Microstructures and phases analysis of the 60Al-40V master alloy produced by aluminothermic process

Abstract

The Al-V binary system is one of the most important sub-systems of the α-β Ti-based alloys designed and used in diverse applications. The Al-V master alloy is used as a semi-finished product in the production of Ti-6Al-4V alloy. The alloy is receiving much attention in the automobile and aerospace industries due to its lightweight and excellent mechanical properties, i.e., the combination of high strength and fatigue resistance at elevated temperatures. In this work, an aluminothermic reaction process was proposed for the production and development of the Al-V master alloy. Al metal and vanadium pentoxide (V2O5) were mixed in a proportion that produced an exothermic reaction capable of generating the master alloy and Al2O3 rich slag. The target was to produce a 60 wt. % Al and 40 wt. % V master alloy through the aluminothermic process. The thermodynamic calculations, as well as the characterisation of the 60Al-40V master alloy, were carried out in order to understand and improve the production process of the master alloy.

The study was aimed at determining the thermodynamic parameters using thermodynamic computer software and analytical characterisation techniques to assist in the development and production of the Al-V master alloy.

The CALPHAD package within Thermo-CalcTM software was used to predict the equilibrium phase diagram, thermodynamic properties such as the Gibbs free energy, enthalpy and /or entropy of formation and activities of constitutive elements in the Al-V system. The aluminothermy produced master alloy was experimentally characterised using techniques such as light optical microscopy (LOM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-Ray diffraction (XRD) analyses. These techniques were employed to assess the viability of the aluminothermic process in terms of microstructure, chemical composition, crystallographic structure, and phases present in the produced master alloy. The differential scanning calorimetry (DSC) and thermogravimetry (TG) thermal analysis techniques were used to determine the phase transformations and phase stability.

The Al-V equilibrium binary phase diagram that was predicted by the Thermo-CalcTM software agreed with some of the phases that were observed in this work, i.e., the following phases were observed as predicted by the Thermo-CalcTM: the Al (fcc), the V (bcc) and three intermetallic compounds namely the Al23V4, Al3V and the Al8V5 phases. The maximum solubility of V in liquid Al of 1.2 wt. % was found at a temperature of 664 °C. The liquidus line was fairly reproduced and the invariant temperatures of the melting temperature of the five intermetallic phases were determined. The value of the integral molar Gibbs energy of mixing was found to be negative, about -25 J/mol-atom for the most stable phase (Al3V), indicating that the reaction was spontaneous and was favoured by the change in the entropy and enthalpy. The activities of both constitutive elements in the Al-V system showed a strong negative deviation from the ideal behaviour which indicates that a good mixing ability existed between components. The enthalpies of formation of all intermetallic phases were negative for the entire composition range. The predicted phase diagram and thermodynamic properties determined in this work were fairly in agreement with what is reported in the literature. The microstructural analysis through both LOM and SEM indicated the dendritic structure, the same morphology was observed after the DSC-TG test. The quantitative determination of the elemental composition of each phase by EDX analysis revealed that the average elemental composition of the alloy was 63 ± 0.05 wt. % and 37± 0.05 wt. % for Al and V, respectively. With this composition, the master alloy produced was close to the theoretical 60Al-40V master alloy. The XRD technique revealed diffraction peaks principally of two intermetallic compounds, namely, the Al3V and Al8V5 phases, and was confirmed by SEM/EDX analysis. Phase transitions of the intermetallic phases formed in the material were observed and their invariant temperatures were determined by the DSC-TG analysis. The DSC-TG trace curves showed peaks during both the heating and cooling process and the peritectic temperatures obtained correspond with the invariant reactions of the intermetallic Al21V2, Al3V and Al8V5 phases as predicted by the Thermo-CalcTM software. The obtained results show that the most stable phase found at high temperature was identified to be the Al3V phase. A good agreement between the calculated phase diagram, using the Thermo-CalcTM software, and experimental data was achieved.