On the prediction of lattice parameter vs. concentration for solid solutions extended by rapid quenching from the melt

Al-2.85 wt% Fe alloy has been subjected to non-equilibrium container-less solidification using a 6.5 m drop tube. Spherical samples were collected and sieved into 7 sizes fractions ranging from 850 μm to 53 μm, with the estimated cooling rates being 150 to 11000 K s^-1 respectively. XRD analysis was employed on all droplet size fractions for identification and evolution of the phases, showing that Al, Al₆Fe and Al₁₃Fe₄ were formed for all sizes while Al₅Fe₂ was observed only in droplets ≤ 150 μm in diameter. Microstructural evaluation was conducted by using SEM and optical microscopy, showing that droplet larger than 300 µm in diameter exhibited distinct morphologies; microcellular, dendritic α-Al with inter-dendritic Al₁₃Fe₄ eutectic and an Al-Al₆Fe eutectic region. With increasing cooling rate, the Al-Al₆Fe region disappears. EDX analysis reveals that increasing the cooling rate increased the dissolved Fe content in α-Al from 0.37 wt% Fe to 1.105 wt% Fe, and correspondingly the eutectic fraction decreased from 49.7 vol.% to 26.7 vol.%. Measurement of the lamellar spacing allowed the eutectic growth velocity and interfacial undercooling to be calculated, wherein the Al-rich boundary of the eutectic coupled zone could be reconstructed. This shows a coupled zone skewed significantly towards the intermetallic side of the eutectic. In order to understand the effect of non-equilibrium the solidification on the mechanical properties micro hardness of the droplets was measured. The micro-hardness has risen from 55.3 HV_0.01 to 66.5 HV_0.01 for $\ge 850\mu \mathrm{m}$ and $\le 75\mu \mathrm{m}$ droplets, respectively.

The present study addressed the change in the microstructure of Al–2.5 wt% Fe binary alloy produced using laser powder bed fusion (L-PBF) technique by thermal exposure at 300 °C, and the associated mechanical and thermal properties were systematically examined as well. Multi-semi-cylindrical patterns corresponding to melt pools in the microstructure were macroscopically observed for the as-manufactured sample. No change in the melt-pool morphology was observed after thermal exposure for 1000 h. Inside the melt pools, a large number of the nanoscale metastable Al₆Fe phase particles were uniformly distributed inside columnar grains of the α-Al matrix containing concentrated solute Fe in supersaturation. The sequential formation and coarsening of stable θ-Al₁₃Fe₄ phases were observed upon exposure to a 300 °C environment, but a considerable amount of nano-sized metastable Al₆Fe phases remained even after 1000 h. Furthermore, the thermal exposure continuously reduced the concentration of solute Fe atoms in the α-Al matrix. No significant grain growth was found in α-Al matrix after 1000 h owing to the pinning effect of the dispersed fine particles on grain boundary migration. These results demonstrate a sluggish change in microstructural morphologies of the Al–2.5 wt% Fe alloy. The quantified microstructural parameters addressed dominant strengthening contributions by the solid solution of Fe element and Orowan strengthening mechanism by fine Al–Fe intermetallics in the L-PBF-produced alloy. The high strength level was sustained even after being exposed to 300 °C for long periods. The superior balance of mechanical properties and thermal conductivity can be achieved in the experimental alloys by taking advantage of the various microstructural parameters related to the Al–Fe intermetallic phases and α-Al matrix.

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  This study focused on additive manufacturing (AM) of the Al–Fe binary alloy samples with a near-eutectic composition of 2.5 mass% Fe using the laser powder bed fusion (LPBF) process. The melt pool depth, relative density, and hardness of LPBF-fabricated Al–2.5Fe alloy samples under different laser power (P) and scan speed (v) conditions were systematically examined. The results provided optimum laser parameter sets (P = 204 W, v ≤ 800 mm s⁻¹) for the fabrication of dense alloy samples with high relative densities >99%. Additionally, Pv^-1/2, which is based on the deposited energy density model, was found to be a more appropriate parameter for additively manufacturing Al–2.5Fe alloy samples, and using it to simplify the relative densities of the samples made the determination of a threshold value for the laser parameters required to fabricate dense alloy samples. The microstructural and crystallographic characterization of the LPBF-built Al–2.5Fe alloy samples revealed a characteristic microstructure consisting of multi-scan melt pools that resulted from local melting and rapid solidification owing to laser irradiation during the LPBF process. Furthermore, a number of columnar grains with a mean width of $\sim$21 μm elongated along the building direction were also observed in the α-Al matrix. Numerous nano-sized particles of the metastable Al₆Fe intermetallic phase with a mean size <100 nm were finely dispersed in the α-Al matrix. The hardness of the refined microstructure produced by the LPBF process was high at $\sim$90 HV, which is more than twofold higher than that of conventionally casted alloys that contain the coarsened plate-shaped Al₁₃Fe₄ intermetallic phase in equilibrium with the α-Al matrix.

Mixtures of high-purity elemental powders of Al with an Fe content of 10 wt% were consolidated at room temperature by means of High-Pressure Torsion (HPT) and subsequently deformed to high levels of strain. Relative density of the consolidated disks was >99% even at low strains. The microstructure observation by transmission electron microscopy revealed that the alloy consisted of an ultrafine grained Al matrix with an average grain size of 145 nm. Supersaturation of Fe was estimated in the matrix to a maximum of ∼1 wt% Fe by X-ray diffraction. The dispersion of secondary Fe-rich particles was quantified by scanning electron microscopy and image processing techniques. Hardness and tensile strength were improved significantly with straining by HPT, in direct correlation with grain refinement and potentiated by dissolution of Fe particles in the matrix. Excellent compromise of strength and ductility was achieved at intermediate levels of imposed strain.

In the present work the structure and morphology of the phases of nanocomposites formed in rapidly solidified Al–Fe alloys were investigated in details using analytical transmission electron microscopy and X-ray diffraction. Nanoquasicrystalline phases, amorphous phase and intermetallics like Al₅Fe₂, Al₁₃F₄ coexisted with α-Al in nanocomposites of the melt spun alloys. It was seen that the Fe supersaturation in α-Al diminished with the increase in Fe content and wheel speed indicating the dominant role of the thermodynamic driving force in the precipitation of Fe-rich phases. Nanoquasicrystalline phases were observed for the first time in the dilute Al alloys like Al–2.5Fe and Al–5Fe as confirmed by high resolution TEM. High hardness (3.57 GPa) was measured in nanocomposite of Al–10Fe alloy, which was attributed to synergistic effect of solid solution strengthening due to high solute content (9.17 at.% Fe), dispersion strengthening by high volume fraction of nanoquasicrystalline phase; and Hall–Petch strengthening from finer cell size (20–30 nm) of α-Al matrix.

Nanocrystalline Al–5 at.% Fe alloy powders produced by mechanical alloying were consolidated by spark plasma sintering. The sintered sample showed high strength >1000 MPa with a large plastic strain of 15% at room temperature and 500 MPa at 350 °C. Microstructure characterizations by transmission electron microscopy and atom probe tomography revealed that the sintered samples are composed of α-Al and Al₆Fe nanocrystalline regions with 90 nm in diameter and a minor fraction of Al₁₃Fe₄ phase and coarsened 0.5–1 μm α-Al grains. This bimodally grained feature is attributed to the relatively large plastic strain for the strength level of 1000 MPa at room temperature.