Accelerated development of soft magnetic materials is vital for addressing the challenges associated with improving the performance of electrical machines, transformer cores, electric vehicles etc. A ...combinatorial assessment of the structural, magnetic and mechanical properties of Co100-xFex (x = 30 to 70) and Ni100-xFex (x = 30 to 70) alloys has been carried out on samples fabricated via laser additive manufacturing (AM). Co100-xFex showed a bcc structure in the composition range studied while Ni100-xFex exhibited either single phase fcc or a mixture of fcc and bcc phases, depending on the composition. The saturation magnetization (Ms) for both Co100-xFex and Ni100-xFex compositionally graded alloys increases monotonically with increasing Fe content while the coercivity (Hc) variation is not monotonic. The Ms value of 199.3 emu/g for Co70Fe30 increases to 248 emu/g for Co30Fe70 alloys. Ni70Fe30 exhibits a Ms of 119.8 emu/g which increases to 168.7 emu/g for Ni30Fe70. The peak hardness is 260 VHN for the Co100-xFex series and 160VHN for the Ni100-xFex series. Such AM processed graded magnetic materials can be used for accelerated experiments to discover novel materials.
•Additive manufacturing was used to fabricate functionally graded soft magnetic materials Co100-xFex and Ni100-xFex.•Co100-xFex showed a bcc structure while Ni100-xFex exhibited either single phase fcc or a mixture of fcc and bcc phases.•Coercivity of Co100-xFex exhibits a non-monotonic change with composition.•Saturation magnetization increases monotonically with Fe content in both Co100-xFex and Ni100-xFex.
The graphene nanoplatelet (GNP) reinforced nickel matrix composites (Ni-GNP) have been processed using two different ball milling approaches, viz, dry ball milling (DM) and solution ball milling ...(SBM), followed by consolidation using spark plasma sintering (SPS) technique. The composites were reinforced with varying GNP concentration (0.5–2 wt%) and were milled for up to 12 hr to investigate the effect of premixing technique, milling duration, and GNP concentration on the grain size, microstructure, the dispersion of GNP in the nickel matrix, and mechanical behavior of these composites. Ni-GNP nanocomposites exhibited improved microhardness and tensile strength compared to pure nickel, primarily attributed to grain refinement and load transfer strengthening due to the uniform dispersion of these GNPs within the nickel matrix, promoting effective load transfer during tensile deformation. Ni-0.5GNP composites processed via dry milling followed by SPS exhibited the highest tensile yield strength of 586 MPa as compared to pure nickel and other Ni-GNP composites. The contribution of each strengthening mechanism in the overall improvement in yield strength of Ni-GNP composites has been qualitatively calculated/quantified and compared with experimentally obtained tensile properties.
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The temperature-dependent from room temperature (RT) to 500°C nanoindentation behavior of a precipitation-strengthened Al
0.3
Ti
0.2
Co
0.7
CrFeNi
1.7
high-entropy alloy (HEA) processed via two ...different additive manufacturing (AM) techniques was investigated in the as-deposited and annealed conditions. The hierarchically heterogeneous microstructures were achieved via simple one-step annealing treatments, exploiting the residual stresses in the AM-processed HEA to partially recrystallize the microstructure, performed remarkably better than the nearly homogeneous microstructures in the as-deposited state. The one-step annealed conditions revealed < 6.6% reduction in hardness values at 500°C compared to RT, while the as-deposited conditions showed a > 18% reduction in the hardness. The one-step annealed conditions also exhibited significantly higher hardness than the as-deposited conditions owing to their L1
2
-strengthened FCC microstructures. Furthermore, serrated yielding or the Portevin-Le Chatlier effect indicative of
microstructural instability
was observed during nanoindentation deformation (at 500°C) for the as-deposited conditions but not for the one-step annealed conditions. This, therefore, signifies the
robustness of the hierarchically heterogeneous microstructures at elevated temperatures
presenting a strong avenue for tuning the HEAs for future nuclear reactor applications.
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A precipitation strengthenable high entropy alloy (HEA), Al0.3CoCrFeNi, was processed via laser-based additive manufacturing (AM), using the laser engineered net shaping (LENS) ...process. The as LENS processed HEA exhibited twice the tensile yield strength, as compared to the conventionally arc-melted and solution treated HEA of the same composition, with a tensile ductility greater than 20%. Subsequent heat-treatments of the AM HEA alloy led to further enhancement of the yield strength while maintaining good tensile ductility. The microstructure of these AM alloys was investigated by coupling transmission electron microscopy (TEM) and atom probe tomography (APT). The near doubling of the yield strength in case of the as AM processed HEA samples, which were devoid of second phase intermetallic precipitates, has been rationalized based on the formation of nanometer-scale Al–Ni rich solute clusters due to the re-heating of the deposited layers during AM. The enhanced yield strength due to these solute clusters has been estimated using a simple cluster-dislocation interaction model involving the coherency strain fields of these nano-clusters. The even higher yield strength in case of the heat-treated AM HEA samples has been quantitatively rationalized employing precipitation strengthening models, based on nanometer scale L12 (gamma prime) precipitates.
•Ni-CNT composites processed via dry milling / solution ball milling methods.•Ni-CNT composites exhibited significant improvement in the yield strength.•Ni-0.5CNT composites exhibited excellent ...combination of strength & ductility.•All possible Ni-CNT composites strengthening mechanisms were discussed.
The multi-walled carbon nanotubes (CNTs) reinforced nickel matrix nanocomposites (Ni-CNT) have been processed via ball milling followed by spark plasma sintering (SPS) process. The CNT content in these nanocomposites has been varied from 0.5 to 2 wt% (approximately from 2 to 8 vol%) to study their effect on the dispersion, microstructure, and mechanical behavior of these composites. Two Ni-CNT composite powders pre-mixing techniques have been employed, namely dry milling (DM) and solution ball milling (SBM), to investigate their effect on the dispersion of CNTs within a nickel matrix. The Ni-CNT powder was milled for different durations (1,2,6 and 12 hrs.) to investigate the milling effect on the grain size and the dispersion of CNTs in the nickel matrix. Ni-CNT nanocomposites exhibited improvement in microhardness and mechanical performance in comparison with pure nickel. Ni-1CNT-DM composites exhibited an excellent combination of the tensile yield strength of 455 MPa and around 13% elongation. This improvement in Ni-CNT nanocomposites is primarily attributed to the uniform dispersion of reinforcement within the nickel matrix, refined grain size, and strong nickel CNT interfacial bonding, which effectively transfers stress during tensile deformation. Various strengthening mechanisms associated with CNT-metal matrix composites have been discussed in detail. We have attempted to quantify the contribution of these strengthening mechanisms using micromechanical models.
A novel porosity-free in situ Ti–B4C composite was fabricated via spark plasma sintering (SPS). Detailed analysis of the phase evolution, coupling results from XRM, XRD and SEM-EBSD-EDS, indicate ...that primarily TiB2 precipitates formed due to the in-situ reactions between the boron-carbide and titanium powders. The precipitation of TiB2 resulted in the formation of a graphitic C-rich thin layer circumscribing the partially reacted B4C particles. Further progression of the in-situ reaction leads to the out-diffusion of the excess carbon from the B4C particles, across the graphite and TiB2 phases, forming TiC upon reacting with the titanium matrix. Therefore, the final microstructure primarily consisted of TiB2, TiC, and partially reacted B4C phases, with small amounts of TiB, α-Ti, and graphite. Furthermore, the microstructure in these SPS processed in situ composites appeared to be the product of a solid-state Ti–B4C diffusion couple, substantially different from their fusion-based additively manufactured counterparts. Nano-indentation tests revealed a remarkably high average hardness of ∼25 GPa for this SPS-processed Ti–B4C composite and comparable (with literature) phase-specific hardness and modulus values for the constituent TiB2, TiC, and B4C phases.
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•Precipitation strengthenable high entropy alloy printed using laser powder bed fusion.•Additive manufacturing process resulted in distinct solidification features.•Yield stress increased by ∼60% due ...to higher L12 phase fraction after annealing.
While additive manufacturing (AM) of single-phase solid solution based high entropy alloys (HEAs) has attracted a lot of recent attention, such processing of precipitation strengthenable multi-phase HEAs has not been actively investigated. This becomes important since AM processing results in each layer experiencing rapid solidification followed by multiple heating-cooling cycles, mimicking in situ heat treatments, potentially influencing the phase evolution. A precipitation strengthenable face centered cubic (FCC) based HEA, Al0.3Ti0.2Co0.7CrFeNi1.7, was printed using laser powder bed fusion (LPBF), resulting in predominantly FCC grains with internal solidification cells, and small pockets of a mixed BCC+B2 phase microstructure in the inter-cellular regions. Early stages of ordered L12 precipitation was also observed within the FCC grains, containing a large fraction of low angle sub-grain boundaries. While the as-built alloy exhibited a yield strength of ∼600 MPa with 15% ductility at room temperature, after ex situ annealing at 800 °C for 5 hrs, the yield strength increased to ∼950 MPa.
Direct laser deposition of a mixture of titanium (Ti) and boron-carbide (B4C), using the laser engineered net shaping process, results in a complex multi-phase Ti matrix composite due to the ...incipient interfacial reactions between liquid titanium and solid/partially melted B4C. Detailed microstructural characterization, at multiple length scales, reveals that titanium boride (TiB) precipitates are the initial reaction product which act as heterogeneous nucleation sites for TiC precipitates. Both the TiB and TiC precipitates in turn act as heterogeneous nucleation sites for fine β-Ti grains, within the retained liquid titanium, which eventually transform into fine equiaxed α-Ti grains comprising the matrix.
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Nickel matrix composites reinforced with graphene exhibiting different morphology (i.e., shape and size) with varying content (0.5 and 1 wt.%) have been prepared using ball-milling and the spark ...plasma sintering technique. The graphene-based reinforcements used in this study were procured from different sources, processed using different techniques, and varied in shape, size, and structural features. The effect of graphene morphology on the microstructure, mechanical properties, and tribological behavior of these composites has been investigated. The results reveal excellent enhancement in the tensile yield strength, reduction in grain size, and coefficient of friction of the composites after incorporating the graphene-based reinforcing material in the nickel matrix. The influence of different types of reinforcement varying in size, surface, and structure on their dispersal and the resultant effect on the properties are discussed. Scanning electron microscopy and energy dispersive spectroscopy were used to investigate the fracture surface and wear tracks for further investigation.
Direct laser deposition of a mixture of 90 wt.% nickel (Ni) and 10 wt.% niobium-carbide (NbC) using the laser engineered net shaping process resulted in the formation of an in-situ Ni-NbC composite ...with a complex hierarchical multi-phase microstructure. Detailed microstructural characterization at multiple length scales revealed that the in-situ composite consisted of a primary dendritic Ni-rich γ phase with an interdendritic γ + NbC eutectic. The interdendritic eutectic γ phase further decomposes via discontinuous precipitation into a lamellar mixture of γ + δ phase (Ni3Nb). The sequence of phase evolution in this complex hierarchical in-situ composite system has also been established in the present study.
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