Laser Additive Manufacturing of Oxide Dispersion-Strengthened Copper–Chromium–Niobium Alloys

: Copper is a key material for cooling of thermally stressed components in modern aerospace propulsion systems, due to its high thermal conductivity. The use of copper materials for such applications requires both high material strength and high stability at high temperatures, which can be achieved by the concept of oxide dispersion strengthening. In the present work, we demonstrate the oxide reinforcement of two highly conductive precipitation-strengthened Cu-Cr-Nb alloys using laser additive manufacturing. Gas-atomized Cu-3.3Cr-0.5Nb and Cu-3.3Cr-1.5Nb (wt.%) powder materials are decorated with Y 2 O 3 nanoparticles by mechanical alloying in a planetary mill and followed by consolidation by the laser additive manufacturing process of laser powder bed fusion (L-PBF). While dense specimens (>99.5%) of reinforced and nonreinforced alloys can be manufactured, oxide dispersion-strengthened alloys additionally exhibit homogeneously distributed oxide nanoparticles enriched in yttrium and chromium next to Cr 2 Nb precipitates present in all alloys examined. Higher niobium contents result in moderate increase of the Vickers hardness of approx. 10 HV0.3, while the homogeneously dispersed nanometer-sized oxide particles lead to a pronounced increase of approx. 30 HV0.3 in material strength compared to their nonreinforced counterparts.


Introduction
Copper is an indispensable material with a wide field of applications in various engineering sectors, such as automotive, aerospace, electronics, and power generation.The versatility of copper and its alloys is commonly attributed to the unique physical properties of high thermal and electrical conductivity.The utilization of copper alloys as heat-transfer media in high-performance components in aerospace [1][2][3][4] and powergeneration applications [4][5][6][7] requires the development of copper alloys with both high strength and high thermal conductivity.Increasing strength by solid solution strengthening requires significant amounts of additional alloying elements, which drastically decreases the thermal conductivity of copper alloys.Among others systems, e.g., Cu-Ni-Mn [8], Cu-Ag [9], Cu-Co [10], Cu-Mg [11,12] and Cu-Fe-P [13], alloy concepts based on the binary Cu-Cr and Cu-Zr systems have been considered for the development of highly conductive high-strength copper alloys [14], which are interesting because they do not contain highly toxic beryllium.[15].This is due to the extremely low room temperature solubility of chromium [16] and zirconium in copper at room temperature, resulting in the precipitation of Cr-rich particles [14] and Cu 5 Zr precipitates [14,[17][18][19].Among others, the commercial copper alloys AMZIRC [14,20], CuCr1Zr, and NARloy-Z [20] are based on these developments.The alloys GRCop-84 and GRCop-42, both developed by NASA [20], are based on the ternary Cu-Cr-Nb system, offering higher strength and improved creep properties than the alloys NARloy-Z and AMZIRC by the formation of C15 structured Cr 2 Nb laves phases [14,[20][21][22][23][24][25].The high affinity of Cr and Nb in combination with low solubility in the copper matrix at room temperature allow the creation of a pure Cu matrix with dispersed Cr 2 Nb phases [26,27] after heat treatment [28].The remarkable creep properties [29] are attributed to the lack of coarsening of the Cr 2 Nb phase in the copper matrix [30].Creep properties are believed to be further improved by the concept of oxide-dispersion strengthening (ODS), where nanoscale oxide particles are dispersed in the metallic matrix.The alloy family of GlidCop [31,32], based on the dispersion of temperature-resistant nanoscale oxides, such as aluminum oxide (Al 2 O 3 ) [14,[33][34][35] and yttrium oxide (Y 2 O 3 ) [36][37][38][39], offers increased strength, particularly in high-temperature atmospheres, and inhibits recrystallization mechanisms [40] without significantly reducing electrical conductivity [41].GRCop (Cu-Cr-Nb), novel Cu-Cr-Nb-Zr [42,43], and GlidCop (ODS copper alloys) [44] are conventionally manufactured via powder metallurgical or rapid solidification manufacturing processes [45][46][47][48], such as additive manufacturing processes [34,49] due to aggregation during the molten stage of Cr and Nb in GRCop alloys or oxide nanoparticles in GlidCop alloys, respectively.
Additive manufacturing processes offer near-net-shape fabrication of complex structures in addition to extremely high solidification conditions.While additive manufacturing of ODS steels using laser-based [50] or electron beam-based [51] additive manufacturing processes have already been demonstrated, additive manufacturing of oxide dispersionstrengthened copper alloys only in singular studies [52][53][54] typically results in highly porous structures [52].This may be attributed to difficult processing of copper and its alloys, due to its low absorption for near-infrared wavelengths [55] and high specific thermal conductivity leading to unstable melt pool behavior [56], frequently resulting in high porosities [57][58][59][60][61] lack of fusion [57,59,62,63] and oxide formation [64] in printed copper components.
Different strategies for processing copper materials, such as reducing the particle sizes used [65][66][67], coating of copper powder particles with native oxide layers [68,69], nickel [70] and carbon [71], the utilization of alternative laser wavelengths in the visible spectrum of green [72-74] and blue light [75] and pulsed laser systems [76], have been examined to improve the processability of copper alloys in laser additive manufacturing processes.Cu-Cr-based alloys have established themselves as high-strength and highly conductive with low alloy content and hardenable by the formation of Cr precipitation [77][78][79].The additional introduction of Zr allows the formation of nanoscale Zr-rich precipitates during aging heat treatments [80][81][82][83][84].Moreover, the formation of Zr-O structures is observed [85,86], leading to the hypothesis that Zr may be used to capture residual oxygen introduced by the powder material or the process atmosphere.Cu-Cr-Nb alloys are also increasingly being manufactured with the laser additive manufacturing technologies of L-DED [87][88][89] and L-PBF [90][91][92][93][94][95], allowing the manufacturing of dense and defect-free parts with outstanding mechanical properties [4,91,96].
However, significant coarsening of Cr 2 Nb precipitations is observed in high-temperature environments [30,92], resulting in rapid degradation of the mechanical properties.In this work, we introduce nanoscale yttrium oxide nanoparticles, which are known to stabilize the microstructure and thus material strength up to high temperatures [97].It is hypothesized that the introduction of nanoscale oxide particles may allow the increase of the service temperatures from approx.700 • C for GRCop alloys [32] up to 800 • C for dispersionstrengthened copper alloys [98] without reducing conductive properties [99].We chose Y 2 O 3 as reinforcement phase due to its higher thermodynamic stability than commonly used Al 2 O 3 in GlidCop alloys [100] and select two Cu-Cr-Nb alloys with different niobium contents to study the influence of increased niobium contents on the formation of Cr 2 Nb precipitates and the chemistry of the introduced yttrium oxide nanoparticles, potentially forming cubic-structured Y 3 NbO 7 , as observed in stainless steels [101].With this approach, we combine the concepts of precipitation-strengthened GRCop and oxide dispersionstrengthened GlidCop families to develop a high-strength, high-temperature-resistant, and highly conductive copper alloy using the AM process of L-PBF.

Powder Preparation
Two Cu-Cr-Nb powder materials with the chemical compositions shown in Table 1 are manufactured via vacuum inert gas atomization (VIGA) using argon gas.Both powder materials are sieved to a particle size fraction of 15-45 µm prior to L-PBF processing.Manufacturing of ODS variants from both alloys with additions of 0.5 wt.% nanoscale Y 2 O 3 (abcr GmbH, Karlsruhe, Germany) requires the fabrication of composite powder materials with oxide nanoparticles attached to micrometer-sized copper particles.Thus, mechanical alloying (MA) is used to manufacture composite powders and performed in a Pulverisette 4 classic line planetary mill (Fritsch GmbH, Idar-Oberstein, Germany) using milling containers and grinding balls (BPR: 5) with a diameter of 5 mm, both made from yttrium-stabilized zirconia (YSZ).The milling procedure is conducted under full argon atmosphere to avoid extensive oxidation of the copper powder particles in three alternating milling (20 min) and pause intervals (10 min) to allow cooling of the powder material.

Laser Powder Bed Fusion (L-PBF)
Laser powder bed fusion is conducted using a custom laboratory-scaled L-PBF machine equipped with a YLR1000WC single-mode fiber laser system (IPG Laser GmbH, Burbach, Germany) emitting laser radiation with a wavelength of 1070 nm.A Gaussian beam profile with a beam diameter of approx.80 µm in the focal plane is created by a galvanometer scanner hurrySCAN20 (SCANLAB GmbH, Puchheim, Germany) coupled with an f-theta lens with a focal length of 255 mm.A schematic of the setup of the machine is depicted in Figure 1a.

Powder Preparation
Two Cu-Cr-Nb powder materials with the chemical compositions shown in Table 1 are manufactured via vacuum inert gas atomization (VIGA) using argon gas.Both powder materials are sieved to a particle size fraction of 15-45 μm prior to L-PBF processing.Manufacturing of ODS variants from both alloys with additions of 0.5 wt.% nanoscale Y2O3 (abcr GmbH, Karlsruhe, Germany) requires the fabrication of composite powder materials with oxide nanoparticles attached to micrometer-sized copper particles.Thus, mechanical alloying (MA) is used to manufacture composite powders and performed in a Pulverisette 4 classic line planetary mill (Fritsch GmbH, Idar-Oberstein, Germany) using milling containers and grinding balls (BPR: 5) with a diameter of 5 mm, both made from yttriumstabilized zirconia (YSZ).The milling procedure is conducted under full argon atmosphere to avoid extensive oxidation of the copper powder particles in three alternating milling (20 min) and pause intervals (10 min) to allow cooling of the powder material.

Laser Powder Bed Fusion (L-PBF)
Laser powder bed fusion is conducted using a custom laboratory-scaled L-PBF machine equipped with a YLR1000WC single-mode fiber laser system (IPG Laser GmbH, Burbach, Germany) emitting laser radiation with a wavelength of 1070 nm.A Gaussian beam profile with a beam diameter of approx.80 μm in the focal plane is created by a galvanometer scanner hurrySCAN20 (SCANLAB GmbH, Puchheim, Germany) coupled with an f-theta lens with a focal length of 255 mm.A schematic of the setup of the machine is depicted in Figure 1a.L-PBF processing is conducted in a full argon atmosphere (residual oxygen < 10 ppm) with a slight overpressure of approx.60 mbar compared to ambient atmosphere.Cubic samples (5 mm 3 ) are manufactured on a CuCr1Zr (CW106C) substrate material with a layer-wise rotating stripe scan strategy (34° per layer) using a hatch distance (hs) of 120 L-PBF processing is conducted in a full argon atmosphere (residual oxygen < 10 ppm) with a slight overpressure of approx.60 mbar compared to ambient atmosphere.Cubic samples (5 mm 3 ) are manufactured on a CuCr1Zr (CW106C) substrate material with a layer-wise rotating stripe scan strategy (34 • per layer) using a hatch distance (h s ) of 120 µm and a layer thickness (t L ) of 30 µm.The used laser power (P L ) of 400 W and scan speed (v s ) of 800 mm/s allow the calculation of the volume energy density (VED) to 138.89 J/mm 3 .
The process parameters are selected based on the process parameter study by Ren et al. [96] The volume energy density is calculated using (1) [102].Five specimens of each material are printed (cf. Figure 1b).
Identical process parameter sets are applied to all powder materials processed and are summarized in Table 2.

Characterization Methods
Printed samples are examined by standard metallographic procedure including watercooled cutting parallel to the build direction, grinding with silicon carbide (SiC) paper and subsequent polishing procedure using a 1 µm diamond polishing suspension.The visual identification of different phases and nanoparticles in the printed specimens is aided by an etching procedure prior to scanning electron microscopy (SEM) examination utilizing an aqueous etchant containing 10 g iron(III) chloride (Fe 3 Cl) and 20 mL hydrogen chloride (HCl) in 100 mL deionized water at room temperature for approx.15 s.The relative density of printed specimens is determined on five polished cross sections prior to etching using the image analysis software ImageJ.
SEM for imagery of powder particles is conducted on a Leo 1455VP (Carl Zeiss Microscopy Deutschland GmbH, Oberkochen, Germany) in secondary electron (SE) mode using an acceleration voltage of 20 kV.SEM images of printed specimens are performed on an Apreo 2C (Thermo Fisher Scientific, Hillsboro, OR, USA) in SE mode and backscattered electron (BSE) mode with 10 kV acceleration voltage.The determination of chemical compositions is performed by energy-dispersive spectroscopy (EDS) measurements with a Bruker XFlash 6L100 detector integrated in the Apreo 2C SEM system.
Hardness measurements are conducted on an automated hardness tester Carat 930 (ATM Qness GmbH, Mammelzen, Germany) in Vickers mode (HV0.3).A regular grid of 6 × 6 indents per sample with a constant distance of 500 µm is performed.

Powder Characterization
Both gas-atomized Cu-Cr-0.5Nb(Figure 2a) and Cu-Cr-1.5Nb(Figure 2b) powder materials are characterized by a predominantly spherical morphology with only few particles showing aspheric morphology.The powder particles show a smooth surface with sporadically present satellite particles attached to the powder particles.
The Cu-3.3Cr-1.5Nbpowder material (Figure 2b) shows a comparably high fraction of satellites, which are regularly encountered in gas-atomized powder materials [103], showing that an increased niobium content influences the gas atomization process.The pronounced formation of satellites might stem from the higher superheat of the copper melt necessary for gas atomization [104], due to higher liquidus temperature with increasing niobium content [105].
After mechanical alloying, the circularity of the gas-atomized powder particles decreases and flake-like powder particles (indicated by the yellow arrow in Figure 3a) are observed occasionally, which are frequently found in mechanically alloyed powder materials [106,107].Additionally, the surface of the powder particles appears to be rougher, which presumably stems from the fracturing and cold-welding events during mechanical alloying processes [108].Mechanically alloyed powder particles are covered with white-appearing particles (Figure 3a,b).Using EDS measurements, enrichments of yttrium and oxygen can be detected, proving the successful deposition of Y 2 O 3 or related compound nanoparticles on the surface of the Cu-Cr-Nb powder particles (Table 3).The Cu-3.3Cr-1.5Nbpowder material (Figure 2b) shows a comparably high fraction of satellites, which are regularly encountered in gas-atomized powder materials [103], showing that an increased niobium content influences the gas atomization process.The pronounced formation of satellites might stem from the higher superheat of the copper melt necessary for gas atomization [104], due to higher liquidus temperature with increasing niobium content [105].
After mechanical alloying, the circularity of the gas-atomized powder particles decreases and flake-like powder particles (indicated by the yellow arrow in Figure 3a) are observed occasionally, which are frequently found in mechanically alloyed powder materials [106,107].Additionally, the surface of the powder particles appears to be rougher, which presumably stems from the fracturing and cold-welding events during mechanical alloying processes [108].Mechanically alloyed powder particles are covered with whiteappearing particles (Figure 3a,b).Using EDS measurements, enrichments of yttrium and oxygen can be detected, proving the successful deposition of Y2O3 or related compound nanoparticles on the surface of the Cu-Cr-Nb powder particles (Table 3).The Cu-3.3Cr-1.5Nbpowder material (Figure 2b) shows a comparably high fraction of satellites, which are regularly encountered in gas-atomized powder materials [103], showing that an increased niobium content influences the gas atomization process.The pronounced formation of satellites might stem from the higher superheat of the copper melt necessary for gas atomization [104], due to higher liquidus temperature with increasing niobium content [105].
After mechanical alloying, the circularity of the gas-atomized powder particles decreases and flake-like powder particles (indicated by the yellow arrow in Figure 3a) are observed occasionally, which are frequently found in mechanically alloyed powder materials [106,107].Additionally, the surface of the powder particles appears to be rougher, which presumably stems from the fracturing and cold-welding events during mechanical alloying processes [108].Mechanically alloyed powder particles are covered with whiteappearing particles (Figure 3a,b).Using EDS measurements, enrichments of yttrium and oxygen can be detected, proving the successful deposition of Y2O3 or related compound nanoparticles on the surface of the Cu-Cr-Nb powder particles (Table 3).Table 3.Chemical composition (in wt.%) of the surface of the mechanically alloyed powder depicted in Figure 2a,b.Averaged values from single-point measurements on 15 randomly selected powder particles.GA: gas-atomized, MA: mechanically alloyed.Overall, a predominantly spherical particle morphology of both gas atomized powder materials was retained after mechanical alloying (Figure 3a,b) and thus the mechanically alloyed powder materials exhibit satisfactory flowability for L-PBF processing.

Alloy
The presence of yttrium-rich oxide nanoparticles is believed to allow the utilization of substantially lower VED compared to other studies, with 277.78 J/mm 3 [60] to produce dense specimens in this study (Table 2) by increasing the absorptivity for the used laser radiation [109] by multiple reflections [110].EDS measurements on the powder particles surface additionally indicate an inhomogeneous distribution of chromium and niobium in the copper matrix of the gas-atomized and mechanically alloyed powder particles, which is confirmed by EDS maps of the powder particles' cross sections (Figure 4a-d).Comparable inhomogeneities were also observed by Seltzman et al. [4,91], but do not result in detrimental effects on the L-PBF printing process or homogeneous distribution of Cr 2 Nb laves phase in the printed copper matrix [91].
radiation [109] by multiple reflections [110].EDS measurements on the powder particles surface additionally indicate an inhomogeneous distribution of chromium and niobium in the copper matrix of the gas-atomized and mechanically alloyed powder particles, which is confirmed by EDS maps of the powder particles' cross sections (Figure 4a-d).Comparable inhomogeneities were also observed by Seltzman et al. [4,91], but do not result in detrimental effects on the L-PBF printing process or homogeneous distribution of Cr2Nb laves phase in the printed copper matrix [91].The printed cubic structures are found to be almost defect-free, exhibiting no cracks and low porosities, resulting in a relative density of 99.7% ± 0.2% and 99.4% ± 0.3% respectively.These results are in accordance with findings of Seltzman et al. [4,91] and Ren et al. [96], both demonstrating the defect-free manufacturing of GRCop-84 and Cu-1.93Cr-0.74Nballoy.Two homogeneously distributed populations of nanosized particles, indicated by yellow and red arrows in Figure 5b,d, are observed in both unreinforced specimens, which can be distinguished by their respective size.
The larger particles, which are marked by yellow arrows (Figure 5b,d) have an approximate size of 100-300 nm and are found to be enriched in chromium.In contrast, the smaller nanoparticle population (red arrows) with sizes below 100 nm shows high amounts of chromium and niobium.This correlates with particle size found in L-PBF processed GRCop-84 alloy of 85-200 nm [4,91].However, due to the small size of both nanoparticle species, EDS measurements were performed on sporadically found clusters of the respective nanoparticle species and therefore large amounts of copper are detected.(Table 4).
The presence of Cr-rich- [14,26] and Cr-Nb-rich precipitates [22,[24][25][26][27][28] are frequently observed in Cu-Cr-Nb alloys.The latter particles may correspond to Cr 2 Nb laves phase, which is also found together with Cr-rich precipitates in laser-directed energy deposition (L-DED) processed Cu-3.4Cr-0.6Nb(at.%) alloy [88].Interestingly, no Cr-rich precipitates are observed in L-PBF processed GRCop-84 alloy [4,88], which contradicts the findings of Yang et al. [112,113] showing faster precipitation of L1 2 -structured Cu 3 Cr from pure Cr precipitates compared to Cr 2 Nb laves phases.However, it is also shown that precipitation of Cr 2 Nb laves phases is enabled at temperatures exceeding 600 • C, consuming Cr-rich precipitations upon formation of Cr 2 Nb [107].We hypothesize that at a fixed Cr/Nb ratio of 2 in combination with high melt pool temperatures of melt pools formed in L-PBF processes favor direct precipitation of Cr 2 Nb from the supersaturate copper matrix or lead to full transformation of Cr-rich precipitates to Cr 2 Nb laves phases.Consequently, exceeding the ratio of Cr/Nb of 2 will allow excess Cr to remain in the copper matrix as Cr or Cu 3 Cr precipitates.

Unreinforced Cu-Cr-Nb Alloys
Figure 5 provides SEM images of cross sections of the unreinforced Cu-3.3Cr-0.5Nb(Figure 5a,b) and Cu-3.3Cr-1.5Nb(Figure 5c,d) in the as-printed state.The microstructure is characterized by a layered structure, showing the melt pool boundaries of the printed layers, characteristic of AM printed parts [111].The printed cubic structures are found to be almost defect-free, exhibiting no cracks and low porosities, resulting in a relative density of 99.7% ± 0.2% and 99.4% ± 0.3% respectively.These results are in accordance with findings of Seltzman et al. [4,91] and Ren et al. [96], both demonstrating the defect-free manufacturing of GRCop-84 and Cu-1.93Cr-0.74Nballoy.Two homogeneously distributed populations of nanosized particles, indicated by yellow and red arrows in Figure 5b,d, are observed in both unreinforced specimens, which can be distinguished by their respective size.The larger particles, which are marked by yellow arrows (Figure 5b,d) have an approximate size of 100-300 nm and are found to be enriched in chromium.In contrast, the smaller nanoparticle population (red arrows) with sizes below 100 nm shows high amounts of chromium and niobium.This correlates with particle size found in L-PBF processed GRCop-84 alloy of 85-200 nm [4,91].However, due to the small size of both nanoparticle species, EDS measurements were performed on sporadically found clusters of the respective nanoparticle species and therefore large amounts of copper are detected.(Table 4).In Figure 6 SEM images of the reinforced Cu-3.3Cr-0.5Nb-0.5Y 2 O 3 (Figure 6a,b) and Cu-3.3Cr-1.5Nb-0.5Y 2 O 3 (Figure 6c,d) are shown.No apparent differences regarding grain morphology or size are observable compared to the unreinforced specimen in Figure 4.However, on a lower scale significant differences in the microstructure evolution become apparent: a large number of sub-100 nm-sized nanoparticles are distributed within the metallic matrix.However, EDS measurements of nanoparticle clusters (Table 5) reveal the presence of two nanoparticle species, marked with yellow and red arrows, respectively (Figure 6b,d).Smaller particles are marked with red arrows and correspond to Cr-Nb-rich precipitates, which were observed in the unreinforced specimen.Interestingly, the larger nanoparticles, which are marked with yellow arrows, are enriched in yttrium, chromium and oxygen.Thus, the addition of Y2O3 nanoparticles seems to promote the formation of complex oxide compounds.The found chromium-rich yttrium oxide nanoparticles may be attributed to the monoclinic-structured compound of YCrO3, which is frequently observed in Cr-alloyed ODS steels [114][115][116] and nickel-based ODS alloys [117].The formation reaction can be described according to (2) [118].
The proposed formation reaction of YCrO3 (1) implies the introduction of additional oxygen to the process zone.While the gas atmosphere during L-PBF processing only contains negligible amounts of oxygen (Table 2), oxygen might be introduced into the printing process from the powder material.Copper powder handled in air is known to form an oxide layer, consisting of CuO [52,53], and thus might carry oxygen into the printing process to provide oxygen to form YCrO3.The EDS measurements performed on the powder particles' surface confirms the presence of oxygen, presumably bound as a thin oxide layer (Table 3).Interestingly, the larger nanoparticles, which are marked with yellow arrows, are enriched in yttrium, chromium and oxygen.Thus, the addition of Y 2 O 3 nanoparticles seems to promote the formation of complex oxide compounds.The found chromium-rich yttrium oxide nanoparticles may be attributed to the monoclinic-structured compound of YCrO 3 , which is frequently observed in Cr-alloyed ODS steels [114][115][116] and nickel-based ODS alloys [117].The formation reaction can be described according to (2) [118].
The proposed formation reaction of YCrO 3 (1) implies the introduction of additional oxygen to the process zone.While the gas atmosphere during L-PBF processing only contains negligible amounts of oxygen (Table 2), oxygen might be introduced into the printing process from the powder material.Copper powder handled in air is known to form an oxide layer, consisting of CuO [52,53], and thus might carry oxygen into the printing process to provide oxygen to form YCrO 3 .The EDS measurements performed on the powder particles' surface confirms the presence of oxygen, presumably bound as a thin oxide layer (Table 3).

Mechanical Properties
Mechanical properties of the printed oxide-reinforced materials and their unreinforced counterparts are characterized by hardness measurements.In Figure 7, representative hardness maps of printed specimens of Cu-3.3Cr-0.5Nb(Figure 7a) and Cu-3.3Cr-1.5Nb(Figure 7b) are depicted.Both materials are characterized by a relatively moderate hardness of 120-140 HV0.3.The hardness is distributed fairly homogeneous over the printed part with hardness variations up to 15 HV0.3.Oxide dispersion-strengthened Cu-3.3Cr-0.5Nb(Figure 7c) and Cu-3.3Cr-1.5Nb(Figure 7d) show considerably higher hardness compared to their unreinforced counterparts, which is in accordance with other metallic systems with dispersed oxide nanoparticles [50,119].

Mechanical Properties
Mechanical properties of the printed oxide-reinforced materials and their unreinforced counterparts are characterized by hardness measurements.In Figure 7, representative hardness maps of printed specimens of Cu-3.3Cr-0.5Nb(Figure 7a) and Cu-3.3Cr-1.5Nb(Figure 7b) are depicted.Both materials are characterized by a relatively moderate hardness of 120-140 HV0.3.The hardness is distributed fairly homogeneous over the printed part with hardness variations up to 15 HV0.3.Oxide dispersion-strengthened Cu-3.3Cr-0.5Nb(Figure 7c) and Cu-3.3Cr-1.5Nb(Figure 7d) show considerably higher hardness compared to their unreinforced counterparts, which is in accordance with other metallic systems with dispersed oxide nanoparticles [50,119].The rather homogeneous nature of the hardness distribution is also reflected by the averaged hardness of the line profiles, shown in Figure 8.Here, the superior hardness levels of oxide-dispersed material compared to the raw copper alloys becomes apparent.Additionally, higher niobium contents are also favorable for higher strength, presumably by higher fractions of nanoscale-precipitated Cr2Nb laves phases, which cannot be resolved with the used SEM system.The rather homogeneous nature of the hardness distribution is also reflected by the averaged hardness of the line profiles, shown in Figure 8.Here, the superior hardness levels of oxide-dispersed material compared to the raw copper alloys becomes apparent.Additionally, higher niobium contents are also favorable for higher strength, presumably by higher fractions of nanoscale-precipitated Cr 2 Nb laves phases, which cannot be resolved with the used SEM system.
Considering the hardness values of extruded [30,120] and L-DED [87] processed Cu-Cr-Nb alloys, L-PBF-manufactured material shows superior hardness despite lower chromium and niobium contents, which typically stems from smaller grains due to higher cooling rates in L-PBF than L-DED or extrusion processes.[50] However, Kini et al. [88] presents notably high hardness values (approx.146 HV) for L-DED-manufactured Cu-3.4Cr-0.6Nb(at.%) material, due to in situ precipitation of Cr 2 Nb laves particles by intrinsic heat treatment (IHT) mechanisms, frequently observed in L-DED processes [121][122][123][124].The higher fraction of precipitated Cr 2 Nb and Cr-rich phases directly leads to increased hardness.Thus, it is hypothesized that a subsequent aging procedure of L-PBF manufactured material will result in further increase in the hardness superior to L-DED-manufactured counterparts.Considering the hardness values of extruded [30,120] and L-DED [87] processed Cu-Cr-Nb alloys, L-PBF-manufactured material shows superior hardness despite lower chromium and niobium contents, which typically stems from smaller grains due to higher cooling rates in L-PBF than L-DED or extrusion processes.[50] However, Kini et al. [88] presents notably high hardness values (approx.146 HV) for L-DED-manufactured Cu-3.4Cr-0.6Nb(at.%) material, due to in situ precipitation of Cr2Nb laves particles by intrinsic heat treatment (IHT) mechanisms, frequently observed in L-DED processes [121][122][123][124].The higher fraction of precipitated Cr2Nb and Cr-rich phases directly leads to increased hardness.Thus, it is hypothesized that a subsequent aging procedure of L-PBF manufactured material will result in further increase in the hardness superior to L-DED-manufactured counterparts.

Conclusions
In the present study, we demonstrated the feasibility of printing oxide dispersionstrengthened Cu-Cr-Nb alloys by the additive manufacturing process of laser powder bed fusion.The addition of nanoscale Y2O3 results in a complex microstructure with populations of Cr-and Nb-enriched nanoparticles next to Cr-rich yttrium oxide nanoparticles, homogeneously dispersed in the metallic copper matrix.The evolution of both nanoparticle species by addition of Y2O3 led to a remarkable increase of the hardness compared to their unreinforced counterparts.Future studies will include in-depth microstructural characterization including TEM investigations to clarify the nanoparticle chemistry and extensive mechanical characterizations using tensile test and creep tests.Additionally, modifications of the alloy system towards refined oxide nanoparticles as well as the effect of higher oxide particle contents are topics for future studies.

Conclusions
In the present study, we demonstrated the feasibility of printing oxide dispersionstrengthened Cu-Cr-Nb alloys by the additive manufacturing process of laser powder bed fusion.The addition of nanoscale Y 2 O 3 results in a complex microstructure with populations of Cr-and Nb-enriched nanoparticles next to Cr-rich yttrium oxide nanoparticles, homogeneously dispersed in the metallic copper matrix.The evolution of both nanoparticle species by addition of Y 2 O 3 led to a remarkable increase of the hardness compared to their unreinforced counterparts.Future studies will include in-depth microstructural characterization including TEM investigations to clarify the nanoparticle chemistry and extensive mechanical characterizations using tensile test and creep tests.Additionally, modifications of the alloy system towards refined oxide nanoparticles as well as the effect of higher oxide particle contents are topics for future studies.

Figure 1 .
Figure 1.(a) Schematic depiction of the L-PBF machine setup.(b) Photograph of five printed specimen of Cu-3.3Cr-0.5Nb-0.5Y 2 O 3 material on a copper substrate.

Figure 4 .
Figure 4. EDS maps of cross sections of randomly selected Cu-3.3Cr-1.5Nbpowder particles in gasatomized state: (a) BSE image showing the presence of darker-appearing particles indicating local enrichment of elements with higher atomic number.EDS maps of copper (b), chromium (c) and niobium (d) confirm the local enrichment of chromium and niobium of dark-appearing particles.

Figure 4 .
Figure 4. EDS maps of cross sections of randomly selected Cu-3.3Cr-1.5Nbpowder particles in gas-atomized state: (a) BSE image showing the presence of darker-appearing particles indicating local enrichment of elements with higher atomic number.EDS maps of copper (b), chromium (c) and niobium (d) confirm the local enrichment of chromium and niobium of dark-appearing particles.

3. 2 .
Figure5provides SEM images of cross sections of the unreinforced Cu-3.3Cr-0.5Nb(Figure5a,b) and Cu-3.3Cr-1.5Nb(Figure5c,d) in the as-printed state.The microstructure is characterized by a layered structure, showing the melt pool boundaries of the printed layers, characteristic of AM printed parts[111].The printed cubic structures are found to be almost defect-free, exhibiting no cracks and low porosities, resulting in a relative density of 99.7% ± 0.2% and 99.4% ± 0.3% respectively.These results are in accordance with findings of Seltzman et al.[4,91] and Ren et al.[96], both demonstrating the defect-free manufacturing of GRCop-84 and Cu-1.93Cr-0.74Nballoy.Two homogeneously distributed populations of nanosized particles, indicated by yellow and red arrows in Figure5b,d, are observed in both unreinforced specimens, which can be distinguished by their respective size.The larger particles, which are marked by yellow arrows (Figure5b,d) have an approximate size of 100-300 nm and are found to be enriched in chromium.In contrast, the smaller nanoparticle population (red arrows) with sizes below 100 nm shows high amounts of chromium and niobium.This correlates with particle size found in L-PBF processed GRCop-84 alloy of 85-200 nm[4,91].However, due to the small size of both nanoparticle species, EDS measurements were performed on sporadically found clusters of the respective nanoparticle species and therefore large amounts of copper are detected.(Table4).The presence of Cr-rich-[14,26] and Cr-Nb-rich precipitates[22,[24][25][26][27][28] are frequently observed in Cu-Cr-Nb alloys.The latter particles may correspond to Cr 2 Nb laves phase, which is also found together with Cr-rich precipitates in laser-directed energy deposition (L-DED) processed Cu-3.4Cr-0.6Nb(at.%) alloy[88].Interestingly, no Cr-rich precipitates are observed in L-PBF processed GRCop-84 alloy[4,88], which contradicts the findings of Yang et al.[112,113] showing faster precipitation of L1 2 -structured Cu 3 Cr from pure Cr precipitates compared to Cr 2 Nb laves phases.However, it is also shown that precipitation

Figure 5 .
Figure 5. SEM images (SE mode) of Cu-3.3Cr-0.5Nballoy in low (a) and high magnification (b) showing the presence of different precipitates (red and yellow arrows).SEM images (SE mode) of Cu-3.3Cr-1.5Nballoy in low (c) and high magnification (d) showing the presence of different precipitates (red and yellow arrows).

Figure 5 .
Figure 5. SEM images (SE mode) of Cu-3.3Cr-0.5Nballoy in low (a) and high magnification (b) showing the presence of different precipitates (red and yellow arrows).SEM images (SE mode) of Cu-3.3Cr-1.5Nballoy in low (c) and high magnification (d) showing the presence of different precipitates (red and yellow arrows).

Figure 6 .
Figure 6.SEM images of oxide-reinforced Cu-3.3Cr-0.5Nballoy in low (a) and high magnification (b) showing the presence of precipitates (red arrows) and dispersoids (yellow arrows).SEM images of oxide-reinforced Cu-3.3Cr-1.5Nballoy in low (c) and high magnification (d) showing the presence of precipitates (red arrows) and dispersoids (yellow arrows).

Figure 6 .
Figure 6.SEM images of oxide-reinforced Cu-3.3Cr-0.5Nballoy in low (a) and high magnification (b) showing the presence of precipitates (red arrows) and dispersoids (yellow arrows).SEM images of oxide-reinforced Cu-3.3Cr-1.5Nballoy in low (c) and high magnification (d) showing the presence of precipitates (red arrows) and dispersoids (yellow arrows).

Table 1 .
Nominal chemical composition of the Cu-Cr-Nb powder materials in wt.%.

Table 1 .
Nominal chemical composition of the Cu-Cr-Nb powder materials in wt.%.

Table 2 .
Process parameters for L-PBF processing of Cu-Cr-Nb samples and their reinforced counterparts.

Table 4 .
Chemical composition (in wt.%) of unreinforced specimens determined by EDS.Values are averaged over 12 single-point measurements on occasionally found clusters or larger particles of the respective nanoparticle species.

Table 5 .
Chemical composition (in wt.%) of reinforced specimens determined by EDS.Values are averaged over 12 single-point measurements on occasionally found clusters or larger particles of the respective nanoparticle species.

Table 5 .
Chemical composition (in wt.%) of reinforced specimens determined by EDS.Values are averaged over 12 single-point measurements on occasionally found clusters or larger particles of the respective nanoparticle species.