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Development of investment casting process has been always a challenge for manufacturers of complex shape parts with thin elements. Particularly, misruns often occur in the as-cast complex shape parts due to the formation of solid skin by freezing of melt in contact with colder ceramic mould. This work presents a new tool for physical simulation of skin formation during investment casting. Special ceramic tubes are designed and fabricated from the material used for the manufacturing of ceramic moulds for investment casting. Melting/solidification experiments are carried out in the thermo-mechanical simulator, where the melt is contained in the ceramic tube, which is heated to the temperature of ceramic mould in investment casting. Detailed microstructural characterization of the solidified specimens is performed; the obtained results predict the thickness of skin and its microstructure. This concept is applied to investment casting of complex shape nozzle guide vanes from the Mar-M247 Ni-based superalloy. Experimental casting trials are performed, and the outcomes of physical simulation tool are validated against experimental results. [ABSTRACT FROM AUTHOR]

This work aims to investigate the influence of solute segregation at particle-matrix interfaces on the twin activity of aged Mg alloys. With that purpose a binary Mg-8Al alloy (wt%) and two ternary Mg-8Al-1 Zn and Mg-8Al-1Ag alloys (wt%) were first homogenized and then aged in order to obtain comparable precipitate distributions. Particle-matrix interfaces in the binary alloy were observed to be clean, while Zn and Ag atomic segregation was clearly present at the interfaces of the ternary alloys. Room temperature micropillar compression tests were carried out in grains with favorable orientations for tensile twinning in the homogenized and the aged conditions for the three materials under investigation. The activation of twinning and slip was then characterized by electron microscopy. This study shows that while in the binary alloy precipitation did not lead to any major changes in the twinning activity and, in particular, it did not prevent the formation of thick twin lamellae encompassing the entire width of the tested micropillars, a drastic reduction of the twin activity took place in the aged ternary alloys. In the latter, straining led to the formation of narrow twin lamellae belonging to a single variant, and occasionally arrested at the interior of the micropillars. We propose that the drastic reduction of the twin volume fraction in the aged ternary alloys may be attributed to the decrease of the particle-matrix interface energy due to Zn and Ag segregation, which would hinder re-nucleation at such interfaces, thus suppressing twin propagation. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The Al–Nb system exhibits a eutectic transformation at , which results in () and () phases. This paper is concerned with the processing of this eutectic by directional solidification. Alloys were prepared by arc melting and directionally solidified in Bridgman-type equipment. The resulting samples were utilized to evaluate the solidification microstructure and morphology regarding the growth conditions. Eutectic microstructures obtained were regular with lamellar morphology. Variations of the growth rate showed that an increase in this parameter causes a decrease in the lamellar spacing. With further increase in the growth rate, eutectic cells were observed as a result of constitutional undercooling. Oxidation tests of eutectic microstructures showed that this alloy suffers severe microstructure instability, with growth kinetics of the oxide scale of linear type. This indicates that the Al in the Nb–Al eutectic alloy is insufficient to form protective oxide films, such as, -. Finally, the heat treatment in argon atmosphere showed that the eutectic alloy presents a high degree of microstructure stability at . [Copyright &y& Elsevier]

Abstract: In situ composite materials obtained by directional growth of eutectic alloys usually show improved properties, that make them potential candidates for high temperature applications. The eutectic alloy found in the Al–Nb system is composed of the two intermetallic phases Al3Nb (D022) and Nb2Al (D8b). This paper describes the directional solidification of an Al–Nb eutectic alloy using a Bridgman type facility at growth rates varying from 1.0 to 2.9 cm/h. Longitudinal and transverse sections of grown samples were characterized regarding the solidification microstructure by using optical and scanning electron microscopy, energy dispersive spectroscopy (EDS) and X-ray diffraction. Despite both phases being intermetallic compounds, the eutectic microstructure obtained was very regular. The results obtained were discussed regarding the effect of the growth rate on the microstructure, lamellar–rod transition and variation of phase volume fraction. [Copyright &y& Elsevier]

The objective of this research is to study the effect of the Zr on the L-PBF processability, microstructure, and microhardness of an AlZnMgCu-Zr alloy. Two AlZnMgCu-0.5 and 1.5 wt% Zr pre-alloyed powders were produced by casting followed by gas atomization. In addition, an excess of Mg and Zn was added to the target compositions to compensate for vaporization during L-PBF. The as-atomized powders and the as-built and heat treated specimens were characterized via scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. Crack-free samples with a relative density of 99.0 ± 0.1% were obtained in the alloy containing 1.5 wt%Zr. The as-built microstructure of this alloy consisted of small equiaxed grains without preferred grain orientation near the melt pool boundary and slightly columnar grains between adjacent melt pools. After a solution and aging heat treatment (T6), the newly developed AlZnMgCu-1.5Zr alloy has an outstanding Vickers micro-hardness of 223 ± 3 HV, which is significantly higher than that of the wrought Al7075-T6 aluminum alloy. This excellent mechanical behavior is attributed to the presence of large Al 3 Zr nucleant particles of up to 500 nm in size that inhibit grain growth during solidification and the subsequent heat treatment, and of a high density of nanoscale second phases (MgZn 2 and Al 3 Zr) located within the aged α-Al grains. • A high strength AlZnMgCu-Zr pre-alloyed powder for L-PBF has been designed. • To compensate for vaporization during L-PBF, the Zn and Mg amount was optimized. • A high microhardness value of 220 HV was obtained after a T6 treatment. • Al 3 Zr precipitates present at GBs provide high thermal stability to printed samples. [ABSTRACT FROM AUTHOR]