Thomas Schmoelzer

In situ study of dynamic recrystallization and hot deformation behavior of a multiphase titanium aluminide alloy

Hot-compression tests were conducted in a high-energy synchrotron x-ray beam to study in situ and in real time microstructural changes in the bulk of a β-solidifying titanium aluminide alloy. The occupancy and spottiness of the diffraction rings have been evaluated in order to access grain growth and refinement, orientation relationships, subgrain formation, dynamic recovery, and dynamic recrystallization, as well as phase transformations. This method has been applied to an alloy consisting of two coexisting phases at high temperature and it was found that the bcc β-phase recrystallizes dynamically, much faster than the hcp α-phase, which deforms predominantly through crystallographic slip underpinned by a dynamic recovery process with only a small component of dynamic recrystallization. The two phases deform to a very large extent independently from each other. The rapid recrystallization dynamics of the β-phase combined with the easy and isotropic slip characteristics of the bcc structure explain the ex...

Phase transition and ordering behavior of ternary Ti–Al–Mo alloys using in-situ neutron diffraction

Abstract Neutron diffraction has been used for in-situ investigations to elucidate the phase transformation behavior of two Mo-containing TiAl alloys with compositions of Ti-44Al-3Mo and Ti-44Al-7Mo (in at.%). Five different phases are present in these alloys. These include three ordered phases at room temperature, namely α2, β0 and γ and two disordered phases, and, which occur at higher temperatures. The sequence of the three phase transformations in each alloy has been determined. The phase transformation and disordering/ordering temperatures were determined on heating and cooling from the diffracted peak intensities. The neutron experiments are particularly sensitive to the order–disorder transitions in TiAl alloys, which are compared with the overall phase fractions obtained from previous high energy X-ray diffraction. Hysteresis and undercooling effects are observed for the various phase transformations and depend on the nature of atomic rearrangements.

Influence of Heat Treatments on the Microstructure of a Multi-Phase Titanium Aluminide Alloy

Abstract Intermetallic titanium aluminides are employed in aircraft engines and automobile engines because of their low density and excellent high-temperature properties. Todays TiAl-based alloys are multi-phase alloys of a complex structure which mainly consist of γ-TiAl, α2-Ti3Al and low fractions of a βo-TiAl phase. An example of such an alloy is the so-called TNM alloy which exhibit a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at %). In this alloy, solidification takes place via the β-phase, with the consequence of a fine-grained and nearly segregation-free microstructure. In spite of that, the cast microstructure also contains coarser grains which can act as crack initiators at room temperature and will reduce the deformation capability during tensile tests. Within the framework of this paper, heat treatment studies were conducted on a cast and hot isostatically pressed material with the primary aim of a microstructural homogenization in order to reduce the crack-initiating microstructural components and, hence, increase its fracture elongation at room temperature. In further heat treatments, microstructures with balanced mechanical properties were adjusted.

Phase Transition and Ordering Temperatures of TiAl-Mo Alloys Investigated by In-Situ Diffraction Experiments

Intermetallic TiAl alloys with a significant volume fraction of the body-centered cubic β-phase at elevated temperatures have proven to exhibit good processing characteristics during hot-working. Being a strong β stabilizer, Mo has gained importance as an alloying element for so-called β/γ-TiAl alloys. Unfortunately, the effect of Mo on the appearing phases and their temperature dependence is not well known. In this work, two sections of the Ti-Al-Mo ternary phase diagram derived from experimental data are shown. These diagrams are compared with the results of in-situ high-temperature diffraction experiments using high-energy synchrotron radiation.

Dynamic Recovery and Recrystallization during Hot-Working in an Advanced TiAl Alloy

Abstract Intermetallic TiAl alloys are light-weight high-temperature materials and intended to partly replace Ni based alloys in jet engines. Due to difficult forming operations, component prices are high and limit the possible field of application. During hot-working, recovery and recrystallization effects determine the microstructural evolution and thereby the mechanical properties of the finished part as well as its behavior during deformation. To study the occurring phenomena, in-situ diffraction experiments with high-energy X-rays were conducted. By means of this method, the dominating processes were identified. The results were validated through electron back scatter diffraction experiments.

Hot Deformation of Cast and Extruded TiAl: An In Situ Diffraction Study

Intermetallic TiAl alloys are a class of innovative high-temperature materials which are developed to replace the substantially denser Ni-base alloys in low-pressure turbine blades of jet engines. By streamlining the production process of these parts, a substantial decrease in production costs can be achieved. To this end, a profound knowledge of the microstructural processes occurring during hot deformation is a prerequisite. To investigate the microstructural development during forming operations, cast and extruded as well as only cast specimens were hot-deformed and the microstructural development investigated in-situ by means of a novel diffraction method. This powder diffraction method utilizes the behavior of individual reflection spots on the Debye-Scherrer rings for deriving the materials response to the deformation imposed. It was found that the behavior of the two specimens is rather similar, although the starting microstructures show pronounced differences.

Morphology of Sintered Cr-Mo Porous Materials

Chromium and molybdenum exhibit continuous solubility in the solid phase region at temperatures of 908°C and above [1]. At lower temperatures, the system exhibits a miscibility gap. Furthermore a congruent minimum in the liquidus boundary exists at 1854°C. Chromium and molybdenum powders with different particle morphologies were mixed and porous green parts were produced by pressing. Sintering experiments were performed at different temperatures and for different chromium to molybdenum ratios. To investigate the evolution of the microstructure, sintering was interrupted at different temperatures and points in time. The microstructure and morphology of the sintered parts was investigated by scanning electron microscopy as well as light optical microscopy. It was found that during sintering, a Cr-Mo solid solution is formed. Depending on the molybdenum content, this induces either shrinking or swelling of the porous parts. Samples exhibited a linear expansion of up to 10% and final porosities of up to 65%.