Additive manufacturing of an ultrastrong, deformable Al alloy with nanoscale intermetallics

Aluminum alloys are widely used in aerospace and automotive applications, and selective laser melting (SLM) offers design flexibility for complex parts. However, many high-strength Al alloys (e.g., AA6061, AA7075) suffer from hot tearing and solidification cracking during additive manufacturing, limiting their processability. Prior AM success has focused on near-eutectic Al–Si and Al–Si–Mg systems that have moderate strength but good hot-cracking resistance. Strategies to mitigate hot cracking include introducing fine, hard particles (e.g., TiN, TiC, TiB₂) or age-hardening precipitates (e.g., Al₃Zr, Al₃Sc, Al₂Cu) to refine grains and strengthen the matrix by impeding dislocations. Despite these advances, peak strengths in AM Al alloys typically remain around 300–500 MPa, with sporadic higher strengths via severe plastic deformation routes that rely on extreme grain refinement. Transition-metal intermetallics (Al–Fe, Al–Co, Al–Ni) are often avoided due to brittleness when coarsely formed in casting (e.g., Al₃Co₂, Al₁₃Fe₄). This work explores whether nanoscale, laminated intermetallics based on Co, Fe, Ni, and Ti can be engineered during LPBF to introduce heterogeneous microstructures that deliver both very high strength and significant plasticity.

• AM of Al alloys has primarily focused on Al–Si and Al–Si–Mg systems due to their hot-tearing resistance and processability, though with medium strengths. High-strength wrought-type Al alloys (AA6061, AA7075) show high hot-cracking susceptibility in LPBF.
• Particle inoculation and precipitation strategies (TiN, TiC, TiB₂; Al₃Zr, Al₃Sc, Al₂Cu) can refine grains, impede dislocations, and reduce columnar grains where cracks initiate.
• Achieved strengths in AM Al alloys are typically 300–500 MPa; higher strengths have been reported via severe plastic deformation or cryo-milling with consolidation, owing to nanoscale grain refinement.
• Transition-metal-bearing intermetallics (Al–Fe, Al–Co, Al–Ni) are generally avoided due to brittleness in cast alloys, though small Fe or Ni additions can increase strength; plasticity and mechanisms remained unclear.
• Rosette-like intermetallic morphologies and complex intermetallics have been reported in certain Al–Ce–Mn AM systems, but associated deformation mechanisms were largely unexplored.

Alloy design and powder: Gas-atomized spherical powder with nominal composition Al₉₂Ti₂Fe₂Co₂Ni₂ (at.%) and particle size −53 +15 µm.

Additive manufacturing (LPBF): SLM 125 HL (IPG fiber laser, λ=1070 nm) in Ar atmosphere (O₂ < 1000 ppm). Parameters: laser power 200–300 W (up to 400 W available), scan speed 1200 mm/s, hatch spacing 100 µm, layer thickness 30 µm, laser spot ~70 µm, 67° layer rotation, build plate preheated to 200 °C. Cylinders (Ø6 × 12 mm) for bulk compression; cubes (10 × 10 × 5 mm) for microstructure, nanoindentation, and micropillar testing.

Structural characterization: XRD (PANalytical Empyrean X'pert PRO MRD, Cu Kα₁, 2θ–ω), SEM (Thermo Fisher Quanta 3D and Teneo FE-SEM, BSE, 30 kV), TEM/STEM/EDS (Thermo Fisher Talos 200X, 200 kV). Crystal orientation mapping with NanoMEGAS ASTAR. APT specimen prep by FIB (Scios 2 DualBeam), tip radius ~50 nm. APT on CAMECA LEAP 5000XS in voltage mode (20% pulse fraction, 50 K, 200 kHz) and laser mode (50 K, 200 kHz, 80 pJ). AP Suite 6.1 for reconstruction/analysis.

Mechanical testing: Nanoindentation (Bruker Hysitron TI Premier, Berkovich tip, displacement control to 800 nm). Mapping over 100 × 100 µm² area with 121 indents (10 µm spacing); progressive multi-segment loading-unloading; hardness and modulus from averages of 10 measurements. Bulk compression: MTS frame, 30 kN load cell, strain rate 1e−3 s−1; ends polished and leveled. In situ micropillar compression in SEM (Quanta 3D) with Hysitron PI 88×R PicoIndenter; pillars FIB-fabricated, height 10 µm, diameter 5 µm (aspect ratio 2:1); flat punches 10 and 20 µm; strain rate 5×10−3 s−1; drift correction 0.2–0.6 nm/s. Post-deformation XTEM prepared from compressed pillars for mechanism analysis.

• Microstructure: LPBF produced heterogeneous melt pool microstructures with interwoven tracks and melt pools ~120 µm wide, ~80 µm deep. Rosette colonies of nanolaminated intermetallics were embedded in an Al-rich matrix. Near melt pool boundaries: fine rosettes (97 vol.%) separated by thin Al (3 vol.%); in melt pool centers: coarse rosettes (36 vol.%), cellular intermetallics (4 vol.%), and Al matrix (60 vol.%).
• Rosette morphology and phases: Fine rosettes have Al₃Ti cores with alternating Al₃Ti and medium-entropy Al₃(Fe,Co,Ni)₂ lamellae, lamella thickness ~20–60 nm (Al₃Ti thinner than Al₃(Fe,Co,Ni)₂). Coarse rosettes show similar chemistry with thicker lamellae ~150–300 nm; cellular boundaries enriched in Al₃(Fe,Co,Ni)₂. Three major phases identified: Al matrix, Al₃Ti (mostly DO₂₂ with some L1₂), and monoclinic Al₃(Fe,Co,Ni)₂ (prototype Al₃Co₂ or Al₃FeNi). Orientation relationship between Al₃Ti and Al₃(Fe,Co,Ni)₂ with low lattice mismatch (~2.1–2.5%).
• Solute distribution (APT): TM solutes in Al grains are low (<1 at.%), with some grain boundary enrichment. Across Al/Al₃(Fe,Co,Ni)₂ interfaces, matrix contains minute solutes (e.g., ~0.20% Ti, 0.30% Fe, 0.03% Co, 0.09% Ni at%), while intermetallic is nearly equiatomic in Fe/Co/Ni (example: 78.06% Al, 0.06% Ti, 5.45% Fe, 7.53% Co, 8.90% Ni). RDFs near unity indicate random distribution (no clustering) in the monoclinic phase.
• Mechanical properties (nanoindentation): Hardness 2.5–4.5 GPa; higher near melt pool boundaries (3.5–4.5 GPa) versus interiors (2.5–3.0 GPa). Young’s modulus 130–150 GPa (higher near boundaries).
• Bulk compression: Cylinders (200 W build) showed ultra-high engineering stress >800 MPa with ~20% compressive strain and ductile barreling; at 250–300 W, flow stress ~550 MPa with 5–20% strain. Analysis indicates uniform deformation up to ~7% true strain (Considère’s criterion) followed by work hardening.
• Micropillar compression: Fine rosette regions reached true flow stress ~1 GPa; coarse rosette regions ~500 MPa. Fine rosette pillars showed wrinkled, wavy surfaces and more uniform co-deformation; coarse rosette pillars exhibited protrusion of rosette precipitates, indicating preferential plastic flow in Al matrix.
• Back stress (HDI): Pronounced hysteresis loops; back stress increased with strain then saturated. Fine rosette regions carried higher back stress (−600 MPa) than coarse (−250 MPa), indicating strong HDI hardening from heterogeneity.
• Post-deformation mechanisms: In coarse rosettes, microcracks formed within Al₃Ti and Al₃(Fe,Co,Ni)₂ but remained confined to single lamellae; abundant dislocations in Al near cellular walls. In fine rosettes, significant crystal rotation and planar defects in Al₃(Fe,Co,Ni)₂; abundant stacking faults on (001), dislocations aligned along SFs, extra diffraction spots suggesting possible local phase changes; low-angle grain boundary (~5°) near (110) observed.
• Strengthening contributions: Solid solution and dislocation strengthening are minor (estimated 25–36 MPa from as-printed dislocation density 4.7×10¹³–1.0×10¹⁴ m⁻²); Orowan secondary. Dominant contributions from HDI back stress due to strain incompatibility across Al–intermetallic and intermetallic–intermetallic interfaces, aided by refined nanoscale lamellae and good lattice matching promoting nanolaminate formation.
• Metastability: Rapid solidification retained some L1₂ Al₃Ti and a partitioned medium-entropy Al₃(Fe,Co,Ni) phase that would decompose at equilibrium, potentially altering mechanical response and enabling defect activity in otherwise brittle intermetallics.

The study addresses the challenge of producing high-strength, deformable AM Al alloys by engineering heterogeneous nanoscale intermetallic rosettes within an Al matrix. Rapid solidification during LPBF promotes Al₃Ti cores and alternating Al₃Ti/Al₃(Fe,Co,Ni)₂ lamellae with low lattice mismatch, refining features to 20–60 nm near melt pool boundaries. This architecture produces strong heterogeneity-driven strain gradients during deformation, generating large hetero-deformation-induced (HDI) back stress that elevates strength and sustains work hardening. Micropillar tests reveal higher flow stresses and back stress in fine rosette regions, consistent with enhanced co-deformation and constrained shear banding. Post-deformation TEM shows confined microcracking in coarse rosettes and unexpected plasticity in monoclinic Al₃(Fe,Co,Ni)₂ via abundant (001) stacking faults and dislocations, suggesting nanoscale confinement and metastability enable defect activity in phases typically brittle in bulk. Together with dislocation plasticity in Al and improved fracture resistance due to laminated interfaces, these mechanisms reconcile high strength (>800 MPa engineering stress in bulk; ~1 GPa true stress in fine rosette pillars) with notable plasticity (~15–20% compression). The results demonstrate that nanoscale laminated medium-entropy intermetallics and heterogeneous microstructures can overcome hot-cracking constraints of conventional high-strength Al alloys and offer a viable route to ultrastrong, deformable AM Al.

A custom Al₉₂Ti₂Fe₂Co₂Ni₂ alloy fabricated by LPBF develops heterogeneous rosette microstructures comprising nanoscale laminated Al₃Ti and medium-entropy Al₃(Fe,Co,Ni)₂ intermetallics. The alloy achieves >800 MPa engineering compressive strength with ~20% plasticity in bulk, and ~1 GPa true flow stress with ≥15% plasticity in fine rosette micropillars. Superior performance arises primarily from HDI back stress generated at hetero-interfaces, co-deformation of matrix and intermetallic nanolaminates, and unexpected defect-mediated plasticity (stacking faults and dislocations) in monoclinic Al₃(Fe,Co,Ni)₂. Rapid solidification aids formation and retention of refined, partially metastable phases with favorable lattice matching. This strategy provides a pathway to design ultrastrong, deformable AM Al alloys via controlled nanoscale intermetallic architectures. Future work should clarify the atomistic origins of stacking fault formation and phase stability in the monoclinic intermetallics and assess tensile behavior and processing–structure–property relationships across broader build conditions.

• Mechanical behavior was primarily assessed under compression; tensile properties and fracture toughness were not reported.
• The mechanistic origin of stacking fault formation and defect activity in monoclinic Al₃(Fe,Co,Ni)₂ remains unresolved and is deferred to future modeling.
• Performance depends on heterogeneous microstructures that vary across melt pools; process-parameter sensitivity (e.g., laser power) affects strength, indicating potential variability.
• Observed microcracks within intermetallic lamellae and potential interface debonding suggest limits to damage tolerance at higher strains or different loading modes.
• Study focuses on a single alloy composition and a limited LPBF parameter window (200–300 W), which may limit generalizability without broader optimization.