Laser Dilatometry Research Articles

Influence of Oxygen on the Kinetics of Omega and Alpha Phase Formation in Beta Ti–V

A detailed understanding of the kinetics of phase formation in beta -stabilised titanium is of decisive importance for the applicability of these materials. However, the complex nature and long timescales of the various transformations, calls for specialized measurement techniques. In this work high-stability isothermal laser dilatometry is used to study the temporal volume changes associated with the various phase formation processes. Distinctly different behaviours between samples of Ti–21 at. pct V with different solute oxygen content could be detected and quantified. Temperature regimes for both diffusionless and diffusion-assisted isothermal omega -formation as well as for omega -to-alpha -transformation were determined. Low oxygen contents promote the diffusionless omega -formation mechanism, but retard the diffusion-assisted one as well as the omega -to-alpha -transformation process. The results confirm recent findings of a clear distinction between the diffusionless and diffusion-assisted isothermal omega formation modes. Modelling of the omega -phase formation applying Austin–Rickett kinetics revealed the temperature-dependent formation rates, on the basis of which the isothermal TTT-diagrams were developed which reflect the strong influence of the oxygen content.

Austenite reversion kinetics and stability during tempering of a Ti-stabilized supermartensitic stainless steel: Correlative in situ synchrotron x-ray diffraction and dilatometry

Correlative physical simulation, synchrotron x-ray diffraction and laser dilatometry were used to characterize the surface and volumetric austenite reversion kinetics and stability in a Ti-stabilized supermartensitic stainless steel. A fast heating rate of 500 °C s−1 was used to minimize any martensite to austenite reversion related to the heating stage. This allowed the characterization of the austenite reversion kinetics and its corresponding thermal stability on cooling for tempering temperatures between 600 and 700 °C. In all cases, a soaking time of 9000 s and a cooling rate of 5 °C s−1 were used. The isothermal transformation was divided in two regimes: At and above 625 °C, the kinetics of the transformation was faster and the austenite equilibrium volume fraction was reached. Below 625 °C, the transformation was slower and incomplete. The reverted austenite was stable during cooling after tempering at and below 610 °C, partially stable for temperatures between 625 and 650 °C, and unstable for temperatures between 670 and 700 °C. The austenite Ni content should be higher than 8 wt % in order to effectively stabilize austenite at room temperature. Correlated bulk (dilatometry) and surface (diffraction) analyses showed very good agreement during the isothermal stage. However, martensitic transformation at the sample surface was evidenced at higher temperatures related to the bulk due to the free surface effect. A reversion TTT diagram and the austenite stability curve were constructed from the in situ x-ray diffraction data, providing tools for microstructural and performance optimization of this material.

Specific volume study of a bulk metallic glass far below its calorimetrically determined glass transition temperature

High-precision LASER dilatometry with a unique measurement setup was applied to the bulk metallic glass alloy ${\mathrm{Zr}}_{52.5}$${\mathrm{Ti}}_{5}$${\mathrm{Cu}}_{17.9}$${\mathrm{Ni}}_{14.6}$${\mathrm{Al}}_{10}$ (Vit105) for specific volume studies. Employing small temperature steps of $\ensuremath{\Delta}T=5$ and 10 K and long measurement times effective heating rates of the order of ${10}^{\ensuremath{-}2}$ K/min were realized and changes of the specific volume were measured with a resolution of $\ensuremath{\Delta}V/V={10}^{\ensuremath{-}6}$ down to 573 K. The temperature regime of these experiments is far below a calorimetrically determined glass transition of ${T}_{g}^{\text{calor}}=659$ K (5 K/min). The experimental results strongly support theoretical predictions for the existence of an ideal glass transition that was observed in this study at ${T}_{g}=628$ K for this bulk metallic glass model alloy.

Flash Sintering of Nanocrystalline Zinc Oxide and its Influence on Microstructure and Defect Formation

We report the sintering behavior of nanocrystalline zinc oxide under external AC electric field between 0 and 160 V/cm. In situ acquisition of density by means of laser dilatometry, evaluation of specimen temperature, real‐time measurement of electric field and current help analyze this peculiar behavior. Field strength and blocking electrodes significantly affect densification and microstructure, which was evaluated in the vicinity of the flash event and for the fully sintered material. High current densities flow through the sample at high electric fields, entailing a sudden increment of the temperature estimated to several hundreds of K and an exaggerated grain growth. In contrast, low current density flows through the sample at lower electric fields, which guarantees normal grain growth and highest final density. Macroscopic photoluminescence measurements give insights into the development of the defect structure. Electric fields are expected to enhance defect mobility, explaining the high densification rates observed during the sintering process.

Mechanical and thermal characterization of a ceramic/glass composite seal for solid oxide fuel cells

Solid oxide fuel cells (SOFCs) require seals that can function in harsh, elevated temperature environments. Comprehensive characterization and understanding of seals is needed for commercially viable SOFCs. The present research focuses on a novel ceramic/glass composite seal that is produced by roller compaction or tape casting of glass and ceramic powders and an organic binder. Upon heat treatment, micro-voids and surface anomalies are formed. Increased heating and cooling rates during the heat treatment resulted in more and larger voids. The first goal of the current research is to suggest an appropriate heating and cooling rate to minimize the formation of microstructural defects. After identifying an appropriate cure cycle, seals were thermally cycled and then characterized with laser dilatometry, X-ray diffraction, and sonic resonance. From these experiments the crystalline phases, thermal expansion, and elastic properties were determined. Subsequently compression testing with an acoustic emission (AE) sensor and post-test microstructural analysis were used to identify the formation of damage. By fully understanding the characteristics of this ceramic/glass composite seal, next generation seals can be fabricated for improved performance.

Softening of polyethylene powders at reactor conditions

Polymer softening is amajor problem in industrial gas phase polymerizations, because it enhances particle agglomeration and reactor wall fouling. It is thus desirable to investigate this phenomenon and identify safe operating conditions. We developed anew experimental method for the characterization of polyolefin softening and proposed atheory qualitatively correlating this phenomenon with the Young modulus. The developed laser dilatometry technique is based on the measurement of polyolefin powder deformations under a constant load at elevated temperatures. Our method enables to measure the softening of polyolefin powders in air and in hydrocarbon gases/vapors, which means that polyolefin softening can be studied at industrially relevant conditions. In the experimental part, we present results obtained for a broad set of polyethylene samples in various atmospheres. The results are in very good agreement with our theoretical expectations and thus verify the proposed effects of polymer properties and penetrant sorption on the softening of polyethylene.

Density gradients and springback: environmental influences

Laser dilatometry was combined with X-ray computed tomography (XRCT) to investigate the relationship between springback and internal density gradients in dry-pressed alumina compacts. Compacts were prepared using the same binder and binder content but different states of plasticization to examine these effects on dimensions and final density distribution. Both density gradients and microstructural-level characteristics exert control over differential springback behavior; the expansion of the low-density (LD) zone is larger than that of the high-density (HD) zone. For these submicron alumina powders, the concentration of binder associated with density gradients leads to potential loss of final dimensional tolerance. A sinusoidal dimensional response, dependent upon the state of the binder, is noted, which changes dramatically with time and environmental exposure. In some cases, dimensional changes occurred due to moisture absorption/desorption in compacts prepared at relative humidities different from ambient conditions.

Bioactive polymer surfaces via supercritical fluids

Supercritical CO 2 (scCO 2) can reversibly swell thermoplastic polymers. We use this phenomenon to enhance the impregnation of a fluorescently-labeled protein into a polymethylmethacrylate (PMMA) matrix. Exposure to scCO 2 provides simultaneous sterilization and foamed morphologies similar to those found in both hard and soft tissues in vivo. This combination of enhanced diffusion, foaming, and sterilization can conceivably be exploited to produce or modify a variety of biomedical materials. In the current study, efforts were made to control the scCO 2-induced foaming in PMMA. Fundamental information regarding solid state swelling (via laser dilatometry) and foaming (via microfocus X-ray computed tomography) were used to explore the interactions between process variables. This is the first application of laser dilatometry to measure scCO 2 swelling of and desorption from a polymer in situ. scCO 2 at 40 °C was also used to impregnate PMMA with a fluorescently-labeled protein, a rhodamine-labeled antibody to immunoglobin. Protein impregnation was verified using confocal microscopy.

Effect of Density Gradients on Dimensional Tolerance During Binder Removal

The present work utilized X‐ray computed tomography (CT) and in situ laser dilatometry to correlate internal structural changes with dimensional variation during thermally driven binder removal. Nonuniform agglomerate arrangement and deformation introduced during compaction provide density gradients that drive binder migration. Simultaneously, density undergoes a slight decrease accompanied by a 1.0% loss in dimensional tolerance. This loss and CT difference images show that capillary forces generated during binder melting can change internal density distribution. This change in density has important implications for the binder removal process.

Elimination, via high-rate laser dilatometry, of structural relaxation during thermal expansion measurement of dental porcelains

Objectives: Thermal expansion measurement of glassy materials is complicated by thermal history effects. Excess volume—trapped in quenched dental porcelains after firing—collapses via structural relaxation on first slow heating during conventional dilatometry, making the thermal expansion coefficient ( α) obtained on first heating unreliable. The purpose of this study was to determine whether porcelain thermal expansion measurement at high thermal rates could minimize the influence of thermal history. Methods: Eight thermal expansion specimens each of six body porcelains and the Component No.1 (leucite-containing) frit prepared according to the patent by Weinstein et al. (US Patent No. 3,052,982) were subjected to three heat–cool conventional dilatometry runs at 3°C/min, while eight thermal expansion specimens of each porcelain were reserved as untreated controls. Eight hollow, cylindrical specimens of the same brands were subjected to three heat–cool laser dilatometer thermal expansion runs at 600°C/min, while eight cylindrical specimens of each porcelain were reserved as untreated controls. Thermal expansion data (25–500°C) of all specimens were subjected to repeated measures analysis of variance. Results: The α obtained on first slow heating was significantly lower than values for succeeding slow heat and cool runs in all porcelains ( P<0.001). High-rate α obtained on first heating was not significantly different from values of succeeding heat and cool runs in all porcelains ( P>0.05). Significance: Conventional dilatometer measurements demonstrated occurrence of structural relaxation, as evidenced by the significant difference in the first heating and subsequent runs. High-rate laser dilatometry eliminated structural relaxation, thereby providing a thermal expansion measurement that is free of interference from thermal history effects.