Enhanced mechanical performance of grain boundary precipitation-hardened high-entropy alloys via a phase transformation at grain boundaries

Grain-boundary(GB)precipitation has a significant adverse effect on plasticity of alloys,which easily leads to catastrophic intergranular failure in safety-critical applications under high external loading.Herein,we report a novel strategy that uses the local stress concentration induced by GB precipitates as a driving force to trigger phase transformation of preset non-equiatomic high-entropy solid-solution phase at GBs.This in situ deformation-induced phase transformation at GBs introduces a well-known effect:transformation-induced plasticity(TRIP),which enables an exceptional elongation to fracture(above 38%)at a high strength(above 1.5 GPa)in a GB precipitation-hardened high-entropy alloy(HEA).The present strategy in terms of"local stress concentration-induced phase transformations at GBs"may provide a fundamental approach by taking advantage of(rather than avoiding)the GB precipitation to gain a superior combination of high strength and high ductility in HEAs.

Conditioning of BPM pickup signals for operations of the Duke storage ring with a wide range of single-bunch current

The Duke storage ring is a dedicated driver for the storage ring based oscillator free-electron lasers（FELs）, and the High Intensity Gamma-ray Source（HIGS）. It is operated with a beam current ranging from about1 mA to 100 mA per bunch for various operations and accelerator physics studies. High performance operations of the FEL and γ-ray source require a stable electron beam orbit, which has been realized by the global orbit feedback system. As a critical part of the orbit feedback system, the electron beam position monitors（BPMs） are required to be able to precisely measure the electron beam orbit in a wide range of the single-bunch current. However, the high peak voltage of the BPM pickups associated with high single-bunch current degrades the performance of the BPM electronics, and can potentially damage the BPM electronics. A signal conditioning method using low pass filters is developed to reduce the peak voltage to protect the BPM electronics, and to make the BPMs capable of working with a wide range of single-bunch current. Simulations and electron beam based tests are performed. The results show that the Duke storage ring BPM system is capable of providing precise orbit measurements to ensure highly stable FEL and HIGS operations.

Design and optimization of a longitudinal feedback kicker cavity for the HLS-Ⅱ storage ring

In the Hefei Light Source （HLS） storage ring, multibunch operation is used to obtain a high luminosity. Multibunch instabilities can severely limit light source performance with a variety of negative impacts, including beam loss, low injection efficiency, and overall degradation of the beam quality. Instabilities of a multibunch beam can be mitigated using certain techniques including increasing natural damping （operating at a higher energy）, lowering the beam current, and increasing Landau damping. However, these methods are not adequate to stabilize a multibunch electron beam at a low energy and with a high current. In order to combat beam instabilities in the HLS storage ring, active feedback systems including a longitudinal feedback system （LFB） and a transverse feedback system （TFB） will be developed as part of the HLS upgrade project, the HLS-Ⅱ storage ring project. As a key component of the longitudinal bunch-by-bunch feedback system, an LFB kicker cavity with a wide bandwidth and high shunt impedance is required. In this paper we report our work on the design of the LFB kicker cavity for the HLS-Ⅱ storage ring and present the new tuning and optimization techniques developed in designing this high performance LFB kicker.