Anomalous grain refinement trends during mechanical milling of Bi2Te3

The structural evolution of nanocrystalline bismuth telluride (Bi2Te3) during mechanical milling is investigated under different milling energies and temperatures. After prolonged milling, the compound evolves toward a steady-state nanostructure that is found to be unusually strongly dependent on the processing conditions. In contrast to most literature on mechanical milling, in Bi2Te3 we find that the smallest steady-state grain sizes are attained under the lowest energy milling conditions. An analysis based on the balance between refinement and recovery in the steady state shows that two regimes of behavior are expected based on the thermo-physical properties of the milled powder. Bi2Te3 lies in a relatively unusual regime where greater impact energy promotes adiabatic heating and recovery more than it does defect accumulation; hence more intense milling leads to larger steady-state grain sizes. Implications for other materials are discussed with reference to a “milling intensity map” that delineates the set of material properties for which this behavior will be observed.United States. Dept. of Energy. Office of Basic Energy Sciences (Award Number DE-SC0001299

High temperature, low neutron cross-section highentropy alloys in the Nb-Ti-V-Zr system

High-entropy alloys (HEAs) with high melting points and low thermal neutron cross-section are promising new cladding materials for generation III+ and IV power reactors. In this study a recently developed high throughput computational screening tool Alloy Search and Predict (ASAP) has been used to identify the most likely candidate single-phase HEAs with low thermal neutron cross-section, from over a million four-element equimolar combinations. The selected NbTiVZr HEA was further studied by density functional theory (DFT) for moduli and lattice parameter, and by CALPHAD to predict phase formation with temperature. HEAs of NbTiVZrx (x = 0.5, 1, 2) were produced experimentally, with Zr varied as the dominant cross-section modifier. Contrary to previous experimental work, these HEAs were demonstrated to constitute a single-phase HEA system; a result obtained using a faster cooling rate following annealing at 1200 °C. However, the beta (BCC) matrix decomposed following aging at 700 °C, into a combination of nano-scale beta, alpha (HCP) and C15 Laves phases

A candidate fusion engineering material, WC-FeCr

A new candidate fusion engineering material, WC-FeCr, has been irradiated with He ions at 25 and 500 °C. Ions were injected at 6 keV to a dose of ~15 dpa and 50 at. % He, simulating direct helium injection from the plasma. The microstructural evolution was continuously characterised in situ using transmission electron microscopy. In the FeCr phase, a coarse array of 3–6 nm bubbles formed. In the WC, bubbles were less prominent and smaller (~2 nm). Spherical-cap bubbles formed at hetero-phase interfaces of tertiary precipitates, indicating that enhanced processing routes to minimise precipitation could further improve irradiation tolerance

Controlling microstructure of nanocrystalline thermoelectrics through powder processing

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2014.220Cataloged from PDF version of thesis.Includes bibliographical references (pages 122-127).Bismuth Telluride and its solid solutions are currently front running thermoelectric materials because of their high figure of merit. When processed via mechanical alloying to obtain nanocrystalline structures, their efficiency is increased dramatically, due to enhanced phonon scattering at grain boundaries. However, the excess free energy of these interfaces renders them inherently susceptible to grain growth, therefore there is a need for materials with enhanced thermal stability. Despite this, little is known about the relevant processing science of these materials with respect to mechanical alloying and powder consolidation. This shortcoming is addressed here via systematic study of the processing-structure relationships that govern these processing operations. Firstly, during mechanical alloying, the primary mechanism of mixing between elemental constituents is revealed, as well as the limitations to subsequent grain refinement. The resultant behaviour is unique in the literature on mechanical alloying, due to the unusual thermal and thermodynamic properties of the compound and its elements, rendering deformation-induced heating effects especially prevalent. Next, during sintering operations of the powders, the kinetics of grain growth and porosity evolution were studied. By quantifying these processes, a thermal budget map for the nanocrystalline compound is constructed, to allow predictive powder and guidance of both processing and device operation at elevated temperatures. Finally, based on the improved understanding in processing science and thermal stability of these materials, a new class of thermally stable composites is engineered, with improved thermal stability, and hence enhanced thermoelectric properties.by Samuel A. Humphry-Baker.Ph. D

Melt-driven mechanochemical phase transformations in moderately exothermic powder mixtures

Usually, mechanochemical reactions between solid phases are either gradual (by deformation-induced mixing), or self-propagating (by exothermic chemical reaction). Here, by means of a systematic kinetic analysis of the Bi–Te system reacting to Bi2Te3, we establish a third possibility: if one or more of the powder reactants has a low melting point and low thermal effusivity, it is possible that local melting can occur from deformation-induced heating. The presence of hot liquid then triggers chemical mixing locally. The molten events are constrained to individual particles, making them distinct from self-propagating reactions, and occur much faster than conventional gradual reactions. We show that the mechanism is applicable to a broad variety of materials systems, many of which have important functional properties. This mechanistic picture offers a new perspective as compared to conventional, gradual mechanochemical synthesis, where thermal effects are generally ignored

Ablation resistance of tungsten carbide cermets under extreme conditions

A cobalt-free tungsten carbide cermet (WC-FeNi) has been subjected to oxyacetylene flame tests to simulate extreme operating conditions such as a worst-case fusion reactor accident. In such an accident, air-ingress to the reactor may impinge on components operating at surface temperatures in excess of 1000 °C, leading to tungsten oxide formation and its subsequent hazardous volatilisation. Here, the most challenging accident stage has been simulated, where the initial air-ingress could lead to extremely rapid air-flow rates. These conditions were simulated using an oxidising oxyacetylene flame. The separation between flame nozzle and sample was varied to permit peak surface temperatures of ~950–1400 °C. When the peak temperature was below 1300 °C, the cermet gained mass due to the dominance of oxide scale formation. Above 1300 °C, the samples transitioned into a mass loss regime. The mass loss regime was dominated by ablation of the scale rather than its volatilisation, which was confirmed by performing a systematic thermogravimetric kinetic analysis. The result was unexpected as in other candidate shielding materials, e.g. metallic tungsten, volatilisation is considered the primary dispersion mechanism. The unusual behaviour of the cermet scale is explained by its relatively low melting point and by the lower volatility of its FeWO4 scale compared to tungsten's WO3 scale. The substantially lower volatility of the WC cermet scale compared to metallic W indicates it may have a superior accident tolerance