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In the present study, we characterized the Fe–Al intermetallic phases formed in interstitial-free (IF) steel hot-dipped in a Zn–6Al–3Mg (wt.%) alloy melt at different temperatures (400, 460 and 500 °C) and dipping times (from 2 to 3600 s). Chemical composition analyses indicated Fe dissolution into the Zn alloy melt even after 2 s of dipping. Microstructural characterization revealed the initial formation of a continuous θ-Fe4Al13 phase layer, followed by the local growth of η-Fe2Al5 phase toward both steel and Zn alloy melt sides during the hot-dipping process. After long-term hot-dipping, further growth of the η phase was accompanied by significant dissolution of Fe into the Zn alloy melt, resulting in the loss of thickness of the IF steel sheets. Altogether, we rationalized the formation mechanism of Fe–Al intermetallic phases and their associated growth at the interface between solid Fe and liquid Zn alloy in terms of a Zn–Al–Mg–Fe quaternary phase diagram, calculated according to thermodynamic database available in the literature., ファイル公開：2021-06-01

We designed two types of Al-based alloy composites reinforced by η-Zn2Mg (hexagonal) and T-Al6Mg11Zn11 (cubic) intermetallic phases using monovariant eutectic reactions in the Al–Mg–Zn ternary system and then attempted to fabricate them using solidification. Thermodynamic assessment revealed two alloy compositions of Al–18Mg–36Zn and Al–22.5Mg–23.5Zn (at%) with the α-Al (fcc) phase reinforced with high fractions (>50%) of the η and T phases. Both alloys exhibited fine two-phase eutectic microstructures consisting of α/η and α/T phases, respectively. The morphology of the α-Al phase in the eutectic colonies varied from lamellae to fibers upon changing the neighboring intermetallic phase. These eutectic microstructures exhibit high stability at an elevated temperature of 300 °C. In addition, the size and spacing of the α-Al phase could be controlled by changing the cooling rate during solidification. The fabricated composites exhibited high hardness values exceeding 220 HV, which were much superior to those of conventional Al alloys. The calculated solidification path and component partitioning in the liquid phase during solidification provided insights into the solidification segregation of Zn element in the Al–18Mg–36Zn alloy. The Zn-enriched regions would enhance the local microstructural change at elevated temperatures, which confirmed by the microstructural observations of the alloy exposed at 300 °C.

This study was undertaken to understand the fundamentals of T-Al6Mg11Zn11 intermetallic phase precipitation in heat-resistant Al–Mg–Zn ternary alloys, with the goal of further strength improvements. The thermodynamic assessment involved three alloy compositions, Al–4Mg–4.5Zn, Al–5Mg–3.5Zn, and Al–7Mg–1.5Zn (at%), with the T phase in a fixed fraction (approximately 6–7.5%) in equilibrium with the α-Al phase at 300 °C. The alloy compositions were set on different tie-lines between the α-Al and T phases in the two-phase region, resulting in varied compositions of each phase. In the three experimental alloys aged at 300 °C, the T phase precipitated on the grain boundaries of the α-Al matrix, and granular precipitates were homogeneously dispersed intragranularly. There was no significant difference in the precipitation morphology. It was demonstrated that the lattice parameters of the T and α-Al phases could be controlled based on the alloy composition and applied heat treatments. However, the lattice parameters have no significant effect on the precipitation morphologies of the thermodynamically stable T phase in the Al–Mg–Zn ternary alloys. The Zn-rich Al–4Mg–4.5Zn alloy exhibited a pronounced softening behavior after 10 h of aging due to the enhanced coarsening of precipitates in the α-Al matrix. The thermal instability of the precipitates was responsible for the transformation sequence from the initially formed metastable phases (η-Zn2Mg associated phases) to the stable T phase. These results provided a fundamental insight that alloy strengthening by introducing thermodynamically stable intermetallic phases is essential for sustaining the strength of heat-resistant Al–Mg–Zn ternary alloys after long-term treatment at elevated temperatures.

An attempt was made to additively manufacture an Al–Fe binary alloy sample with a hypereutectic composition of 15 wt% Fe using a laser powder bed fusion (LPBF) process. The LPBF-built Al–15Fe alloy sample exhibited a microstructure consisting of a number of melt pools in which regions had locally melted and rapidly solidified due to scanning laser irradiation during the LPBF process. The α-Al (fcc) matrix consists of a number of elongated grains with a mean size of approximately 10 μm. These microstructural features correspond well to previous results of Al alloys additively manufactured by the LPBF process. It was found that relatively coarsened stable θ-Al13Fe4 phases with a length of a few micrometers were localized along the melt pool boundaries. Numerous spherical particles of a metastable Al–Fe intermetallic phase were finely distributed within the nanoscale eutectic microstructure consisting of α-Al and metastable Al6Fe phases inside the melt pools. The metastable phase formation corresponds well to the previous results on the rapidly solidified Al–Fe alloys. The refined multiple intermetallic phases produced by the LPBF process contribute to a high hardness of approximately 200 HV. The refined microstructure appeared stable at an elevated temperature of 300 °C. The high microstructural stability would sustain sufficient strength in a hostile environment for long-term periods of service at elevated temperatures above 200 °C. The present results were utilized to discuss the formation sequence of multiple Al–Fe intermetallic phases in rapid solidification by the LPBF process.

We designed three Al-based cast alloys strengthened by T-Al6Mg11Zn11 intermetallic phases using a eutectic reaction (liquid → α(fcc) + T) in the Al–Mg–Zn ternary system and then fabricated them using solidification processes. Thermodynamic assessment provided target alloy compositions of Al–26.5Zn–21.5 Mg (at%), Al–23.5Zn–22.5 Mg (at%), and Al–13Zn–28Mg (at%) with the α -Al (fcc) phase in equilibrium with the T phase with high fractions (>60%). All of the alloys exhibited regularly arrayed fibers of the α phase embedded in a T-phase matrix in α/T two-phase eutectic microstructures. A slight effect of phase compositions on the fibrous eutectic morphology was observed, whereas the volume fraction of the T phase could be controlled according to the thermodynamic assessments. The fibrous α phases had faceted interfaces of {111} planes with a surrounding T-phase matrix, suggesting a low interfacial energy of the α/T interfaces. The fabricated eutectic alloys exhibited a high Vickers hardness (>230 HV) and high yield strength at elevated temperatures above 250 °C. The Mg-rich alloy with a composition of Al–13Zn–28Mg (at%) exhibited a relatively lower hardness, responsible for the low hardness of the T phase with a Mg-rich composition. The Mg-rich alloy exhibited the plasticity at a lower temperature of 200 °C but comparatively lower strength at elevated temperatures above 250 °C. These results indicated that chemical composition strongly affected the mechanical properties of the T phase in the eutectic alloys. The present results provide important insights into control of the mechanical properties of Al-based eutectic alloys reinforced by the T intermetallic phase.

Long-scale Al–Ti intermetallic foam was fabricated by self-propagating high-temperature synthesis (SHS) mode combustion reaction with the help of exothermic agent and preheating treatment. The effect of exothermic agent and preheating condition on the SHS mode combustion reaction foaming of the long-scale Al–Ti intermetallic foam was investigated. Maximum temperature of combustion reaction, reaction propagation speed and porosity of specimen were increased by increasing the amount of exothermic agent. A correspondence relation between the distribution of porosity and the distribution of the maximum temperature throughout the self propagation direction was indicated. Long-scale Al–Ti foam which had 60–70% porosity and uniformed porosity distribution was earned with 623 and 673 K homogenous preheating. It was due to the appearance of Al 3 Ti liquid phase throughout the observation point of the specimen during the combustion reaction by the preheating treatment.

Porous Al–Ti intermetallics and Al3Ti/Al composites were fabricated by combustion foaming process. Fundamentally, aluminum and titanium powders were blended (Al/Ti mole blending ratios ranging from 0.33 to 10.0). Additionally, boron carbide (B4C) powder was blended as exothermic agent since the heat of TiC and TiB2 formation is much higher than those of Al–Ti intermetallics. The blended powder compact was heated to induce the combustion reaction. Isotropic closed pores were produced by this process. By adding 10 vol.% exothermic agent, the porosity exceeded 60% in a wide range of Al/Ti ratios (3.0–10.0). It was confirmed that high heat of reaction of exothermic agent assisted the complete conversion of aluminum and titanium powders to Al3Ti.

Porous Al3Ti/Al composites were fabricated by the combustion foaming process and the effect of elemental powder size on porosity and cell morphology was investigated. High porosity was obtained by using the small titanium powders. Pores also became larger and more spherical by using the small titanium powders. Pore size became smaller and porosity became higher by decreasing the aluminum powder size. This tendency was remarkably observed on the specimens with high Al/Ti ratio (7.0).

Open-cellular porous TiAl was synthesized by a chemical reaction between titanium and aluminum with a space holder powder (NaCl). Self-propagation of the combustion reaction occurred throughout the specimen when the NaCl addition was below 40vol%, while such a propagating behavior was not observed with higher NaCl additions. TiAl was synthesized after the reaction. Pores were formed in places where the NaCl particles were present. This process shows the hierarchical structure with a bimodal size distribution.