Properties and structural characteristics of Ti–Nb–Al alloys

　　The Ti–Nb system has been extensively investigated for scientific and applied reasons [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Special properties such as superconductivity and shape memory effect (SME) together with a considerable dumping phenomenon due to peaks in the internal friction, justify the interest for Ti–Nb alloys [1], [2]. The multiplicity of phase transformations in this system is associated with complex structures and a broad range of mechanical and thermal properties [3], [4], [5], [6], [7], [8]. Binary alloys with β phase, body centered cubic (BCC) structure, obtained after quenching, have been widely studied for most Nb concentrations of useful interest [3], [4], [5], [6].

　　Ternary alloys were also object of investigations. For some industrial applications it is common practice to add Al as an α, hexagonal closed-packed (HCP) Ti structure stabilizer. The Al addition increases both the mechanical and thermal resistance of the alloy [10], [11]. This advantageous effect of Al addition has been reported in other Ti alloys. Previous works on Ti–Cr, Ti–Mo and Ti–V systems [12], [13], [14] have shown the additional influence of Al as an α stabilizer in the presence of a strong β stabilizer, such as Cr, Mo or V.

　　Since Nb is also a strong β stabilizer, the objective of the present paper was to expand the knowledge on the combined α and β phases effects using Al as an addition to Ti–Nb. These effects were investigated in properties directly related to the SME, such as the elastic modulus, E, and the internal friction, Q−1. Alloys with 10–40% in weight of Nb and 2–15% in weight of Al were chosen for the investigation. These ranges of composition investigated correspond to the relevant structural transformations and property modifications, which occurs in the Ti–Nb–Al system.

　　It has been known that quenched alloys in the binary Ti–Nb system present a metastable α′ martensitic structure for Nb contents up to 12% in weight [3], [4], [5]. This martensite has the same HCP structure of α-Ti. With increasing Nb content, the value of the elastic modulus, E = 110 GPa, for pure Ti, decreases sharply and reaches 60 GPa for the 12% Nb alloy.

　　Above 12% Nb the α′-HCP martensite undergoes a rhombic distortion, giving rise to a typical α″-orthorhombic martensite [2], [3], [4], [5]. At the same time, β-BCC and its precursor ω phase, both metastable, are also formed. The ω phase has a highly distorted transitional hexagonal lattice between α and β. In particular, the corresponding increase that occurs in the value of E, which reaches 90 GPa for 32% Nb, was attributed to the participation of the ω phase [3]. An increase in hardness, from 12 to 32% Nb, was also associated with the occurrence of ω [2], [3], [4], [10], [11].

　　Between 32 and 42% Nb the amount of ω decreases while increasing the relative proportion of β. As a consequence, E decreases once again to its lowest value of 60 GPa [3], [4]. In this range of percentages, the α″ martensitic structure is still the main structure. However, for Nb contents above 42%, the structure tends to transform into single β and the value of E goes up again. For instance, at 50% Nb, it reaches E = 70 GPa.

　　With all these changes taking place in the binary Ti–Nb system, an important question to be addressed refers to the effect of adding an α stabilizer. Therefore the present work analyzes the effect of Al as a third element added up to a maximum of 15% into Ti–Nb alloys with a limit of 40% Nb.