Thermodynamic investigation of phase equilibria in Al–Si–V system

Abstract

The Al–Si–V system is assessed to understand the phase equilibria of V-containing (3d transition metal) Al–Si alloys, which are generally of great importance for technical applications. Four annealed alloys were prepared to study the phase equilibria of Al-rich corner (V ≤ 23.3 at.%, Si ≤ 57.6 at.%) in the temperature range of 500–930 °C. The microstructure and phase constituents of the samples were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectrometer (EDS). The existence of ternary phase τ (Al_0.6Si_1.4V, TiSi₂-type) was confirmed with the composition ranging from 17.98 to 18.59 at.% Al and 48.89–49.20 at.% Si. However, it did not equilibrate with fcc(Al) below 868 °C, which was determined by differential scanning calorimetry (DSC). The Si₃V₅ appeared in Al_58.5Si_18.3V_23.2 and Al_64.7Si_20.2V_15.1 alloys annealed at 500 °C. It nearly disappeared after 3000 h-annealing and its thermodynamic stability was discussed. According to the measured phase relationships, the thermodynamic description of Al–Si–V system was optimized combing with the enthalpies of formation of binary and ternary compounds obtained by density functional theory (DFT). The Si–V system was modified to obtain the congruent liquidus of Si₃V₅ and enthalpy of formation of Si₃V₅ and Si₂V with experimental data. By comparing the calculated phase equilibria and phase transitions with experimental data, it showed that a good agreement was reached. The description of Al–Si–V system could be used to guide the development of Al–Si alloys.

Introduction

The Al–Si alloys are widely used for automotive parts, such as engine blocks, cylinder heads, chassis, and driveline due to their good castability, low density, excellent thermal conductivity, high strength and low thermal expansion coefficient [[1], [2], [3]]. However, the microstructure of conventional cast Al–Si alloys contains large brittle Fe-containing intermetallic and coarse Si phase, both of which are detrimental for mechanical properties [4,5]. The role of transition metals in cast Al–Si alloys has been a subject of many studies because of the significantly improved hardness and compressive strength [[6], [7], [8], [9]]. Among the transition metals, vanadium has low diffusivity and limited solid solubility in Al–Si alloys. The intermetallic formed during solidification process would promote the strength of cast Al–Si alloys. However, it is still controversial which phase plays the important role. Pathak et al. [10] and Sahoo and Pathak [11] found that addition of V in Al–Fe–Si alloys led to the formation of ultrafine Al₁₃(Fe, V)₃Si, which hindered the growth of large brittle intermetallic compounds and coarse Si phases. The results of Yaneva et al. [12,13] revealed that a quaternary silicide as Al_13.18(Fe, V)_1.84Si with lattice parameter of 1.2578 nm formed in low silicon alloys and a new phase nucleated and grew to be polyhedral particles as a fine α₁₃ (α-AlFeSi) quaternary intermetallic when Si concentration exceeded 3 wt%. In addition, Kasprzak et al. [9] and Meng et al. [14,15] indicated that V stabilized the phase of Al₃V or Al₂₁V₂, which served as the site of heterogeneous nucleation and improved the hardness and yield strength of Al alloys. These investigations suggest that the phase constitution and phase formation in V-containing Al–Si alloys are not clear, which is obstructing our understanding of V affects. Therefore, the phase equilibria of Al–Si–V system needs to be investigated to explain the refinement mechanism in Al–Si alloy with the addition of V.

There are few reports concerning the thermal stability of intermetallic and phase equilibria in the Al–Si–V system. Gebhardt and Joseph [16] proposed a partial liquidus projection, a partial Scheil diagram and several vertical sections of Al–Si–V system up to 6.7 at.% V. The results showed that the solubility of Si was 0.3 at.% in Al₂₁V₂ and 1.5 at.% in Al₂₃V₄, whereas Al₄₅V₇ had little solubility of Si. They also determined the three-phase fields Al₃V + Al₂₃V₄ + Si₂V and Fcc(Al) + Al₂₃V₄ + Si₂V in the Al–Si–V system. The phase relations in V-rich part concerning the superconducting phase SiV₃ were provided by Müller [17]. It showed that the solubility of Al in SiV₃ was 3.5 at.% at 1000 °C and no ternary phase was found. Huber et al. [18,19] studied the phase equilibria and phase transitions of Al–Si–V system up to 50 at.% V in the temperature range from 500 to 850 °C. The electron probe microanalysis (EPMA) results indicated that the solubility of Si in Al₄₅V₇ was higher than that of other Al–V compounds, whereas the Al₂₃V₄ and Al₂₁V₂ showed no solubility of Si. A TiSi₂-type ternary compound τ-Al_0.6Si_1.4V was discovered and the structure was determined to be a space group of Fddd, a = 8.091 Å, b = 4.697 Å and c = 8.501 Å by means of single crystal X-ray diffraction (XRD). The analysis of the morphologies of cast samples suggested that the extension of the Si₃V₅ crystallization field towards the Al-rich corner. Except the experimental investigations, few thermodynamic descriptions of the Al–Si–V system was reported.

Lack of thermodynamic investigation of the Al-rich corner in Al–Si–V system is the barrier to explain the phase transformation and refinement mechanism of V addition in Al–Si alloys. Therefore, the present work focuses on the experimental determination and thermodynamic analysis of the phase equilibria in the Al-rich corner of Al–Si–V system. The phase constitution and phase relationship in each sample ranging from 500 to 930 °C are determined by XRD, scanning electron microscopy (SEM) and differential scanning calorimetry (DSC).