Tilen Balaško

This study examines the solidification behaviour of Al–Cr–La alloys, reflecting the increasing use of rare-earth elements to enhance the properties of aluminium alloys. The research aims to determine how the new ternary phase Al ₂₀ Cr ₂ La, formed by the addition of La, influences the solidification process. This fundamental understanding is essential before these alloys can be developed for high-performance applications. Ten alloys with varying Cr (0.03–2.0 at%) and La (0.03–1.0 at%) contents were produced using an arc melter. After annealing at 600 °C for 600 h, their solidification and microstructure were analysed using DSC and SEM. In alloys with higher Cr content, solidification begins with the Al ₄₅ Cr ₇ phase, followed by the ternary Al ₂₀ Cr ₂ La phase, then (Al), and finally a mixture of (Al) + Al ₁₁ La ₃ . Thus, the first series of alloys (samples A–E with varying Cr contents) exhibit four phases: Al ₄₅ Cr ₇ , Al ₂₀ Cr ₂ La, (Al), and Al ₁₁ La ₃ . (Al) is present in the matrix and mixed with Al ₁₁ La ₃ , while the ternary Al ₂₀ Cr ₂ La phase is observed around the binary Al ₄₅ Cr ₇ phase. Second series (samples F–J with varying La contents), solidification also begins with the binary Al ₄₅ Cr ₇ phase, followed by the ternary Al ₂₀ Cr ₂ La phase and (Al), and finally the (Al) + Al ₁₁ La ₃ mixture. In this case, the microstructure of samples with increased La content (F–J) consists of the ternary Al ₂₀ Cr ₂ La phase, the (Al) + Al ₁₁ La ₃ mixture, and an (Al) matrix.

The phase equilibria of the ternary phase system Al-Cr-La were investigated in the Al-rich corner at 600°C, with minimum Al content of 98.77 at.%. The formed phases, transformation temperatures (specifically liquidus and solidus) and microstructure evolution were experimentally determined by isothermal annealing at 600°C for 600 hours and Differential Scanning Calorimetry (DSC) analysis. Scanning Electron Microscope (SEM) equipped with Energy-Dispersive X-ray Spectroscopy (EDXS) was employed to completely characterize the samples. The equilibrium microstructure state consists of the (Al), the binary phase \mathrm{Al_{45}} <![CDATA[ \mathrm{Cr_7} and the ternary phase \mathrm{LaCr_2} > \mathrm{Al_{20}} in the samples with increased Cr content (from 0.03 up to 1.12 at.%), while it consists of the (Al), the mixture (Al) + \mathrm{Al_{11}} <![CDATA[ \mathrm{La_3} and the ternary \mathrm{LaCr_2} > \mathrm{Al_{20}} phase in the samples with increased La content (from 0.03 up to 0.45 at.%). It was observed that even the small amount of alloying elements leads to the formation of ternary phase.

This study examines the solidification behaviour of Al-Cr-La alloys, reflecting the increasing use of rare-earth elements to enhance the properties of aluminium alloys. The research aims to determine how the new ternary phase Al ₂₀ Cr ₂ La, formed by the addition of La, influences the solidification process. This fundamental understanding is essential before these alloys can be developed for high-performance applications. Ten alloys with varying Cr and La contents were produced using an arc melter. After annealing, their solidification and microstructure were analysed using DSC and SEM. In alloys with higher Cr content, solidification begins with the Al ₄₅ Cr ₇ phase, followed by the ternary Al ₂₀ Cr ₂ La phase, then (Al), and finally a mixture of (Al) + Al ₁₁ La ₃ . Thus, these samples (A–E) exhibit four phases: Al ₄₅ Cr ₇ , Al ₂₀ Cr ₂ La, (Al), and Al ₁₁ La ₃ . (Al) is present in the matrix and mixed with Al ₁₁ La ₃ , while the ternary Al ₂₀ Cr ₂ La phase is observed around the binary Al ₄₅ Cr ₇ phase. In samples F–J, solidification also begins with the binary Al ₄₅ Cr ₇ phase, followed by the ternary Al ₂₀ Cr ₂ La phase and (Al), and finally the (Al) + Al ₁₁ La ₃ mixture. In this case, the microstructure of samples with increased La content (F–J) consists of the ternary Al ₂₀ Cr ₂ La phase, the (Al) + Al ₁₁ La ₃ mixture, and an (Al) matrix.

The high-strength 7xxx Al alloys are frequently used due to their excellent properties. To achieve these properties, heat treatment is crucial. In this study, the influence of La on the microstructure evolution of Al–Zn–Mg–Cu alloys during solution annealing, the first step of heat treatment, was investigated. The results showed that La additions significantly affect the microstructure by increasing in the eutectic solidus temperature, influencing the melting enthalpy of Mg₂Si/LaAlSi, promoting the formation of LaAlSi and reducing Mg₂Si, increasing the melting enthalpy of α-Al, shortening the time to reach the eutectic melting peak and preventing a significant increase in grain size. A 10 h solution heat treatment was recommended, with minimal benefits after 12 h. 0.15 wt% La was the minimum to prevent an increase in grain size. La altered the Al₄₅Cr₇ phase and formed a new Al₂₀Cr₂La phase with larger dimensions and sharper edges. Faster cooling rates refine the grain size due to precipitation along the grain boundaries. The content of the T-phase (Mg₃₂(Al, Cu, Zn)₄₉) decreased with increasing annealing time. Prolonged annealing promoted the diffusion of Mg and Zn from the eutectic phases, which led to their dissolution in the matrix. Zn diffused out first, followed by Mg. Prolonged annealing favored the formation of the Al₇Cu₂Fe phase over the Al₁₃Fe₄ phase.

The outstanding properties of the high-strength 7xxx aluminium alloys are achieved through critical heat treatment. This research focussed on the significant effects of lanthanum addition on the microstructural changes of EN AW-7175 alloy during solution annealing, the first stage of heat treatment. The main objective was to elucidate these microstructural changes and in particular to determine how lanthanum influences the evolution of the different phases. To investigate the microstructural evolution, the alloys were first produced. After casting, a heat treatment, i.e. solution annealing, was carried out with holding times of 1 to 24 hours. Differential scanning calorimetry (DSC) was then carried out to analyse the characteristic temperatures and enthalpies of the samples. And finally, metallographic analysis was performed, which included scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to analyse the microstructure. The results showed that lanthanum profoundly changes the microstructure by increasing the eutectic solidus temperature, affecting the melting behaviour of Mg 2Si and promoting the formation of LaAlSi (while suppressing Mg 2Si) and increasing the melting enthalpy of the aluminium matrix. Lanthanum also accelerated the eutectic melting kinetics and effectively controlled the grain growth with an optimum annealing time of 10 hours and a minimum lanthanum concentration of 0.15 wt% to stabilise the grain size. In addition, lanthanum was involved in the development of chromium-containing phases leading to the formation of a novel Al 20Cr 2La phase. Processing parameters such as faster cooling rates led to grain refinement due to precipitation along the grain boundaries. Longer annealing times caused a decrease in the T-phase content due to the diffusion of Mg and Zn into the matrix (Zn diffuses faster) and favoured the formation of the intermetallic phase Al 7Cu 2Fe over Al 13Fe 4.

Aluminium Matrix Composites (AMC) are advanced materials composed of an aluminium alloy reinforced with ceramic or metallic particles, fibres, or whiskers to enhance properties such as strength, stiffness, wear resistance, and thermal stability. These composites offer a high strength-to-weight ratio, improved mechanical performance, and corrosion resistance, making them ideal for aerospace, automotive, and defence applications. The purpose of the research was to synthesise, homogenise and characterise Al-Mg 2Si composite samples. We plotted the equilibrium phase diagram and Scheil curves using the Thermo-Calc software. Based on the results of the thermodynamic simulations of equilibrium and non-equilibrium solidification, we prepared the appropriate raw materials and melted them in a BN-coated steel crucible in an induction furnace. The most appropriate ratio was calculated to be Mg/Si = 1.74. Due to the high casting temperature, shrinkage and gas porosity occur in the samples. The cooling rate during solidification should be as high as possible to achieve the appropriate microstructure. We also performed simple thermal analysis (ETA) and plotted cooling curves in the Origin software and differential scanning calorimetry (DSC). Microstructural characterisation was also carried out to define the type, proportion and morphology of phases in the as-cast and homogenised state. According the thermodynamic calculation under all conditions the primary phase Mg 2Si solidifies first, then the Al 13Fe 4 phase and finally the eutectic (a Al + Mg 2Si). It is also possible that the eutectic (a Al + Mg 2Si + b Si) solidifies last. The results of the DSC and metallographic analysis show the primary Mg 2Si phase, the eutectics (a Al + Al 13Fe 4), (a Al + Mg 2Si) and (a Al + Mg 2Si + b Si) in the as-cast samples. The solidification of the investigated composite takes place in the range between 750 ºC and 540 ºC. Upon the homogenisation process, the rounding of the edges of the primary solidified Mg 2Si particles, and the rounding and growth of the eutectic Mg 2Si phase was observed in the microstructure.

The transformation of wüstite (FeO) in the oxide layer formed during high temperature oxidation (600 °C and 700 °C) on hot-worked tool steel was investigated. Wüstite plays an important role in the oxide layer of these steels used for hot working. However, understanding its transformation behavior during cooling is crucial for controlling the final oxide layer structure. Slow cooling rates have a significant influence on the final wüstite content, resulting in inaccurate representations of the composition of the oxide layer at temperatures above 570 °C. The aim of this study was to determine the influence of cooling rate on the wüstite content in the oxide layer after high temperature oxidation. It was found that for hot-worked steel samples oxidized at 700 °C or higher, a cooling rate of more than 1000 °C min⁻¹ is required to suppress the eutectoid transformation and maintain the realistic wüstite content. At lower temperatures (570 °C–600 °C), a cooling rate of more than 100 °C min⁻¹ is required to achieve the wüstite content observed at oxidation temperatures in the oxide layer. Overall, the hematite and magnetite contents also vart with the cooling rate, which is associated with changes in the wüstite content.

The high-strength aluminium alloys of the 7xxx series, which belong to the Al–Zn–Mg–Cu system, are known for their exceptional properties and are often used for applications where strength, fatigue resistance, stress corrosion resistance and wear resistance are required. The alloy EN AW-7175 requires an understanding of the precipitation process during solidification and cooling, where different phases are formed depending on the Zn/Mg ratio and cooling rate. Microstructural defects in the as-cast state affect the mechanical properties, prompting the investigation of La additions to refine the microstructure and improve the mechanical properties. In this study, the influence of La additions on the solidification and microstructure of the alloy EN AW-7175 in the as-cast state is investigated. Thermodynamic calculations, DSC and SEM analyses were performed. Samples with La additions (0.05–0.17 mass%) were compared with a reference sample (0 mass% La). La additives have only a minimal effect on the liquidus temperature and show minor differences in the solidus temperature in equilibrium calculations. The solidification interval decreases slightly compared to the reference sample, which is consistent with the Scheil simulations. The DSC results show reduced liquidus and solidus temperatures, while the solidification interval remains largely unchanged by the addition of La. The addition of La alongside Al₁₃Fe₄, Mg₂Si and the eutectic α(Al) + σ[Mg(Zn, Cu, Al)₂] leads to the formation of two new La-based phases: Al₂₀Cr₂La and LaSi₂. Al₂₀Cr₂La modifies the Al₄₅Cr₇ phase and solidifies first, while LaSi₂ modifies Mg₂Si. As the La content increases, the Mg₂Si content decreases until it completely disappears at a La content of more than 0.1 mass%. On the contrary, according to the literature, the grain size increases somewhat with a higher La content.

The alloying elements present in both tool steel and aluminium are influenced in the dissolution of tool steels in molten aluminum alloys. This study aims to predict how the alloying elements in the tool steel, particularly nickel (Ni) and molybdenum (Mo), impact the formation of the interaction layer between the tool steel and the molten aluminium alloy. The interaction layer between the investigated tool steels Dievar or RavnexHD and the aluminium alloys Al99.9 and Al99.7 consists of an intermetallic and a composite layer. The interaction layer between the investigated tool steels and the AlSi12 aluminium alloy consists of three different interaction layers. The hot-work tool steel RavnexHD shows the better solubility resistance in various molten aluminium alloys. The main alloying element in the tool steels investigated that has an influence on the dissolution of tool steel in molten aluminium is Ni, which is present in higher amounts in RavnexHD. Ni incorporate in the matrix, and inhibits the diffusion of iron and aluminium through the matrix. Mo in Dievar tool steel forms carbides, allowing easier diffusion of these elements through the Fe-matrix. The resilience in molten aluminium alloys is demonstrated by Ravnex HD tool steel for hot work.