Ultra-fine grained Nd–Fe–B by high pressure reactive milling and desorption

This paper reports on the grain refinement in dynamic hydrogenation disproportionation desorption and recombination (d-HDDR) processed Nd-rich Nd₂Fe₁₄B and stoichiometric Nd₂Fe₁₄B powders using high pressure reactive milling (HPRM) followed by a subsequent desorption and recombination. In contrast to the dynamic-HDDR processed anisotropic powder with a grain size of the Nd₂Fe₁₄B phase of 300 nm, the new approach yields a further reduction of the Nd₂Fe₁₄B₁ grain size to less than 70 nm. Nd-rich Nd₂Fe₁₄B powder produced by HPRM and subsequent desorption exhibits a coercivity μ_0iH_c=1.35 T and a remanence of 0.80 T. In the stoichiometric material, the reduction of the Nd-content leads to an increase in remanence to 0.85 T. Additionally, it is demonstrated that highly anisotropic powders can also be obtained by dynamic-HDDR processing of stoichiometric Nd₂Fe₁₄B powders.     

Fracture mechanisms in bulk metallic glassy materials

We find that the failure of bulk metallic glassy (BMG) materials follows three modes, i.e., shear fracture with a fracture plane significantly deviating from 45 degrees to the loading direction, normal tensile fracture with a fracture plane perpendicular to the loading direction, or distensile fracture in a break or splitting mode with a fracture plane parallel to the loading direction. The actually occurring type of failure strongly depends on the applied loading mode and the microstructure of the material. Extensive evidence indicates that the Tresca fracture criterion is invalid, and for the first time, three fracture criteria are developed for isotropic materials with high strength, such as advanced BMGs or the newly developed bulk nanostructural materials.     

Magnetization reversal in textured NdFeB-Fe composites observed by domain imaging

Textured composite samples consisting of Nd_13.6Fe_73.6Ga_0.6Co_6.6B_5.6 (MQU-F™) and micron-sized Fe particles with weight ratios from 100:0 to 70:30 have been prepared by hot deformation. Microstructure studies revealed a layered structure of both phases with the layer normal parallel to the pressing direction. Magnetic measurements showed single-phase hysteresis curves for all samples when measured along the pressing direction, which is also the easy axis of magnetization. Coercivity decreased drastically from 1.32 T for pure NdFeB samples to 0.154 T for a sample with 30 wt% Fe. Magneto-optical Kerr microscopy with a digitally enhanced imaging technique has been used to examine the evolution of magnetic domains in the hard and soft phase during demagnetizing a sample consisting of 70 wt% NdFeB and 30 wt% Fe. It is shown that demagnetization takes place via domain rearrangements within the soft phase, which lead to and support the nucleation of reversed interaction domains at phase boundaries. Also nucleation of interaction domains within the hard magnetic phase could be revealed.     

Local vs. global redundancy – trade-offs between resilience against cascading failures and frequency stability

When designing or extending electricity grids, both frequency stability and resilience against cascading failures have to be considered amongst other aspects of energy security and economics such as construction costs due to total line length. Here, we compare an improved simulation model for cascading failures with state-of-the-art simulation models for short-term grid dynamics. Random ensembles of realistic power grid topologies are generated using a recent model that allows for a tuning of global vs local redundancy. The former can be measured by the algebraic connectivity of the network, whereas the latter can be measured by the networks transitivity. We show that, while frequency stability of an electricity grid benefits from a global form of redundancy, resilience against cascading failures rather requires a more local form of redundancy and further analyse the corresponding trade-off.     

The impact of model detail on power grid resilience measures

Extreme events are a challenge to natural as well as man-made systems. For critical infrastructure like power grids, we need to understand their resilience against large disturbances. Recently, new measures of the resilience of dynamical systems have been developed in the complex system literature. Basin stability and survivability respectively assess the asymptotic and transient behavior of a system when subjected to arbitrary, localized but large perturbations in frequency and phase. To employ these methods that assess power grid resilience, we need to choose a certain model detail of the power grid. For the grid topology we considered the Scandinavian grid and an ensemble of power grids generated with a random growth model. So far the most popular model that has been studied is the classical swing equation model for the frequency response of generators and motors. In this paper we study a more sophisticated model of synchronous machines that also takes voltage dynamics into account, and compare it to the previously studied model. This model has been found to give an accurate picture of the long term evolution of synchronous machines in the engineering literature for post fault studies. We find evidence that some stable fix points of the swing equation become unstable when we add voltage dynamics. If this occurs the asymptotic behavior of the system can be dramatically altered, and basin stability estimates obtained with the swing equation can be dramatically wrong. We also find that the survivability does not change significantly when taking the voltage dynamics into account. Further, the limit cycle type asymptotic behaviour is strongly correlated with transient voltages that violate typical operational voltage bounds. Thus, transient voltage bounds are dominated by transient frequency bounds and play no large role for realistic parameters.