The formation of stray grains (SGs) remains a critical and pervasive challenge hindering the additive manufacturing (AM) of single crystals for high-temperature aerospace applications. Here, we elucidate the mechanism underlying SG formation during the AM of Ni-based single-crystal alloys, through integrating in situ synchrotron imaging/diffraction, ex situ characterization, and multi-physics modeling. In contrast to the conventional understanding that attributes SG formation solely to thermal effects, we demonstrate that SG originates from subgrain rotation driven by heterogeneous dislocation activity. We further reveal that dislocation-induced SG formation can be regulated by substrate orientations, in which the Gini coefficient derived from dislocation distributions is proposed to serve as the physics-based predictive metric for SG susceptibility. Specifically, high-symmetry orientations exhibiting low Gini coefficients suppress SGs via more uniform dislocation distribution. This study advances the understanding of SG formation under extreme nonequilibrium solidification processes, thereby guiding the fabrication of high-quality AM single-crystal components for aerospace applications.

The first water-cooling nano multilayer Kirkpatrick-Baez mirror system in the Structural Dynamics beamline (ID23) at High Energy Photon Source (HEPS) has been implemented. An Invar gantry is engineered to achieve a balance between light weight and stability. The cooling system, including eutectic gallium-indium (eGaIn), copper braids and a cooling water circuit with a multi-bend copper pipe in a compact space, decouples movement and mitigates vibration. The mirror cooling holders introduce less than 0.5 nm RMS height error and 0.1 µrad RMS slope error in the mirror surface shapes. A series of stability tests is applied to verify the mechanism structure. In a 1 h test assessment using a water-cooling flow of 4.5 L min-1, the system demonstrated a positional stability of 5.96 nm and an angular stability of 86.76 nrad from 1 Hz to 500 Hz. The focal spot size of 13.39 nm × 15.15 nm (H × V) at a photon energy of 21.8 keV demonstrates the system's performance.

The inherently high susceptibility to hot cracking of AA7075 alloy poses significant challenges in its Laser Powder Bed Fusion (LPBF) manufacturing process, thereby impeding its widespread adoption in aerospace and automotive industries. However, the laser-matter interactions and melt pool dynamics in laser Additive Manufacturing (AM) remain obscure, particularly in how cracks initiate and propagate during the process. In the present study, in-situ high-speed X-ray imaging technique was employed to characterize the crack formation and elimination during the laser remelting process on LPBF fabricated AA7075 substrates. The remelting process resembles scanning on preceding layers during LPBF. The microstructure of substrates and the processing parameters were investigated. Two modes of crack formation were unveiled: one initiating from the inherent crack defects, propagating upwards through vulnerable areas, and the other originating within the final depression zone of the melt pool, extending downwards across the vulnerable areas. On a substrate with rich defects, cracks tend to initiate from the inherent cracks. Another crucial finding is that the rupture of gas pores can induce fluctuations in the melt pool, leading to the elimination of cracks. The efficacy of crack elimination within the melt pool is highly influenced by variations in the inherent microstructure of the substrates and the applied processing parameters. These results will provide enlightening insight into crack reduction and elimination in LPBF.

Laser-induced projectile impact testing (LIPIT) based on synchrotron imaging is proposed and validated. This emerging high-velocity, high-strain microscale dynamic loading technique offers a unique perspective on the strain and energy dissipation behavior of materials subjected to high-speed microscale single-particle impacts. When combined with synchrotron radiation imaging techniques, LIPIT allows for in situ observation of particle infiltration. Two validation experiments were carried out, demonstrating the potential of LIPIT in the roentgenoscopy of the dynamic properties of various materials. With a spatial resolution of 10 µm and a temporal resolution of 33.4 µs, the system was successfully realized at the Beijing Synchrotron Radiation Facility 3W1 beamline. This innovative approach opens up new avenues for studying the dynamic properties of materials in situ.

Laser metal additive manufacturing technology is capable of producing components with complex geometries and compositions that cannot be realized by conventional manufacturing methods. However, a large number of pores generated during the additive manufacturing process greatly affect the mechanical properties of the additively manufactured parts, and the mechanism of such pore generation has not been revealed by direct observation clearly. Here, we report the mechanism of pore generation in the laser direct energy deposition process as revealed by {\it in-situ} high-speed high-resolution synchrotron X-ray imaging. We found that dissolution and re-precipitation of external gases and precipitation of metal vapors are the two main mechanisms of pore formation. We further explored the effects of different process parameters on the generation of pores and optimized the process to suppress pore generation. This work provides important insights into the formation of porosity defects during laser metal additive manufacturing, and can provide guidance for related process optimization.

Understanding the dynamic process of epitaxial microstructure forming in laser additive manufacturing is very important for achieving products with a single crystalline texture. Here, we perform in situ, real-time synchrotron Laue diffraction experiments to capture the microstructural evolution of nickel-based single-crystal superalloys during the rapid laser remelting process. In situ synchrotron radiation Laue diffraction characterises the crystal rotation behaviour and stray grain formation process. With a complementary thermomechanical coupled finite element simulation and molecular dynamics simulation, we identify that the crystal rotation is governed by the localised heating/cooling heterogeneity-induced deformation gradient and recognise that the sub-grain rotation caused by rapid dislocation movement could be the origin of granular stray grains at the bottom of the melt pool.

In situ transient synchrotron Laue x-ray diffraction based on high-energy and broadband x rays under high strain-rate tensile loading was developed at a superconducting wiggler beamline at the Beijing Synchrotron Radiation Facility. A split-Hopkinson tensile bar is utilized to realize this dynamic loading condition, while the transient Laue x-ray diffraction captures the transient internal structure of monocrystalline materials. Plastic deformation of a monocrystalline nickel specimen was investigated to prove the ability of this instrumentation in the characterization of a dynamic response of monocrystalline materials during a high strain-rate impact process with 5 µs time resolution.

The ultrafast photoinduced strain (UPS) resulting from the coupling of piezoelectric and photovoltaic effects in ferroelectric has been focused in the last decade, endowing them with extensive applications including ultrafast optical memories, sensors and actuators with strain engineering. The mechanism of screening of the depolarization field by photoinduced carriers is generally accepted for UPS in ferroelectrics, while the thermal component of the strain is usually diluted as the offset and has not been systematically confronted, leading to unnecessary confusion. Herein, both the positive and negative thermal expansion effects in composite ferroelectric epitaxial films are investigated by use of high-repetition-rate ultrafast X-ray diffraction, along with the piezoelectric and photovoltaic effects. The coupling of the positive/negative thermal effects and the piezoelectric/photovoltaic effects in ultrafast strain is evidenced and can be regulated. The opposite lattice responses due to different thermal effects of the samples with different axial ratios are observed. The maximum UPS is up to 0.24%, comparable to that of conventional ferroelectric. The interaction between the thermal and ferroelectric effects in the induced strain could promote the diversified applications with the coupling of light, heat and electricity.

The upgrade of the laser pump time-resolved X-ray probes, namely time-resolved X-ray absorption spectroscopy (TR-XAS) and X-ray diffraction (TR-XRD), implemented at the Beijing Synchrotron Radiation Facility, is described. The improvements include a superbunch fill, a high-efficiency fluorescence collection, an efficient spatial overlap protocol and a new data-acquisition scheme. After upgrade, the adequate TR-XAS signal is now obtained in a 0.3 mM solution, compared with a 6 mM solution in our previous report. Furthermore, to extend application in photophysics, the TR-XAS probe is applied on SrCoO2.5 thin film. And for the first time, TR-XAS is combined with TR-XRD to simultaneously detect the kinetic trace of structural changes in thin film.

The Energy Recovery Linac (ERL)can produce continuous electron bunches with low emittance at high average current.The photocathode electron gun needs high repetition rate,high average power drive laser system.Adopting advanced fi-ber laser technologies,especially using Yb-doped photonic crystal fiber in the laser system,a high repetition frequency,high aver-age power laser system was set up.By using chirped pulse amplification (CPA)and optimizing the design,a 100 MHz oscillator and a 1.3 GHz oscillator were integrated into one laser system.The structure of the whole laser system can be simplified and easy to operate.The second harmonic generation (SHG)efficiencies of the laser system at two different repetition rates have already reached 50％ and 30％,respectively.In addition,more than 5 W green light has been achieved,which meets the needs of test platform of photocathode at IHEP.

A new setup and commissioning of transient X-ray absorption spectroscopy are described, based on the high-repetition-rate laser pump/X-ray probe method, at the 1W2B wiggler beamline at the Beijing Synchrotron Radiation Facility. A high-repetition-rate and high-power laser is incorporated into the setup with in-house-built avalanche photodiodes as detectors. A simple acquisition scheme was applied to obtain laser-on and laser-off signals simultaneously. The capability of picosecond transient X-ray absorption spectroscopy measurement was demonstrated for a photo-induced spin-crossover iron complex in 6 mM solution with 155 kHz repetition rate.

The setup of dynamic synchrotron radiation dichroism spectroscopy (SRCD) is discussed by using continuous flow probe with microfluidic mixer, developed at 4B8 ultraviolet vacuum spectroscopy beamline at Beijing Synchrotron Radiation Facility. The quartz microfluidic mixer was fabricated with deep ion etching with channel depth down to 44. 5 microns. The mixer is designed on serpentine configuration to reach the effective mixing. The mixing efficiency was evaluated based on fluorescence image of the mixing solution under practical high viscous solution condition. Dynamic SRCD measurement with mixer was validated after implementing focus position feedback, since wavelength dispersion of the focusing lens results in the changes of focus spot position during wavelength scan. The time range covers from 4. 5 to 270 ms at present under 500 (L . min' flow rate within the wide observation channel. The performance of the,time-resolved SRCD is demonstrated in snapshoting the folding of cytochrome c back to 54% at 4. 5 millisecond after completely mixing.

Background:Time-resolved experiments could provide a dynamical insight into the structure of materials. The ultrafast X-ray diffraction (UXRD) technique is a common way in time-resolved experiment with the laser pump and the X-ray probe method.Purpose: This study aims to perform the UXRD experiments with the high repetition rate pump laser.Methods: A picosecond resolution UXRD setup on high repetition rate laser pump at Beijing Synchrotron Radiation Facility (BSRF) was built and a test with the sample SrRuO3 (SRO) grown on the substrate SrTiO3 (STO) was performed at the repetition rate of 310 kHz. Some exploratory experiments on the static heating problem accompanied by high repetition rate photoexcitation were discussed briefly.Results: Compared with the previous 1-kHz repetition rate measurement, the signal to noise ratio (SNR) of those at 310 kHz was improved dramatically. And a cooling system was necessary for the overheating problem.Conclusion: Those UXRD experiments at BSRF were performed at high repetition rate, which laid a foundation for the future work.

We investigate the transient photoexcited lattice dynamics in a layered perovskite Mott insulator Sr2IrO4 film by femtosecond X-ray diffraction using a laser plasma-based X-ray source. The ultrafast structural dynamics of Sr2IrO4 thin films are determined by observing the shift and broadening of (0012) Bragg diffraction after excitation by 1.5 eV and 3.0 eV pump photons for films with different thicknesses. The observed transient lattice response can be well interpreted as a distinct three-step dynamics due to the propagation of coherent acoustic phonons generated by photoinduced quasiparticles (QPs). Employing a normalized phonon propagation model, we found that the photoinduced angular shifts of the Bragg peak collapse into a universal curve after introducing normalized coordinates to account for different thicknesses and pump photon energies, pinpointing the origin of the lattice distortion and its early evolution. In addition, a transient photocurrent measurement indicates that the photoinduced QPs are charge neutral excitons. Mapping the phonon propagation and correlating its dynamics with the QP by ultrafast X-ray diffraction (UXRD) establish a powerful way to study electron-phonon coupling and uncover the exotic physics in strongly correlated systems under nonequilibrium conditions.

The implementation of a laser pump/X-ray probe scheme for performing picosecond-resolution X-ray diffraction at the 1W2B wiggler beamline at Beijing Synchrotron Radiation Facility is reported. With the hybrid fill pattern in top-up mode, a pixel array X-ray detector was optimized to gate out the signal from the singlet bunch with interval 85 ns from the bunch train. The singlet pulse intensity is ∼2.5 × 10(6) photons pulse(-1) at 10 keV. The laser pulse is synchronized to this singlet bunch at a 1 kHz repetition rate. A polycapillary X-ray lens was used for secondary focusing to obtain a 72 µm (FWHM) X-ray spot. Transient photo-induced strain in BiFeO3 film was observed at a ∼150 ps time resolution for demonstration.

It is difficult to find commercial laser products meeting the requirements of ERL photocathode gun. Therefore, a drive laser system is being developed at Institute of High Energy Physics (IHEP), which comprises two kinds of laser seeds, 1.3 GHz and 100 MHz at repetition rate. Fiber amplifiers were being constructed and a test system was set up. The output power of the main amplifier can reach up to 27 W. With a 5-mm-long LBO crystal, the green second harmonic laser of 6 W was achieved in the preliminary validation experiment. ©, 2015, Editorial Office of High Power Laser and Particle Beams. All right reserved.

The Electro-optical sampling delay scanning technique can be used for electron beam bunch length measurement.A novel non-synchronous delay scanning technique based on the electro-optical sampling measurements is presented.Based on Beijing Free Electron Laser（BFEL）,the electron beam bunch length was measured with the electro-optical sampling technique for the first time in China.The result shows that the electron beam bunch length at BFEL is about 5.6±1.2 ps.

Theoretical analyses have been done on the cross correlation between pulses of the femtosecond laser sequence. Experimental results are also derived using a cross-correlation setup, which can be switched between intensity cross-correlation mode and interferometric cross-correlation mode. The results show that the pattern of adjacent pulses cross-correlation is similar to that of auto-correlation. In the future, the method can be used in advanced light sources based electron accelerators to stabilize signals and to locate the cavity mirror precisely.