Causal structure and degenerate phase boundaries

Timelike and null hypersurfaces in the degenerate space-times in the Ashtekar theory are defined in the light of the degenerate causal structure proposed by Matschull. Using the new definition of null hypersufaces, the conjecture that the "phase boundary" separating the degenerate space-time region from the non-degenerate one in Ashtekar's gravity is always null is proved under certain circumstances.Comment: 13 pages, Revte

Physical-metallurgical properties and micro-milling machinability evaluation of high entropy alloy FeCoNiCrAlx

High-entropy alloy (HEA) belongs to the emerging multi-principal alloy with excellent mechanical-physical properties. The material machinability is critical for cutting planning, especially for novel materials with various chemical compositions and mechanical properties. The machinability of high-entropy alloys mainly depends on the physical-metallurgical properties and cutting conditions. This work investigated the physical-metallurgical properties and micro-machinability of HEA FeCoNiCrAlx (x = 0.1, 0.5, 1) with vacuum arc melting preparation. Experimental results indicated that the difference of Al element content affected the chemical element distribution, phase composition, microstructure, and microhardness of prepared HEA FeCoNiCrAlx. FeCoNiCrAl0.1 appeared single face-center-cubic (FCC) structure, while the increase in Al element content led to dual face-center-cubic and body-center-cubic (FCC + BCC) structure for FeCoNiCrAl0.5 and FeCoNiCrAl1. The average microhardness values were approximately 183 HV, 294 HV, and 461 HV for FeCoNiCrAl0.1, FeCoNiCrAl0.5, and FeCoNiCrAl1, respectively. The increase in Al element content led to poor material machinability, in which FeCoNiCrAl0.1 had better machinability due to lower micro-milling forces, more stable cutting process, lower specific cutting energy, better surface qualities and smaller tool wear. This work combined the prepared material properties and micro-machinability evaluation to guide HEA design and select practical machining parameters

A Theoretical and Experimental Investigation of High-Frequency Ultrasonic Vibration-Assisted Sculpturing of Optical Microstructures

Ultrasonic vibration-assisted cutting (UVAC) has been regarded as a promising technology to machine difficult-to-machine materials. It allows for a sub-micrometer form accuracy and surface roughness in the nanometer range. In this paper, high-frequency vibration-assisted sculpturing is used to efficiently fabricate quadrilateral microlens array with sharp edges, instead of using slow-slide-servo diamond turning with vibration. The machining principle of diamond sculpturing, the cutting dynamics of ultrasonic vibration, and the tool edge on the theoretical form error between the designed structure and the machined structure were investigated for this technique. Then, the quadrilateral microlens array was machined by means of conventional sculpturing (CS) and high-frequency ultrasonic vibration-assisted sculpturing (HFUVAS), respectively, followed by a study of the cutting performances including form accuracy, the surface morphology of the machined structure, and the tool wear. Results showed that conventional sculpturing fabricated microlens array with poor form accuracy and surface finish due to couple effect of material adhesion and tool wear, while the high-frequency ultrasonic vibration-assisted sculpturing achieved optical application level with sub-micrometer form accuracy and surface roughness of nanometer due to reduction of material adhesion and tool wear resulted from high-frequency intermittent cutting

A Theoretical and Experimental Investigation of High-Frequency Ultrasonic Vibration-Assisted Sculpturing of Optical Microstructures

Ultrasonic vibration-assisted cutting (UVAC) has been regarded as a promising technology to machine difficult-to-machine materials. It allows for a sub-micrometer form accuracy and surface roughness in the nanometer range. In this paper, high-frequency vibration-assisted sculpturing is used to efficiently fabricate quadrilateral microlens array with sharp edges, instead of using slow-slide-servo diamond turning with vibration. The machining principle of diamond sculpturing, the cutting dynamics of ultrasonic vibration, and the tool edge on the theoretical form error between the designed structure and the machined structure were investigated for this technique. Then, the quadrilateral microlens array was machined by means of conventional sculpturing (CS) and high-frequency ultrasonic vibration-assisted sculpturing (HFUVAS), respectively, followed by a study of the cutting performances including form accuracy, the surface morphology of the machined structure, and the tool wear. Results showed that conventional sculpturing fabricated microlens array with poor form accuracy and surface finish due to couple effect of material adhesion and tool wear, while the high-frequency ultrasonic vibration-assisted sculpturing achieved optical application level with sub-micrometer form accuracy and surface roughness of nanometer due to reduction of material adhesion and tool wear resulted from high-frequency intermittent cutting

Preliminary Study of the Magnetic Perturbation Effects on the Edge Density Profiles and Fluctuations Using Reflectometers on EAST

The resonant magnetic perturbation (RMP) coils have been successfully designed and installed on the Experimental Advanced Superconducting Tokamak (EAST). Using the reflectometer systems, the density profile and the density fluctuations during magnetic perturbations (MPs) phase have been investigated. During the experiments, two different cases are studied separately: steady MPs and rotating MPs. In both cases, a strongly density pump-out has been observed. In the steady MPs cases, an enhancement of the low frequency (<60 kHz) density fluctuations in H-mode phase has been observed. The plasma density boundary out-shifts ~ 5% caused by the MPs. The pedestal density gradient is reduced by 50%, while the radial location nearly stays unchanged. In the rotating MPs, the line-averaged density, the Dα emission at the divertor region and the spectrum of the density fluctuations are modulated. The results suggest that the low frequency (<60 kHz) density fluctuations may contribute to the strong density pump-out