Fusion-based additive manufacturing (AM) has significantly grown to fabricate Nickel-based superalloys with design freedom across multiple length scales. Several phenomena such as feedstock/energy ...source/melt pool interactions, solidification and phase transformations occur during fusion-based AM processes of Nickel-based superalloys, which determine the ultimate microstructure and mechanical performance of the built parts. In this review, we elaborate a comprehensive discussion on AM Nickel-based superalloys and influential factors including feedstock characteristics (powder morphology, chemistry, contamination, flowability, recycling) and AM processing (parameters, and powder spreading/wall/balling/spattering effects) on their microstructure (micro-segregation, phases formations and grain structures), defect generation (sub-surface/internal defects, microcracks, surface roughness, and residual stress). Furthermore, the mechanical properties of AM Nickel-based superalloys such as tensile, creep and fatigue at room/elevated temperatures are analyzed in accordance with the initial, and post processing effects. Additionally, the commonly utilized modeling approaches in literature to predict the microstructure and mechanical behavior of these alloys are highlighted. Finally, the current challenges and mitigation approaches for future research are identified considering the gaps in the AM Nickel-based superalloys.
This article presents the high temperature tensile and creep behaviors of a novel high entropy alloy (HEA). The microstructure of this HEA resembles that of advanced superalloys with a high entropy ...FCC matrix and L1
ordered precipitates, so it is also named as "high entropy superalloy (HESA)". The tensile yield strengths of HESA surpass those of the reported HEAs from room temperature to elevated temperatures; furthermore, its creep resistance at 982 °C can be compared to those of some Ni-based superalloys. Analysis on experimental results indicate that HESA could be strengthened by the low stacking-fault energy of the matrix, high anti-phase boundary energy of the strengthening precipitate, and thermally stable microstructure. Positive misfit between FCC matrix and precipitate has yielded parallel raft microstructure during creep at 982 °C, and the creep curves of HESA were dominated by tertiary creep behavior. To the best of authors' knowledge, this article is the first to present the elevated temperature tensile creep study on full scale specimens of a high entropy alloy, and the potential of HESA for high temperature structural application is discussed.
The effects of Al and Ti contents on the microstructures and high temperature properties of AlxCo1.5CrFeNi1.5Tiy (x, y values in molar ratio, x + y = 0.5) high entropy alloys have been investigated. ...With an increase in Al content and a decrease in Ti concentrations, precipitated phases in the FCC high entropy γ matrix can evolve from η to γ′ to β phases. Some of these alloys can possess high hardness values at elevated temperatures due to γ′ precipitations, and continuous Cr2O3 can form on the surfaces of these alloys for protection against oxidation. This study can serve as a guideline for future high entropy alloy design for high temperature applications.
Display omitted
•AlxCo1.5CrFeNi1.5Tiy high entropy alloys have been investigated.•Increase in Al/Ti ratio can have the following effects.•Intermetallics in FCC matrix evolve from η to γ′ to β phases.•The γ′ precipitates can result high hardness at high temperatures.•Continuous Cr2O3 can form for protection against oxidation.
Tensile creep, which is one of the most important deformation modes for high temperature applications, is rarely reported for refractory high entropy alloys (RHEAs). In the present study, the optical ...floating zone (OFZ) technique was used to fabricate HfNbTaTiZr with grain size larger than 1 mm on average; tensile creep tests under vacuum at 1100-1250°C and stepwise loading of 5–30 MPa were conducted. The stress exponents and creep activation energies were determined to be 2.5–2.8 and 273 ± 15 kJ mol–1, respectively. The stress exponents determined have suggested solute drag creep behavior, and deformation was governed by a/2 type dislocations. To elucidate the effect of alloying constituents on solute drag creep, intrinsic diffusion coefficients of all elements were determined by simulation, and theoretical minimum creep strain rates were compared with those of experimental values. Analysis suggests that creep rate of HfNbTaTiZr appears to be controlled by Ta, which possesses the lowest intrinsic diffusivity and contributes the most to drag dislocations. To our knowledge, this work is the first to report tensile creep deformation mechanism of HfNbTaTiZr, especially up to 1250°C.
Display omitted
In this work, a precipitation strengthened high entropy alloy was subjected to thermo-mechanical process in order to homogenize the grain microstructure. Tensile tests from room temperature to ...1000 °C were conducted; microstructures were observed by scanning electron microscope and transmission electron microscope. Formation of cellular precipitate along grain boundaries was observed and could be related to hot tensile ductility drop at 750 °C (1023 K). Experimental analysis has indicated that driving force for the formation of cellular precipitates could be resulted from the chemical instability of supersaturation after annealing and migration of grain boundaries, and this phenomenon could be suppressed either through alloy design to increase gamma-prime solvus, and to hinder the migration of grain boundaries. This study serves as a guideline to design composition and thermo-mechanical process for precipitation strengthened high entropy alloys.
•A precipitation strengthened high entropy alloy was subjected to thermos-mechanical process.•Chemical instability of supersaturation and migration of grain boundaries can cause formation of cellular precipitate.•Tensile ductility drop occurred around 750 °C in present high entropy alloy due to cellular precipitate.•Cellular precipitate does not affect tensile ductility at room temperature.•The formation of cellular precipitate can be suppressed by alloy design and cooling rate during heat-treatment.
In this study, the grain boundary evolution of equiatomic CoCrFeMnNi, CoCrFeNi, and FeCoNi alloys after one-step recrystallization were investigated. The special boundary fraction and twin density of ...these alloys were evaluated by electron backscatter diffraction analysis. Among the three alloys tested, FeCoNi exhibited the highest special boundary fraction and twin density after one-step recrystallization. The special boundary increment after one-step recrystallization was mainly affected by grain boundary velocity, while twin density was mainly affected by average grain boundary energy and twin boundary energy.
A hierarchical microstructure strengthened high entropy superalloy (HESA) with superior cost specific yield strength from room temperature up to 1,023 K is presented. By phase transformation pathway ...through metastability, HESA possesses a hierarchical microstructure containing a dispersion of nano size disordered FCC particles inside ordered L1
precipitates that are within the FCC matrix. The average tensile yield strength of HESA from room temperature to 1,023 K could be 120 MPa higher than that of advanced single crystal superalloy, while HESA could still exhibit an elongation greater than 20%. Furthermore, the cost specific yield strength of HESA can be 8 times that of some superalloys. A template for lighter, stronger, cheaper, and more ductile high temperature alloy is proposed.
Abstract
Although refractory high entropy alloys (RHEAs) have shown potentials to be developed as structural materials for elevated temperature applications, most of the reported oxidation behaviours ...of RHEA were associated with short term exposures for only up to 48 hours, and there is a lack of understanding on the oxidation mechanism of any RHEA to-date. In this work, by using thermogravimetric analysis, isothermal oxidation was conducted on a novel RHEA at 1000 °C and 1100 °C for up to 200 hours, which is an unprecedented testing duration. The external oxide layer strongly influenced the weight gain behaviours, and it consisted of CrTaO
4
-based oxide with some dispersion of Al
2
O
3
and Cr
2
O
3
. At 1000 °C, the inability to form dense CrTaO
4
-based oxide layer resulted an exponential dependence of weight gain throughout 200 hours. At 1100 °C, mass gain curve showed two parabolic dependences associated with the formation of protective CrTaO
4
-based oxide layer and the weight gain after 200 hours was 4.03 mg/cm
2
, which indicates that it is one of the most oxidation resistant RHEAs comparing to literature data to-date. This work can also provide insights on how to further develop RHEA to withstand long term oxidation at elevated temperatures.
The refractory high entropy alloy (RHEA) has shown great potentials for high temperature applications beyond modern Ni-based superalloy. However, its oxidation behaviours are rarely reported and ...understood. In this work, the oxidation behaviours of a novel RHEA “NV1”, Cr-17.6Al-20.3Mo-15.2Nb-2.9Si-13.4Ta-5.4Ti (in at%), were examined at 1200, 1300, and 1400 °C up to 100 h. At 1200 °C, the oxidation kinetics curve yielded toward parabolic behaviour owing to the formation of a rutile-type complex oxide layer with Al2O3 and Cr2O3 dispersions; breakaway oxidation contributed by Cr2O3 evaporation occurred at 1300 °C; a single power-law behaviour governed the oxidation kinetics curve at 1400 °C, and mullite was identified within the oxide layer. This work provides guidelines for understanding the oxidation mechanisms and improving oxidation resistance of RHEA at elevated temperature.
•A new refractory high entropy alloy were oxidised up to 1400 °C for 100 h.•Microstructural evolution strongly affected the oxidation of the studied alloy.•(Al, Cr)(Nb, Ta)O4 external oxide layer was protective against oxidation.•Strong Ti-rich nitride presence was observed deep beneath the oxidised regions.
PDF
