Room-Temperature Thermoelectric Performance of n-Type Multiphase Pseudobinary Bi2Te3-Bi2S3 Compounds: Synergic Effects of Phonon Scattering and Energy Filtering.

Bismuth telluride-based alloys possess the highest efficiencies for the low-temperature-range (<500 K) applications among thermoelectric materials. Despite significant advances in the efficiency of p-type Bi2Te3-based materials through engineering the electronic band structure by convergence of multiple bands, the n-type pair still suffers from poor efficiency due to a lower number of electron pockets near the conduction band edge than the valence band. To overcome the persistent low efficiency of n-type Bi2Te3-based materials, we have fabricated multiphase pseudobinary Bi2Te3-Bi2S3 compounds to take advantages of phonon scattering and energy filtering at interfaces, enhancing the efficiency of these materials. The energy barrier generated at the interface of the secondary phase of Bi14Te13S8 in the Bi2Te3 matrix resulted in a higher Seebeck coefficient and consequently a higher power factor in multiphase compounds than the single-phase alloys. This effect was combined with low thermal conductivity achieved through phonon scattering at the interfaces of finely structured multiphase compounds and resulted in a relatively high thermoelectric figure of merit of ∼0.7 over the 300-550 K temperature range for the multiphase sample of n-type Bi2Te2.75S0.25, double the efficiency of single-phase Bi2Te3. Our results inform an alternative alloy design to enhance the performance of thermoelectric materials.

Parent grain reconstruction from partially or fully transformed microstructures in MTEX.

A versatile generic framework for parent grain reconstruction from fully or partially transformed child microstructures has been integrated into the open-source crystallographic toolbox MTEX. The framework extends traditional parent grain reconstruction, phase transformation and variant analysis to all parent-child crystal symmetry combinations. The inherent versatility of the universally applicable parent grain reconstruction methods and the ability to conduct in-depth variant analysis are showcased via example workflows that can be programmatically modified by users to suit their specific applications. This is highlighted by three applications, namely α'-to-γ reconstruction in a lath martensitic steel, α-to-ß reconstruction in a Ti alloy, and a two-step reconstruction from α' to ɛ to γ in a twinning and transformation-induced plasticity steel. Advanced orientation relationship discovery and analysis options, including variant analysis, are demonstrated via the add-on function library ORTools.

Discriminating ß, α and α″ phases in metastable ß titanium alloys via segmentation: A combined electron backscattering diffraction and energy-dispersive X-ray spectroscopy approach.

Annealed metastable ß titanium (Ti) alloys comprise body-centred-cubic ß and hexagonal-close-packed α phases and possibly, orthorhombic α″ martensite that forms on quenching or deformation. Electron backscattering diffraction is amongst the most popular methods for characterising such multi-phase microstructures. However, the crystallographic similarity between α and α″ martensite renders unambiguous discrimination of these phases via electron backscattering patterns (EBSPs) virtually impossible; thereby limiting the use of EBSD in characterising ß-Ti alloys. In this study, we demonstrate that α and α″ martensite are primarily misindexed due to an indiscernible difference between these phases along their [1¯10]α and [Formula: see text] zone axes. Furthermore, the slight compositional difference between α and α″ is insufficient to discriminate these phases using on-the-fly energy-dispersive X-ray spectroscopy (EDS) spectrum matching. Consequently, a segmentation method was developed that relies on a combination of reindexed EBSPs and grain-median EDS elemental data to unambiguously discriminate ß, α and α″ martensite in metastable ß Ti alloys. All steps are implemented in an open-source and freely available computer program called phaseSegmenter that makes use of the MTEX toolbox in MATLAB. The program is readily applicable to Ti alloys containing α', α″ or massively transformed α as well as other phase transforming alloy systems with similar phase discrimination issues.

A Combination of Keyhole GTAW with a Trapezoidal Interlayer: A New Insight into Armour Steel Welding.

The ballistic performance of armour steel welds using austenitic filler materials is poor on account of the disparity in the mechanical properties of the weld and base metals. Consequently, a novel Keyhole Gas Tungsten Arc Welding process with a trapezoidal AISI309 austenitic stainless steel interlayer was developed to tailor chemical composition and microstructure by controlling the solidification sequence. Results show that the dilution rate in the weld metal region can reach up to 43.5% by placing a specially designed interlayer in between the base metal, providing a major scope for microstructure modification. Detailed weld analysis was undertaken by X-ray diffraction, optical and secondary and transmission electron microscopy, energy dispersive spectroscopy and electron back-scattering diffraction. The results from Vickers hardness indents and Charpy impact toughness testing at -40 °C show that the properties of the weld metal region are comparable to that of the base metal. This is ascribed to the weld metal comprising a two phase microstructure of martensite and retained austenite, which contribute to improvements in strength and toughness, respectively. Furthermore, the tailored chemical composition, microstructure and low temperature phase transformation in the weld metal may reduce the tendency toward both solidification cracking and hydrogen assisted cold cracking.

Transmission Kikuchi diffraction versus electron back-scattering diffraction: A case study on an electron transparent cross-section of TWIP steel.

The present case study compares transmission Kikuchi diffraction (TKD) with electron back-scattering diffraction (EBSD) on the same area of an electron transparent cross-section of a twinning induced plasticity steel. While TKD expectedly provides better clarity of internal defect substructures in the band contrast map, EBSD returns orientation data that approaches the quality of the TKD map. This was rationalised by Monte Carlo simulations of the electron energy spreads, which showed that due to the geometry-based compromises associated with adapting a conventional EBSD detector (which is off-axis with respect to the incident electron beam) to TKD, a broadening in the electron energy distribution of the forward-scattered electrons collected on the detector phosphor screen, is unavoidable. In this circumstance, the values of the full-widths at half-maximum of the energy distributions for TKD and EBSD are of the same order. It follows that EBSD on electron transparent cross-sections may be a viable alternative to TKD when: (i) conventional EBSD detectors are adapted to TKD and, (ii) sample microstructures comprise features whose sizes do not mandate the application of TKD.

A correlative approach to segmenting phases and ferrite morphologies in transformation-induced plasticity steel using electron back-scattering diffraction and energy dispersive X-ray spectroscopy.

Using a combination of electron back-scattering diffraction and energy dispersive X-ray spectroscopy data, a segmentation procedure was developed to comprehensively distinguish austenite, martensite, polygonal ferrite, ferrite in granular bainite and bainitic ferrite laths in a thermo-mechanically processed low-Si, high-Al transformation-induced plasticity steel. The efficacy of the ferrite morphologies segmentation procedure was verified by transmission electron microscopy. The variation in carbon content between the ferrite in granular bainite and bainitic ferrite laths was explained on the basis of carbon partitioning during their growth.