Relative role of enthalpy and entropy in the stabilization of senary FCC Al–Co–Cr–Fe–Ni–Mn high entropy alloys was investigated via a high throughput combinatorial solid-to-solid diffusion couple ...approach. Many off-equiatomic compositions of FCC Al p Co q Cr r Fe s Ni t Mn u were generated by the diffusing Al and Ni in equiatomic Co20Cr20Fe20Ni20Mn20 alloy, i.e., the Al48Ni52 vs Co20Cr20Fe20Ni20Mn20 diffusion couple, annealed at 900°, 1000°, 1100°, and 1200 °C. Above 1000 °C, the solubility limit of Al in off-equiatomic Al p Co q Cr r Fe s Ni t Mn u alloy was determined to be higher than the solubility limit of Al in equiatomic Al x CoCrFeNiMn alloy. Compositions corresponding to the highest solubility limit of Al in off-equiatomic Al p Co q Cr r Fe s Ni t Mn u alloy exhibited a lower free energy of mixing, i.e., higher thermodynamic stability, than equiatomic Al x CoCrFeNiMn compositions, at 1100 °C and above. Therefore, the role of enthalpy was estimated to be significant in achieving higher thermodynamic stability in off-equiatomic alloys, since they always have lower entropy of mixing than their equiatomic counterparts. The magnitude of interdiffusion coefficients of individual elements in Al–Co–Cr–Fe–Ni–Mn alloys were compared to the interdiffusion coefficients in relevant quinary, quaternary, and ternary solvent-based alloys. Interdiffusion coefficients were not necessarily lower in FCC Al–Co–Cr–Fe–Ni–Mn alloys; therefore no sluggish diffusion was observed in FCC HEA, but diffusion of individual elements in BCC Al–Co–Cr–Fe–Ni–Mn alloy followed the sluggish diffusion hypothesis except for Ni. All compositions in the FCC Al–Co–Cr–Fe–Ni–Mn alloy were observed to comply with existing empirical single phase formation rules in high entropy alloys.
Aluminum alloy microlattices have been increasingly used in automotive, aerospace, packaging, defense, machinery, and construction industries due to their superior physical and mechanical properties ...such as high specific strength and energy absorption capacity. However, design and fabrication of microlattice structures remains a challenge because the structure-property relationship in aluminum microlattices has not been established. To address this issue, AlSi10Mg microlattices with different unit cell structures, number of unit cells, and strut diameters were designed and then fabricated by selective laser melting (SLM). The specific energy, compressive strength, and failure modes of the AlSi10Mg microlattices were examined. Experimental results have shown that the AlSi10Mg microlattices fabricated by SLM exhibited a maximum specific compressive strength of 83.113 MPa·g−1 cm3, which is higher than most metallic and non-metallic microlattices reported in the literature. During compression tests, four different failure modes, including contact region crushing, consecutive diagonal cracks at 45° to the loading direction, elastic/plastic buckling and plastic deformation, and single diagonal crack at 45° to the loading direction, were observed. In addition, the microlattices with different unit cells exhibited different strength-density relationships due to these different failure modes.
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•AlSi10Mg microlattices with high strength-to-weight ratio by selective laser melting (SLM)•Printability and mechanical behavior of AlSi10Mg microlattices•Design-mechanical behavior-microstructural characteristics relationship•Four failure modes observed during the compression tests
Dense and crack-free Al-6Zn-2Mg (wt%) alloys with 1 wt% (Sc + Zr) addition were additively manufactured by laser powder bed fusion (LPBF) using gas atomized powders. As-built microstructure consisted ...of small equiaxed grains near the melt pool boundary and columnar grains between adjacent melt pools. Alloying of Sc + Zr promoted the formation of Al3(Sc,Zr) particles, which contributed to the grain refinement. The alloy exhibited outstanding tensile properties (i.e., 418 ± 3 MPa yield strength, 436 ± 3 MPa tensile strength and 11 ± 1% elongation) after heat treatment. The results demonstrate that high strength aluminum alloy can be fabricated by LPBF through alloy design and microstructural control.
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Consistent manufacturing of volumetrically dense engineering components, free of solidification cracks by laser powder bed fusion (LPBF), has been demonstrated for Al-Si alloys such as AlSi10Mg and ...Al12Si. The success in LPBF of these alloys is attributed to the near eutectic composition with a small freezing range. To illuminate this observation, cracking susceptibility was examined from Scheil-Gulliver solidification modeling by calculating the hot cracking susceptibility, |dT/dfS1/2|. To validate the findings from hot cracking susceptibility calculations, six binary Al-Si alloys, whose compositions were strategically chosen at hypo-, near-, and hyper-eutectic compositions, were gas atomized into alloy powders, and processed by LPBF. Only Al-Si alloys with 1.0 and 2.0 wt.% Si were found to exhibit cracking, which was predicted by relatively large magnitudes of |dT/dfS1/2|. Either as particles or with a eutectic structure, Si segregation at the intercellular boundaries was observed to define the sub-grain cellular structure. For selected compositions, measurement of the cellular structure allowed for estimation of the cooling rate to be 106 to 107 K•s−1. Excluding the alloys with solidification cracking, an increase in tensile strength and the corresponding decrease in ductility were observed with an increase in Si concentration, which were attributed to the formation of a cellular structure and the amount of Al-Si eutectic found at the intercellular boundaries.
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High entropy and sluggish diffusion “core” effects were investigated in an FCC Al-Co-Cr-Fe-Ni alloy by examining the nonequiatomic compositions generated by the concentration profiles within the ...solid-to-solid diffusion couple, Al
48
Ni
52
vs
Co
25
Cr
25
Fe
25
Ni
25
, annealed at 900 °C, 1000 °C, 1100 °C, and 1200 °C. The average effective interdiffusion coefficients of individual components and the maximum solubility limit of Al in nonequiatomic Al-Co-Cr-Fe-Ni alloys were determined as a function of temperature. The magnitudes of the average effective interdiffusion coefficients in Al-Co-Cr-Fe-Ni alloys were compared to the interdiffusion coefficients in relevant ternary and quaternary alloys. The solubility limit of Al in nonequiatomic Al
p
Co
q
Cr
r
Fe
s
Ni
t
alloys was compared to that of Al in equiatomic Al
x
CoCrFeNi determined from the equilibrium pseudo-binary phase diagram. A reduction in the magnitude of interdiffusion coefficients was not observed for individual components in Al-Co-Cr-Fe-Ni alloys. The maximum solubility of Al in nonequiatomic Al
p
Co
q
Cr
r
Fe
s
Ni
t
alloys was observed to be higher than that in equiatomic Al
x
CoCrFeNi alloys at a temperature of 1100 °C or above. Correspondingly, the free energy of mixing for nonequiatomic Al
p
Co
q
Cr
r
Fe
s
Ni
t
alloys was determined to be lower than that of equiatomic Al
x
CoCrFeNi alloys at a temperature of 1100 °C or above. At a temperature of 1100 °C or above, the role of enthalpy of mixing was estimated to be significant in achieving higher thermodynamic stability of the nonequiatomic Al
p
Co
q
Cr
r
Fe
s
Ni
t
alloy than in the equiatomic Al
x
CoCrFeNi alloy for the compositions corresponding to the highest solubility limit for Al. The compositions of nonequiatomic Al
p
Co
q
Cr
r
Fe
s
Ni
t
alloys were observed to follow the existing empirical rules for the formation of single phase in high entropy alloys (HEAs).
The microstructure and mechanical properties of AlSi10Mg alloy manufactured via selective laser melting process were investigated. The effect of post heat treatment on microstructure and mechanical ...properties was also examined. Direct aging (DA) process without solution heat treatment was employed as a post heat treatment to prepare specimens. Specimens were also arranged via the conventional solution heat treatment + precipitate hardening (T6) for comparison. Initial microstructure observation results revealed that the as-built alloy showed the unique characteristic of SLM material’ microstructure having cellular structure together with molten pool. The cellular structure boundaries were found to contain eutectic Si and a small amount of nano size Si particles inside the cellular structure. After the solid solution + T6 heat treatment, the unique microstructural characteristic of the SLM as-built material disappeared, whereas the Si particles became as coarse as several μm. On the other hand, the DA heat treated alloy, even after the heat treatment, maintained the molten pool and cellular structure and contained a lot of Si precipitates (nano size) inside the cellular structure. Room temperature tensile results showed that the DA alloy possessed superior tensile strength (310 MPa) compared with the as-built and T6 alloys. The elongation of the DA alloy was 6.2%, which was about 3% lower than that of the as-built alloy. However, compared with the conventional precipitate hardening Al-Si alloy, the DA alloy's elongation was found higher. Tensile fracture surface observation found that in the as-built specimen, fine dimples were formed and cellular structure played a role in increasing the strength of the alloy. In the DA alloy, a lot of precipitated Si particles played as a major reinforcing phase. From the results above, the DA was found as a suitable heat treatment method capable of enhancing the strength of SLM AlSi10Mg alloy, and at the same time, granting a decent level of elongation.
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•Post-heat treatment of direct aging for additively manufactured Al alloy was suggested.•The unique microstructure characteristic of direct aging heat treated SLM Al alloy was well analyzed.•Direct aging effectively improved the mechanical properties of SLM Al alloy.
The increasing potential for additive manufacturing technology continuously drives the need for printable materials, including novel aluminum alloys. Ce is considered an economically feasible ...addition that is beneficial to corrosion resistance and high temperature performance of aluminum alloys. In this study, a binary Al–10Ce alloy was additively manufactured by laser powder bed fusion (LPBF) using gas-atomized powders for the first time. Initial investigation was carried out to determine the optimal LPBF parameters for the Al–10Ce alloy, which was found using the laser power of 350 W and scan speed of 1400 mm/s. Alloy samples with nearly full density and outstanding printability were obtained. Room temperature tensile tests of the as-built Al–10Ce alloy yielded 222 ± 2 MPa in yield strength, 319 ± 1 MPa in tensile strength and 10.8 ± 0.1% in elongation. This is superior to the cast counterpart reported in the literature. A uniform microstructure was observed throughout the alloy, and it primarily consisted of extremely fine-scale eutectic Al and Al
11
Ce
3
intermetallic ribbons, arranged in a skeleton pattern. This fine microstructure would have originated from the rapid cooling inherent to the LPBF process, and strongly suggests the presence of the Orowan strengthening mechanism. This study demonstrated that the binary Al–10Ce alloy is a promising base composition with good printability that brings new possibilities for future ternary and higher-order alloy design for LPBF.
The high entropy effect and sluggish diffusion effect postulate that the alloys with higher thermodynamic stability exhibit anomalously slow diffusion kinetics. This postulation assumed that the ...enthalpy of mixing and excess entropy in random single-phase solid solution based high entropy alloys (HEAs) are negligible and configurational entropy is the dominant contributor towards the overall thermodynamic stability. The highly stable HEAs were assumed to exhibit very rugged potential energy landscape characterized by larger potential energy fluctuations, which slows down the diffusion kinetics. These assumptions, however, were challenged by several investigations. Therefore, this work investigates the dependence of tracer diffusion kinetics on thermodynamic stability parameters by systematically investigating the underlying assumptions of high entropy and sluggish diffusion effects. The effect of enthalpy of mixing and excess entropy on the overall thermodynamic stability of several FCC HEAs was also examined. Findings suggest that: (1) FCC HEAs exhibit non-ideal solid solution behavior because enthalpy of mixing and excess entropy contributes significantly towards the overall thermodynamic stability; (2) contrary to sluggish diffusion effect, higher entropy do not always result in higher potential energy fluctuations; and (3) tracer diffusion in FCC alloys is composition dependent and exhibit no direct correlation with the thermodynamic stability parameters.
Additive manufacturing (AM) technology for metallic alloys such as laser powder bed fusion (LPBF) brings tremendous opportunities for development of novel alloys specifically designed for AM that ...would desensitize the inherent process variability and requires a refined understanding of processing–structure–property relationship that would contribute to future alloy development. In this study, two different alloys, Al-6Zn-2Mg and Al-6Zn-2Mg-0.7Sc-0.3Zr in wt. pct, representing traditional and AM-specific novel aluminum alloys, respectively, were manufactured by LPBF technique using pre-alloyed gas atomized powders. The Al-Zn-Mg ternary alloys exhibited cracks at various LPBF processing parameters, primarily along the grain boundaries of the large columnar grains that extended across multiple melt pools. The severity of cracking in LPBF Al-Zn-Mg alloys was process-dependent and could be correlated to the change in alloy composition due to evaporation of Zn and Mg with high vapor pressure. The Scheil–Gulliver non-equilibrium solidification calculations showed that the Al-Zn-Mg alloys with lower Zn and Mg concentrations had smaller solidification range (
i.e.
, Δ
T
) and steepness values (
i.e.
,
d
T
/
d
f
s
1
/
2
near
f
s
1
/
2
=
1
), which corresponded to a lower cracking severity. On the other hand, no cracks were observed in Al-Zn-Mg-Sc-Zr alloys, although their solidification range and steepness values were similar to the ternary Al-Zn-Mg alloys. The microstructure of Al-Zn-Mg-Sc-Zr alloys exhibited a much refined heterogeneous microstructure containing small equiaxed and columnar grains within each melt pool, owing to the heterogeneous nucleation upon primary Al
3
(Sc,Zr) particles. Process-dependent microstructure was observed as a result of variation in thermal gradient and cooling rate associated with LPBF parameters. Findings from this study provide guidance for the future design of AM-specific aluminum alloys and insights into the microstructural control by LPBF.
Laser powder bed fusion (LPBF) is a popular additive manufacturing (AM) technique that has demonstrated the capability to produce sophisticated engineering components. This work reports the ...crack-free fabrication of an SS316L/IN718 bimetallic structure via LPBF, along with compositional redistribution, phase transformations and microstructural development, and nanohardness variations. Constituent intermixing after LPBF was quantitatively estimated using thermo-kinetic coefficients of mass transport and compared with the diffusivity of Ni in the austenitic Fe-Ni system.
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