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Research on powder-based additive manufacturing of aluminium alloys is rapidly increasing, and recent breakthroughs in printing of defect-free parts promise substantial movement beyond traditional ...

The effects of adding 0.08 at.% Cr or 1 at.% Zn to a dilute Al-0.11Zr-0.005Er-0.02Si at.% alloy are studied in terms of the precipitation behavior of Al3Zr (L12-structure) nanoprecipitates and the resulting alloy's creep resistance. Although Cr and Zn additions do not affect measurably the precipitation kinetics or coarsening resistance, the modified alloys exhibit changes in dislocation creep resistance at 300 °C: the creep threshold stress is decreased by 17% (2 MPa) in the Zn-modified alloy and increased by 25% (3 MPa) in the Cr-modified alloy. This is attributed to a modification of the lattice parameter of Al3Zr(L12), which affects the ease with which matrix dislocations climb over the nanoprecipitates. The Zn-modified alloy exhibits Al3Zr(L12) nanoprecipitates containing 6–7 at.% Zn, as determined by atom-probe tomography, which reduces the lattice parameter by 0.17% as a result of Zn substituting for Al on its sublattice, as calculated utilizing density functional theory. The Al3Zr(L12) nanoprecipitates in the Cr-modified alloy contain 0.10–0.20 at.% Cr (1.2–2.5 times more than the matrix) and 0.28–0.51 at.% Er (a 60- to 100-fold enrichment). Erbium, which increases the lattice parameter misfit of Al3Zr(L12) with the Al matrix, is confirmed to be particularly potent in increasing the creep resistance of aluminum alloys containing L12 nanoprecipitates. An alloy design methodology for creep resistance is also validated, whereby precipitate compositions measured by APT are input to DFT calculations to determine their effects on lattice parameter misfit.

Strengthening mechanisms at ambient and elevated temperatures are studied in a cast Al-0.11Zr-0.02Si-0.005Er (at.%) alloy with a 2.86 at.% Ni addition, containing: (i) incoherent Al3Ni microfibers formed during eutectic solidification; and (ii) coherent, equiaxed Al3Zr (L12-structure) nanoprecipitates created on subsequent aging. Strengthening contributions from microfibers and nanoprecipitates are cooperative at ambient temperature, over the full range of Al3Zr precipitation during under-, peak-, and over-aging states. In contrast, during compressive creep testing at 300 °C, the binary eutectic Al-Al3Ni alloy is not further strengthened by the Al3Zr nanoprecipitates, reflecting their lower number density (5.8 × 1022 m−3) in the regions between Al3Ni microfibers, where load transfer and/or microfiber/dislocation interactions provide strengthening. Also, when the Al-0.11Zr-0.02Si-0.005Er (at.%) alloy is modified with very low Ni concentrations of 0.07 at.%, without Al3Ni microfiber formation, the precipitation kinetics of Al3Zr(L12) are unaffected and negligible amounts of Ni are measured in the nanoprecipitates. The binary Al-2.86Ni at.% alloys with Al3Ni eutectic microfibers, with and without Al3Zr nanoprecipitates, are significantly more creep resistant at 300 °C than dilute Al-Sc or Al-Zr alloys strengthened solely by Al3Zr or Al3Sc nanoprecipitates. Unlike Al-Zr alloys, their upper service temperature is, however, limited to ∼400 °C, above which Al3Ni coarsening becomes rapid.

Dilute Al Sc alloys ( Ni alloys (Al-6 wt.% Ni) derive their high strength at elevated temperature from Al3Ni microfibers formed during solidification. Here, we investigate ternary Al 6Ni-0.2Sc and Al 6Ni-0.4Sc alloys with both types of strengthening precipitates (Al3Sc and Al3Ni) and compare them to binary Al 6Ni, Al-0.2Sc and Al-0.4Sc alloys with a single population of precipitates. Kinetics of Al3Sc and Al3Ni precipitation and coarsening are studied via hardness measurements during isochronal and isothermal aging; the two phases resist coarsening up to 475 and 350 °C, respectively, upon short term exposures (1 h isochronal steps). No noticeable effect of Sc is observed on the Al3Ni micro-fiber composition and hardening in the alloy. Similarly, Ni does not significantly affect the hardening provided by the Al3Sc precipitates, despite the presence of 0.14–0.17 at.% Ni in the Al3Sc nano-precipitates. The strengthening contributions of the Al3Ni and Al3Sc phases at ambient temperature are cumulative in the ternary alloys. For creep deformation at 300 °C, all alloys show a creep threshold stress, indicating that both types of precipitates impede dislocation motion. The ternary Al Ni Sc alloys show higher creep threshold stresses than their binary counterparts, also consistent with cumulative strengthening effects of precipitation strengthening (from Al3Sc nano-precipitates) and load transfer (from Al3Ni micro-fibers).

The precipitation behavior of an Al-0.08Zr-0.08Hf-0.045Er at.% alloy with micro-additions of Si and/or Fe was investigated using microhardness and electrical conductivity measurements in conjunction with scanning electron microscopy and atom-probe tomography. Hardening is achieved through the formation of a high number density (~1023 m−3) of coarsening-resistant, nanoscale L12 trialuminide precipitates containing Zr, Hf, Er, and Si. Simultaneous additions of 300 at. ppm Si and 400 at. ppm Fe produce an alloy with the fastest precipitation kinetics and highest microhardness after homogenization at 640 °C for 24 h followed by 90 days aging at 350 °C, due to: (i) scavenging of Er by Fe in the form of primary precipitates, thus reducing Er-stimulated precipitation of coarse Zr- and Hf-rich precipitates during homogenization; and (ii) the accelerating effects of Si on the precipitation kinetics of the nanometric L12 trialuminide. Removal of the homogenization step results in accelerated precipitation kinetics during aging due to an increased supersaturation of L12-forming elements, Zr, Hf, and Er. During isothermal aging of a non-homogenized Al-0.08Zr-0.08Hf-0.045Er-0.03Si-0.04Fe (at.%) alloy at 400 °C, a peak microhardness of 500 MPa is maintained for up to 90 days. Atom-probe tomography displays a high number density of nanometric L12 precipitates with an Er-rich core and homogeneously distributed Zr and Hf, with Hf concentrations ~1.5 times higher at the matrix/nanoprecipitate heterophase interface than in the core (~5 vs. ~3.5 at.%). The presence of Hf in the nanoprecipitates does not, however, affect their precipitation kinetics or coarsening resistance.

An Al–Cu alloy micro-alloyed with Mn and Zr (ACMZ) was examined to understand the thermal stability and strengthening mechanism of metastable θ'-Al2Cu precipitates with interfacial segregation after prolonged thermal exposure. The microstructure was characterized at multiple scales with techniques including synchrotron x-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy, and atom probe tomography. The θ' precipitates did not exhibit measurable coarsening after thermal exposure at 300°C for 5000 h. Kinetic effects of Mn and Zr interfacial segregation, which dominate over thermodynamic effects under these conditions, were necessary to understand the complete inhibition of precipitate coarsening. The θ' phase fraction was stable during the 5000 h exposure. This stable phase fraction was regarded as the metastable equilibrium value and was smaller than that predicted by the θ' solvus line of the ACMZ alloy. As expected from the observed phase stability, the alloy hardness also remained stable during the 5000 h exposure. An Orowan mechanism alone was inadequate to explain θ' precipitate strengthening. Additional strengthening mechanisms by θ' precipitates specifically related to the transformation strain may explain the observed hardness values.

The compressive creep behavior of hot-pressed Mg1.96Al0.04Si0.97Bi0.03, a promising thermoelectric material, is investigated at 500 °C. At stress levels between 81 and 212 MPa, dislocation creep with stress exponent n = 7.6 ± 0.3 is observed. No diffusional creep is observed, likely attributable to a dispersion of ~1 μm Bi-, Al-, and O- rich particles which pin grain boundaries. Mg1.96Al0.04Si0.97Bi0.03 exhibits similar creep behavior to previously studied silicides, but is significantly more creep resistant than other thermoelectric materials, PbTe and Bi2Te3. This makes Mg1.96Al0.04Si0.97Bi0.03 an excellent material for thermoelectric power generation systems subjected to high stresses and temperatures.

High-temperature Al-9Cu-6Ce and Al-9Cu-6Ce-1Zr (wt%) alloys were designed for fabrication with laser powder bed fusion additive manufacturing (AM). An ultra-fine eutectic structure comprising FCC-Al and particles of a previously unidentified Al8Cu3Ce intermetallic phase was obtained with an inter-particle spacing of approximately 280 nm. The inherent hot-tearing resistance of the eutectic alloys resulted in > 99.5% relative density. A thermodynamic model suggested improved hot-tearing resistance of the present alloys relative to the benchmark AM AlSi10Mg alloy. The Al-Cu-Ce alloy exhibited superior thermal stability with approximately 75% of the as-fabricated hardness retained after 200 h exposure at 400 °C, owed to the coarsening resistance of the intermetallic particles. The Al-Cu-Ce-Zr alloy age-hardened through precipitation of nanoscale Al3Zr precipitates. The aged microstructure was stable at 350 °C with a 13% higher hardness after 200 h exposure compared to the as-fabricated condition. The combined influence of ultra-fine spacing and coarsening resistance of the intermetallic particles resulted in the higher yield strength of the Al-Cu-Ce and Al-Cu-Ce-Zr alloys compared to AM AlSi10Mg and Scalmalloy at temperatures greater than 200 °C. This work essentially demonstrates that thermally stable Al alloys with exceptional mechanical properties can be produced by additive manufacturing.

The deformation and failure mechanisms of Al-9Cu-6Ce (wt%) based alloys fabricated with laser powder bed fusion were investigated from room temperature to 400°C. The yield and ultimate tensile strengths decreased monotonically with increase in temperature, but the tensile elongation dipped unexpectedly at elevated temperatures and exhibited a minimum at 300°C. The dip in tensile elongation occurred with a concomitant dip in strain-rate sensitivity (SRS) of deformation. The as-fabricated alloy microstructure was heterogeneous, and the heat affected zone (HAZ) underneath the melt pool boundary was prone to strain localization. At 300 °C, the reduced SRS promoted the progression of strain localization in the HAZ leading to failure initiation and the dip in tensile elongation. A higher SRS or strain-hardening rate at other temperatures improved the tensile elongation by slowing the progression of strain localization in the HAZ such that failure initiated by other mechanisms elsewhere in the microstructure. Notably, the tensile elongation was limited by the defect structure only in a narrow temperature range (150 - 200 °C) while at other temperatures it was limited by the inherent microstructural features. This investigation exemplifies unexpected deformation and failure mechanisms possible in heterogeneous microstructures that result from additive manufacturing.

This study investigates the creep behavior of a cast, coarse-grained Al-12.5 wt.% Ce (Al-2.7 at.% Ce) alloy, consisting of an eutectic microstructure (α-Al with ~11 vol.% submicron Al11Ce3 “Chinese script” platelets) with ~3 vol.% primary, micron-scale Al11Ce3 plates. Upon aging at 322 °C for 8 weeks or at 400 °C for 12 weeks, the microhardness of the alloy remains unchanged, demonstrating excellent coarsening resistance of the strengthening Al11Ce3 phase. In addition, no coarsening of Al11Ce3 is observed metallographically after 3 weeks under compressive loads of 13–70 MPa at 260–350 °C. When tested to failure under a constant tensile stress of 23 MPa at 300 °C, the alloy shows primary, secondary and tertiary creep regimes, and fails after 19 days at 17% tensile strain, demonstrating both high creep ductility and high creep resistance. Under compressive and tensile creep conditions, the alloy exhibits high apparent stress exponents (n = 9–11), which translate into threshold stresses for dislocation creep of 34, 22, and 14 MPa at 260, 300 and 350 °C, respectively. The creep resistance of Al-12.5 wt.% Ce is higher than that of Al-Sc-Zr-based alloys (with ~0.3 vol.% of coherent nanoprecipitates) and similar to cast, eutectic Al-6 wt.% Ni (with ~11 vol.% of incoherent Al3Ni micro-fibers). For as-cast grain sizes of 2–3 mm, Al-12.5 wt.% Ce exhibits a transition from dislocation creep to diffusional creep at strain rates of ~10−7 s−1, with a threshold stress of 19 MPa in compression at 260 °C and 5 MPa in tension at 300 °C.