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•Investigation of the phase behavior of a metallic glass forming alloy.•Heat capacity measurement by Stochastic Temperature Modulated DSC.•The use of Fast Differential Scanning ...Calorimetry for heat capacity determination.•Determination of the phase diagram including metastable modifications.•Approximations of the Gibbs free energy difference between liquid and solids.
The Au-based bulk metallic glass-forming alloy Au70Cu5.5Ag7.5Si17 is the first atomic system in which monotropic polymorphism has been experimentally found. In this study, strategies for measuring thermodynamic quantities such as heat capacity, transformation enthalpy and entropy, and Gibbs free energy for stable and metastable phases are evaluated based on this system. The transformation enthalpies between phases are determined via differential scanning calorimetry (DSC); the corresponding heat capacities are measured via stochastic temperature-modulated DSC (TOPEM); and the equilibrium melting temperatures of metastable phases, only detectable at high rates, are determined via fast DSC (FDSC). Whereas the slow heating rate of conventional DSC limits the characterization of metastable phases, the combination of both DSC and FDSC allows for their detailed heat-capacity determination. Furthermore, various approximation functions that have previously been used to determine the temperature dependence of the thermodynamic driving force for nucleation and crystallization are compared with the experimental data.
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•Three nanolayered polymorphs are found in a comb-like di-alkoxylated polyester.•Flash DSC is used to study the liquid crystalline (LC) state in quenched samples.•Transitions from ...modification B to A and from A to LC are studied by DSC and XRD.•Crystal orientation is observed in case of ram extruded polymer fibers.•Oriented nanolayered states are also seen in case of polymer coated glass fibers.
Structure formation in an alkoxylated polyester having rigid backbone and flexible side chains composed of 10 alkyl carbons per side chain is studied using X-ray diffraction, conventional DSC and flash DSC techniques. Two crystalline states-modification B and modification A are observed with their occurrence depending on the thermal treatment. Additionally, a liquid crystalline phase is also detected above the melting temperature of modification A. While modification B is the thermodynamically preferred phase at low temperatures consisting of crystalline side chains, modification A is observed on heating above 120 °C and shows a disordered packing of the side chains. Rapid cooling using Fast Differential Scanning Calorimetry (FDSC) yields a liquid crystalline state wherein a long range ordered layered morphology exists, however, the π − π stacking of the backbones is absent. The molecular orientation of backbones under the influence of shear fields as well as on a glass fiber surface is also studied. Both modifications A and B show backbone orientation along the shear fields in an extruded sample while a preferential edge-on orientation is inferred on the glass fiber surface.
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•Quantitative predictions with a physical model of AMZ4 BMG crystallization induced by the LPBF process over a wide range of crystallized fractions.•FEM simulations with a proper TTT ...diagrams measured by Flash DSC upon heating on the exact same powder used for LPBF experiments.•Highlight of two thermal effects operating at different scales: a global one, originating from heat accumulation in the whole sample, and a local one, due to the effect of each individual laser track pass.•Comparison experiments vs FEM simulations -> good agreement on the crystallized fraction.•Validation of the model on an optimized sample with 0% crystallization.
One of the main challenges in the fabrication of bulk metallic glasses (BMGs) via laser-based additive manufacturing (AM) is undesirable crystal phase formation, which usually deteriorates the mechanical properties of the BMG fabricated parts. Understanding the crystallization process therefore helps to manufacture parts with desirable properties. In this study, partially crystallized Zr-based BMG (AMZ4) samples were fabricated via laser powder-bed fusion (LPBF). Samples with a different time delay between each adjacent laser tracks were produced to vary the thermal history during the manufacturing process. Two characteristic thermal effects were decoupled, a global one and a local one. The global thermal effect originates from heat accumulation in the whole sample, increasing the overall sample temperature, reducing local cooling rates from the melt and changing thermal cycles in the heat-affected zones (HAZs). The local thermal effect refers to the contribution of each individual laser-track pass, happening even in the absence of the global effect. As the time delay is increased, the sample has more time to dissipate heat, which implies a reduced influence of the global thermal effects, and therefore lower crystalline fractions. The experiments were designed such as to allow for detailed validations of the thermal fields predicted by a Finite Element (FEM) model of the LPBF process. These were indeed used as an input to predict the crystallized fraction in each AMZ4 sample, using previously measured TTT diagrams. For the first time, quantitative predictions with a numerical model could be made over a wide range of crystallized fractions, and were in good agreement with those measured by DSC. To validate the model, a 0 % crystallized fraction was also simulated, corresponding to optimized printing conditions despite the high oxygen content (>1000 ppm) of the AMZ4 chosen for the experiments. It therefore represents a reliable tool for finding optimal processing parameters of BMGs known to be challenging to print.
The temperature dependence of Gibbs free energy difference between the supercooled liquid and the crystalline phase, Δ
G
c
, plays a fundamental role for the description of crystallization processes ...especially at high supercooling as occurring with many technical processes. In the literature, many different approximations for Δ
G
c
(
T
) can be found. A test of these models with polymer data from the ATHAS database shows that none of them can be used as a general model. We present a new model for Δ
G
c
(
T
) of polymers, which was successfully tested on ten different polymers. The result indicates that our approach might be generally valid for all polymers in the temperature range between the equilibrium melting temperature and the glass transition temperature.
•Introducing a method for determination of the thermal short stability of polymers.•This is important at short thermal stress during application and processing.•The method contains thermal exposure ...steps and analysis steps.•The influence of the exposure on the crystallization behavior is evaluated.•This method based on fast scanning calorimetry (Flash DSC).
Thermogravimetric analysis (TGA) is a standard technique to measure the thermal stability of polymeric materials. This technique is not sensitive for degradation steps which are not related to mass loss. However, such reactions can significantly influence the mechanical behavior of material. In this contribution we introduce the technique of stability estimation by crystallization analysis (SECA) and pseudo TGA which uses differential scanning calorimetry (DSC).
SECA measures the influence of decomposition on crystallization kinetics. This technique is very sensitive to decomposition. Using fast scanning calorimetry, SECA determines the short time thermal stability of semi-crystalline polymers. This property is essential for fast polymer processing like selective laser sintering or welding.
Amorphous calcium carbonate (ACC) is the least stable polymorph of calcium carbonates. It has been identified to play an important role in nature (e.g., biomineralization and speleothem formation), ...where it acts as a precursor for the transformation to more stable polymorphs such as calcite. Furthermore, the use of ACC in technical applications requires a robust understanding of the material’s properties. We present the first study that reveals the existence of a glass transition for synthetic and anhydrous ACC. The glass transition occurs at 339 °C. Such measurements are impossible with conventional differential scanning calorimetry (DSC) due to the high tendency of ACC to crystallize. Fast scanning DSC with heating rates of 500 °C/s and higher, however, can be used to separate the endothermic glass transition signature from the exothermic crystallization event since crystallization is shifted to higher temperatures. This allows the detection and quantification of the glass transition for ACC. These observations indicate that ACC is a structural glass and are especially significant because the synthesis of ACC, precipitation from a solution followed by lyophilization, contrasts with the more conventional and well-known route of glass formation–the rapid cooling of a melt. Moreover, we prove that a structural glass can be produced from a simple single-component carbonate system.
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