In Situ Observation of Various Phase Transformation Paths in Nb‐Rich TiAl Alloys during Quenching with Different Rates

Abstract

Intermetallic g-TiAl based alloys are a class of novel, light-weight structural materials with attractive mechanical properties for advanced high-temperature applications. Due to their low density (4 g cm À3 ), their high yield and creep strength up to 800 8C and their good oxidation resistance they have the potential to replace the heavier Ni-based superalloys (8 g cm À3 ) in industrial and in aviation gas turbines as well as in automobile engines. [1] Conventional titanium aluminide alloys consist of tetragonal g-TiAl (L1 0 structure; P 4/m m m) and small small amounts of hexagonal a 2 -Ti 3 Al (D0 19 structure; P 6 3 /m m c). Through special heat treatments various microstructures can be established in these two phase alloys to optimize their mechanical properties. [2] The most restricting factor for a broad industrial implementation of titanium aluminides is their low ductility that also limits their workability. A promising design strategy to overcome the brittleness and to improve the hot workability is to induce the formation of more ductile phases by adding ternary alloying elements. The body-centered cubic (bcc) high-temperature b-Ti(Al) phase (A2 structure; I m 3 m) can act as a ductilizing phase in TiAl alloys because it provides a high number of independent slip systems. In recent years several authors have reported that stabilizing the b phase by alloying elements such as Nb, Mo, Ta, or V, significantly improves the hot workability. [3][4][5] Additionally, novel types of microstructures can be achieved exploiting the ternary solid state transformations. [3,4] In spite of this progress, the exact pathway of phase transformations and thus the evolution of microstructures in b phase containing TiAl alloys are not fully understood up to now. At lower temperatures, the disordered bcc b phase can transform to ordered cubic b o -TiAl phase (B2 structure; P m 3 m). However, calculated and experimental transition temperatures show large discrepancies. [6,7] In high-Nb containing TiAl alloys b and/or b o can decompose to ordered hexagonal v o -Ti 4 Al 3 Nb phase (B8 2 structure; P 6 3 /m m c). [8,9] The formation of an orthorhombic phase (B19 structure; P m m a) is reported in Al-lean and Nb-rich TiAl alloys and is interpreted as a transition structure between the cubic b and/ or b o and the orthorhombic O-Ti 2 AlNb phase (C m c m). [4] The crystallographic data of all phases mentioned above and relevant for this work are listed in Table 1.Ordered phases, such as b o and v o , are often assumed to be detrimental to ductility due to their low crystal symmetry. Otherwise the orthorhombic O phase is known to be relatively ductile and even v o containing TiAl alloys show good plastic formability at 800 8C. [8] Thus, with respect to alloy design and processing, it is of high importance to know which kind of additional phase will be formed and which further phase transformations occur during processing and service.In recent years intermetallic g-TiAl based alloys with additional amounts of the ternary b phase have a...

Ordered α2 to ωo phase transformations in high Nb-containing TiAl alloys

“…In the disordered x phase, the 1a and 2d Wykoff positions are occupied by the same type of atom [11]. On the other hand, the a 2 -Ti 3 Al phase in the D0 19 structure is a main phase in TiAl intermetallics, in which the Wykoff positions 2c and 6 h are occupied by Al and Ti atoms, respectively. In disordered a phase, the 2c positions are occupied by the same type of atom.…”

“…As a result, efforts have been made to eliminate the b o (x) phase by thermomechancial processing or heat treatments. In recent years, the ordered x phases in high Nb-TiAl alloys have garnered increasing attention due to their variable structures and morphologies, which can be induced by different heat treatment processes [10][11][12][13][14][15][16][17][18][19][20]. Commonly, the designation x is restricted to the disordered x phase (space group P6/ mmm) in Ti based alloys.…”

In Situ Synchrotron Study of B19 Phase Formation in an Intermetallic γ‐TiAl Alloy

“…[6,7] In addition to these main constituents, several phases with lower symmetry are mentioned in literature. Among these are hexagonal ω-related phases [9] and orthorhombic phases. [10,11] Abe et al [10] reported on an orthorhombic phase with B19 structure (Pmma) that forms very fine precipitates upon rapid cooling in a Ti-48 Al alloy.…”

In Situ High-Energy X-ray Diffraction during Hot-Forming of a Multiphase TiAl Alloy

“…The phase diagram of Nb rich γ-TiAl based alloys is rather complex and still under research [25,26] even though it is composed of only three elements. At temperatures up to service temperature (about 700 °C to 800 °C ) γ-TiAl alloys mainly consist of the tetragonal γ-TiAl phase (L10 structure, P4/mmm) and the hexagonal α2-Ti3Al phase (D019 structure, P63/mmc).…”