In Situ Diffraction Experiments for the Investigation of Phase Fractions and Ordering Temperatures in Ti‐44 at% Al‐(3‐7) at% Mo Alloys

Abstract

During the last 20 years, research and development efforts have been undertaken to develop g-TiAl based alloys as a replacement for Ni-based superalloys for high-temperature applications in turbine blades of advanced aero engines and turbo-chargers of automotive combustion engines. [1] Intermetallic g-TiAl based alloys are intended for use in the temperature range from 600 to 900 8C and there is growing demand for alloys which can be deformed easily by cost-effective forming operations. One possible approach is to design alloys with microstructures in which homogenously distributed b and g-phase are the main constituents. [2] Therefore, these alloys are named b/g-alloys.[3] Since Mo is a strong b stabilizer the ternary system Ti-Al-Mo is well suited for studying this type of alloys. [4] Despite the importance of Mo as an alloying element, the knowledge of its effect on phase diagram, transformation temperatures, and order/ disorder transition temperatures is limited. In many advanced multi-phase TiAl alloys three intermetallic phases, g, a 2 , and b o , are the dominating microstructural constituents, all of which are ordered at room temperature. At elevated temperatures the ordered hexagonal a 2 -phase (D0 19 ) disorders to a (A3) and the ordered cubic b o -phase (B2) disorders to the body-centered cubic b-phase (A2) while the g-phase (L1 0 ) remains ordered up to its dissolution temperature (T g ). The phase fractions present have a strong influence on the mechanical properties of the material and on the processing characteristics at hot-working temperatures. High b/b o -phase contents, for instance, improve the deformation behavior during hot-working but simultaneously decrease the creep resistance if prevailing at service temperature. [5] Additionally, the room temperature ductility is negatively affected by high b o -phase fractions. [6] In the present work, sections of the ternary phase diagram Ti-Al-Mo, obtained by thermodynamic calculations and COMMUNICATION Being a strong b stabilizer, Mo has gained importance as an alloying element for so-called b/g-TiAl alloys. Intermetallic TiAl-based alloys which contain a significant volume fraction of the body-centered cubic b-phase at elevated temperatures have proven to exhibit good processing characteristics during hot-working. Unfortunately, the effect of Mo on the appearing phases and their temperature dependence is not well known. In this work, sections of the Ti-Al-Mo ternary phase diagram derived from thermodynamic calculations as well as experimental data are presented. The phase transition temperatures stated in these phase diagrams are compared with the results of high-temperature diffraction studies using high-energy synchrotron radiation. Additionally, the disordering temperature of the b o -phase is determined. 306 wileyonlinelibrary.com ß

Phase Equilibria and Phase Transformations in Molybdenum-Containing TiAl Alloys

“…Further investigations were performed to determine the phase transition temperatures of alloy Ti-44Al-3Mo [3]. By means of HEXRD the 2 temperature was determined to be at ~1205°C, which is significantly higher than that predicted by the calculated phase diagram (~1058°C) in Figure 1a.…”

“…Thus, the ( 2 + + )-phase field region below the 2 temperature as shown in Figure 1b cannot be correct. Phase evolution as a function of temperature resulting from in-situ HEXRD experiments of Ti-44Al-7Mo [3]. Circles stands for the -phase, whereas triangles and squares represent the -and -phase, respectively.…”

“…According to this diagram, the following solidification pathway is expected for Ti-44Al-3Mo: L L+ + + + 2 + + , while alloy Ti-44Al-7Mo solidifies and transforms according to L L+ + + + + . In-situ HEXRD was also used to determine the disordering temperature of the o -phase during continuous heating [3]. According to these investigations disordering takes place for Ti-44Al-3Mo at approximately 1240°C.…”

“…Based on the fact that Mo is a strong stabilizer, the ternary system Ti-Al-Mo is well-suited for studying this type of alloys. In this paper, which is based on a recent study [3], representative sections of the ternary Ti-Al-Mo phase diagram, derived from thermodynamic calculations and experimental results, are presented. In principle, advanced multi-phase TiAl alloys contain three intermetallic phases, namely , 2 and o , which are all ordered at ambient temperature.…”

In Situ Characterization Techniques Based on Synchrotron Radiation and Neutrons Applied for the Development of an Engineering Intermetallic Titanium Aluminide Alloy

“…For in situ investigations of the phase transformation behavior of titanium aluminides, high-energy X-ray diffraction (HEXRD) has evolved into a powerful and versatile tool [48,50,51,[63][64][65][66]. At synchrotron radiation sources, a white X-ray beam is produced by the deflection of charged particles, either electrons or positrons, by means of a bending magnet or an insertion device, i.e., a wiggler or an undulator, as shown in Figure 4 [67].…”