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All sourcesNASA NTRSDOE OSTI
NASA NTRS2014

Comparison of Directionally Solidified Samples Solidified Terrestrially and Aboard the International Space Station

This article reports research that has been carried out under the aegis of NASA as part of a collaboration between ESA and NASA for solidification experiments on the International Space Station (ISS). The focus has been on the effect of convection on the microstructural evolution and macrosegregation in hypoeutectic Al-Si alloys during directional solidification (DS). Terrestrial DS-experiments have been carried out at Cleveland State University (CSU) and under microgravity on the International Space Station (ISS). The thermal processing-history of the experiments is well defined for both the terrestrially processed samples and the ISS-processed samples. As of this writing, two dendritic metrics was measured: primary dendrite arm spacings and primary dendrite trunk diameters. We have observed that these dendrite-metrics of two samples grown in the microgravity environment show good agreements with models based on diffusion controlled growth and diffusion controlled ripening, respectively. The gravity-driven convection (i.e., thermosolutal convection) in terrestrially grown samples has the effect of decreasing the primary dendrite arm spacings and causes macrosegregation. Dendrite trunk diameters also show differences between the earth- and space-grown samples. In order to process DS-samples aboard the ISS, the dendritic seed crystals were partially remelted in a stationary thermal gradient before the DS was carried out. Microstructural changes and macrosegregation effects during this period are described and have modeled.

Angart, S.↗
DOE OSTI2023

Microstructural refinement in ultrasonically modified A356 aluminum castings

Two A356 aluminum alloys (Al-Si-Mg), one with 0.09 wt.% Fe and one with 0.91 wt.% Fe, were cast in a graphite mold with the simultaneous application of local ultrasonic intensification to refine the as-cast microstructure. Ultrasonication during casting transformed the morphology of primary Al grains from dendritic (~140-290 microns in size) to globular (~33-36 microns in size). The alloy with high Fe exhibited globular grains at distances up to 45 mm away from the ultrasound probe, while the alloy with low Fe exhibited globular grains at distances only up to 6 mm away from the ultrasound probe. Near the location of the ultrasound probe (< 2 mm away), a second non-dendritic microstructural morphology was observed with fine aluminum grains (~9-25 microns in size). This unique fine-grained morphology has not been previously reported, contains a greater concentration of Si relative to the globular microstructure, and may be a large, fully eutectic region. Ultrasonication during casting also transformed the morphology of the ß-Al 5 FeSi phase particles (which are deleterious to the strength and ductility of the alloy) in the high Fe alloy from needle-like to rectangular, which could enable the greater use of secondary Al alloys. Thermodynamic simulations conducted to calculate the solidification paths of the two alloys studied predict that the ß-Al 5 FeSi phase begins to form earlier in the alloy with high Fe. Finally, data suggest that the ß-Al 5 FeSi phase (which is more abundant in alloys with high Fe content) may enhance ultrasonically-induced grain refinement.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Strontium Effects on the Formation of Iron-Intermetallic Phases in Secondary Al–9Si–0.6Fe Alloys

The presence and morphology of Fe-containing intermetallic phases affect the mechanical properties of aluminum alloys, especially in secondary Al–Si-based cast alloys. Although strontium (Sr) addition of 50 to 500 ppm is known to refine the needle-type eutectic silicon structure, the influence of Sr on the formation of Fe-intermetallic phases remains unclear. The present work investigates the combined additions of Sr and Mn to Al–9Si–0.6Fe–0.35Mg (All compositions are in wt pct except otherwise stated.) alloys on the formation of Fe-intermetallic phases at different solidification rates from ~ 1.5 to ~ 60 °C/s. Long and branched-type AlFeSi phase with size ranging from 50 to 120 µm are more common when solidified at the rate of 1.5 °C/s regardless of Sr and Mn additions. However, at the fast solidification rate of 60 °C/s, a 60 ppm Sr addition significantly reduced the average length of needle-shaped AlFeSi phase to less than 3 to 5 µm. Thermodynamic simulations have been performed using CALculation of PHAse Diagrams (CALPHAD) models to predict the formation of various phases and their possible interactions during solidification. The results indicated that the combination of a high solidification rate and about 60 ppm of Sr is beneficial to refining the δ-Al 3 FeSi 2 phase in Al–Si–Mg alloys containing 0.6 pctFe. As a result, this unexpected finding of Fe-intermetallic refinement by low Sr addition (~60 ppm) provides an important guide in designing secondary alloys for sustainable casting applications.

36 MATERIALS SCIENCE↗
DOE OSTI2024

The Beneficial Effect of Iron in Aluminum-Cerium-Based Cast Alloys

Iron (Fe) has been considered a major impurity since it is detrimental to the mechanical properties of many cast aluminum alloys due to the formation of Fe-containing brittle intermetallic phases. Fe is found naturally as an impurity in bauxite ore, resulting in Fe contamination of aluminum alloys with increasing contamination from current recycling practices. The Al–Ce–Fe system was investigated using CALPHAD (CALculation of PHAse Diagrams) modeling and experimental casting techniques. It was found that additions of Fe to the Al–Ce system are beneficial to the strength (slightly) and ductility (significantly) of the ternary alloys, which is attributed to the formation of fine metastable Al 8 CeFe 2 phase with aggregate morphology and equilibrium Al 10 CeFe 2 phase, suppressing coarse proeutectic Al 11 Ce 3 phase in near-eutectic Al–Ce alloys. Heat treatment study showed that the metastable Al 8 CeFe 2 phase transforms to predicted equilibrium Al 10 CeFe 2 phase at 500 °C, with essentially no intermetallic or grain coarsening; thus, the alloy displayed excellent property retention. The Al–Ce–Fe alloy system offers opportunities for sustainable, recyclable alloy development using low-cost Fe and low-cost cerium (a byproduct of rare-earth extraction).

42 ENGINEERING↗
DOE OSTI2020

Materials Data on AlSi by Materials Project

AlSi is Tetraauricupride structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. Al is bonded in a body-centered cubic geometry to eight equivalent Si atoms. All Al–Si bond lengths are 2.73 Å. Si is bonded in a body-centered cubic geometry to eight equivalent Al atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Al4Si by Materials Project

Al4Si crystallizes in the trigonal R-3m space group. The structure is three-dimensional. there are six inequivalent Al sites. In the first Al site, Al is bonded to nine Al and three equivalent Si atoms to form distorted AlAl9Si3 cuboctahedra that share corners with six equivalent SiAl6 cuboctahedra, corners with twelve AlAl9Si3 cuboctahedra, edges with six equivalent SiAl6 cuboctahedra, edges with eighteen AlAl9Si3 cuboctahedra, and faces with twelve AlAl9Si3 cuboctahedra. There are three shorter (2.84 Å) and six longer (2.85 Å) Al–Al bond lengths. All Al–Si bond lengths are 2.73 Å. In the second Al site, Al is bonded to twelve Al atoms to form AlAl12 cuboctahedra that share corners with three equivalent SiAl6 cuboctahedra, corners with nine AlAl9Si3 cuboctahedra, edges with three equivalent SiAl6 cuboctahedra, edges with twenty-one AlAl9Si3 cuboctahedra, and faces with eighteen AlAl9Si3 cuboctahedra. There are six shorter (2.85 Å) and three longer (2.88 Å) Al–Al bond lengths. In the third Al site, Al is bonded to twelve Al atoms to form AlAl12 cuboctahedra that share corners with three equivalent SiAl6 cuboctahedra, corners with nine AlAl9Si3 cuboctahedra, edges with three equivalent SiAl6 cuboctahedra, edges with twenty-one AlAl9Si3 cuboctahedra, and faces with eighteen AlAl9Si3 cuboctahedra. There are three shorter (2.84 Å) and six longer (2.85 Å) Al–Al bond lengths. In the fourth Al site, Al is bonded to twelve Al atoms to form AlAl12 cuboctahedra that share corners with three equivalent SiAl6 cuboctahedra, corners with nine AlAl9Si3 cuboctahedra, edges with three equivalent SiAl6 cuboctahedra, edges with twenty-one AlAl9Si3 cuboctahedra, and faces with eighteen AlAl9Si3 cuboctahedra. There are a spread of Al–Al bond distances ranging from 2.84–2.88 Å. In the fifth Al site, Al is bonded to twelve Al atoms to form AlAl12 cuboctahedra that share corners with three equivalent SiAl6 cuboctahedra, corners with nine AlAl9Si3 cuboctahedra, edges with three equivalent SiAl6 cuboctahedra, edges with twenty-one AlAl9Si3 cuboctahedra, and faces with eighteen AlAl9Si3 cuboctahedra. There are three shorter (2.84 Å) and six longer (2.85 Å) Al–Al bond lengths. In the sixth Al site, Al is bonded to twelve Al atoms to form AlAl12 cuboctahedra that share corners with three equivalent SiAl6 cuboctahedra, corners with nine AlAl9Si3 cuboctahedra, edges with three equivalent SiAl6 cuboctahedra, edges with twenty-one AlAl9Si3 cuboctahedra, and faces with eighteen AlAl9Si3 cuboctahedra. There are a spread of Al–Al bond distances ranging from 2.84–2.88 Å. Si is bonded to six equivalent Al atoms to form distorted SiAl6 cuboctahedra that share corners with eighteen AlAl9Si3 cuboctahedra, edges with six equivalent SiAl6 cuboctahedra, and edges with eighteen AlAl9Si3 cuboctahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Al4Si19 by Materials Project

Al4Si19 is beta beryllia-like structured and crystallizes in the triclinic P1 space group. The structure is three-dimensional. there are eight inequivalent Al3+ sites. In the first Al3+ site, Al3+ is bonded to four Si+0.63- atoms to form AlSi4 tetrahedra that share a cornercorner with one AlSi4 tetrahedra and corners with eleven SiSi4 tetrahedra. There are a spread of Al–Si bond distances ranging from 2.48–2.51 Å. In the second Al3+ site, Al3+ is bonded to four Si+0.63- atoms to form AlSi4 tetrahedra that share corners with two AlSi4 tetrahedra and corners with ten SiSi4 tetrahedra. There are a spread of Al–Si bond distances ranging from 2.46–2.54 Å. In the third Al3+ site, Al3+ is bonded to four Si+0.63- atoms to form AlSi4 tetrahedra that share corners with two AlSi4 tetrahedra and corners with ten SiSi4 tetrahedra. There are a spread of Al–Si bond distances ranging from 2.49–2.51 Å. In the fourth Al3+ site, Al3+ is bonded to four Si+0.63- atoms to form AlSi4 tetrahedra that share corners with two AlSi4 tetrahedra and corners with ten SiAlSi3 tetrahedra. There are a spread of Al–Si bond distances ranging from 2.46–2.52 Å. In the fifth Al3+ site, Al3+ is bonded to four Si+0.63- atoms to form AlSi4 tetrahedra that share corners with four AlSi4 tetrahedra and corners with eight SiSi4 tetrahedra. There are a spread of Al–Si bond distances ranging from 2.49–2.56 Å. In the sixth Al3+ site, Al3+ is bonded to four Si+0.63- atoms to form AlSi4 tetrahedra that share corners with three AlSi4 tetrahedra and corners with nine SiAl2Si2 tetrahedra. There are a spread of Al–Si bond distances ranging from 2.47–2.57 Å. In the seventh Al3+ site, Al3+ is bonded to four Si+0.63- atoms to form AlSi4 tetrahedra that share corners with two AlSi4 tetrahedra and corners with ten SiSi4 tetrahedra. There are a spread of Al–Si bond distances ranging from 2.44–2.53 Å. In the eighth Al3+ site, Al3+ is bonded to four Si+0.63- atoms to form AlSi4 tetrahedra that share corners with two AlSi4 tetrahedra and corners with ten SiSi4 tetrahedra. There are three shorter (2.49 Å) and one longer (2.52 Å) Al–Si bond lengths. There are thirty-eight inequivalent Si+0.63- sites. In the first Si+0.63- site, Si+0.63- is bonded to one Al3+ and three Si+0.63- atoms to form corner-sharing SiAlSi3 tetrahedra. There are one shorter (2.37 Å) and two longer (2.38 Å) Si–Si bond lengths. In the second Si+0.63- site, Si+0.63- is bonded to two Al3+ and two Si+0.63- atoms to form corner-sharing SiAl2Si2 tetrahedra. Both Si–Si bond lengths are 2.37 Å. In the third Si+0.63- site, Si+0.63- is bonded to two Al3+ and two Si+0.63- atoms to form corner-sharing SiAl2Si2 tetrahedra. There are one shorter (2.39 Å) and one longer (2.40 Å) Si–Si bond lengths. In the fourth Si+0.63- site, Si+0.63- is bonded to one Al3+ and three Si+0.63- atoms to form SiAlSi3 tetrahedra that share corners with two AlSi4 tetrahedra and corners with ten SiSi4 tetrahedra. There are one shorter (2.35 Å) and two longer (2.37 Å) Si–Si bond lengths. In the fifth Si+0.63- site, Si+0.63- is bonded to two Al3+ and two Si+0.63- atoms to form SiAl2Si2 tetrahedra that share corners with two AlSi4 tetrahedra and corners with ten SiAl2Si2 tetrahedra. There are one shorter (2.35 Å) and one longer (2.37 Å) Si–Si bond lengths. In the sixth Si+0.63- site, Si+0.63- is bonded to four Si+0.63- atoms to form SiSi4 tetrahedra that share corners with three AlSi4 tetrahedra and corners with nine SiAlSi3 tetrahedra. There are a spread of Si–Si bond distances ranging from 2.35–2.38 Å. In the seventh Si+0.63- site, Si+0.63- is bonded to four Si+0.63- atoms to form SiSi4 tetrahedra that share corners with four AlSi4 tetrahedra and corners with eight SiAl2Si2 tetrahedra. There are a spread of Si–Si bond distances ranging from 2.36–2.40 Å. In the eighth Si+0.63- site, Si+0.63- is bonded to four Si+0.63- atoms to form SiSi4 tetrahedra that share corners with three AlSi4 tetrahedra and corners with nine SiAlSi3 tetrahedra. There are a spread of Si–Si bond distances ranging from 2.36–2.39 Å. In the ninth Si+0.63- site, Si+0.63- is bonded to one Al3+ and three Si+0.63- atoms to form SiAlSi3 tetrahedra that share corners with two AlSi4 tetrahedra and corners with ten SiAlSi3 tetrahedra. Both Si–Si bond lengths are 2.37 Å. In the tenth Si+0.63- site, Si+0.63- is bonded to one Al3+ and three Si+0.63- atoms to form SiAlSi3 tetrahedra that share corners with two AlSi4 tetrahedra and corners with ten SiAl2Si2 tetrahedra. There are one shorter (2.35 Å) and one longer (2.37 Å) Si–Si bond lengths. In the eleventh Si+0.63- site, Si+0.63- is bonded to four Si+0.63- atoms to form SiSi4 tetrahedra that share corners with four AlSi4 tetrahedra and corners with eight SiAl2Si2 tetrahedra. There are a spread of Si–Si bond distances ranging from 2.36–2.40 Å. In the twelfth Si+0.63- site, Si+0.63- is bonded to one Al3+ and three Si+0.63- atoms to form SiAlSi3 tetrahedra that share corners with four AlSi4 tetrahedra and corners with eight SiAl2Si2 tetrahedra. There are a spread of Si–Si bond distances ranging from 2.33–2.39 Å. In the thirteenth Si+0.63- site, Si+0.63- is bonded to four Si+0.63- atoms to form SiSi4 tetrahedra that share corners with two AlSi4 tetrahedra and corners with ten SiSi4 tetrahedra. There are a spread of Si–Si bond distances ranging from 2.36–2.40 Å. In the fourteenth Si+0.63- site, Si+0.63- is bonded to four Si+0.63- atoms to form SiSi4 tetrahedra that share corners with four AlSi4 tetrahedra and corners with eight SiAlSi3 tetrahedra. There are a spread of Si–Si bond distances ranging from 2.36–2.40 Å. In the fifteenth Si+0.63- site, Si+0.63- is bonded to two Al3+ and two Si+0.63- atoms to form SiAl2Si2 tetrahedra that share a cornercorner with one AlSi4 tetrahedra and corners with eleven SiAl2Si2 tetrahedra. There are one shorter (2.33 Å) and one longer (2.34 Å) Si–Si bond lengths. In the sixteenth Si+0.63- site, Si+0.63- is bonded to four Si+0.63- atoms to form SiSi4 tetrahedra that share corners with four AlSi4 tetrahedra and corners with eight SiAlSi3 tetrahedra. There are a spread of Si–Si bond distances ranging from 2.36–2.40 Å. In the seventeenth Si+0.63- site, Si+0.63- is bonded to one Al3+ and three Si+0.63- atoms to form SiAlSi3 tetrahedra that share corners with three AlSi4 tetrahedra and corners with nine SiAl2Si2 tetrahedra. There are one shorter (2.36 Å) and one longer (2.38 Å) Si–Si bond lengths. In the eighteenth Si+0.63- site, Si+0.63- is bonded to four Si+0.63- atoms to form SiSi4 tetrahedra that share corners with two AlSi4 tetrahedra and corners with ten SiAl2Si2 tetrahedra. There are a spread of Si–Si bond distances ranging from 2.36–2.38 Å. In the nineteenth Si+0.63- site, Si+0.63- is bonded to one Al3+ and three Si+0.63- atoms to form SiAlSi3 tetrahedra that share corners with two AlSi4 tetrahedra and corners with ten SiSi4 tetrahedra. There are one shorter (2.34 Å) and one longer (2.38 Å) Si–Si bond lengths. In the twentieth Si+0.63- site, Si+0.63- is bonded to two Al3+ and two Si+0.63- atoms to form SiAl2Si2 tetrahedra that share corners with two AlSi4 tetrahedra and corners with ten SiAl2Si2 tetrahedra. In the twenty-first Si+0.63- site, Si+0.63- is bonded to four Si+0.63- atoms to form SiSi4 tetrahedra that share corners with two AlSi4 tetrahedra and corners with ten SiAlSi3 tetrahedra. The Si–Si bond length is 2.36 Å. In the twenty-second Si+0.63- site, Si+0.63- is bonded to one Al3+ and three Si+0.63- atoms to form distorted SiAlSi3 tetrahedra that share a cornercorner with one AlSi4 tetrahedra and corners with eleven SiAl2Si2 tetrahedra. In the twenty-third Si+0.63- site, Si+0.63- is bonded to four Si+0.63- atoms to form SiSi4 tetrahedra that share corners with four AlSi4 tetrahedra and corners with eight SiAlSi3 tetrahedra. The Si–Si bond length is 2.37 Å. In the twenty-fourth Si+0.63- site, Si+0.63- is bonded to one Al3+ and three Si+0.63- atoms to form distorted corner-sharing SiAlSi3 tetrahedra. In the twenty-fifth Si+0.63- site, Si+0.63- is bonded to two Al3+ and two Si+0.63- atoms to form SiAl2Si2 tetrahedra that share a cornercorner with one AlSi4 tetrahedra and corners with eleven SiSi4 tetrahedra. In the twenty-sixth Si+0.63- site, Si+0.63- is bonded to one Al3+ and three Si+0.63- atoms to form distorted SiAlSi3 tetrahedra that share a cornercorner with one AlSi4 tetrahedra and corners with eleven SiAl2Si2 tetrahedra. The Si–Si bond length is 2.37 Å. In the twenty-seventh Si+0.63- site, Si+0.63- is bonded to four Si+0.63- atoms to form SiSi4 tetrahedra that share corners with five AlSi4 tetrahedra and corners with seven SiAl2Si2 tetrahedra. In the twenty-eighth Si+0.63- site, Si+0.63- is bonded to two Al3+ and two Si+0.63- atoms to form corner-sharing SiAl2Si2 tetrahedra. In the twenty-ninth Si+0.63- site, Si+0.63- is bonded to four Si+0.63- atoms to form SiSi4 tetrahedra that share corners with four AlSi4 tetrahedra and corners with eight SiSi4 tetrahedra. The Si–Si bond length is 2.38 Å. In the thirtieth Si+0.63- site, Si+0.63- is bonded to one Al3+ and three Si+0.63- atoms to form distorted SiAlSi3 tetrahedra that share a cornercorner with one AlSi4 tetrahedra and corners with eleven SiAlSi3 tetrahedra. In the thirty-first Si+0.63- site, Si+0.63- is bonded to two Al3+ and two Si+0.63- atoms to form corner-sharing SiAl2Si2 tetrahedra. In the thirty-second Si+0.63- site, Si+0.63- is bonded to four Si+0.63- atoms to form SiSi4 tetrahedra that share corners with three AlSi4 tetrahedra and corners with nine SiAlSi3 tetrahedra. The Si–Si bond length is 2.36 Å. In the thirty-third Si+0.63- site, Si+0.63- is bonded to one Al3+ and three Si+0.63- atoms to form distorted SiAlSi3 tetrahedra that share a cornercorner with one AlSi4 tetrahedra and corners with eleven SiAl2Si2 tetrahedra. In the thirty-fourth Si+0.63- site, Si+0.63- is bonded to two Al3+ and two Si+0.63- atoms to form SiAl2Si2 tetrahedra that share a cornercorner with one AlSi4 tetrahedra and corners with eleven SiAl2Si2 tetrahedra. In the thirty-fifth Si+0.63- site, Si+0.63- is bonded to one Al3+ and three Si+0.63- atoms to form distorted SiAlSi3 tetrahedra that share a cornercorner with one AlSi4 tetrahedra and corners with eleven SiSi4 tetrahedra. The Si–Si bond length is 2.37 Å. In the thirty-sixth Si+0.63- site, Si+0.63- is bonded to four Si+0.63- atoms to form SiSi4 tetrahedra that share corners with four AlSi4 tetrahedra and corners with eight SiSi4 tetrahedra. In the thirty-seventh Si+0.63- site, Si+0.63- is bonded to one Al3+ and three Si+0.63- atoms to form corner-sharing SiAlSi3 tetrahedra. The Si–Si bond length is 2.36 Å. In the thirty-eighth Si+0.63- site, Si+0.63- is bonded to four Si+0.63- atoms to form SiSi4 tetrahedra that share corners with three AlSi4 tetrahedra and corners with nine SiAlSi3 tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Effect of Cooling Rate on Nano-Eutectic Formation in Laser Surface Remelted and Rare Earth Modified Hypereutectic Al-20Si Alloys

Laser Surface Remelting (LSR) was applied to arc-melted Al-20Si-0.2Sr, Al-20Si-0.2Ce, and Al-20Si hypereutectic alloys to refine microstructures. Experiments revealed that microstructures in the melt pool varied from fully eutectic to a mixture of Al dendrites and inter-dendritic eutectic. We calculated cooling rates using the Eagar-Tsai model and correlated cooling rates with characteristic microstructures, revealing that a cooling rate on the order of 104 K/s could lead to maximized fully eutectic microstructure morphology. Due to rapid solidification, the Si composition in the LSR eutectic was measured at 18.2 wt.%, higher than the equilibrium eutectic composition of 12.6 wt.%Si. Compared to Al-20Si, Ce addition had no significant effect on the volume fraction of the fully eutectic structure but refined Si fibers to approximately 30 nm in diameter. Sr addition did not further refine the diameter of eutectic Si fibers compared to Al-20Si but increased the volume fraction of the fully eutectic microstructure morphology. The refinement ratio (φ) of the Si fiber diameter from the bottom of the melt pool to the surface for the three alloys was similar, at around 28%. The established correlation between the cooling rate and the size and morphology of the microstructure within the melt pool will enable tailoring of the microstructure in laser-processed as well as deposited alloys for high strength and plasticity.

36 MATERIALS SCIENCE↗