Changes In Alloy Composition Research Articles

Effect of Alloying Elements on the High-Temperature Yielding Behavior of Multicomponent γ'-L12 Alloys.

The exceptional mechanical properties of Ni-based high entropy alloys are due to the presence of ordered L12 (γ') precipitates embedded within a disordered matrix phase. While the strengthening contribution of the γ' phase is generally accepted, there is no consensus on the precise contribution of the individual strengthening mechanisms to the overall strength. In addition, changes in alloy composition influence several different mechanisms, making the assessment of alloying conditions complex. Multicomponent L12-ordered single-phase alloys were systematically developed with the aid of CALPHAD thermodynamic calculations. The alloying elements Co, Cr, Ti, and Nb were chosen to complexify the Ni3Al structure. The existence of the γ' single phase was validated by microstructure characterization and phase identification. A high-temperature compression test from 500 °C to 1000 °C revealed a positive temperature dependence of strength before reaching the peak strength in the studied alloys NiCoCrAl, NiCoCrAlTi, and NiCoCrAlNb. Ti and Nb alloying addition significantly enhanced the high-temperature yield strengths before the peak temperature. The yield strength was modeled by summing the individual effects of solid solution strengthening, grain boundary strengthening, order strengthening, and cross-slip-induced strengthening. Cross-slip-induced strengthening was shown to be the key contributor to the high-temperature strength enhancement.

Revealing the elusive role of water vapor in the oxidation behavior of a Mn-Si containing NiCr alloy at 950 °C

Detailed nanoscale characterizations revealed the formation of a considerably higher fraction of SiO2 particles in the external Cr2O3 layer on a NiCr alloy. In addition, atom probe tomography revealed substantially lower segregation of Mn at the Cr2O3 grain boundaries in wet air compared to dry air exposures, where the intriguing formation of Mn-rich clusters was observed in the Cr2O3 grains for the very first time. These findings provided additional evidence regarding the impact of distinct transport processes through the Cr2O3 layer on the resulting oxidation rates and subsurface alloy compositional changes.

High-velocity laser accelerated deposition (HVLAD): An experimental study

A versatile solid-state cladding technique named high-velocity laser accelerated deposition (HVLAD), which is capable of creating highly uniform coverage on various surfaces, is reported here. The method is a non-thermally driven, mechanically based process that relies on laser peening (LP) technology which has been used for commercial applications for decades. In HVLAD, a high-intensity laser is used to accelerate, propel, and deposit small areas of a thin film in a controlled step-by-step manner onto a substrate. This process can be repeated until a desired thickness of material is deposited onto the substrate. This technique does not generate any large variations in temperature between the thin film and the substrate material. Instead, it employs intense laser pulses with energies up to 50 J for a duration of 8–50 ns to create a plasma generated pressure wave which accelerates the thin film in the order of a few hundred meters per second into the substrate material. Two cladding methods were evaluated in this study including the confined method, which incorporates a confining water media and a vinyl tape overlay, and the unconfined method which does not. High speed and thermal imaging cameras showed no macrolevel melting or significant temperature increase in the clad or substrate material during the cladding process, however signs of microlevel melting and re-solidification were observed at the interface primarily in specimens cladded using the unconfined method. Grain structures in adjacent areas to the clad location remained preserved using both methods, and no signs of any phase or alloy composition changes, except for at the interface, were observed. The effects of a confining media (water) and vinyl tape in the process were studied both analytically and experimentally and a change in the deposition mechanism was observed. With further research and development this technique has the ability to be used to deposit a variety of coatings without any limitations dictated by their strength on complex geometries in a cost-effective manner. The HVLAD process can be implemented to deposit corrosion, wear, thermal, and even impact-resistant coatings with strong bonding on a wide range of substrates.

Selective oxidation and possible mode of porous oxide formation on welded Al alloy at high-temperature condition

High-temperature exposure leads to a non-protective oxide scale that involves selective oxidation of a welded aluminum alloy. This process resulted in a change of alloy composition, especially in the subsurface of the welded zone which is also known as fusion metal. This paper focused on a phase transformation in the welded A6061 Al alloy. It has been assumed that phase transformation involves the formation of new phases and the dissolution of current phases, depending on the system being welded. In this study, the welded A6061 Al alloy was subjected to a high-temperature oxidation test in the hot air furnace at 600 °C for 40 h. The oxidized samples were studied using a LEO 1450 model scanning electron microscope (SEM) is typically equipped with an energy dispersive X-ray analyzer (EDX) to determine the oxide flake morphology, oxides cross-sectional images, and pre-existing structures. Meanwhile, phase identification analysis was carried out using a Bruker AXS: D8 Advance X-ray diffractometer (XRD). The result shows that the selective oxidation of Al from welded A6061 alloy forms MgO on the alloy surface. The MgO was suggested to inhibit further oxidation underneath the oxide scale and allowed the oxygen to diffuse into the internal welded substrate and react. This could lead to the formation of Al2O3. Inward diffusion of Al and Mg causes the formation of MgAl2O4 spinel and Al2O3 alumina which leads to the internal degradation of the welded substrate. A thick outer oxide layer is formed, consisting of the oxides of the base metal (MgO, Al2O3), with layers of different oxide combinations (MgO or Al2O3 or MgO + Al2O3, forming MgAl2O4). Below this layer is a region depleted of the alloying element Al.

Alloying Elements Effect on the Recrystallization Process in Magnesium-Rich Aluminum Alloy.

This paper addresses the study of the complex effect of alloying elements (magnesium, manganese, copper and zirconium) on changes in magnesium-rich aluminum alloy composition, fine and coarse particle size and number, recrystallization characteristics and mechanical properties. The data obtained made it possible to analyze change in the chemical composition, sizes of intermetallic compounds and dispersoids depending on alloying elements content. The effect of the chemical composition on the driving force and the number of recrystallization nuclei was studied. It was established that the addition of alloying elements leads to grain refinement, including through the activation of a particle-stimulated nucleation mechanism. As a result, with Mg increase from 4 to 5%, addition of 0.5% Mn and 0.5% Cu, the grain size decreased from 72 to 15 µm. Grain refinement occurred due to an increase in the number of particle-stimulated nuclei, the number of which at minimal alloying rose from 3.47 × 1011 to 81.2 × 1011 with the maximum concentration of Mg, Mn, Cu additives. The retarding force of recrystallization, which in the original alloy was 1.57 × 10−3 N/m2, increased to 5.49 × 10−3 N/m2 at maximum alloying. The influence of copper was especially noticeable, the introduction of 0.5% increasing the retarding force of recrystallization by 2.39 × 10−3 N/m2. This is due to the fact that copper has the most significant effect on the size and number of intermetallic particles. It was established that strength increase without ductility change occurs when magnesium, manganese and copper content increases.

High thermal conductivity of highly alloyed Mg-Zn-Cu alloy and its mechanism

The content of alloying elements in most thermally conductive magnesium alloys is as low as possible, and this will lead to lack of alloying effect. In this paper, a Mg-xZn-xCu(x = 1,3,5, wt%) alloy was studied with comparatively high alloying element contents. The thermal conductivity of this alloy declines very slowly with the increment of element concentration. Even with the additive amount of 10 wt% of alloying elements, this alloy can attain a high thermal conductivity of 136.5 W/(m·K). After T6 treatment, the thermal conductivity of Mg-5Zn-5Cu alloy increases to 141.6 W/(m·K). The CuMgZn intermetallic compound are specially prepared to study the thermal conduction mechanism. The results show that the formation of CuMgZn ternary compound in Mg-Zn-Cu alloy consumes massive solute atoms, which makes the thermal conductivity of the alloy insensitive to alloy composition changes.

The effect of crack length and maximum stress on the fatigue crack growth rates of engineering alloys

The fatigue crack growth rate (FCGR) curve of metallic alloys is usually divided into three regions. Region II is often referred to as the Paris regime and is usually modelled with a power law relationship with a single exponent. Regions I and III are located at the beginning and end of the FCGR curve, respectively, and are frequently modelled with asymptotic relationships. In this paper we hypothesize that fatigue crack growth is governed by power law behaviour at all crack lengths and all stress intensity factor ranges (ΔK). To accommodate for the changes in the FCGR slope at regions I - III mathematical pivot points are introduced in the Paris equation. Power law behaviour with the presence of pivot points enables direct fitting of the crack length vs. cycles (a-N) curve to obtain the FCGR as a function of ΔK. This novel approach is applicable to small and long crack growth curves and results in accurate multilinear FCGR curves that are suitable for reconstruction of the measured a-N curves. The method is subsequently applied to i) different alloys to show local changes in the FCGR curve for changes in alloy composition and heat treatments, ii) naturally increasing ΔK testing of microstructurally small cracks to obtain accurate small crack FCGR data. The comparison with accurate long crack data shows that small cracks are faster, but the transition from region I to region II occurs at a specific fatigue crack growth rate which results in an apparent shift in ΔK at the transition. iii) Long cracks, which show that the FCGR increases with maximum stress for a given ΔK and stress ratio when the maximum stress approaches the yield stress. The maximum stress phenomenon becomes important in the case of fatigue testing, where the initial crack lengths are usually small and maximum stresses are high. It is concluded that for long cracks the phenomenon explains why the Paris equation is applicable rather at low maximum stress, while the Frost-Dugdale equation is more applicable at high maximum stress.

Effect of composition and phase diagram features on printability and microstructure in laser powder bed fusion: Development and comparison of processing maps across alloy systems

Additive manufacturing (AM) has gained considerable academic and industrial interest due to its ability to produce parts with complex geometries with the potential for local microstructural control. However, due to the large number of material and process variables associated with AM, optimization of alloying compositions and process parameters to achieve desired properties is an arduous task. There is a fundamental gap in understanding how changes in process variables and alloy composition and thermodynamics affect additively manufactured parts. The present systematic study sheds light on the effects of alloying composition and corresponding phase diagram features on the printability and solidification microstructures of four binary nickel-based alloys, namely, Ni-20 at% Cu, Ni-5 at% Al, Ni-5 at% Zr, and Ni-8.8 at% Zr. These compositions are selected to represent binary isomorphous, weak solute partitioning, strong solute partitioning, and eutectic alloying conditions, respectively. Single track and bulk experiments are conducted to quantify the effects of varying material thermodynamic properties such as solidification temperature ranges, alloy melting temperatures, and other solidification conditions on resultant microstructures across the laser powder bed fusion (L-PBF) parameter space. A simple framework for developing processing maps detailing porosity formation and microsegregation across the laser power – scan speed parameter space is established and validated for each of these alloys to determine how material properties affect printability and microstructure in L-PBF. This knowledge will be vital in optimizing alloy chemistry and process parameters to design alloys specifically for additive manufacturing, as well as to provide a path toward local microstructure control.

Effects of the Addition of Co or Ni Atoms on Structure and Magnetism of FeB Amorphous Alloy: Ab Initio Molecular Dynamics Simulation.

The effects of the substitution of Fe by Co or Ni on both the structure and the magnetic properties of FeB amorphous alloy were investigated using first-principle molecular dynamics. The pair distribution function, Voronoi polyhedra, and density of states of Fe80−xTMxB20 (x = 0, 10, 20, 30, and 40 at.%, TM(Transition Metal): Co, Ni) amorphous alloys were calculated. The results show that with the increase in Co content, the saturation magnetization of Fe80−xCoxB20 (x = 0, 10, 20, 30, and 40 at.%) amorphous alloys initially increases and then decreases upon reaching the maximum at x = 10 at.%, while for Fe80−xNixB20 (x = 0, 10, 20, 30, and 40 at.%), the saturation magnetization decreases monotonously with the increase in Ni content. Accordingly, for the two kinds of amorphous alloys, the obtained simulation results on the variation trends of the saturation magnetization with the change in alloy composition are in good agreement with the experimental observation. Furthermore, the relative maximum magnetic moment was recorded for Fe70Co10B20 amorphous alloy, due to the induced increased magnetic moments of the Fe atoms surrounding the Co atom in the case of low Co dopant, as well as the increase in the exchange splitting energy caused by the enhancement of local atomic symmetry.

Microstructure and composition dependence of mechanical characteristics of nanoimprinted AlCoCrFeNi high-entropy alloys

Molecular dynamics is applied to explore the deformation mechanism and crystal structure development of the AlCoCrFeNi high-entropy alloys under nanoimprinting. The influences of crystal structure, alloy composition, grain size, and twin boundary distance on the mechanical properties are carefully analyzed. The imprinting load indicates that the highest loading force is in ascending order with polycrystalline, nano-twinned (NT) polycrystalline, and monocrystalline. The change in alloy composition suggests that the imprinting force increases as the Al content in the alloy increases. The reverse Hall–Petch relation found for the polycrystalline structure, while the Hall–Petch and reverse Hall–Petch relations are discovered in the NT-polycrystalline, which is due to the interactions between the dislocations and grain/twin boundaries (GBs/TBs). The deformation behavior shows that shear strain and local stress are concentrated not only around the punch but also on GBs and adjacent to GBs. The slide and twist of the GBs play a major in controlling the deformation mechanism of polycrystalline structure. The twin boundary migrations are detected during the nanoimprinting of the NT-polycrystalline. Furthermore, the elastic recovery of material is insensitive to changes in alloy composition and grain size, and the formability of the pattern is higher with a decrease in TB distance.

Thermodynamics of formation and evaporation of lead-tin alloys

Only a few works have been devoted to thermodynamic studies of the lead-tin system by methods including the volatile components evaporation process. When the binary system is separated into metals by distillation, the volatile component is removed from the alloy and the low-volatile component accumulates in the bottom products, that is, there are alloy composition changes over the entire concentration range. It is necessary to know the boundaries position of the melt and vapor coexistence fields on the state diagram, especially for solutions beneficiated with non-volatile metal to assess the quality of the vapor phase by the content of the low-volatile component. In this regard, the study has been completed with the purpose to clarify the values of the thermodynamic functions of the formation and evaporation of lead-tin melts required to calculate the boundaries of the liquid and vapor coexistence fields on the state diagram that enables us to judge the amount of a low-volatile component in the vapor phase under equilibrium conditions. The thermodynamic activity of lead was calculated, as well as the numerical integration of the Gibbs-Duhem equation using the substitution proposed by Darken is the thermodynamic activity and pressure of saturated tin vapor Based on the values of the saturated lead vapor pressure, determined by the boiling point method (isothermal version) for alloys predominantly of the lead edge of the phase diagram. The thermodynamic constants thus obtained will add to the base of physicochemical data and will be used to calculate the boundaries of the vapor-liquid equilibrium fields on the phase diagram, allowing to determine the possibility and completeness of the distillation separation of metals.

High-throughput ion irradiation of additively manufactured compositionally complex alloys

Several advanced nuclear reactor designs promise efficiency and safety improvements over the current reactor fleet but are limited by the current set of ASME code-qualified materials. Novel alloys including high-entropy alloys (HEAs), and more broadly compositionally complex alloys (CCAs), have shown promising irradiation-tolerance. However, the vast range of alloy compositions adds to an already time-consuming alloy development process. In this study, to accelerate the development of novel alloys for nuclear applications, a high-throughput (HTP) methodology has been employed. Additive manufacturing has been used to produce a compositional array of unary, binary, ternary, and quaternary alloys, including several CCAs, which span the Cr-Fe-Mn-Ni composition space. The compositional array was homogenized at 1000°C for 24 hours and each sample was irradiated using 4-MeV Ni2+ ions at room temperature to a peak damage of 50 dpa, as estimated using SRIM, at the University of Wisconsin Ion Beam Laboratory. A custom XY stage was built to accommodate the large compositional array and half of each sample was masked during irradiation enabling both the irradiated and unirradiated properties of each alloy to be characterized side-by-side. Each alloy was characterized using X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and nanoindentation. CALPHAD simulations spanning the entire Cr-Fe-Mn-Ni composition space at 1000°C were performed to compare predicted equilibrium phases with phases identified experimentally from the unirradiated regions of each alloy. Nanoindentation measurements indicate radiation-induced hardening ranging from ~1-1.5 GPa in each of Cr-Fe-Mn-Ni CCAs, which is relatively insensitive to modest changes in alloy composition and comparable to hardening observed in neutron irradiated Cr-Fe-Mn-Ni CCAs in the literature. Overall, a substantial time savings was realized by employing HTP synthesis, irradiation, and characterization in this study compared to conventional techniques, the implications of which are discussed.

Electrochemical dissolution of Pt-Pd-Ru alloys

The electrochemical dissolution of Pt-Pd-Ru ternary alloys has been studied in 0.5 M H2SO4 aqueous solutions by cyclic voltammetry and the electrochemical quartz crystal microbalance (EQCM). The amounts of dissolved metals from Pt, Pd and Ru electrodes as well as from their alloys during voltammetric polarization to various upper potential limits have been determined. Under the conditions of a single voltammetric cycle in the range 50–1360 mV vs. SHE (0.01 V/s) Pt-Pd-Ru alloys are more resistant to dissolution than both pure Pd and Ru, but more prone to dissolution than pure Pt. The effect of continuous voltammetric cycling in a fixed potential range on alloy dissolution, composition changes and real surface area modifications was also examined. During the repeated polarization to the potentials of surface oxidation the alloys become impoverished in Ru and the surface roughness decreases. Prior hydrogen electrosorption probably leads to an increase in the surface homogeneity, which results in a greater alloy stability during prolonged voltammetric polarization to high potentials.

The Effect of the Transformation of ε-Fe2-3N into γ′-Fe4N Phase on the Fatigue Strength of Gas-Nitrided Pure Iron

The effect of the iron nitride phases, ε-Fe2-3N and γ′-Fe4N, on the fatigue strength was investigated. Pure iron was used to observe only the effect of nitride, excluding the effects of factors, such as residual stress, depending on the alloy composition and microstructural change according to working on the fatigue strength. In this work, ε and γ′ phases were respectively grown at a time on the surface of the pure iron specimens using the appropriate nitriding potential KN, the mixture rates of ammonia and hydrogen gases, at same temperature of 570 °C according to the Fe-N Lehrer diagram. Another γ′ phase was prepared by first growing the ε phase and then transformed from ε phase into γ′ phase by changing the KN at the same temperature of 570 °C in the 2-stage gas nitriding. The fatigue strengths of the iron nitride consisted of ε and γ′ phases, γ′ phase, and γ′ phase grown by the 2-stage gas nitriding were evaluated, respectively. As a result, first, it can be seen that the diffusion layer of ε phase was deeper than γ′ phase, but fatigue strength was lower. On the other hand, fatigue strength of both the γ′ phases are higher than that of the ε, and the fatigue strength of γ′ phase nitride grown by 2-stage gas nitriding was almost similar to that of γ′ phase nitride grown at a time, i.e., fatigue strength was not significantly related to diffusion depth and depended on nitride phases in this study. Secondly, we cannot clearly conclude that there was the difference in fatigue strength according to the thickness of nitride layer consisted of γ′ phase. However, it is clear that when ε phase was transformed to γ′ phase, fatigue strength had the same level as γ′ phase formed at one time.

An innovative multi-component fluoropolymer-based coating on outdoor patinated bronze for Cultural Heritage: Durability and reversibility

Abstract The application of protective coatings for the conservation of outdoor metallic artworks is currently considered the best approach actively employed. This paper focuses on the evaluation of the protectiveness of an optimised fluorosilane (FA-MS) coating with low occupational hazard impact and high inhibition efficiency. FA-MS coating was applied on a typical historical as-cast bronze (Cu-Sn-Zn-Pb), artificially patinated by synthetic acid rain dropping, to simulate natural unsheltered patinas. Subsequently, the patinated bronze was coated by brushing and then artificially aged by synthetic rain dropping. In order to investigate the coating-patina system, a multi-analytical approach was applied including scanning electron microscopy coupled with elemental analysis (FEG-SEM with EDS), microstructural analysis of cross-sections produced by Focused Ion Beam milling (FIB) and X-ray Photoelectron Spectroscopy (XPS). The coating homogenously covered the whole patinated surface producing a transparent layer few μms thick. The weathering solutions were also analysed for the assessment of metallic cations release. FA-MS showed excellent protective performances for the patinated bronze exposed to simulated outdoor conditions. Very high values of inhibiting efficiency in terms of metal release in the weathering solutions (up to 99%) were measured. In order to investigate all the necessary requirements for application in conservation field, the durability of FA-MS was also tested on a real bronze artistic casting, in terms of limiting the alloy composition changes (measured by in situ XRF) over time. A good durability of the coating was observed during the ongoing field exposure test. Lastly, reversibility of FA-MS was investigated by applying different mechanical cleaning methods, such as brushing and blasting. The reversibility was assessed by the evaluation of the morphology (colour change, surface roughness and optical microscopy) and the composition (SEM-EDS and Py-GC-MS chromatography) of the cleaned surfaces. Cleaning by brushing proved to be an efficient method for removing this coating without significantly altering the bronze patina.

Process-Dependent Composition, Microstructure, and Printability of Al-Zn-Mg and Al-Zn-Mg-Sc-Zr Alloys Manufactured by Laser Powder Bed Fusion

Additive manufacturing (AM) technology for metallic alloys such as laser powder bed fusion (LPBF) brings tremendous opportunities for development of novel alloys specifically designed for AM that would desensitize the inherent process variability and requires a refined understanding of processing–structure–property relationship that would contribute to future alloy development. In this study, two different alloys, Al-6Zn-2Mg and Al-6Zn-2Mg-0.7Sc-0.3Zr in wt. pct, representing traditional and AM-specific novel aluminum alloys, respectively, were manufactured by LPBF technique using pre-alloyed gas atomized powders. The Al-Zn-Mg ternary alloys exhibited cracks at various LPBF processing parameters, primarily along the grain boundaries of the large columnar grains that extended across multiple melt pools. The severity of cracking in LPBF Al-Zn-Mg alloys was process-dependent and could be correlated to the change in alloy composition due to evaporation of Zn and Mg with high vapor pressure. The Scheil–Gulliver non-equilibrium solidification calculations showed that the Al-Zn-Mg alloys with lower Zn and Mg concentrations had smaller solidification range (i.e., ΔT) and steepness values (i.e., $$ \left| {{\text{d}}T / {\text{d}}f_{\text{s}}^{1/2} } \right| $$ near $$ f_{\text{s}}^{1/2} = 1 $$), which corresponded to a lower cracking severity. On the other hand, no cracks were observed in Al-Zn-Mg-Sc-Zr alloys, although their solidification range and steepness values were similar to the ternary Al-Zn-Mg alloys. The microstructure of Al-Zn-Mg-Sc-Zr alloys exhibited a much refined heterogeneous microstructure containing small equiaxed and columnar grains within each melt pool, owing to the heterogeneous nucleation upon primary Al3(Sc,Zr) particles. Process-dependent microstructure was observed as a result of variation in thermal gradient and cooling rate associated with LPBF parameters. Findings from this study provide guidance for the future design of AM-specific aluminum alloys and insights into the microstructural control by LPBF.

Effect of mass evaporation on measurement of liquid density of Ni-based superalloys using ground and space levitation techniques

Loss of mass due to evaporation during molten metal levitation processing significantly influences the evaluation of density, viscosity and surface tension during thermophysical property measurement. Since there is no direct way to track the evaporation rate during the process, this paper describes a mathematical approach to track mass loss and quantify any changes in alloy composition as a function of time and temperature. The Ni-based super alloy CMSX-4 Plus (SLS) was investigated and a model was developed to predict the dynamic loss of mass with time and track the potential for composition shifts throughout each thermal cycle based on the Langmuir’s equation for ideal solution behavior. Results were verified by post-test chemical analysis of key elemental constituents including Al, Cr, Ti, and Co where the error in composition for each element was less than 1% when the activity of aluminum in solution was fixed at zero – effectively eliminating evaporation of aluminum for ground-based electrostatic levitation (ESL) testing in vacuum. This model predicts the mass evaporation for Al and Co within ±6 % errors for CMSX-4 plus samples processed in ESL. Application of this technique to the space tests using the ESA ISS-EML facility shows that by conducting experiments in an inert shielding-gas environment, composition shifts due to differential relative evaporation become negligible and the composition is maintained within the desired limits. By tracking overall mass loss during testing the influence of evaporation on density measurements is discussed.

Modelling the alloy element composition change in NiTi achieved through laser induced vaporization

The use of pulse laser processing to join or modify NiTi shape memory alloys has become an area of extreme interest in recent years. Until now, an in-depth investigation into the vaporization mechanisms associated with alloy composition changes during laser processing has not been done. This work systematically examines the effect of laser pulse parameters on the change in alloy composition. It was found that during conduction mode vaporization, the Ti-rich surface oxide limits the vaporization rate of Ni. This allows for less nickel loss and more control over the final composition. Additionally, keyhole formation during laser processing resulted in significantly higher rates of Ni loss, but also a greater amount of variance and reduced control of the final composition. Models are proposed that successfully predict the final bulk composition based on laser pulse parameters.

Selective electron beam melting of Al0.5CrMoNbTa0.5 high entropy alloys using elemental powder blend

High Entropy Alloys (HEAs) is a novel promising class of multi-component materials which may demonstrate superior mechanical properties useful for high-temperature applications. Despite the high potential of HEAs, their production is complicated, using pre-alloyed powders in powder metallurgy route. This significantly complicates development and implementation of refractory BCC solid solution based HEAs. The present paper reports on experiments aiming at production of Al0.5CrMoNbTa0.5 multi-principle alloy using powder bed beam based additive manufacturing. Samples were manufactured using Selective Electron Beam Melting (SEBM) additive manufacturing technique from a blend of elemental powders aiming at achieving microstructure with high configurational entropy. Though it was not possible to achieve completely homogeneous microstructure, the as-printed material was composed of the zones with two multi-component solid solutions, which differed only by Al content confirming in situ alloying. The process parameters optimization was not carried out and the as-print material contained a notable amount of residual porosity. It was possible to reach lower porosity level using heat treatment at 1300 °C for 24 hours, however undesirable alloy composition changes took place. The main conclusion is that the production of the Al0.5CrMoNbTa0.5 multi-principle alloy from elemental powder blends using SEBM technique is achievable, but the process parameter optimization rather than post-process heat treatment should be performed to reduce the porosity of samples.

Elemental segregation in an AlCoCrFeNi high-entropy alloy – A comparison between selective laser melting and induction melting

Additive manufacturing of a high-entropy alloy, AlCoCrFeNi, was studied with selective laser melting from gas atomized powder. A wide process parameter window in the SLM process was investigated but it was impossible to produce crack-free samples, attributed to stresses that originate during the building processes. The microstructure and elemental segregation in the SLM samples were compared with induction-melted AlCoCrFeNi. The induction-melted sample crystallizes in randomly oriented large grains (several hundred microns). Dendritic and inter-dendritic areas with slightly different chemical composition can be observed. Within these areas a spinodal decomposition occurs with a separation into FeCr- and NiAl-rich domains. Further spinodal decomposition within the FeCr-rich regions into Cr- and Fe-rich domains was observed by atom probe tomography.In contrast, the SLM-samples crystallizes in much smaller grains (less than 20 μm) with a dendrite-like substructure. These dendrite-like features exhibit distinct chemical fluctuations on the nm-scale. During annealing more pronounced chemical fluctuations and the formation of Cr-rich and Cr-poor regions can be observed. The difference in microstructure and spinodal decomposition between the induction-melted and SLM samples is attributed to the significantly higher cooling rate for SLM. This study shows that, by using different synthesis pathways, it is possible to modify the microstructure and segregation of element within alloys. This can be used to tune the materials properties, if the cracking behavior is handled e.g. by change of alloy composition to minimize phase transformations or use of a heating stage.