In Situ Characterization of a Nb and Mo Containing γ‐TiAl Based Alloy Using Neutron Diffraction and High‐Temperature Microscopy

Abstract

Intermetallic titanium aluminides (Ti-Al) are a class of novel, light-weight materials with attractive mechanical properties at high temperatures, making them desirable for applications in the automotive and aerospace industries. [1,2] Because of the small ''deformation window'' in thermomechanical parameter space hot-working of g-TiAl alloys is a complex and difficult task and, therefore, isothermal forming processes are favored. In order to increase the deformation window novel Nb and Mo containing g-TiAl based alloys, so-called TNM TM alloys, were developed, which solidify via the atomic-species-disordered b-phase space and also exhibit an adjustable b-phase volume fraction. [3,4] The disordered b-phase which exhibits a bcc lattice provides a sufficient number of independent slip systems. It therefore improves the deformability at elevated temperatures. For detailed information on TNM alloys, e.g., development strategy, processing, microstructure, and mechanical properties the reader is referred to recent papers. [4][5][6][7] Due to high volume fractions of disordered b-phase at temperatures above the a-transus temperature TNM alloys can be hot-die forged under near conventional conditions, which means that conventional forging equipment with minor and inexpensive modifications can be used. [4,8] At service temperature (T < 750 8C), however, the b-phase shows an ordered B2-structure, designated b 0 . At room temperature, three major phases are present in the investigated TNM alloy: g-TiAl revealing an ordered tetragonal L1 0 -structure, a 2 -Ti 3 Al forming an ordered hexagonal DO 19 -structure and b with an ordered cubic B2-structure. Note, minor phases, such as borides, play also important metallurgical roles, but will not be considered in this study. At elevated temperatures, both a 2 and b 0 show disordering, i.e., a 2 ! a, a disordered hcpstructure and b 0 ! b, a disordered bcc-structure. Volume fractions and arrangement of these phases in the microstructure are decisive for hot-working and determine the mechanical properties in service. In situ studies provide real-time information on the phase composition and evolution from the bulk of a material, thus delivering important information for defining hot-working parameters and for designing effective heat treatments in multi-phase materials. Neutron diffraction performed on TiAl alloy is sensitive to the atomic order of these phases. In the present study, we have investigated the ordering/disordering of the b 0 /b and a 2 /a phases as well as the dissolution/formation of the g-phase atIn recent times, novel titanium aluminides containing the bcc b-phase at high temperatures are being developed for improved hot-working capabilities, however, predictions of the phase diagrams are merely uncertain. Here we present in-situ neutron studies, which are particularly sensitive to the atomic disorder in the ordered phases. Complementary laser scanning confocal microscopy is employed for in-situ microstructural investigations.932

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

“…Combined time-resolved and multi-dimensional diffraction in a so-called materials oscilloscope [38] allows determination of the microstructural evolution of multi-phase titanium aluminides during thermo-mechanical processing [7,39]. In situ neutron diffraction is complementary to X-ray studies, due to the negative and positive scattering lengths of Ti and Al, respectively, and is therefore ultimately sensitive to the atomic order in a crystal structure [40,41].…”

“…Moreover, studies of microstructural evolution, allowing for the segregation of the elements in a multi-phase field, such as the co-existing γ-and α 2 -phases in titanium aluminides, require larger sample volumes. Furthermore, the chemistry of titanium alloys may change at the surface layer, such as the depletion of aluminium, as observed in a vacuum [40] or influenced by the pressure medium, as well as uni-axial stress components [48].…”

Quantum Beam Science—Applications to Probe or Influence Matter and Materials

“…The complementarity in scattering contrast is exemplified by titanium aluminides, where the Ti scattering length for neutrons is negative while Al is positive, emphasizing large structure factors for superstructure reflections that describe atomic order. This is in contrast to X-rays, for which both scattering lengths are positive, rendering them sensitive to the overall structural packing [25]. Concepts of various quantum beams may be similar, such as the dynamical theory of diffraction [26,27], which can be used to trace crystal defects with neutrons [28], high-energy X-rays [29], and electrons [30], but on very different scales.…”

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

“…Thirdly, SEM and X-ray diffraction using laboratory X-ray sources usually suffer from the restriction to the surface or to near-surface areas of the specimens. These areas might in some cases not yield sufficient grain statistics or, following extended heat treatments, be affected by the formation of an α case, an Al-depleted near-surface region of several µm in thickness [48][49][50]. Transmission electron microscopy (TEM) is also restricted to small sample volumes.…”

“…Diffraction and scattering techniques using high-energy X-rays and neutrons have been shown to overcome most of the problems mentioned above [48,49,51,52]. Having been used as a tool for material characterization since the early beginnings of research activities on TiAl alloys [46,[53][54][55][56][57][58][59], they have successfully promoted the development of the alloy in various aspects ( Figure 2).…”