Structure and properties of special-purpose alloys after annealing

Introduction

Special-purpose Al–Si–based alloys doped with copper, nickel, and other elements are finding broad application across contemporary industries, particularly in engine building, instrument engineering, electronics, and aerospace [1 – 4]. In many functional units, these alloys are often in contact with components made of various steels or ceramics; therefore, their properties must match those of contacting materials in the coefficient of linear thermal expansion (CLTE), ensuring dimensional stability and, when required, vacuum-tight joints. In addition to a regulated CLTE value, special-purpose alloys must exhibit high wear resistance and low density, while their specific mechanical properties are comparable to those of medium-carbon structural steels [5; 6]. Because many precision components operate over a wide temperature range, special-purpose alloys must retain stable properties even at the upper operating limits. To ensure this stability, aluminum–silicon alloys of hypereutectic composition are alloyed with refractory elements, modified, and subjected to heat treatment [7 – 9].

Previous studies have shown that hypereutectic Al – Si alloys containing copper in amounts comparable to or exceeding the silicon content exhibit a low and stable CLTE across the wide temperature range [10].

In this context, the present study investigated the effect of isochronous annealing at 100 – 900 °C (holding time 11 h, air cooling) on the microstructure, density, and microhardness of the Al – 30 % Si – 50 % Cu alloy.

Materials and methods

The study used silumins containing 30 % Si and 50 % Cu. The starting materials for alloy preparation were aluminum grade A7, silicon grade Kr0, and copper grade M1.

Aluminum A7 was melted first, followed by sequential additions of silicon and copper in amounts of 30 and 50 %, respectively. Once the alloying elements were fully dissolved, the melt was treated with fine-fraction wet dolomite at 880 °C. The melt was then held, and casting was carried out at 1100 °C into a cold aluminum chill mold.

Heat treatment at 100, 250, and 350 °C was performed in SNOL–3.5.3.5.3.5/3.5–I2M resistance furnaces with a working chamber of 350×350×350 mm and a temperature variation of ±5 °C. Heat treatment at higher temperatures (500 – 900 °C) was carried out in SNOL–1.6.2.5.1/9–I3 resistance furnaces with a chamber size of 160×250×100 mm and a temperature deviation of ±5 °C within the working range.

Structural analysis of the Al – 30 % Si – 50 % Cu alloy samples was performed using a KYKY EM6900 Std scanning electron microscope (SEM) at the Laboratory of Electron Microscopy and Image Processing, SibGIU, in secondary and backscattered electron modes (SE + BSE) at an accelerating voltage of 25 – 30 kV, working distance of 15 – 18 mm, and magnifications ranging from 200 to 1000^×. To analyze elemental distribution among structural components, micro-X-ray spectral analysis (MXSA) was performed using an energy-dispersive spectroscopy (EDS) module.

Density was determined by hydrostatic weighing on WA-21 analytical balances with an accuracy of 0.0001 g. Microhardness was measured on an HVS-1000 digital microhardness tester under a load of 0.245 N (25 gf).

Results and discussion

SEM is widely used to address specific research and technological problems because of its high resolution and the reliability of the results obtained [2 – 5]. Owing to their high depth of field, SEM enables detailed examination of the structure of heterophase alloys with pronounced surface microrelief at high magnifications and, importantly, allows precise observation of eutectic structures (see Figure).

Microstructural analysis of the highly alloyed Al – 30 % Si – 50 % Cu alloy at different magnifications showed that its structure is primarily defined by plate-shaped primary silicon crystals (PSC). Between the PSC regions are areas of ternary eutectic (α + Si + CuAl₂) with a fine needle-like structure formed at the final stage of solidification. Elemental mapping of the polished surface was performed to examine how elements were distributed among the structural components. The results showed that silicon is mainly concentrated in the PSC, with a smaller fraction found in the eutectic. Copper is predominantly concentrated in the ternary eutectic of the Al – 30 % Si – 50 % Cu alloy, while aluminum is evenly distributed throughout the eutectic. A slight increase in iron concentration was observed in the eutectic as needle-shaped phases.

According to MXSA performed at various points within the eutectic and along the scan line (see Figure, a), copper is the predominant element (57 – 80 %), while the silicon content does not exceed 35 %, and aluminum accounts for 8 – 13 %. The highest copper content (80 %) was observed in dark regions, whereas the lowest (57 %) was found in needle-like crystals up to 1 μm in size. The dark regions correspond to equilibrium (CuAl₂) and nonequilibrium (Cu₄Al₉ and CuAl) intermetallic phases.

It was established that the distinctive feature of the high-copper Al – 30 % Si – 50 % Cu alloy is its high thermal stability, which allows for prolonged annealing not only at 400 – 500 °C (as in binary silumins) but also at 700 – 900 °С.

Electron microscopy revealed that prolonged annealing at 710 °C transforms the eutectic morphology from fine-needle to finely dispersed, while the eutectic silicon particles acquire a rounded shape (see Figure, b). The topographic contrast highlights the surface relief of the sample. The dark rounded crystals correspond to silicon, while the remaining eutectic matrix mainly consists of copper (53 – 68 %) and aluminum (8 – 18 %).

After high-temperature annealing at 900 °C, relatively large silicon crystals (up to 10 μm) with well-defined facets are formed in the eutectic (see Figure, c). In the bright intercrystalline regions, copper segregates (up to 75 %), accompanied by 17 – 20 % aluminum. In some areas, all alloying elements are detected, confirming the presence of the ternary eutectic (α + Si + CuAl₂).

The effect of annealing temperature on the density (ρ) and microhardness (μ) of the Al – 30 % Si – 50 % Cu alloy was investigated (see Table). Microhardness was measured on the eutectic, and the results were averaged over at least four measurements.

The data show that increasing annealing temperature results in a decrease in both density and microhardness. The initial density and microhardness of the alloy are 4.4113 g/cm³ and 413.6 HV, respectively, while after annealing at 900 °C, these values decrease to 4.2067 g/cm³ and 345.5 HV. The slight (≤5 %) decrease in density can be attributed to greater hydrogen uptake from the furnace atmosphere and the acceleration of diffusion processes. The decrease in microhardness (up to 16 %) is associated with eutectic silicon coagulation and increased heterogeneity in the distribution of alloying elements.

Conclusions

The distinctive feature of the high-copper Al – 30 % Si – 50 % Cu alloy is its high thermal stability, which enables prolonged annealing in the 700 – 900 °C range, a condition unacceptable for binary silumins.

Electron microscopy showed that, as the annealing temperature increases, the eutectic silicon particles change shape and undergo coagulation, which is accompanied by redistribution and segregation of copper in localized regions of the eutectic. After annealing at 710 and 900 °C, upward diffusion processes further intensify the alloy’s heterogeneity. In addition, increasing the annealing temperature causes a slight decrease in density and microhardness, which is also attributed to the enhanced diffusion rate.