Abstract
A thermodynamic analysis of the synthesis of face-centred cubic (fcc) and body-centred cubic (bcc) solid solutions of Ti-based alloys produced by mechanical alloying was performed. Four Ti-based alloys were analysed: (i) Ti-13Ta-3Sn (at.%), (ii) Ti-30Nb-13Ta (at.%), (iii) Ti-20Nb-30Ta (wt. %) and (iv) Ti-33Nb-4Mn (at.%). The milled powders were characterized by X-ray diffraction, and the crystallite size and microstrain were determined using the Rietveld and Williamson–Hall methods. The Gibbs free energy of mixing for the formation of a solid solution of the three ternary systems (Ti-Ta-Sn, Ti-Nb-Ta and Ti-Nb-Mn) was calculated using an extended Miedema’s model, applying the Materials Analysis Applying Thermodynamics (MAAT) software. The values of the activity of each component were determined by MAAT. It was found that increasing the density of crystalline defects, such as dislocations and crystallite boundaries, changed the solubility limit in these ternary systems. Therefore, at longer milling times, the Gibbs free energy increases, so there is a driving force to form solid solutions from elemental powders. Finally, there is agreement between experimental and thermodynamic data confirming the formation of solid solutions.
Highlights
The main metallurgical characteristics of titanium and titanium alloys are the following: a combination of high strength, stiffness, toughness, low density, good corrosion resistance and biocompatibility [1].Titanium alloys are used widely in engineering applications such as aeronautical/aerospace, chemical and medical among other high-performance applications [2]
The alloy was milled using a planetary mill (Retch PM400, Haan, Germany) at 250 rpm and using the following conditions: (i) jar/balls of Yttrium stabilized ZrO2 (YSZ, volume of the jar was 250 mL), (ii) agate balls of two diameters, 10 and 5 mm, with a constant ball-to-powder ratio of 10:1, (iii) the jars were filled with ultra-pure argon gas, (iv) 2 wt. % of stearic acid was used as a process control agent (PCA) to prevent cold welding and (v) the powders were milled for different periods ranging from 5 to 100 h and for each specific milling time, powder mixtures were collected in a dry box under Ar atmosphere and dispersed in hexane to prevent oxidation during handling
There is agreement between X-ray powder diffraction (XRD) patterns and thermodynamic data related to the formation of free energy of mixing, enthalpy and entropy of mixing and activities can be obtained using the Materials Analysis Applying Thermodynamics (MAAT) software
Summary
The main metallurgical characteristics of titanium and titanium alloys are the following: a combination of high strength, stiffness, toughness, low density, good corrosion resistance and biocompatibility [1]. Titanium alloys are used widely in engineering applications such as aeronautical/aerospace, chemical and medical among other high-performance applications [2]. Ti-6Al-4V alloy is the most commonly used and accounts for almost 50% of all the alloys used in engineering applications [3,4,5]. Ti2 AlNb alloys have received considerable attention as new potential structural materials in advanced gas turbine jet engines [6]. Other alloys used or evaluated extensively in aerospace, missile and space as well as other high-performance applications include Ti-6V-2Sn-2Zr2Cr-2Mo-Si, Ti-6Al-6V-2Sn, Ti-10V-2Fe-3A1 and. The chemical industry has been interested principally in the commercially pure
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