Experimental studies and thermodynamic simulation of phase transformations in high Nb containing γ-TiAl based alloys

Abstract

Solid-state phase transformations and phase transition temperatures in Ti-45 at.% Al and Ti-45 at.% Al-(5, 7.5, 10) at.% Nb alloys were analyzed experimentally and compared to thermodynamic calculations. Results from scanning electron microscopy, high-energy and conventional X-ray diffraction as well as differential scanning calorimetry were used for the characterization of the prevailing phases and phase transformations. For the prediction of phase stabilities and phase transition temperatures, thermodynamic calculations using the CALPHAD method were conducted. In order to achieve better agreement between calculated and experimental results, a commercially available database was modified using our own results from thermo-physical measurements and annealing treatments.

Hydrostatic Compression Behavior and High-Pressure Stabilized β-Phase in γ-Based Titanium Aluminide Intermetallics

“…Both alloys have been manufactured at the GKSS research center, Geesthacht, Germany, in a similar way and in consecutive batches [28] in order to study compositional effects only. The thermo-calorimetric and in situ diffraction studies undertaken by Chladil et al [29] and Yeoh et al [31] have been combined [30]. For Ti-45Al-7.5Nb-xC alloys, increasing carbon additions of x {0, 0.25, 0.5}% increase the eutectoid temperature T eu to {1432, 1453, 1473} K, respectively, but otherwise, the transformation behavior is the same, specifically the γ-solvus or α-transus remains stable at T α = 1565 K. These expected transition temperatures T eu and T α are marked in Figure 10 for the Ti-45Al-7.5Nb-0.25C used in this study.…”

“…Although niobium is a β-phase stabilizer, β-containing phase fields were not detected in concentrations up to 7.5 at. % [31], while higher concentrations above 8% Nb rendered a detectable amount of this phase [30,75]. Cγ and Cα are the phase fractions of γ and α/α2, respectively, (Cγ + Cα = 1), and are represented by the black markers and long-short-dashed line in Figure 9.…”

“…A variety of alloys based on TiAl with additions of niobium has been developed in the so-called TNB alloy series [5,[25][26][27], where Nb substitutes Ti sites and eventually stabilizes the bcc-based β-phases. In the present study, we have investigated the TNB composition Ti-45Al-7.5Nb-0.25C, which has been produced through a powder-metallurgical route by Gerling et al [28] using gas-atomization and subsequent hot-isostatic pressing for 2 h at 200 MPa and 1553 K. It is obtained from the same series as extensively characterized by Chladil et al [29,30] and Yeoh et al [31], showing globular γ-TiAl/α 2 -Ti 3 Al grains with an average size of about 15 µm (see Figure 1b in [29]). In situ investigations revealed conventional transformation behavior for this Ti-45Al-7.5Nb-0.25C alloy, with T eu = 1453 K and T α = 1565 K, with an additional peritectic (α + β)-phase field at T per~1 500 K, but no (α + β + γ)-phase field has been observed.…”

In situ high-energy X-ray diffraction study and quantitative phase analysis in the α+γ phase field of titanium aluminides

“…The investigated Ti-45Al-7.5Nb-0.5C sample (composition in atomic %) is part of a series [8] of different high Nb containing -TiAl based alloys which have been produced using a powder metallurgical approach, guaranteeing a homogeneous chemical distribution of the constituting elements. Pre-alloyed powder of the alloy was produced by means of gas atomization in the PIGA-facility (Plasma Melting Induction Guiding Gas Atomization) at the GKSS Research Center [9].…”

Phase Equilibria and Phase Transformations in Molybdenum-Containing TiAl Alloys

“…The calculated temperature is about 1060°C, whereas the experimental data from literature indicate an increase of this reaction to 1165°C. However, it should be mentioned that previous thermodynamic calculations of transition temperatures for other highly -stabilized TiAl alloys also showed a strong deviation from experimental data, especially with respect to the eutectoid temperature [13]. Moreover, in the experimental phase diagram (Figure 1b), the lower boundary of the single -phase field region, particularly with regards to the transformation point in which the -phase meets with the three other phase-field regions, is shifted on the one hand to a lower temperature and on the other hand to a significantly higher Mo concentration.…”