Interplay of laser power and pore characteristics in selective laser melting of ZK60 magnesium alloys:A study based on in-situ monitoring and image analysis

This study offers significant insights into the multi-physics phenomena of the SLM process and the subsequent porosity characteristics of ZK60 Magnesium(Mg)alloys.High-speed in-situ monitoring was employed to visualise process signals in real-time,elucidating the dynamics of melt pools and vapour plumes under varying laser power conditions specifically between 40 W and 60 W.Detailed morphological analysis was performed using Scanning-Electron Microscopy(SEM),demonstrating a critical correlation between laser power and pore formation.Lower laser power led to increased pore coverage,whereas a denser structure was observed at higher laser power.This laser power influence on porosity was further confirmed via Optical Microscopy(OM)conducted on both top and cross-sectional surfaces of the samples.An increase in laser power resulted in a decrease in pore coverage and pore size,potentially leading to a denser printed part of Mg alloy.X-ray Computed Tomography(XCT)augmented these findings by providing a 3D volumetric representation of the sample internal structure,revealing an inverse relationship between laser power and overall pore volume.Lower laser power appeared to favour the formation of interconnected pores,while a reduction in interconnected pores and an increase in isolated pores were observed at higher power.The interplay between melt pool size,vapour plume effects,and laser power was found to significantly influence the resulting porosity,indicating a need for effective management of these factors to optimise the SLM process of Mg alloys.

Effect of solution treatment on the microstructure,phase transformation behavior and functional properties of NiTiNb ternary shape memory alloys fabricated via laser powder bed fusion in-situ alloying

Post-heat treatment is commonly employed to improve the microstructural homogeneity and enhance the mechanical performances of the additively manufactured metallic materials.In this work,a ternary(NiTi)91Nb9(at.%)shape memory alloy was produced by laser powder bed fusion(L-PBF)using pre-alloyed NiTi and elemental Nb powders.The effect of solution treatment on the microstructure,phase transformation behavior and mechanical/functional performances was investigated.The in-situ alloyed(NiTi)91Nb9 alloy exhibits a submicron cellular-dendritic structure surrounding the supersaturated B2-NiTi matrix.Upon high-temperature(1273 K)solution treatment,Nb-rich precipitates were precipitated from the supersaturated matrix.The fragmentation and spheroidization of the NiTi/Nb eutectics occurred during solution treatment,leading to a morphological transition from mesh-like into rod-like and sphere-like.Coarsening of theβ-Nb phases occurred with increasing holding time.The martensite transformation temperature increases after solution treatment,mainly attributed to:(i)reduced lattice distortion due to the Nb expulsion from the supersaturated B2-NiTi,and(ii)the Ti expulsion from theβ-Nb phases that lowers the ratio Ni/Ti in the B2-NiTi matrix,which resulted from the microstructure changes from non-equilibrium to equilibrium state.The thermal hysteresis of the solutionized alloys is around 145 K after 20%pre-deformation,which is comparable to the conventional NiTiNb alloys.A short-term solution treatment(i.e.at 1273 K for 30 min)enhances the ductility and strength of the as-printed specimen,with the increase of fracture stress from(613±19)MPa to(781±20)MPa and the increase of fracture strain from(7.6±0.1)%to(9.5±0.4)%.Both the as-printed and solutionized samples exhibit good tensile shape memory effects with recovery rates>90%.This work suggests that post-process heat treatment is essential to optimize the microstructure and improve the mechanical performances of the L-PBF in-situ alloyed parts.

Deformation behavior of Mg-Y-Ni alloys containing different volume fraction of LPSO phase during tension and compression through in-situ synchrotron diffraction

The deformation behavior of the as-extruded Mg-Y-Ni alloys with different volume fraction of long period stacking ordered(LPSO)phase during tension and compression was investigated by in-situ synchrotron diffraction.The micro-yielding,macro-yielding,tension-compression asymmetry and strain hardening behavior of the alloys were explored by combining with deformation mechanisms.The micro-yielding is dominated by basal slip of dynamic recrystallized(DRXed)grains in tension,while it is dominated by extension twinning of non-dynamic recrystallized(non-DRXed)grains in compression.At macro-yielding,the non-DRXed grains are still elastic deformed in tension and the basal slip of DRXed grains in compression are activated.Meanwhile,the LPSO phase still retains elastic deformation,but can bear more load,so the higher the volume fraction of hard LPSO phase,the higher the tensile/compressive macro-yield strength of the alloys.Benefiting from the low volume fraction of the non-DRXed grains and the delay effect of LPSO andγphases on extension twinning,the as-extruded alloys exhibit excellent tension-compression symmetry.When the volume fraction of LPSO phase reaches∼50%,tension-compression asymmetry is reversed,which is due to the fact that the LPSO phase is stronger in compression than in tension.The tensile strain hardening behavior is dominated by dislocation slip,while the dominate mechanism for compressive strain hardening changes from twinning in theα-Mg grains to kinking of the LPSO phase with increasing volume fraction of LPSO phase.The activation of kinking leads to the constant compressive strain hardening rate of∼2500 MPa,which is significantly higher than the tensile strain hardening rate.

In-situ Micro-CT analysis of deformation behavior in sandwich-structured meta-stable beta Ti−35Nb alloy

Beta Ti−35Nb sandwich-structured composites with various reinforcing layers were designed and produced using additive manufacturing(AM)to achieve a balance between light weight and high strength.The impact of reinforcing layers on the compressive deformation behavior of porous composites was investigated through micro-computed tomography(Micro-CT)and finite element method(FEM)analyses.The results indicate that the addition of reinforcement layers to sandwich structures can significantly enhance the compressive yield strength and energy absorption capacity of porous metal structures;Micro-CT in-situ observation shows that the strain of the porous structure without the reinforcing layer is concentrated in the middle region,while the strain of the porous structure with the reinforcing layer is uniformly distributed;FEM analysis reveals that the reinforcing layers can alter stress distribution and reduce stress concentration,thereby promoting uniform deformation of the porous structure.The addition of reinforcing layer increases the compressive yield strength of sandwich-structured composite materials by 124%under the condition of limited reduction of porosity,and the yield strength increases from 4.6 to 10.3 MPa.

In situ Hydrothermal Oxidation of Ternary FeCoNi Alloy Electrode for Overall Water Splitting

Exploring noble metal-free catalyst materials for high efficient electrochemical water splitting to produce hydrogen is strongly desired for renewable energy development.In this article,a novel bifunctional catalytic electrode of insitu-grown type for alkaline water splitting based on FeCoNi alloy substrate has been successfully prepared via a facile one-step hydrothermal oxidation route in an alkaline hydrogen peroxide medium.It shows that the matrix alloy with the atom ratio 4∶3∶3 of Fe∶Co∶Ni can obtain the best catalytic performance when hydrothermally treated at 180℃for 18 h in the solution containing 1.8 M hydrogen peroxide and 3.6 M sodium hydroxide.The as-prepared Fe_(0.4)Co_(0.3)Ni_(0.3)-1.8 electrode exhibits small overpotentials of only 184 and 175 mV at electrolysis current density of 10 mA cm^(-2)for alkaline OER and HER processes,respectively.The overall water splitting at electrolysis current density of 10 mA cm^(-2)can be stably delivered at a low cell voltage of 1.62 V.These characteristics including the large specific surface area,the high surface nickel content,the abundant catalyst species,the balanced distribution between bivalent and trivalent metal ions,and the strong binding of in-situ naturally growed catalytic layer to matrix are responsible for the prominent catalytic performance of the Fe_(0.4)Co_(0.3)Ni_(0.3)-1.8 electrode,which can act as a possible replacement for expensive noble metal-based materials.

Investigations on the tensile deformation of pure Mg and Mg–15Gd alloy by in-situ X-ray synchrotron radiation and visco-plastic self-consistent modeling

In this study,the texture evolutions of two Mg materials during tension are explored.In-situ X-ray synchrotron and Visco-Plastic SelfConsistent(VPSC) modeling are employed to investigate the different deformation modes between pure Mg and Mg-15Gd(wt.%) alloy.These two materials with a strong extrusion texture show large different slip/twinning activity behaviors during tensile deformation.The basal(a) slip has the highest contribution to the initial stage of plastic deformation for pure Mg.During the subsequent plastic deformation,the prismatic slip is dominant due to the strong ED//(100) fiber texture.In contrast,the deformation behavior of Mg-15Gd alloy is more complex.Twinning and basal slip are dominant at the early stage of plastic deformation,but further deformation results in the increased activation of prismatic and pyramidal slips.In comparison to pure Mg,the ratios of the critical resolved shear stress(CRSS) between non-basal slip and basal slip of the Mg-15Gd alloy are much lower.

Industrial-Scale Polypropylene–Polyethylene Physical Alloying Toward Recycling

Polypropylene(PP)and polyethylene(PE)play central roles in our daily life.However,their immiscibility presents a major hurdle in both industry and academia when recycling them into alloys with favorable mechanical properties.Moreover,typical compatibilizer-enabled approaches are limited due to increased environmental concerns.Herein,inspired by a traditional Chinese technique,we report a facile,industry-scale methodology that produces a PP/PE binary blend with a highly ordered honeycomb nanostructure without any additives.Due to its nanostructure,the blend exhibits enhanced tensile properties in com-parison with the parent components or with a sample prepared using an internal mixer.This approach has potential for applications not only in immiscible polymer blending,but also in non-sorting,compatibilizer-free waste plastics recycling.Through this technique,we expect that an environmentally friendly and sustainable plastic wastes recycling avenue can be found,and great economic benefits can be gained.

In situ TEM investigation of electron irradiation and aging-induced high-density nanoprecipitates in an Mg-10Gd-3Y-1Zn-0.5Zr alloy

In-situ electron irradiation and aging are applied to introduce high-density precipitates in an Mg-10Gd-3Y-1Zn-0.5Zr(GWZ1031K,wt.%)alloy to improve the hardness.The results show that the hardness of the Mg alloy after irradiation for 10 h and aging for 9 h at 250℃ is 1.64 GPa,which is approximately 64% higher than that of the samples before being treated.It is mainly attributed to γ'precipitates on the basal plane after irradiation and the high-density nanoscale β'precipitates on the prismatic plane after aging,which should be closely related to the irradiation-induced homogenous clusters.The latter plays a key role in precipitation hardening.This result paves a way to improve the mechanical properties of metallic materials by tailoring the precipitation through irradiation and aging.

Efficient and stable PtFe alloy catalyst for electrocatalytic methanol oxidation with high resistance to CO

Direct methanol fuel cells(DMFC) are widely considered to be an ideal green energy conversion device but their widespread applications are limited by the high price of the Pt-based catalysts and the instability in terms of surface CO toxicity in long-term operation.Herein,the PtFe alloy nanoparticles(NPs) with small particle size(~4.12 nm) supported on carbon black catalysts with different Pt/Fe atomic ratios(Pt_(1)Fe_(2)/C,Pt_(3)Fe_(4)/C,Pt_(1)Fe_(1)/C,and Pt_(2)Fe_(1)/C) are successfully prepared for enhanced anti-CO poisoning during methanol oxidation reaction(MOR).The optimal atomic ratio of Pt/Fe for the MOR is 1:2,and the mass activity of Pt_(1)Fe_(2)/C(5.40 A mg_(Pt)^(-1)) is 13.5 times higher than that of conventional commercial Pt/C(Pt/C-JM)(0.40 A mg_(Pt)^(-1)).The introduction of Fe into the Pt lattice forms the PtFe alloy phase,and the electron density of Pt is reduced after forming the PtFe alloy.In-situ Fourier transform infrared results indicate that the addition of oxyphilic metal Fe has reduced the adsorption of reactant molecules on Pt during the MOR.The doping of Fe atoms helps to desorb toxic intermediates and regenerate Pt active sites,promoting the cleavage of C-O bonds with good selectivity of CO_(2)(58.1%).Moreover,the Pt_(1)Fe_(2)/C catalyst exhibits higher CO tolerance,methanol electrooxidation activity,and long-term stability than other Pt_(x)Fe_(y)/C catalysts.

On the micromechanism of superior strength and ductility synergy in a heterostructured Mg-2.77Y alloy

Heterostructured metals and alloys are a new class of materials in which mechanical behaviors between the heterogeneous regions are significantly different,and the mechanical properties of bulk materials are superior to the superposition of individual regions.In this paper,three distinct types of heterostructures were constructed in Mg-2.77Y(wt.%)alloy by applying simple thermomechanical processing.Namely,Type I:the non-recrystallized grains of several tens of microns were embedded in the micron-scaled recrystallized grains that were distributed along shear bands and dispersed near grain boundaries;Type II:the aggregations of micron-scaled recrystallized grains were surrounded by the non-recrystallized grains;Type II:the micron-scaled recrystallized grains dominated the microstructure,and the non-recrystallized regions with diameters of tens of micrometers were surrounded by those fine recrystallized grains.Mechanical tests showed that the material with type III heterostructure had the optimal combination of yield strength and uniform elongation.This is attributed to its remarkable hetero-deformation induced(HDI)strengthening and dislocation strengthening.At the initial stage of plastic deformation(engineering strain below 4%),the rapid accumulation of geometrically necessary dislocations(GNDs)at the interfaces between recrystallized and non-recrystallized regions and between neighboring recrystallized grains lead to the significant HDI strengthening.As deformation proceeded,the HDI strengthening effect gradually decreased,and the traditional dislocation strengthening that was caused by GNDs accumulation at grain boundaries became significant.In-situ electron back-scattered diffraction(EBSD)testing revealed that the non-basal slip in the non-recrystallized regions became more remarkable in the late stage of deformation,which improved ductility and strain hardening of the alloy.These findings provide new insight into the design of high-performance hexagonal close-packed structural materials by using the concept of HDI strengthening.

SYNTHESIS AND CHARACTERIZATION OF POLYPROPYLENE/MONTMORILLONITE NANOCOMPOSITES VIA AN in-situ POLYMERIZATION APPROACH

Polypropylene/montmorillonite (PP/MMT)nanocomposites were prepared by in-situ polymerization using aMMT/MgCl_2/TiCl_4-EB Ziegler-Natta catalyst activated by triethylaluminum(TEA). The enlarged layer spacing of MMT wasconfirmed by X-ray wide angle diffraction (WAXD), demonstrating that MMT were intercalated by the catalyst components.X-ray photoelectron spectrometry (XPS) analysis proved that TiCl_4 was mainly supported on MgCl_2 instead of on the surfaceof MMT The exfoliated structure of MMT layers in the PP matrix of PP/MMT composites was demonstrated by WAXDpatterns and transmission electron microscopy (TEM) observation. The higher glass transition temperature and higher storage modulus of the PP/MMT composites in comparison with pure PP were revealed by dynamic mechanical analysis (DMA).

Investigation of the Micro-Mechanics of an Extruded Precipitation-Strengthened Magnesium Alloy under Cyclic Loading

Precipitation strengthening is a crucial microscopic mechanism for enhancing the strength of magnesium alloys. In order to elucidate the influence of precipitation on the microscopic deformation mechanisms and macroscopic mechanical response of magnesium alloys under cyclic loading conditions, we employed a crystal plasticity model to analyze the stress-strain curves, specific crystal plane diffraction intensities, and the temporal evolution of various microscopic deformation mechanisms and twinning volume fractions for an extruded magnesium alloy, AXM10304, containing coherent precipitates. The research findings indicate that precipitation does not fundamentally alter the microscopic mechanisms of this alloy. However, it hinders twinning during the compression stage, mildly promotes detwinning during the tension stage, and enhances tension secondary hardening by elevating the difficulty of activation of the prismatic slip.

In-situ observation of porosity formation during directional solidification of Al-Si casting alloys

In-situ observation of porosity formation during directional solidification of two Al-Si alloys (7%Si and 13%Si) was made by using of micro-focus X-ray imaging.In both alloys,small spherical pores initially form in the melt far away from the eutectic solid-liquid (S/L) interface and then grow and coagulate during solidification.Some pores can float and escape from the solidifying melt front at a relatively high velocity.At the end of solidification,the remaining pores maintain spherical morphology in the near eutectic alloy but become irregular in the hypoeutectic alloy.This is attributed to different solidification modes and aluminum dendrite interactions between the two alloys.The mechanism of the porosity formation is briefly discussed in this paper.

Microstructural evolution of copper-titanium alloy during in-situ formation of TiB_2 particles

Bulk Cu-Ti alloy reinforced by TiB2 nano particles was prepared using in-situ reaction between Cu 3.4%Ti and Cu-0.7%B master alloys along with rapid solidification and subsequent heat treatment for 1-10 h at 900 ℃. High-resolution transmission electron microscopy （HRTEM） characterization showed that primary TiB2 nano particles and TiB whiskers were formed by in-situ reaction between Ti and B in the liquid copper. The formation of TiB whiskers within the melt led to coarsening of TiB2 particles. Primary TiB2 particles were dispersed along the grain boundaries and hindered grain growth at high temperature, while the secondary TiB2 particles were formed during heat treatment of the alloy by diffusion reaction of solute titanium and boron inside the grains. Electrical conductivity and hardness of the composite were evaluated during heat treatment. The results indicated that the formation of secondary TiB2 particles in the matrix caused a delay in hardness reduction at high temperature. The electrical conductivity and hardness increased up to 8 h of heat treatment and reached 33.5% IACS and HV 158, respectively.

Quasi in-situ EBSD analysis of twinning-detwinning and slip behaviors in textured AZ31 magnesium alloy subjected to compressive-tensile loading

Twinning and detwinning behavior,together with slip behavior,are studied in a textured AZ31 magnesium alloy under compressive and tensile strains along the rolling direction(RD)after each interrupted mechanical test via quasi in-situ electron backscattered diffraction technique.The results show that twinning firstly takes place under the compressive strain along the RD.With the increasing compressive strain,{1012}tensile twins firstly nucleate,then propagate,and finally thicken.While under a reversed tensile strain along the RD,detwinning occurs.No nucleation happens during detwinning.Thus,tensile twins can detwin at lower tensile strain,followed by thinning,shortening,and vanishing.Slips are also activated to accommodate the plastic deformation.In the matrix,prismatic slip can only dominate at relatively high strains.Otherwise,basal slip dominates.While in the twins,prismatic slip can activate at lower strains,which is ascribed to the texture reorientation.

Homogenization of twin-roll cast AA8079 aluminum alloy studied by in-situ TEM

AA8079 is a commonly used stock material for manufacturing thin packaging foils.The primary alloying elements Fe and Si can form binary and tertiary intermetallics.In-situ TEM simulating homogenization annealing process of the as-cast material was used to analyze the real-time changes of the shape,type,and distribution of these particles.They affect the mechanical properties of the final product and susceptibility of the material to the formation of pinholes and other macroscopic defects.Another set of as-cast samples were annealed in a regime simulating industrial treatment in combination with measurements of resistivity to validate the results of the in-situ experiment.The results show clear temperature intervals of recovery,matrix desaturation,and phase transformations occurring in several stages:spheroidization of the original particles above 450℃,nucleation of new particles at 475℃,particles coarsening above 525℃,and an entire dissolution of the original particles above 550℃.

In-situ layered double hydroxides on Mg−Ca alloy:Role of calcium in magnesium alloy

Mg−Al layered double hydroxides(LDHs),produced on cast Mg−xCa(x=0.5,0.8,2.0,wt.%)alloys by an in-situ growth method,showed good corrosion resistance compared to the bare magnesium substrate.The influence mechanism of the second phase(Mg_(2)Ca)on LDHs production was investigated.Increasing Ca content increased the amount of Mg_(2)Ca,decreasing the grain size and the corrosion rate of the alloys.The increased amount of the second phase particles and the grain refinement promoted the growth of LDHs,and thus led to the decreasing of corrosion rate of the Mg−xCa alloys with LDHs.A higher Mg_(2)Ca amount resulted in forming fluffy LDHs.Due to the dual effects of the second phase(Mg_(2)Ca)for LDHs growth and microgalvanic corrosion,LDHs/Mg−0.8Ca showed the lowest corrosion rate.

SEM in-situ investigation on fatigue cracking behavior of P/M Rene95 alloy with surface inclusions

The low-cycle fatigue behavior of powder metallurgy Rene95 alloy containing surface inclusions was investigated by in-situ observation with scanning electron microscopy （SEM）. The process of fatigue crack initiation and early stage of propagation behavior indicates that fatigue crack mainly occurs at the interface between the inclusion and the matrix. The effect of inclusion on the fatigue crack initiation and the early stage of crack growth was very obvious. The fatigue crack growth path in the matrix is similar to the shape of inclusion made on the basis of fatigue fracture image analysis. The empiric relation between the surface and inside crack growth length, near a surface inclusion, can be expressed. Therefore, the fatigue crack growth rate or life of P/M Rene95 alloy including the inclusions can be evaluated on the basis of the measurable surface crack length parameter. In addition, the effect of two inclusions on the fatigue crack initiation behavior was investigated by the in-situ observation with SEM.

In-situ analysis of slip transfer and heterogeneous deformation in tension of Mg-5.4Gd-1.8Y-1.5Zn alloy

Slip transfer is influential in determining damage nucleation of polycrystalline material.The interactions between dislocations and grain boundaries(GBs)was investigated using in-situ tension test in a multi-directionally forged Mg-5.4Gd-1.8Y-1.5Zn(wt%)alloy.It was found that strain accommodation of individual grains by means of slip occurred more easily than slip transfer when several slip systems were operable.The basal-basal slip transfer occurred when the GB misorientation was smaller than 34.2°,whereas basal-pyramidal type took place when the crystallographic misorientation was larger than 48.8°.The product of Luster-Morris m factor and the sum of the Schmid factors of the two correlated slip systems indicated that the threshold for basal-basal slip transfer may exist,however,basal-pyramidal slip transfer shows no such threshold and is more complicated.These results presented here demonstrated that besides the geometrical alignment,the deformation details(such as the number of operable slip systems)and stress state in each individual grain must be considered.

Comparative study on high temperature deformation behavior and processing maps of Mg-4Zn-1RE-0.5Zr alloy with and without in-situ sub-micron sized TiB2 reinforcement

Mg-4Zn-1RE-0.5Zr (ZE41) Mg alloy is extensively used in the aerospace and automobile industries.In order to improve the applicability and performance,this alloy was engineered with in-situ Ti B2reinforcement to form Ti B2/ZE41 composite.The high temperature deformation behavior and manufacturability of the newly developed Ti B2/ZE41 composite and the parent ZE41 Mg alloy were studied via establishing constitutive modeling of flow stress,deformation activation energy and processing map over a temperature range of 250℃-450℃ and strain rate range of 0.001 s-1-10 s-1.The predicted flow stress behavior of both materials were found to be well consistent with the experimental values.A significant improvement in activation energy was found in Ti B2/ZE41 composite (171.54 k J/mol) as compared to the ZE41 alloy (148.15 k J/mol) due to the dispersed strengthening of in-situ Ti B2particles.The processing maps were developed via dynamic material modeling.A wider workability domain and higher peak efficiency (45%) were observed in Ti B2/ZE41 composite as compared to ZE41 alloy (41%).The Dynamic recrystallization is found to be the dominating deformation mechanism for both materials;however,particle stimulated nucleation was found to be an additional mode of deformation in Ti B2/ZE41 composite.The twinning and stress induced cracks were observed in both the materials at low temperature and high strain rate.A narrow range of instability zone is found in the present Ti B2/ZE41 composite among the existing published literature on Mg based composites.The detailed microstructural characterization was carried out in both workability and instability domains to establish the governing deformation mechanisms.