. The comparison with classical literature curves showed a good agreement with them, although the calorimetric technique proved to be more accurate for these determinations. Dilatometry was able to detect the end of hydride dissolution from concentrations around 60 wppm-H up to 650 wppm-H, where the eutectoid reaction: α + δ→α + β takes place (at 550oC). We assume that this ability is a good indicator of the aptitude of the technique to measure dimensional changes in the given hydrogen concentration range. Then, the expansion of Zircaloy-4 homogeneously hydrided samples was measured at 300oC, the typical operating temperature of a nuclear power reactor, obtaining a relative expansion of 2.21 * 10-4% per wppm-H. Considering the relative expansion observed for Zircaloy-4 at room temperature due to hydriding, starting from a hydrogen free sample, the total relative expansion rate is calculated to be 5.21 * 10-4% per wppm-H.

Hydrogen Pick up in Zircaloy-4: Effects in the Dimensional Stability of Structural Components under Nuclear Reactor Operating Conditions

In the present work, the expansion coefficient due to hydrogen incorporation was measured for the axial direction of a Zircaloy-4 cooling channel, similar to that installed in the Atucha I PHWR, Argentina, trying to simulate the nuclear power reactor operating conditions. As a first step, the solubility curve of hydrogen in Zircloy-4 was determined by two techniques: differential scanning calorimetry and differential dilatometry. The comparison with classical literature curves showed a good agreement with them, although the calorimetric technique proved to be more accurate for these determinations. Dilatometry was able to detect the end of hydride dissolution from concentrations around 60 wppm-H up to 650 wppm-H, where the eutectoid reaction: α + δ→α + β takes place (at 550oC). We assume that this ability is a good indicator of the aptitude of the technique to measure dimensional changes in the given hydrogen concentration range. Then, the expansion of Zircaloy-4 homogeneously hydrided samples was measured at 300oC, the typical operating temperature of a nuclear power reactor, obtaining a relative expansion of 2.21 * 10-4% per wppm-H. Considering the relative expansion observed for Zircaloy-4 at room temperature due to hydriding, starting from a hydrogen free sample, the total relative expansion rate is calculated to be 5.21 * 10-4% per wppm-H.

Kinetics of austenitization under uniaxial compressive stress in Fe–2.96 at.% Ni alloy

Differential dilatometry has been employed to study the kinetics of the massive ferrite (α) → austenite (γ) transformation upon isochronal heating (i.e. austenitization) of the substitutional Fe–2.96 at.% Ni alloy subjected to a range of applied constant uniaxial compressive stresses. A phase-transformation model, involving site saturation, interface-controlled (continuous) growth and incorporating an impingement correction for an intermediate of the cases of ideally periodically and of ideally randomly dispersed growing particles, has been employed to extract the interface-migration velocity of the α/γ interface and the transformation-induced deformation energy taken up by the specimen. The value obtained for the energy corresponding with the elastic and plastic deformation associated with the accommodation of the α/γ volume misfit depends on the austenite fraction and increases distinctly with an increase in the applied uniaxial compressive stress, which is compensated by, in particular, an increase in the chemical driving force corresponding to an increase in the onset temperature. The opposite effects of an applied uniaxial compressive stress on the α → γ transformation and on the γ → α transformation can be discussed as the outcome of constrained plastic deformation due to transformation-induced strain.

Thermal expansion of zirconia–zirconium titanate materials obtained by slip casting of mixtures of Y-TZP–TiO 2

The objective of this work was to establish the optimum conditions to get zirconia materials with different proportions of zirconium titanate by reaction sintering of ZrO 2 stabilized with 3 mol% of Y 2O 3 (Y-TZP) and TiO 2. The green bodies were fabricated from stable colloidal suspensions of the powders by slip casting in plaster moulds. The rheological characterization of the suspensions allowed the establishment of the optimum green processing conditions. Reaction sintering was performed at 1500 °C during 2 h, and the obtained materials have been characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy with energy dispersive X-ray microanalysis (FE-SEM-EDX). Under these conditions, zirconium titanate materials with tetragonal zirconia could not be obtained because a solid solution of cubic zirconia with titania and yttria is formed at 1500 °C. The thermal expansion of the materials was determined by differential dilatometry from room temperature up to 850 °C, demonstrating that the incorporation of zirconium titanate reduces the thermal expansion of zirconia.

The effect of Cu addition on the precipitation and redissolution in Al–Zn–Mg alloy by the differential dilatometry

With the aim to determine the effect of the addition of micro-constituents (copper and magnesium, essentially the copper) on the phenomena of phase transformation and kinetic of precipitation in an Al–Zn–Mg alloy we proceeded to study the effect of addition of Cu and Mg by dilatometry and measurements of microhardness. The results show that the addition of copper modifies appreciably the number and perhaps the nature of some phases that precipitate in this alloy system. We showed that the number of the types of the equilibrium η phase increases and that for the underaged states this equilibrium η phase seems to precipitate from the η′ intermediate phase.

Austenite–ferrite transformation kinetics under uniaxial compressive stress in Fe–2.96 at.% Ni alloy

The effect of an applied constant uniaxial compressive stress on the kinetics of the austenite (γ) → ferrite (α) massive transformation in the substitutional Fe–2.96 at.% Ni alloy upon isochronal cooling has been studied by differential dilatometry. All imposed stress levels are below the yield stress of austenite and ferrite in the temperature range of the transformation. An increase in compressive stress results in a small but significant increase of the onset temperature of the γ → α transformation and a decrease of the overall transformation time. A phase transformation model, involving site saturation, interface-controlled growth and incorporation of an appropriate impingement correction, has been employed to extract the interface-migration velocity of the γ/α interface. The interface-migration velocity for the γ → α transformation is approximately constant at fixed uniaxial compressive stress and increases with increasing applied uniaxial compressive stress. Furthermore, the value obtained for the energy corresponding with the elastic and plastic deformation associated with the accommodation of the γ/α volume misfit depends on the transformed fraction and decreases significantly as the applied uniaxial compressive stress increases. An understanding of the observed effects is obtained, recognizing the constraints imposed on the phase transformation due to the applied stress.

Hydrogen terminal solid solubility determinations in Zr–2.5Nb pressure tube microstructure in an extended concentration range

In the present work the temperature of terminal solid solubility of hydrogen in the zirconium alloy Zr–2.5Nb was measured for material taken from a CANDU type pressure tube. The measurements were made in a wide concentration range (50–463 wppm) using two techniques, differential scanning calorimetry and differential dilatometry, with the additional purpose of checking the coincidence of the TSS points in both techniques. The work was complemented by microstructural studies pursuing the (β-Zr) → (α-Zr + β-Zr enr + β-Nb) transformation. This transformation is accelerated if the material remains at temperatures higher than 400 °C, which corresponds to a hydrogen solubility of about 180 wppm. Thus, microstructural studies were made by transmission electron microscopy and energy dispersive analysis, which allowed to relate changes in the solubility with these microstructural transformations.

Structural evolution during non-isothermal ageing of a dilute Al–Cu alloy by dilatometric analysis

Non-isothermal ageing of an Al–4, 5% Cu alloy has been studied by using differential dilatometry, differential scanning calorimetry (DSC) and high-temperature X-ray diffraction (HTXRD). Six anomalies have been detected on the differential thermodilatometry curve (DTD). The GP zones formation and dissolution are traduced by two small contractions followed by one expansion. The precipitation of θ′-phase induces a big expansion and its dissolution reaction produces an important contraction. Moreover, θ-phase formation accompanies the precipitation, as well as the dissolution reactions of the θ′-phase. The θ-phase dissolution which dissolves the Cu atoms in the Al lattice is accompanied by a thermal dilatation of the Al lattice. The obtained values of the activation energy associated to the main precipitation reaction, by using Kissinger and Ozawa methods, are similar to that found for the precipitation reaction of θ′-phase.

Characterization of precipitation and phase transformations in Al–Zn–Mg alloy by the differential dilatometry

The phenomena of phase transformations are very complex particularly in the case of the Al–Zn–Mg alloys because of the various parameters which influence them (heat treatments, chemical composition, types of elements of addition, relationship between different chemical constituents…). In order to contribute to the comprehension of these phenomena we have made a study of phase transformations by using differential dilatometry with some microhardness measurements. The sequence of precipitation for this system of alloys is as follows: solid solution → Guinier–Preston zones (ZGP) → intermediate η′ phase → equilibrium η phase. The analysis of the results suggests principally the formation of GPII zones on one hand and, on the other hand, at the beginning of its formation the equilibrium η phase consists of several types of precipitates and for sufficiently long time there remains one type.