Laser metal deposition of refractory high-entropy alloys for high-throughput synthesis and structure-property characterization

Progress in materials development is often paced by the time required to produce and evaluate a large number of alloys with different chemical compositions.This applies especially to refractory high-entropy alloys(RHEAs),which are difficult to synthesize and process by conventional methods.To evaluate a possible way to accelerate the process,high-throughput laser metal deposition was used in this work to prepare a quinary RHEA,TiZrNbHfTa,as well as its quaternary and ternary subsystems by in-situ alloying of elemental powders.Compositionally graded variants of the quinary RHEA were also analyzed.Our results show that the influence of various parameters such as powder shape and purity,alloy composition,and especially the solidification range,on the processability,microstructure,porosity,and mechanical properties can be investigated rapidly.The strength of these alloys was mainly affected by the oxygen and nitrogen contents of the starting powders,while substitutional solid solution strengthening played a minor role.

A novel method of utilizing static mixer to obtain mixing homogeneity of multi-species powders in laser metal deposition

Real-time mixing of multi-species powder challenges Laser Metal Deposition(LMD)of Functionally Graded Materials(FGMs).The current work proposes a novel method of using a static mixer to realize rapid,uniform multi-species powder mixing.Firstly,copper powder and 316L stainless steel powder are selected to complete the powder mixing observation experiment with Scanning Electron Microscope(SEM)and Energy Dispersive Spectrometer(EDS).Secondly,computational fluid dynamics and particle mixing simulation models are used to analyze the flow field and particle motion characteristics in the static mixer.Finally,LMD experiment and metallo-graphic observation are carried out with 316L stainless steel powder and WC powder to verify the feasibility of the static mixer.This study provides a theoretical and practical basis for powder mixing in laser processing with a static mixer.The conclusions can also be applied to other processing fields requiring real-time and uniform mixing of multi-species powders.

Microstructure and Size‑Dependent Mechanical Properties of Additively Manufactured 316L Stainless Steels Produced by Laser Metal Deposition

Metal additive manufacturing(AM),as a disruptive technology in the feld of fabricating metallic parts,has shown its ability to design component with macrostructural complexity.However,some of these functionally complex structures typically contain a wide range of feature sizes,namely,the characteristic length of elements in AM-produced components can vary from millimeter to meter-scale.The requisite for controlling performance covers nearly six orders of magnitude,from the microstructure to macro scale structure.Understanding the mechanical variation with the feature size is of critical importance for topology optimization engineers to make required design decisions.In this work,laser metal deposition(LMD)is adopted to manufacture 316L stainless steel(SS)samples.To evaluate the efect of defects and specimen size on mechanical properties of LMD-produced samples,fve rectangular sample sizes which ranged from non-standard miniature size to ASTM standard sub-sized samples were machined from the block.Tensile test reveals that the mechanical properties including yield strength(YS),ultimate tensile strength(UTS),and elongation to failure(εf)are almost the identical for samples with ASTM standard size.Whilst,relatively lower YS and UTS values,except forεf,are observed for samples with a miniature size compared with that of ASTM standard samples.Theεf values of LMD-produced 316L SS samples show a more complex trend with sample size,and are afected by three key infuencing factors,namely,slimness ratio,cluster of pores,and occupancy location of lack of fusion defects.In general,theεf values exhibit a decreasing trend with the increase of slimness ratio.Microstructure characterization reveals that the LMD-produced 316L samples exhibited a high stress status at low angle grain boundaries,whilst its location changed to high angle grain boundaries after plastic deformation.The grain size refnement and austenite-to-martensite phase transformation occurred during plastic deformation might be responsible for the very high YS and UTS attained in this study.The experimental works carried out in this study is expected to provide a guideline for evaluating the mechanical properties of LMD-produced parts with complex structure,where critical parameter such as a certain slimness ratio has to be considered.

Comparison of Vacuum Metal Deposition and 1,2-lndandione/Ninhydrin Reagent Method for the Development of Fingerprints on Renminbi

It is extremely difficult to develop fingerprints from the surface of currency.There are studies reporting that the high vacuum metal deposition(VMD)method can be used to detect fingerprints on certain types of currency notes.Both VMD and 1,2-indandione/ninhydrin techniques are employed to visualize latent fingermarks on porous surfaces,such as paper.The current study explores whether the VMD method or 1,2-indandione/ninhydrin reagent method is more effective in the development of fingerprints on remninbi(RMB).Uncirculated,circulated,and water-exposed RMB was utilized in this study,along with five donors who ranged in their age and potential to leave fingermarks.Samples were aged for a determined period(for uncirculated and circulated RMB,times were 1,3,5,10,and 35 days;for watei^exposed RMB,exposure time was 1 day)and then treated with VMD and 1,2-indandione/ninhydrin.The results suggested that the 1,2-indandione/ninhydrin reagent yielded a better effect for both circulated and uncirculated RMB.For the RMB exposed to water,VMD performed better and gave limited results in terms of fingerprint development,which could serve as a reference for actual forensic cases.

A comparative study on microstructure,nanomechanical and corrosion behaviors of AlCoCuFeNi high entropy alloys fabricated by selective laser melting and laser metal deposition

The present study investigated the microstructure,nanomechanics,and corrosion behavior of AlCoCuFeNi high entropy alloys fabricated by selective laser melting(SLM)and laser metal deposition(LMD).The microstructure of SLM-processed specimens was mainly composed of columnar-grained BCC matrix(^90μm in width)and Cu-rich twinned FCC phase.The columnar grains grew epitaxially along the building direction and exhibited a strong{001}texture.In comparison,a coarse columnar-grained BCC matrix(^150μm in width)with a stronger<001>texture,rod-like B2 precipitates,and large core-shell structured FCC phases were formed in the LMD-processed specimens due to the higher heat accumulation effect.Consequently,the LMD-processed specimens showed a lower hardness,wear resistance,and corrosion resistance,but higher creep resistance and reduced Young's modulus than the SLM-processed specimens.Hot cracks occurred in both types of specimens,which could not be completely suppressed due to Cu segregation.

Microstructure and Mechanical Properties of B-Bearing Austenitic Stainless Steel Fabricated by Laser Metal Deposition In-Situ Alloying

In the field repair application of laser metal deposition(LMD),the kinds of powder materials that can be used are limited,while the equipment components are made of various materials.Hence many components have to be repaired with heterogeneous materials.However,it is difficult to match the mechanical properties between the repaired layer and the substrate due to the diff erent materials.Based on the high flexibility of raw materials and processes in LMD,an in-situ alloying method is proposed herein for tailoring the mechanical properties of LMDed alloy.Using diff erent mixing ratios of Fe314 and 316 L stainless steel powders as the control parameter,the microstructure and mechanical properties of B-bearing austenitic stainless steel fabricated by LMD in-situ alloying with diff erent proportions of Fe314 and 316 L particles were studied.With the increase in the concentration of 316 L steel,the volume fraction of the eutectic phase in deposited B-bearing austenitic stainless steel reduced,the size of the austenite dendrite increased,the yield strength and ultimate tensile strength decreased monotonically,while the elongation increased monotonically.Moreover,the fracture mode changed from quasi-cleavage fracture to ductile fracture.By adding 316 L powder,the yield strength,tensile strength,and elongation of deposited B-bearing austenitic stainless steel could be adjusted within the range of 712 MPa–257 MPa,1325 MPa–509 MPa,and 8.7%–59.3%,respectively.Therefore,this work provides a new method and idea for solving the performance matching problem of equipment components in the field repair.

Annealing before gate metal deposition related noise performance in AlGaN/GaN HEMTs

For a further improvement of the noise performance in A1GaN/GaN HEMTs, reducing the relatively high gate leakage current is a key issue. In this paper, an experiment was carried out to demonstrate that one method during the device fabrication process can lower the noise. Two samples were treated differently after gate recess etching： one sample was annealed before metal deposition and the other sample was left as it is. From a comparison of their Ig-Vg characteristics, a conclusion could be drawn that the annealing can effectively reduce the gate leakage current. The etching plasma-induced damage removal or reduction after annealing is considered to be the main factor responsible for it. Evidence is given to prove that annealing can increase the Schottky barrier height. A noise model was used to verify that the annealing of the gate recess before the metal deposition is really effective to improve the noise performance of AIGaN/GaN HEMTs.

Mechanical and microstructural characterization of additive manufactured Inconel 718 alloy by selective laser melting and laser metal deposition

The direct comparison of the microstructure and tensile properties of Inconel 718 fabricated by selective laser melting (SLM) or laser metal deposition (LMD) has been carried out. In the as-built state, LMD-fabricated specimens show lower tensile yield strength and fracture elongation than SLM-fabricated specimens due to the coarser solidification microstructure, including grains, cellular dendrites and Laves phases. This is mainly because the cooling rate of the LMD process is 2 to 3 orders lower than that of the SLM process. Upon the same heat treatment, both yield strengths of SLMand LMD-fabricated specimens are enhanced significantly. Notably, LMD-fabricated specimens exhibit simultaneous improvement in the strength and ductility, which is mainly attributed to the presence of small granular Laves phases and uniformly distributed nanoscale c00 strengthening phases. The results could serve as a guidance for selecting suitable postheat treatment routes for specific additive manufacturing process to attain excellent strength-ductility synergy.

Impact of Pulsed Laser Parameters and Scanning Pattern on the Properties of Thin-Walled Parts Manufactured Using Laser Metal Deposition

The quality of parts manufactured using laser metal deposition(LMD),similar to other additive manufacturing methods,is influenced by processing parameters.Such parameters determine geometric stability,favorable microstructures,and good mechanical properties.This study aimed to investigate the effects of pulsed laser parameters(duty cycle and pulse frequency)and scanning patterns(unidirectional and bidirectional patterns)on the properties of parts fabricated using LMD.Results show that the properties of the LMD-fabricated parts are obviously influenced by pulsed laser parameters and scanning patterns.Using the unidirectional scanning pattern in both pulsed laser parameters enhances the properties of the thin-walled parts prepared using LMD.An increase in duty cycle can improve geometric stability,increase grain size,and reduce microhardness.Furthermore,the geometric stability does not vary considerably with the use of different frequencies,but the microstructure of fabricated parts shows various grain sizes with different pulse frequencies.In addition,the microhardness increases as the frequency increases from 13.33 to 50 Hz.In general,the influence of the duty cycle on geometric properties is greater than that of frequency.Meanwhile,the impact of frequency on microhardness is greater than that of the duty cycle.

Low-leakage-current AlGaN/GaN HEMTs on Si substrates with partially Mg-doped GaN buffer layer by metal organic chemical vapor deposition

High-performance low-leakage-current A1GaN/GaN high electron mobility transistors （HEMTs） on silicon （111） sub- strates grown by metal organic chemical vapor deposition （MOCVD） with a novel partially Magnesium （Mg）-doped GaN buffer scheme have been fabricated successfully. The growth and DC results were compared between Mg-doped GaN buffer layer and a unintentionally onμe. A 1μ m gate-length transistor with Mg-doped buffer layer exhibited an OFF-state drain leakage current of 8.3 × 10-8 A/mm, to our best knowledge, which is the lowest value reported for MOCVD-grown A1GaN/GaN HEMTs on Si featuring the same dimension and structure. The RF characteristics of 0.25-μ m gate length T-shaped gate HEMTs were also investigated.

Selective Area Growth and Characterization of GaN Nanorods Fabricated by Adjusting the Hydrogen Flow Rate and Growth Temperature with Metal Organic Chemical Vapor Deposition

CaN nanorods are successfully fabricated by adjusting the flow rate ratio of hydrogen （H2）/nitrogen （N2） and growth temperature of the selective area growth （SAG） method with metal organic chemical vapor deposition （MOCVD）. The SAG template is obtained by nanospherical-lens photolithography. It is found that increasing the flow rate of 1-12 will change the CaN crystal shape from pyramid to vertical rod, while increasing the growth temperature will reduce the diameters of GaN rods to nanometer scale. Finally the CaN nanorods with smooth lateral surface and relatively good quality are obtained under the condition that the H2：N2 ratio is 1：1 and the growth temperature is 1030℃. The good crystal quality and orientation of GaN nanorods are confirmed by high resolution transmission electron microscopy. The cathodoluminescence spectrum suggests that the crystal and optical quality is also improved with increasing the temperature.

In-situ wafer bowing measurements of GaN grown on Si(111) substrate by reflectivity mapping in metal organic chemical vapor deposition system

In this work, the wafer bowing during growth can be in-situ measured by a reflectivity mapping method in the 3×2 Thomas Swan close coupled showerhead metal organic chemical vapor deposition（MOCVD） system. The reflectivity mapping method is usually used to measure the film thickness and growth rate. The wafer bowing caused by stresses（tensile and compressive） during the epitaxial growth leads to a temperature variation at different positions on the wafer, and the lower growth temperature leads to a faster growth rate and vice versa. Therefore, the wafer bowing can be measured by analyzing the discrepancy of growth rates at different positions on the wafer. Furthermore, the wafer bowings were confirmed by the ex-situ wafer bowing measurement. High-resistivity and low-resistivity Si substrates were used for epitaxial growth. In comparison with low-resistivity Si substrate, Ga N grown on high-resistivity substrate shows a larger wafer bowing caused by the highly compressive stress introduced by compositionally graded Al Ga N buffer layer. This transition of wafer bowing can be clearly in-situ measured by using the reflectivity mapping method.

Growth and Characterization of Doped CeO_2 Buffers on Ni-W Substrates for Coated Conductors Using Metal Organic Deposition Method

CeO2 and Ce0.8M0.2O2-d films （M = Mn, Y, Gd, Sm, Nd and La） with （00l） preferred orientation have been prepared on biaxially textured Ni-W substrates by metal organic decomposition （MOD） method. The factors influencing the formation of cracks on the surface of these CeO2 and doped CeO2 films on Ni-W substrates were explored by X-ray diffraction （XRD）, scanning electron microscopy （SEM） analysis, atomic force microscopy （AFM） and differential scanning calorimetry （DSC）. The results indicate that many factors, such as the change of the ionic radii of doping cations, the transformation of crystal structure and the formation of oxygen vacancies in lattices at high annealing temperature, may be related to the formation of cracks on the surface of these films. However, the crack formation shows no dependence on the crystal lattice mismatch degree of the films with Ni-W substrates. Moreover, the suppression of surface cracks is related to the change of intrinsic elasticity of CeO2 film with doping of cations with a larger radius. SEM and AFM investigations of Ce0.8M0.2O2-d （M = Y, Gd, Sm, Nd and La） films reveal the dense, smooth and crack-free microstructure, and their lattice parameters match well with that of YBCO, illuminating that they are potentially suitable to be as buffer layer, especially as cap layer in multi-layer architecture of buffer layer for coated conductors.

Growth and characterization of AlN epilayers using pulsed metal organic chemical vapor deposition

We report the growth of Al N epilayers on c-plane sapphire substrates by pulsed metal organic chemical vapor deposition（MOCVD）. The sources of trimethylaluminium（TMAl） and ammonia were pulse introduced into the reactor to avoid the occurrence of the parasitic reaction. Through adjusting the duty cycle ratio of TMAl to ammonia from 0.8 to 3.0, the growth rate of Al N epilayers could be controlled in the range of 0.24 m/h to 0.93 m/h. The high-resolution x-ray diffraction（HRXRD） measurement showed that the full width at half maximum（FWHM） of the（0002） and（10-12） reflections for a sample would be 194 arcsec and 421 arcsec, respectively. The step-flow growth mode was observed in the sample with the atomic level flat surface steps, in which a root-mean-square（RMS） roughness was lower to 0.2 nm as tested by atomic force microscope（AFM）. The growth process of Al N epilayers was discussed in terms of crystalline quality, surface morphology,and residual stress.

Superior material qualities and transport properties of InGaN channel heterostructure grown by pulsed metal organic chemical vapor deposition

Pulsed metal organic chemical vapor deposition is introduced into the growth of InGaN channel heterostructure for improving material qualities and transport properties. High-resolution transmission electron microscopy imaging shows the phase separation free InGaN channel with smooth and abrupt interface. A very high two-dimensional electron gas density of approximately 1.85 x 10^13 cm-2 is obtained due to the superior carrier confinement. In addition, the Hall mobility reaches 967 cruZ/V-s, owing to the suppression of interface roughness scattering. Furthermore, temperature-dependent Hall measurement results show that InGaN channel heterostructure possesses a steady two-dimensional electron gas density over the tested temperature range, and has superior transport properties at elevated temperatures compared with the traditional GaN channel heterostructure. The gratifying results imply that InGaN channel heterostructure grown by pulsed metal organic chemical vapor deposition is a promising candidate for microwave power devices.

Improved Semipolar(11(2|-)2) GaN Quality Grown on m-Plane Sapphire Substrates by Metal Organic Chemical Vapor Deposition Using Self-Organized SiN_x Interlayer

The effect of a self-organized SiNs interlayer on the defect density of （1122） semipolar GaN grown on 7n-plane sapphire is studied by transmission electron microscopy, atomic force microscopy and high resolution x-ray diffrac- tion. The SiNx interlayer reduces the c-type dislocation density from 2.5 ×10^10 cm^-2 to 5 ×10^8 cm 2. The SiNx interlayer produces regions that are free from basal plane stacking faults （BSFs） and dislocations. The overall BSF density is reduced from 2.1×10^5 cm-1 to 1.3×10^4 cm^-1. The large dislocations and BSF reduction in semipolar （1122） GaN with the SiNx, interlayer result from two primary mechanisms. The first mechanism is the direct dislocation blocking by the SiNx interlayer, and the second mechanism is associated with the unique structure character of （1122） semipolar GaN.

Fabrication of InAlGaN/GaN High Electron Mobility Transistors on Sapphire Substrates by Pulsed Metal Organic Chemical Vapor Deposition

Nearly lattice-matched InAIGaN/GaN heterostructure is grown on sapphire substrates by pulsed metal organic chemical vapor deposition and excellent high electron mobility transistors are fabricated on this heterostructure. The electron mobility is 1668.08cm2/V.s together with a high two-dimensional-electron-gas density of 1.43 × 10^13 cm-2 for the InAlCaN/CaN heterostructure of 2Onto InAlCaN quaternary barrier. High electron mobility transistors with gate dimensions of 1 × 50 μm2 and 4μm source-drain distance exhibit the maximum drain current of 763.91 mA/mm, the maximum extrinsic transconductance of 163.13 mS/mm, and current gain and maximum oscillation cutoff frequencies of 11 GHz and 21 GHz, respectively.

High-crystalline GaSb epitaxial films grown on GaAs(001) substrates by low-pressure metal–organic chemical vapor deposition

Orthogonal experiments of Ga Sb films growth on Ga As（001） substrates have been designed and performed by using a low-pressure metal–organic chemical vapor deposition（LP-MOCVD） system. The crystallinities and microstructures of the produced films were comparatively analyzed to achieve the optimum growth parameters. It was demonstrated that the optimized Ga Sb thin film has a narrow full width at half maximum（358 arc sec） of the（004） ω-rocking curve, and a smooth surface with a low root-mean-square roughness of about 6 nm, which is typical in the case of the heteroepitaxial single-crystal films. In addition, we studied the effects of layer thickness of Ga Sb thin film on the density of dislocations by Raman spectra. It is believed that our research can provide valuable information for the fabrication of high-crystalline Ga Sb films and can promote the integration probability of mid-infrared devices fabricated on mainstream performance electronic devices.

GaInP/GaInAs/GaInNAs/Ge Four-Junction Solar Cell Grown by Metal Organic Chemical Vapor Deposition with High Efficiency

We directly grow a lattice matched GalnP/GalnAs/GalnNAs/Ge （1.88 eVil .42 eVil .05 eV/0.67eV） four-junction （4J） solar cell on a Ge substrate by the metal organic chemical vapor deposition technology. To solve the current limit of the GalnNAs sub cell, we design three kinds of anti-reflection coatings and adjust the base region thickness of the GalnNAs sub cell. Developed by a series of experiments, the external quantum efficiency of the GalnNAs sub cell exceeds 80%, and its current density reaches 11.24 mA/cm2. Therefore the current limit of the 4J solar cell is significantly improved. Moreover, we discuss the difference of test results between 4J and GalnP/GalnAs/Ge solar cells under the 1 sun AMO spectrum.

Effects of specific surface area of metallic nickel particles on carbon deposition kinetics

Carbon deposition on nickel powders in methane involves three stages in different reaction temperature ranges. Temperature programing oxidation test and Raman spectrum results indicated the formation of complex and ordered carbon structures at high deposition temperatures. The values of I（D）/I（G） of the deposited carbon reached 1.86, 1.30, and 1.22 in the first, second, and third stages, respectively. The structure of carbon in the second stage was similar to that in the third stage. Carbon deposited in the first stage rarely contained homogeneous pyrolytic deposit layers. A kinetic model was developed to analyze the carbon deposition behavior in the first stage. The rate-determining step of the first stage is supposed to be interfacial reaction. Based on the investigation of carbon deposition kinetics on nickel powders from different resources, carbon deposition rate is suggested to have a linear relation with the square of specific surface area of nickel particles.