Microstructure of a spark-plasma-sintered Fe2VAl-type Heusler alloy for thermoelectric application

The influence of microstructure on thermoelectricity is increasingly recognized. Approaches for microstructural engineering can hence be exploited to enhance thermoelectric performance, particularly through manipulating crystalline defects, their structure, and composition. Here, we focus on a full-Heusler Fe2VAl-based compound that is one of the most promising thermoelectric materials containing only Earth-abundant, non-toxic elements. A Fe2VTa0.05Al0.95 cast alloy was atomized under a nitrogen-rich atmosphere to induce nitride precipitation. Nanometer- to micrometer-scale microstructural investigations by advanced scanning electron microscopy and atom probe tomography (APT) are performed on the powder first and then on the material consolidated by spark-plasma sintering for an increasing time. APT reveals an unexpected pick-up of additional impurities from atomization, namely W and Mo. The microstructure is then correlated with local and global measurements of the thermoelectric properties. At grain boundaries, segregation and precipitation locally reduce the electrical resistivity, as evidenced by in-situ four-point probe measurements. The final microstructure contains a hierarchy of structural defects, including individual point defects, dislocations, grain boundaries, and precipitates, that allow for a strong decrease in thermal conductivity. In combination, these effects provide an appreciable increase in thermoelectric performance

Ultrashort Sintering and Near Net Shaping of Zr-Based AMZ4 Bulk Metallic Glass

The GeniCore Upgraded Field Assisted Sintering Technology U-FAST was applied to the sintering of a commercial Zr-based bulk metallic glass powder AMZ4. The XRD, SEM and DSC analysis of the sintered compacts showed the benefit of the U-FAST method as an enabler for the production of fully amorphous samples with 100% relative density when sintering at 420 °C/480 s (693 K/480 s) and 440 °C/ 60 s (713 K/480 s). The hardness values for fully amorphous samples, over HV1 519, surpass cast materials and 1625 MPa compressive strengths are comparable to commercial cast products. The advantage of the U-FAST technology in this work is attributed to the high heating and cooling rates inherent to ultra-short pulses, which allow to maintain metastable structures and achieve better temperature control during the process. Increasing sintering temperature and time led to the crystallization of the materials. The geometry and material of the dies and punch determine the thermal inertia and pressure distribution inside the compacts, thus affecting the properties of the near net shape NNS compacts made using the U-FAST device

Atomisation of Ti-6Ta-1.5Zr-0.2Ru-5Cu (wt%) for additive manufacturing for biomedical applications

The use of titanium alloys is growing fast as people have longer life expectancies and small, customised, biomedical implants, especially in dental applications, encourage the use of additive manufacturing (AM) to shape them. The Ti-6Ta-1.5Zr-0.2Ru-5Cu (wt%) alloy has been identified as a potential alloy for biomedical applications. Since laser powder bed fusion (L-PBF) requires starting powders to be spherical and within a 10-100 μm size range, the Ti-6Ta-1.5Zr-0.2Ru-5Cu (wt%) powder was ultrasonically atomised and then analysed by a Malvern Mastersizer, XRD and SEM-EDX to ascertain that it met the requirements of L-PBF

Effects of a New Type of Grinding Wheel with Multi-Granular Abrasive Grains on Surface Topography Properties after Grinding of Inconel 625

Finishing operations are one of the most challenging tasks during a manufacturing process, and are responsible for achieving dimensional accuracy of the manufactured parts and the desired surface topography properties. One of the most advanced finishing technologies is grinding. However, typical grinding processes have limitations in the acquired surface topography properties, especially in finishing difficult to cut materials such as Inconel 625. To overcome this limitation, a new type of grinding wheel is proposed. The tool is made up of grains of different sizes, which results in less damage to the work surface and an enhancement in the manufacturing process. In this article, the results of an experimental study of the surface grinding process of Inconel 625 with single-granular and multi-granular wheels are presented. The influence of various input parameters on the roughness parameter (Sa) and surface topography was investigated. Statistical models of the grinding process were developed based on our research. Studies showed that with an increase in the cutting speed, the surface roughness values of the machined samples decreased (Sa = 0.9 μm for a Vc of 33 m/s for a multigranular wheel). Observation of the grinding process showed an unfavorable effect of a low grinding wheel speed on the machined surface. For both conventional and multigranular wheels, the highest value for the Sa parameter was obtained for Vc = 13 m/s. Regarding the surface topography, the observed surfaces did not show defects over large areas in the cases of both wheels. However, a smaller portion of single traces of active abrasive grains was observed in the case of the multi-granular wheel, indicating that this tool performs better finishing operations

Spherical powders: Control over the size and morphology of powders for additive manufacturing and enriched stable isotope nuclear targets

Metal powders are a fundamental starting point for fabricating many types of nuclear targets. Elemental powder properties can differ drastically between batches, even when using the same method. Therefore, the variation in morphology and the size of metal powders can cause variable quality and produce inconsistent results with what are otherwise proven target manufacturing techniques. Additive manufacturing has additional requirements for higher quality and more uniform feedstock. The production of spheroidized powders with uniform, reproducible properties and a narrow size distribution represents unexplored opportunities for experiments. These opportunities include experimenting with solid metals that can now flow like liquids, new options for powder handling and dispensing, and new target fabrication methods using additive manufacturing. The Stable Isotope Materials and Chemistry Group at Oak Ridge National Laboratory obtained an AMAZEMET rePowder ultrasonic metal atomization tool for creating limited batches of fully dense, free flowing, spherical powders with a narrow size distribution of extremely rare materials. Early results are presented with materials that were produced. The team explores the anticipated limits of this instrument with extremely rare materials (e.g., enriched stable isotopes) and highlights research into new fabrication techniques that provide additional options benefitting the international nuclear target community