Locally Adapted Microstructures in an Additively Manufactured Titanium Aluminide Alloy Through Process Parameter Variation and Heat Treatment

Electron beam powder bed fusion (PBF-EB/M) has been attracting great research interest as a promising technology for additive manufacturing of titanium aluminide alloys. However, challenges often arise from the process-induced evaporation of aluminum, which is linked to the PBF-EB/M process parameters. This study applies different volumetric energy densities during PBF-EB/M processing to deliberately adjust the aluminum contents in additively manufactured Ti–43.5Al–4Nb–1Mo–0.1B (TNM-B1) samples. The specimens are subsequently subjected to hot isostatic pressing (HIP) and a two-step heat treatment. The influence of process parameter variation and heat treatments on microstructure and defect distribution are investigated using optical and scanning electron microscopy, as well as X-ray computed tomography (CT). Depending on the aluminum content, shifts in the phase transition temperatures can be identified via differential scanning calorimetry (DSC). It is confirmed that the microstructure after heat treatment is strongly linked to the PBF-EB/M parameters and the associated aluminum evaporation. The feasibility of producing locally adapted microstructures within one component through process parameter variation and subsequent heat treatment can be demonstrated. Thus, fully lamellar and nearly lamellar microstructures in two adjacent component areas can be adjusted, respectively.     

Influence of Two-Step Heat Treatments on Microstructure and Mechanical Properties of a β-Solidifying Titanium Aluminide Alloy Fabricated via Electron Beam Powder Bed Fusion

Additive manufacturing technologies, particularly electron beam powder bed fusion (PBF-EB/M), are becoming increasingly important for the processing of intermetallic titanium aluminides. This study presents the effects of hot isostatic pressing (HIP) and subsequent two-step heat treatments on the microstructure and mechanical properties of the TNM-B1 alloy (Ti–43.5Al–4Nb–1Mo–0.1B) fabricated via PBF-EB/M. Adequate solution heat treatment temperatures allow the adjustment of fully lamellar (FL) and nearly lamellar (NL-β) microstructures. The specimens are characterized by optical microscopy and scanning electron microscopy (SEM), X-ray computed tomography (CT), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD). The mechanical properties at ambient temperatures are evaluated via tensile testing and subsequent fractography. While lack-of-fusion defects are the main causes of failure in the as-built condition, the mechanical properties in the heat-treated conditions are predominantly controlled by the microstructure. The highest ultimate tensile strength is achieved after HIP due to the elimination of lack-of-fusion defects. The results reveal challenges originating from the PBF-EB/M process, for example, local variations in chemical composition due to aluminum evaporation, which in turn affect the microstructures after heat treatment. For designing suitable heat treatment strategies, particular attention should therefore be paid to the microstructural characteristics associated with additive manufacturing.     

Influence of Electron Beam Powder Bed Fusion Process Parameters on Transformation Temperatures and Pseudoelasticity of Shape Memory Nickel Titanium

Electron beam powder bed fusion (PBF-EB) is used to manufacture dense nickel titanium parts using various parameter sets, including the beam current, scan speed, and postcooling condition. The density of manufactured NiTi parts is investigated in relation to the linear energy input. The results imply that the part density increases with increasing linear energy density to over 98% of the bulk density. With a constant energy input, a combination of low power and low scan speed leads to denser parts. This is attributed to lower electrostatic repulsive forces from lower number density of the impacting electrons. After manufacturing, the densest parts with distinct parameter sets are categorized into three groups: 1) high power with high scan speed and vacuum slow cooling, 2) low power with low scan speed and vacuum slow cooling, and 3) low power with low scan speed and medium cooling rate in helium gas. Among these, a faster cooling rate suppresses phase transformation temperatures, while vacuum cooling combinations do not affect the phase transformation temperatures significantly. Herein, all the printed parts exhibit almost 8% pseudoelasticity regardless of the process parameters, while the parts cooled in helium have a higher energy dissipation efficiency (1 − η), which implies faster damping of oscillations.     

Influence of contour scans on surface roughness and pore formation using Scalmalloy® manufactured by laser powder bed fusion (PBF-LB)

The scandium modified aluminium alloy Scalmalloy® is specifically developed for the use in laser-based powder bed fusion (PBF-LB). It is supposed to show potential in the production of lightweight structures due to its high specific strength compared to other aluminium alloys. A limiting factor is the high surface roughness of additively manufactured parts, which has a negative influence on its mechanical properties, especially under cyclic loads. In order to reduce the surface roughness, methods of design of experiments (DoE) are applied to develop contour parameters. Additionally, the formation of pores in keyhole-mode welding and strategies to reduce the porosity in the contour area are investigated. The surface roughness of vertical walls can be reduced down to Ra < 7 μm using contour scans with a line energy E_L >0.9 J mm⁻¹ but keyhole pores start to form applying E_L >0.6– 0.75 J mm⁻¹. Two contour parameter sets in different E_L-ranges are developed that can be used to reduce the surface roughness compared to parameter sets without contour scans, without increasing the porosity in the contour area. Their impact on the mechanical properties has to be further investigated.     

Using Selective Electron Beam Melting to Enhance the High-Temperature Strength and Creep Resistance of NiAl–28Cr–6Mo In Situ Composites

By increasing the density of interfaces in NiAl–CrMo in situ composites, the mechanical properties can be significantly improved compared to conventionally cast material. The refined microstructure is achieved by manufacturing through electron beam powder bed fusion (PBF-EB). By varying the process parameters, an equiaxed or columnar cell morphology can be obtained, exhibiting a plate-like or an interconnected network of the (Cr,Mo) reinforcement phase which is embedded in a NiAl matrix. The microstructure of the different cell morphologies is investigated in detail using scanning electron microscope, transmission electron microscopy, and atom probe tomography. For both morphologies, the mechanical properties at elevated temperatures are analyzed by compression and creep experiments parallel and perpendicular to the building direction. In comparison to cast NiAl and NiAl–(Cr, Mo), the yield strength of the PBF-EB fabricated specimens is significantly improved at temperatures up to 1,027 °C. While the columnar morphology exhibits the best improved mechanical properties at high temperatures, the equiaxial morphology shows nearly ideal isotropic mechanical behavior, which is a substantial advantage over directionally solidified material.     

Dependence of Superconducting Critical Temperature of A15-Type V3Ga on Composition

The composition dependence of the superconducting critical temperature T _c has been investigated for A15-type V₃Ga compounds. The T _c is determined by the specific heat and the magnetic flux transmission ratio through the sample and discussed in terms of the powder X-ray diffraction. The long-range order parameter and the lattice parameter of V₃Ga are evaluated quantitatively by the Rietveld method at each composition, which is evaluated exactly by the electron probe microanalysis (EPMA) measurements. The well-ordered homogeneous alloying structures. V _1–x Ga _x compounds are found to have nearly the same T _c as that for the stoichiometric composition between 0.23<0.25.     

Preparation and investigation on glasses in the Te46As32Ge10Si12 and Te41As37Ge10Si12 systems

Thin films of Te_46–xAs_32+xGe₁₀Si₁₂ (x=0,5) of different thicknesses are deposited on glass substrate by vacuum evaporation. X-ray diffraction revealed the formation of amorphous films. The value of the optical band gap, E _g, is found to increase with the thickness of the films and with increasing As content. The films are heat treated at different elevated temperatures from 298 to 423 K. The values of E _g are found to decrease with increasing temperature of heat treatment. The band tail, E _e, obey Urbach's empirical relation.     

Heat transfer from metallic filaments (whiskers) into helium under overcritical pressure for temperatures between 3.7 and 7.2 K

The heat transfer coefficient α, near the critical temperature, T_cO, was determined for several whiskers from the In-Pb alloy system. For this purpose the hysteresis of the voltage-temperature (V-T_B) transition curves at fixed currents, I, and of the V-I characteristics at fixed helium bath temperature, T_B, was determined. The advantage of using measurements made with whiskers is that there is no heat transfer to a substrate and negligible heat transfer to the contracts. The only heat transfer is that to the surrounding helium.     

Pulsed metall inert gas (MIG) welding and its effects on the microstructure and element distribution of an aluminum matrix reinforced with SiC composite material

This study aims to demonstrate the effects of pulsed current on the welding pool and fusion zone microstructures of the aluminum 2014 alloy matrix composite material reinforced with 14 and 20 vol% SiC particles. A programmable synergic controlled MIG welding machine with pulsed power supply was used. One hundred Ampere and 120 Ampere pulsed current values were used to determine the effect of heat input on microstructures. A 1 mm diameter SG‐AlSi₅ wire was used as filler material. The microstructures were studied using a scanning electron microscope (SEM) with energy dispersive X‐ray (EDX) spectroscopy, and the phase analyses were performed via X‐ray diffraction analyzer (XRD). The study showed that increasing the SiC rate has a greater effect on the formation of Al₄C₃ phase than increasing the heat input values. Al₄C₃ formation was not formed as a needle‐like structure.     

Optical and thermal properties of poly(ethylene oxide) doped with MnCl2 salt

The optical and thermal properties of prepared poly(ethylene oxide)/MnCl₂ films were studied as a function of MnCl₂ concentration at room temperature. The observed optical energy gap (E_opt) and energy gap tail (ΔE) were determined from the measured absorption spectra. It was found that the optical energy gap decreases with MnCl₂ concentration, and the absorption coefficient reduces sharply at concentration of 5 wt% MnCl₂ compared with the neat PEO. Differential scanning calorimetry (DSC) measurement shows that the heat of fusion increases with the MnCl₂ concentration, while the melting point decreases. Correlation between the observed optical energy gap and heat of fusion is presented.     

Dielectric properties,microstructure and diffuse transition of Al-doped Ba(Zr<Subscript>0.2</Subscript>Ti<Subscript>0.8</Subscript>)O<Subscript>3</Subscript> ceramics

Pure and Al-doped BaZr_0.2Ti_0.8O₃ (short for BZT) ceramics are prepared via the conventional solid state reaction method. The microstructures, dielectric properties, and diffuse transition of Al-doped BaZr_0.2Ti_0.8O₃ ceramics were investigated. These results indicate that aluminum ions enter the unit cell maintaining the perovskite structure of solid solution. The addition of aluminum leads to the change of the Curie temperature. The dielectric loss of the Al-doped BZT ceramics is higher than that of pure BZT ceramics, and increases as aluminum content increases. The diffuseness of the phase transition of Al-doped BZT ceramics weakens with the increasing of aluminum content. There is no obvious frequency dispersion around the dielectric constant peaks for Al-doped BZT ceramics. The coercive electric field (E _C) increases as Al content increases, and the remanent polarization (P _r) of Al-doped BZT ceramics is lower than that of pure BZT ceramics.     

Pinning potential transport measurements of Bi2Sr2CaCu2Ox thin films

Flux creep characteristics ofc-axis-oriented Bi₂Sr₂CaCu₂O _x thin films grown by the method of electron beam evaporation of stoichiometric target and subsequent annealing have been investigated in the absence of external magnetic field. A decrease of the pinning potential with decreasing temperature is obtained fromI–V curves. This result is explained by the presence of the spatially nonlinear pinning potential.     

Fatigue Behavior of Ti-6Al-4V

Low-cycle-fatigue texts in vacuum and air were performed. Under cyclic loading the Ti-6Al-4V showed both cyclic hardening and cyclic softening depending on heat treatment, stress amplitude, and microstructure. Plastic deformation of the β-phase in the unaged condition due to stress induced martensitic transformation caused cyclic hardening. Cyclic softening was observed if the α-phase hardened by coherent Ti₃Al particles was plastically deformed. Equiaxed microstructures exhibited a stronger cyclic softening than lamellar structures. This behavior could be explained by the pronounced texture of the equiaxed microstructures, whereas the lamellar structures were texture-free. The fatigue life was influenced by the cyclic softening process mainly in the low-cycle-fatigue regime. The fatigue life at normalized stress amplitude (σ_a/σ_y) was shorter for microstructures with strong cyclic softening as compared to microstructures with lower cyclic softening.     

A simple modification of the Hodgkin and Huxley equations explains type 3 excitability in squid giant axons

The Hodgkin and Huxley (HH) model predicts sustained repetitive firing of nerve action potentials for a suprathreshold depolarizing current pulse for as long as the pulse is applied (type 2 excitability). Squid giant axons, the preparation for which the model was intended, fire only once at the beginning of the pulse (type 3 behaviour). This discrepancy between the theory and experiments can be removed by modifying a single parameter in the HH equations for the K⁺ current as determined from the analysis in this paper. K⁺ currents in general have been described by I_K=g_K(V−E_K), where g_K is the membrane''s K⁺ current conductance and E_K is the K⁺ Nernst potential. However, I_K has a nonlinear dependence on (V−E_K) well described by the Goldman–Hodgkin–Katz equation that determines the voltage dependence of g_K. This experimental finding is the basis for the modification in the HH equations describing type 3 behaviour. Our analysis may have broad significance given the use of I_K=g_K(V−E_K) to describe K⁺ currents in a wide variety of biological preparations.     

Microstructural Characterization of Lamellar Features in TiAl by FIB Imaging

A novel experimental procedure is introduced to determine phase fractions and the distribution of individual phases of TiAl‐based two‐phase alloys using the focused ion beam (FIB) technique. Two γ‐titanium aluminide alloys with a fine‐grained duplex and a nearly lamellar microstructure are examined. The special FIB‐based preparation procedure results in high contrast ion beam‐induced images for all investigated alloys and allows to quantify the phase contents easily by automated microstructural analysis. Fine two‐phase structures, e.g. lamellar colonies in γ‐TiAl, can be imaged in high resolution with respect to different phases. To validate the FIB‐derived data, we compare them to results obtained with another method to determine phase fractions, electron back‐scatter diffraction (EBSD). This direct comparison shows that the FIB‐based technique generally provides slightly higher α₂‐fractions, and thus helps to overcome the limited lateral resolution near grain boundaries and interfaces associated with the conventional EBSD approach. Our study demonstrates that the FIB‐based technique is a simple, fast, and more exact way to determine high resolution microstructural characteristics with respect to different phase constitutions in two‐phase TiAl alloys and other such materials with fine, lamellar microstructures.     

Soft-Magnetic Behavior of Fe-Based Nanocrystalline Alloys Produced Using Laser Powder Bed Fusion

Herein, an extensive experimental study is presented on the influence of the major process parameters of the laser powder bed fusion (LPBF) technique on the bulk density and soft-magnetic properties of Fe-based bulk metallic glasses (BMGs). For this purpose, 81 samples are manufactured using the combinations of different process parameters, that is, layer thickness (t: 50–70 μm), laser power (P: 70–130 W), laser scan speed (v: 900–1100 mm s⁻¹), and hatch spacing (h: 20–40 μm). High bulk density (≥99%) is achieved utilizing low P and v combined with low h and t in order to decrease energy input to the powder, preventing cracks associated with the brittle nature of BMGs. Furthermore, it is indicated that h = 30 μm and v = 1000 mm s⁻¹ play a determining role in acquiring high saturation magnetization (≥200 Am² kg⁻¹). Due to the laser scanning nature of the process, two distinct microstructures evolve, melt-pool (MP) and heat-affected zone (HAZ). According to thermal modeling performed in this study, laser power has the major effect on the thermal development in the microstructure (thermal gradient evolved between the two hatches and the cooling rate from MP through HAZ).     

Post-deposition heat treatment of co-deposited Cr3C2 and AISI 410 stainless steel using the coaxial laser deposition technique

Cr₃C₂ ceramic powder is added in varying amounts to AISI 410 stainless steel powder to develop AISI 400 based alloys with varying chromium and carbon content using the coaxial laser deposition technique operating at parameter sets which guarantee full melting of the constituent powder particles. Theoretical isothermal curves for the in situ generated alloys are correlated with the as-deposited and heat-treated microstructures using electron microscopy, X-ray and electron backscatter diffraction techniques. It is concluded that with an increased carbon loading in the mixture, post-deposition heat treatment involving full re-austenitising and tempering is necessary in order to reduce the effect of solute trapping which negatively affects the material mechanical properties.     

Effect of heat treatment on the structural and optical properties of CuInTe2 thin films

The CuInTe2 thin films were prepared by thermal vacuum evaporation of the bulk compound. The structural and optical properties in the temperature range 300–47 K of thin films grown on glass substrates and annealed in vacuum were studied. The films were investigated by X-ray diffraction and electron microscope techniques. The calculated lattice constants for CuInTe₂ powder were found to bea=0.619 nm andc –1,234 nm. From the reflection and transmission data, the optical constants, refractive indexn, absorption index,k, and the absorption coefficient, , werw computed. The optical energy gap was determined for CuInTe₂ thin films heat treated at different temparatures for different periods of time. It was found thatE _g increases with both increasing temperature and time of annealing.     

Titanium aluminide (Ti-48Al) powder synthesis,size refinement and sintering

Titanium aluminide based alloys have shown significant potential in high temperature applications, but the high production cost of TiAl considerably limits its utilisation. Although the use of powder metallurgy processes can reduce the cost by minimising post-machining, an economical powder production route is still required. Therefore, in the present study a pre-alloyed Ti-48Al powder is developed using an elemental Ti and Al powder blend prepared using a simple vacuum heat treatment. A formation model of the intermetallic phases (i.e. TiAl, Ti₃Al, TiAl₂, TiAl₃) during powder synthesis is proposed. In order to improve the sinterability, various milling methods (i.e. ball, attrition and shatterbox milling) are examined to reduce the particle size. The sintered microstructures, particularly the two-phased (α₂-Ti₃Al γ-TiAl) lamellar structures are also investigated. Improved densification is achieved at 1300 °C, held for 2 h, using the manufactured powder, compared to the elemental powder blend (～55%). With higher sintering temperatures or longer hold periods, increased density TiAl components are possible.     

Validation of a new pertinent packing coefficient to estimate flow properties of pharmaceutical powders at a very early development stage, by comparison with mercury intrusion and classical flowability methods

The present study compares four characterisation techniques, such as packing and rearrangement under pressure methods or shear cell measurement methods, used to evaluate powder flow properties. The reduction of the powder bed volume under low pressures is analysed using mercury porosimetry and two compressibility methods (uniaxial press and volumenometer). Flow functions, deduced from shear cell measurements, are determined using a Johanson Indicizer^TM Tester. The examination of the reduction of the powder bed volume leads to new parameters such as the packing coefficient (C _t) and the volume of mercury intruded (V _hg). The packing coefficient appears to be a reliable approximation of powder flow properties, whatever cohesive or free flowing : it is actually well correlated with shear cell measurements and it is more accurate than classical flowability tests recommended by the European Pharmacopoeia. Furthermore, this method is easy to use and consumes a small amount of powders (<1 g). All together, this method is able to give—very early in the development—a quite accurate estimation of powder flow properties of new drug substances. This may be very helpful for an early determination of the optimum particle granulometry or for a rapid development of a feasible industrial process.