High-entropy alloys have become ideal candidates for wear- and corrosion-resistant coatings materials because of their unique structure and excellent physical properties. Owing to their outstanding corrosion resistance and formability, FeCoCrNi high-entropy alloys are used extensively in corrosion-resistant coatings. However, their comparatively low mechanical strength does not satisfy the requirements for wear- and corrosion-resistant coatings. Adding high-melting-point Mo can promote the formation of Mo-rich phases in the alloy, thereby improving its mechanical strength and crevice corrosion resistance. This study aims to investigate the microstructure, forming process, and corrosion resistance of FeCoCrNiMo by fabricating FeCoCrNiMo high-entropy alloy coatings on 45 steel round bars or 316L stainless steel primer using extremely high-speed laser cladding (EHLC) technology. This study is expected to provide some essential technicalities for high-entropy FeCoCrNiMo alloy coatings that can be applied under different and complex wear and corrosion conditions.

In this study, scanning electron microscopy (SEM) in conjunction with backscatter imaging and energy dispersive X-ray spectroscopy (EDS) was employed to characterize the microstructure and composition of the coatings. X-ray diffraction (XRD) was adopted to ascertain the physical phase composition of the coatings, where Co was utilized as the target material. Transmission electron microscopy (TEM) samples of the coatings were prepared using a focused ion beam. The microstructure of the coating was characterized using high-resolution scanning TEM, and microhardness measurements were performed on the polished coating surfaces using a Vickers hardness tester. Electrochemical and neutral salt spray experiments were conducted to evaluate the corrosion resistance of the coatings.

A reduction in the linear velocity from 15 to 5 m/min results in a decrease in the number of cracks in the cladding coated on the 45 steel substrate. Additionally, a transition from reticulated to striated cracks is observed, as shown in Fig. 5. By using 316L stainless steel as a primer and reducing the linear speed to 5 m/min, cold cracks are effectively mitigated, as illustrated in Fig. 7. Therefore, one can reasonably conclude that the heat-affected zone of the 45 steel substrate undergoes a martensitic transformation, which increases the tensile stress within the coating and resultes in reticulated or striped peritectic cracking. Nevertheless, the induction of the 316L primer reveals no alteration in the microstructure of the coatings, which is dominated by the typical lamellar eutectic microstructure, as shown in Fig. 7. In contrast to the microstructure of the coatings created under higher linear velocities, the basic microstructural characteristics remain unaltered. However, a reduction of 15 percentage points is detected in the face-centered cubic (FCC) phase (Fig. 9), which is assumed to have contributed to the modest decrease in the Vickers hardness, although the latter remained at a prominently high level.

This study employed extremely high-speed laser cladding technology to fabricate a FeCoCrNiMo high-entropy alloy coating. Subsequently, a detailed analysis of its microstructure, phase composition, forming process, and corrosion resistance was performed. The findings indicate that the coatings comprise primarily an FCC matrix phase enriched in Fe, Co, and Ni, with a body-centered cubic (BCC) precipitated phase enriched in Mo and Cr. The lower and middle regions of the coating feature columnar crystals of the FCC phase, intersperse with alternating submicron BCC/FCC lamellar eutectic structures among the dendrites. Additionally, the heat-affected zone of the 45 steel substrate undergoes a martensitic transformation, thus increasing the tensile stress within the coating and resulting in reticulated or striped peritectic cracking. Using 316L stainless steel as a primer as well as reducing the line speed effectively mitigates these cracks and maintains high hardness levels. By contrast, the upper region is dominated by equiaxed crystals with similar alternating lamellar eutectic microstructures. Compared with a standard 304 stainless steel coating, the high-entropy alloy coating exhibits a higher self-corrosion potential by 0.130 V, a significantly lower self-corrosion current density by one-sixth, and a 235-fold increase in the coating film resistance, thus suggesting substantially enhanced corrosion resistance. In conclusion, the fine and uniform FCC/BCC lamellar eutectic microstructure at the top of the coating is believed to have contributed significantly in improving the corrosion resistance.

Lasers plays a crucial role in various scientific experiments in fields such as quantum communication, high-resolution atomic spectroscopy, cold-atom physics, and optical clocks. The stability of laser power significantly influences experimental results. For instance, in strontium atomic optical lattice clocks, stabilizing the power of more than a dozen laser beams is required to achieve a clock frequency with fractional stability on the order of 10^-18. High-frequency fluctuations in laser power can reduce the signal-to-noise ratio, thereby compromising the stability of frequency standards, whereas low-frequency fluctuations can impact the long-term stability of atomic clocks. Consequently, reliable laser power stabilization technology is indispensable. Furthermore, the portable or space-based applications of cutting-edge experimental devices, such as optical clocks, increase the demand for higher integration, flexibility, and response speed in laser power stabilization. In this study, we leverage the high-speed and low-amplitude noise characteristics of a previously developed high-bandwidth direct digital synthesizer (DDS) circuit and construct a proportional-integral controller in a field-programmable gate array (FPGA) to directly modify the output amplitude of the DDS for laser power feedback control. This approach eliminates the need for an external voltage-controlled attenuator, thereby improving integration and minimizing high-frequency noise interference.

Mainstream methods for laser power stabilization can be classified into two types: internal loop control and external loop control. This study selected the latter, using an acousto-optic modulator (AOM) that enables flexible control of output power without affecting the laser output frequency. A portion of the laser beam was split and directed onto a photodiode for power detection. The photodiode output was digitized using a 16-bit analog-to-digital converter (ADC) and compared to a target value to generate an error signal, which was then processed using an incremental digital proportional-integral (PI) controller. Based on the PI output, the amplitude of the output signal from the DDS was directly adjusted in reverse and applied to the AOM, achieving feedback control of laser power without the need for an additional digital-to-analog converter (DAC) or voltage-controlled attenuator. To prevent excessive ringing caused by loop delay during the laser startup from a fully off state, an output offset is preset to the target value with a specific delay before enabling PI feedback. The closed-loop system was tested by measuring the output radio frequency (RF) signal from the DDS using an RF power detector, simulating the photodiode's function and evaluating electronic noise in a closed-loop configuration without optics.

A 160-minute test evaluated the long-term performance of the closed-loop laser power stabilization in the time domain. The peak-to-peak values of relative laser power drift were found to be 7.1% and 0.0076% in the open- and closed-loop states, respectively (Fig.6). This result shows a significant improvement in laser power stability. Frequency domain measurements indicated that the relative intensity noise power spectral density at 1 Hz was suppressed from -60.1 dBc/Hz in the open loop to -111.2 dBc/Hz in the closed loop, approaching electronic noise levels up to 10 kHz (Fig.5). Regarding the transition from a fully off state to a stabilized laser power, the method of presetting the output offset and delaying the PI controller’s activation achieved a rise time of approximately 3 μs. In contrast, the traditional PI controller required approximately 7 μs under the same conditions (Fig. 7).

This study presents a laser power stabilization method based on high-bandwidth DDS, which directly adjusts the DDS output signal amplitude to control the AOM driving power, thereby eliminating the need for a voltage-controlled attenuator or DAC. Compared to traditional laser power stabilization methods, our method leverages the high-speed response and low-amplitude noise characteristics of the high-bandwidth DDS, while optimizing the FPGA-based digital PI controller to shorten the turn-on time of the laser. Using an 813-nm laser system, we achieved a suppression of the relative intensity noise power spectral density to below -111.2 dBc/Hz between 1 Hz and 10 kHz, with long-term drift reduced to approximately 0.0076% over a 160-minute period. The technique of presetting the output offset and delaying the activation of the PI controller enables the laser to turn on and reach a stabilized power level within approximately 3 μs. This method demonstrates high integration, rapid response, and low noise, fulfilling the laser power stabilization requirements for a wide range of experimental configurations, including space-based or portable atomic optical clocks. Moreover, this approach significantly simplifies the feedback loop, making it more suitable for complex experimental environments.