Niobium - Nickel - Titanium

Abstract

The knowledge of phase equilibria in Nb-Ni-Ti system is crucial due to its utility in the development and understanding of several technologically important alloys, such as Ti and Nb containing Ni-base superalloys, Nb containing shape memory alloys based on TiNi, etc. Niobium rich ternary alloys of this system can act as hydrogen permeation membrane for separation and purification of hydrogen [2006Luo, 2008Kis]. Alloying NiTi with Nb is known to improve its ductility and expands thermal hysteresis [2004He1, 2004He2, 2006He1, 2006He2], which in turn enables the material to be deformed at low temperatures and yet be safely transported at ambient temperatures [1994Cai]. Literature concerning phase relations, crystal structure and thermodynamics is summarized in Table 1. This system was critically reviewed by [1991Gup, 2008Gup], covering experimental data available until year 2000. One of the earliest studies of the system is concerning the section TiNi3-NbNi3 by [1958Kor]. The authors suggested an isomorphous quasibinary section with an azeotropic minimum at 1285°C and ~19 at.% Ti. [1958Gla] remarked that the continuous series solid solution between TiNi3 and NbNi3 cannot be true since these phases are not isostructural. [1961Dwi] stated that continuous solid solubility at high temperatures is possible only if both TiNi3 and NbNi3 disorder to form a phase with same crystal structure, albeit the author had no experimental evidence to support this. [1966Pry] established partial isothermal sections at 1000°C and 900°C by microstructural and XRD studies, pertaining to the region NbNi-TiNi-Ni. They reported existence of four ternary phases, viz. P1 (~Ti12Nb13Ni75), P2 (~Ti20Nb5Ni75), P3 (~Ti29Nb15Ni56), and P4 (~Ti5Nb15Ni80). According to [1966Pry], alloys annealed at 900, 1000 and 1100°C, lying on the join TiNi3-NbNi3, indicated that there is no continuous solid solubility and the section is quasibinary. According to these studies at 1100°C the solubility of Nb in TiNi3 is about 3 at.% and that of Ti in NbNi3 is about 5 at.%. [1966Pry] also reported that two of the ternary phases (P1 and P2) lie along the TiNi3-NbNi3 section. The ternary phase P1 occurs between 8-15 at.% Nb and P2 occurs between 5-6 at.% Nb in this section. The phase P3 is present in the region TiNi3-NbNi3-TiNi-NbNi. The phase P4 which is present in the TiNi3-NbNi3-Ni region is nearly stoichiometric. The authors claim that this phase forms incongruently. [1966Vuc] also reported the existence of ternary phases along the TiNi3-NbNi3 join, some of which are transitional in nature. [1984Voz] investigated the precipitation behavior of Ti-60Nb (mass%) alloys on addition of 1 to 3 mass% Ni. The alloys were quenched from 1000°C and aged for 30 h between 400 to 800°C. Precipitation of (Ti,Nb)2Ni in (Ti) matrix was observed. According to [1991Zha, 2000Zha] vacuum annealing a Ti44Nb9Ni47 alloy at 1100°C for 10 h resulted in a microstructure consisting of ($Nb), $’ TiNi, and (Ti,Nb)2Ni phase having about 8 at.% Nb. Same alloy in the as-cast state also exhibited similar microstructural constituents [2000Jia]. [1992Pia] studied the effect of Nb addition on the microstructure of Ni-Ti alloys. They investigated three alloys Ti50–x/2Ni50–x/2Nbx, Ti50Ni50–xNbx and Ti50–xNi50Nbx in a wide composition range and reported the onset of eutectic structure on niobium addition to NiTi alloy. [1992Pri] presented an isothermal section at 900°C. [1994Jia] has studied the partitioning of Nb between ((Ni) and Ti2Ni (0) between 1000-1200°C employing diffusion couples. Their results indicate that Nb partitions more into ((Ni). [1995Shi] reported isothermal section at 900, 800, and 700°C employing diffusion couples and composition measurements employing EPMA. They could confirm the presence of the ternary compounds P1, P2 and P3 and also reported a new ternary compound P5 (~Ti15Nb40Ni45) lying on the TiNi-Nb7Ni6 join. However, [1995Shi] could not detect the ternary phase P4 due to the choice of the diffusion couple. [1997Gua] and [2000Gua] reported isothermal sections at 800 and 900°C respectively, using diffusion couples and EPMA. [1995Pri] reported a partial isothermal section for the region Nb-NbNi-NiTi-Ti at 900°C, where the presence of the ternary phase P3 was detected. They also presented a liquidus projection for the region. [1995Pri] also proposed that the Nb-TiNi section is quasibinary with a monovariant eutectic reaction estimated to occur at about 1170°C having liquid composition around 36Ti-26Nb-38Ni (at.%) producing TiNi and ($Nb). [1997Uey] studied the phases present in a Ti10Nb15Ni75