Possibility of Strengthening Aluminum Using Low-Symmetry Phases of the Fe-Al Binary System
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This study analyzes the effects of manufacturing parameters such as the temperature and heating time on the shape and size of high-aluminum-phase precipitates produced from a 95.5 wt pct Al and 4.5 wt pct Fe powder mixture. The samples were prepared by powder metallurgy, and the changes in the heat flux during the simulation of the sintering process by differential thermal analysis were recorded to determine the temperature of the transformation. To identify the resulting structures, microscopic observations (scanning electron microscope) and chemical composition analyses were performed simultaneously. The distribution and shape of the resulting high-aluminum-phase precipitates were determined. Microhardness measurements were also performed, and Meyer’s law and Kick’s similarity law were applied to the results to analyze the influence of the high-aluminum phases on the reinforcement of the aluminum matrix.
Modern design solutions require lighter and more durable materials. Composites consisting of light metals and other types of materials in various configurations have become increasingly feasible, thus creating new design possibilities. The most commonly known and used light metal is aluminum and its alloys. Pure aluminum is characterized by a low density (2.7 g/cm3), which is its greatest advantage, and a high elongation value of up to 35 pct. However, it also has poor mechanical properties, such as low hardness (18 HB) and strength (70 MPa). To improve these properties, various elements are added to aluminum to modify the phase structure during either the crystallization step or heat treatment.
The two types of aluminum alloys include casting alloys, which mainly contain silicon, and wrought alloys. The latter group can be further subdivided into manganese and magnesium alloys, which are strengthened by work hardening, and copper and zinc alloys, which are strengthened by precipitation hardening. The properties of aluminum can be significantly enhanced by strengthening mechanisms, and the fabrication of alloys which, together with its low density, has resulted in its wide use in various industries.
Despite the existence of a wide range of well-known aluminum alloys since the 1930s, numerous studies of reinforcement growth and the effects of various factors, such as the size and distribution of the precipitates, on the resulting mechanical properties have been and are still being performed. The study in this field is twofold. On one hand, research efforts are focused on modifying the production processes of well-known alloys by introducing new technologies, such as powder metallurgy, which was used to produce Al-Cu alloys, or by tuning the chemical composition to produce reinforcing phases in the aluminum matrix. For example, Varin prepared Al-Zr particles in an aluminum matrix to increase the hardness of the material. On the other hand, cheaper materials that are easier to use in the synthesis, such as iron, which is commonly used in construction materials, can be employed as reinforcing components. In his study, Srivastava showed that an Fe content ranging from 1.67 to 11.2 pct in aluminum-iron alloys significantly improves the alloy strength and tribological properties, due to the precipitation of the low-symmetry, high-aluminum phase FeAl3.[4,5] The undeniable potential of these particles to reinforce aluminum alloys is due to the complexity of the crystal structure, which gives rise to high hardness and abrasion resistance. In addition, these particles are cheaper than intermetallic phases containing other elements that are useful in construction. However, the production process must be modified to obtain high-aluminum precipitates with a suitable morphology and distribution throughout the material. The traditional production methods for alloys containing intermetallic phases are mainly based on melting and casting, which usually results in a coarse structure with low plasticity and brittle fracture. The resulting “technology gap” has been filled with production techniques based on powder metallurgy. In addition to its many well-known advantages, this technology enables the constitutive reinforcement of the alloy matrix by in situ methods, which, unlike ex situ particle reinforcement, leads to materials with high thermodynamic stability, i.e., the improved boundary between the matrix and reinforcing particles hinders potential degrading chemical reactions.
Selected Properties of High Aluminum Phases from the Fe-Al Binary System
Cell Volume (Å3)
triclinic P1 (1)
orthorhombic Cmcm (63)
monoclinic C2/m (12)
Although the solubility of iron in aluminum is negligible (e.g., 0.012, 0.024, and 0.04 pct at 550 °C, 590 °C, and 630 °C, respectively), only a very low content (several hundred ppm) of this element is needed to significantly inhibit dislocation movement in pure aluminum by affecting its strengthening mechanisms and creep resistance. Therefore, to enhance the Al(Fe) matrix strengthening, it was proposed that the Fe4Al13 phase (FeAl3) could be used, because it limits warp deformation near the surface of each particle due to its elastic interactions with the matrix and its restraint of the dislocation motion. In this method of reinforcement, a hydrostatic stress field is generated around the reinforcing particles, which should be sufficiently large and uniformly distributed in the matrix volume and have similar, preferably spherical, shapes to ensure their efficiency. Achieving these requirements during eutectic crystallization is very difficult; therefore, in this study, we attempted to reinforce the aluminum matrix with low-symmetry Fe-Al phases produced in situ by diffusion through a mixture of aluminum and iron powders during a powder metallurgy process conducted in the eutectic temperature region.
2 Materials and Methods
The prepared powders were mixed in a turbulent mill for 30 minutes and then compacted to a cylinder with a diameter of 3 mm and height of 5 mm by pressing the sample on one side with a pressure of 700 MPa to ensure a sequential equilibrium phase transition during the heating and cooling processes.
To record the changes in the heat flux during the phase change of the compacted samples, differential thermal analysis was performed to simulate the sintering process by heating from 610 °C to 700 °C in 1 hour. Calorimetric analysis was performed to identify the precipitates and determine the temperature at which no reflections from the melting and crystallization of the test mixture are observed. At this temperature, the separation processes only occur by diffusion, which simplifies the subsequent technological processes and allows the reinforcing phase to be distributed throughout the entire volume of the alloy.
The microstructure, particularly the morphology and chemical composition of the resulting precipitates, was observed using a scanning electron microscope equipped with an energy-dispersive X-ray spectroscopy chemical composition analyzer. The results suggest that phase decomposition occurs during the analyzed process. To accurately determine the volumes of the high-aluminum phases produced in the aluminum matrix from the Fe-Al system, computer-assisted quantum digitization was performed using the NIS ELEMENT software.
Microhardness measurements were performed to determine the changes in the degree of hardening as a function of the annealing temperature. In particular, 200, 300, and 500 G loads were employed, the HV hardness was defined for a constant diagonal of 20 μm, and Meyer’s law and Kick’s similarity law were applied to the data. Using this approach, the effects of the elastic deformations on the hardness value and the Meyer’s coefficient (strength factor), which characterizes the method of the material reinforcement,[6,18,20] were determined.
3 Results and Discussion
3.1 Powder Characterization
It should be noted that at these sintering temperatures, the difference between HV20 μm and HV0.1, which is due to the effects of the elastic stresses generated in the matrix network, is assumed to be at a maximum. These observations are confirmed by the minimum value of the Meyer coefficient (strengthening), which varies around n = 1.7. It is known that a decrease in the “n” parameter corresponds to an increase in the strengthening rate of the material, and n = 1.6 is the boundary between brittle and plastic materials. Increasing the process temperature above 630 °C results in the gradual disappearance of the resilient matrix stresses due to the melting and crystallization processes, and HV20μm and HV0.1 are essentially the same at 650 °C and higher. At the same time, a noticeable increase in the Meyer’s coefficient is observed; at 650 °C, it reaches the limit of n = 2, at which the load does not affect the hardness value, meaning that the material is very difficult to strengthen under the measurement conditions. A further increase in the process temperature to 700 °C results in n > 2, which is rare and results from brittle fracture (separation) at the boundary of the hard plastic matrix.
It is possible to reinforce aluminum with high-aluminum phases of the Fe-Al binary system.
The strengthening effect is most influenced by the morphology and distribution of the resulting phases, which depend on the temperature of the process.
The most visible increase in the hardness is obtained when the samples are sintered at 620 °C to 630 °C, which enables the generation of the highest number of finely dispersed particles in the aluminum matrix by diffusion.
Under the studied process conditions, the resulting precipitates are identified as the metastable Fe2Al9 phase, which has been rarely described in the literature.
It appears that the highest strengthening effect should be obtained by using iron powder with the least granulation.
Future study in this area will focus on obtaining other Fe-Al phases that strengthen the aluminum matrix, including the stable-phase Fe4Al13 (FeAl3), mainly by changing the heating rate and sintering time.
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