Andreas Stark

Neutrons and Synchrotron Radiation in Engineering Materials Science

PART I: GENERAL MICROSTRUCTURE AND PROPERTIES OF ENGINEERING MATERIALS INTERNAL STRESSES IN ENGINEERING MATERIALS TEXTURE AND TEXTURE ANALYSIS IN ENGINEERING MATERIALS PHYSICAL PROPERTIES OF PHOTONS AND NEUTRONS RADIATION SOURCES GENERATION AND PROPERTIES OF NEUTRONS PRODUCTION AND PROPERTIES OF SYNCHROTRON RADIATION PART II: METHODS INTRODUCTION TO DIFFRACTION METHODS FOR INTERNAL STRESS ANALYSES STRESS ANALYSIS BY ANGLE-DISPENSIVE NEUTRON DIFFRACTION STRESS ANALYSIS BY ENERGY-DISPERSIVE NEUTRON DIFFRACTION RESIDUAL STRESS ANALYSIS BY MONOCHROMATIC HIGH-ENERGY X-RAYS RESIDUAL STRESS ANALYSIS BY WHITE HIGH ENERGY X-RAYS REFLECTION MODE TRANSMISSION MODE DIFFRACTION IMAGING FOR MICROSTRUCTURE ANALYSIS BASICS OF SMALL-ANGLE SCATTERING METHODS SMALL-ANGLE NEUTRON SCATTERING DECOMPOSITION KINETICS IN COPPER-COBALT ALLOY SYSTEMS: APPLICATIONS OF SMALL-ANGLE X-RAY SCATTERING New Developments in Neutron Tomography NEUTRON AND SYNCHROTRON -RADIATION-BASED IMAGING FOR APPLICATIONS IN MATERIALS SCIENCE - FROM MACRO- TO NANOTOMOGRAPHY mu-TOMOGRAPHY OF ENGINEERING MATERIALS DIFFRACTION ENHANCED IMAGING PART III: NEW AND EMERGING METHODS 3D X-RAY DIFFRACTION MICROSCOPE 3D MICRON-RESOLUTION LAUE DIFFRACTION QUANTITATIVE ANALYSIS OF THREE-DIMENSIONAL PLASTICS STRAIN FIELD USING MARKERS AND X-RAY ABSORPTION TOMOGRAPHY COMBINED DIFFRACTION AND TOMOGRAPHY PART IV: INDUSTRIAL APPLICATIONS DIFFRACTION-BASED RESIDUAL STRESS ANALYSIS APPLIED TO PROBLEMS IN THE AIRCRAFT INDUSTRY OPTIMIZATION OF RESIDUAL STRESSES IN CRANKHAFTS

Phase transformation kinetics during continuous heating of a β-quenched Ti–10V–2Fe–3Al alloy

The effect of heating rate on the phase transformation kinetics of a Ti–10V–2Fe–3Al metastable β titanium alloy quenched from the β field is investigated by fast in situ high energy synchrotron X-ray diffraction and differential scanning calorimetry. The initial microstructure is formed by α″ martensite and fine ωath particles distributed in the retained β-phase matrix. The phase transformation sequence varies with the heating rate as revealed by analysis of the continuous evolution of crystallographic relationships between phases. At low temperatures an athermal reversion of α″ martensite into β takes place. This reversion occurs to a larger extent with increasing heating rate. On the other hand, diffusion–driven precipitation and growth of the ω phase is observed for lower heating rates accompanying the reverse martensitic transformation. Furthermore, the results show that the stable α phase can form through three different paths: (a) from the ω phase, (b) from α″ martensite, and (c) from the β phase.

Influence of rare-earth addition on the long-period stacking ordered phase in cast Mg–Y–Zn alloys

The microstructure and thermal stability of the Mg97Y2Zn1 (at.%) alloy, modified with the addition of 0.5 at.% of gadolinium or neodymium, have been examined by synchrotron radiation diffraction during in situ differential scanning calorimetry. The microstructure of the three alloys consists of magnesium dendrites with the Long Period Stacking Ordered (LPSO) phase at interdendritic regions. Rare-earth atoms substitute yttrium atoms in the LPSO phase, promoting the formation of the 14H structure. Lattice parameters of the LPSO do not change significantly with the rare-earth addition. However, they reduce the melting point of the LPSO phase, especially in the case of neodymium addition.

Neutron and synchrotron probes in the development of Co-Re-based alloys for next generation gas turbines with an emphasis on the influence of boron additives

Nickel-based superalloys are the materials of choice in the hot section of current gas turbines, but they are reaching temperature limits constrained by their melting temperature range. Co–Re alloy development was prompted by a search for new materials for future gas turbines, where the temperature of application will be considerably higher. Addition of the very high melting point refractory metal Re to Co can increase the melting range of Co alloys to much higher temperatures than the commercial Co alloys in use today. The alloy development strategy is first discussed very briefly. In this program, model ternary and quaternary compositions were studied in order to develop a basic understanding of the alloy system. In situ neutron and synchrotron measurements (small and wide angle) at high temperatures were extensively used for this purpose and some selected results from the in situ measurements are presented. In particular, the effect of boron doping in Co–Re–Cr alloys and the stability of the TaC precipitates at high temperatures were investigated. A fine dispersion of TaC precipitates strengthens some Co–Re alloys, and their stability at the application temperature is critical for the long-term creep properties.

Study of the Solidification of AS Alloys Combining In Situ Synchrotron Diffraction and Differential Scanning Calorimetry

In situ synchrotron diffraction experiments were performed during Differential Scanning Calorimetry (DSC) of AS31, AS33 and AS35 alloys. The samples were encapsulated in stainless steel crucibles during the measurement using an empty crucible as the reference. The samples were heated up to 680°C, melted and solidified in the beginning of the experiment in order to fill the crucible. This short cycle was followed by three subsequent cycles between 400°C and 680°C with 5, 10 and 20 K/min heating and cooling rates with 5 min of holding time in the molten state. The diffraction patterns were recorded every 6 s during the DSC program by a Perkin-Elmer XRD 1622 Flatpanel detector including an acquisition time of 3 s and the collection of reference images. The endothermic and exothermic peaks are in correlation with the dissolution and formation of new diffraction patterns, respectively. During cooling from the liquid state, first, α-Mg dendrites solidify, followed by the formation of Mg2Si and Mg17Al12 intermetallics. The results are correlated with those obtained by thermodynamic simulations performed with the software Pandat.

Texture Formation during Hot-Deformation of High-Nb Containing γ-TiAl Based Alloys

In this study texture and microstructure formation in high-Nb containing TiAl alloys during lab-scale compression experiments and “near conventional” forging on an industrial scale are investigated. The deformation temperatures range from 700 °C up to temperatures close to the α transus temperature (Tα = 1295 °C). Depending on the deformation conditions, the texture of the tetragonal γ-TiAl phase is formed by pure deformation components, components related to dynamic recrystallization, or transformation components. This changing corresponds with microstructural observations. The hexagonal phases α2-Ti3Al and α-Ti(Al) show a similar texture as it is known for Ti and Ti-base alloys after compressive deformation at elevated temperatures. In contrast to the γ texture, no significant change of the α/α2 texture was observed in the investigated temperature range. In the alloy with a composition of Ti-45Al-10Nb (in at.%) even deformation textures of ternary intermetallic phases, as the hexagonal ωo-Ti4Al3Nb and the cubic βo-TiAl(Nb) phase, respectively, were measured and analyzed.

A Study of Recrystallization and Phase Transitions in Intermetallic Titanium Aluminides by In-Situ High-Energy X-Ray Diffraction

High-energy synchrotron X-ray diffraction is a novel and powerful tool for bulk studies of materials. In this study, it is applied for the investigation of an intermetallic γ-TiAl based alloy. Not only the diffraction angles, but also the morphology of reflections on the Debye-Scherrer rings are evaluated in order to approach lattice parameters and grain sizes as well as crystallographic relationships. An in-situ heating cycle from room temperature to 1362 °C has been conducted starting from massively transformed γ-TiAl which exhibits high internal stresses. With increasing temperature the occurrence of strain relaxation, chemical and phase separation, domain orientations, phase transitions, recrystallization processes, and subsequent grain growth can be observed. The data obtained by high-energy synchrotron X-ray diffraction, extremely rich in information, are interpreted step by step.

’Quenching and Partitioning’ - An In Situ Approach to Characterize the Process Kinetics and the Final Microstructure of TRIP-Assisted Steel

The ‘Quenching and Partitioning’ (Q&P) concept aims to increase the strength level of conventional TRIP-assisted advanced high strength steel (AHSS) by replacing ferritic constituents by tempered martensite. The Q&P heat treatment process involves austenitization and interrupted quenching followed by carbon partitioning from martensite to austenite at elevated temperatures. The final microstructure is traditionally investigated at room temperature after metallographic preparation by microscopy and x-ray analysis with laboratory tubes. Besides other disadvantages the established characterization methods are not adequate to observe the development of the microstructure during Q&P treatment. In the present work the microstructural evolution during Q&P processing was monitored by in-situ diffraction experiments using very hard (100 keV) synchrotron x-ray radiation. Debye-Scherrer rings were recorded as a function of time and temperature during the heat treatment in a state-of-the-art dilatometer (type Bähr DIL805AD) at the Engineering Materials Science beamline HARWI-II (HZG outstation at Deutsches Elektronensynchrotron (DESY), Hamburg). The diffraction patterns contain quantitative information on the phases present in the sample (for more details cf. Abstract Carmele et al, this conference). The evolution of the austenite phase fraction during the partitioning treatment at the quench temperature (1-step Q&P) is discussed exemplarily for a Si-based TRIP steel with additions of Ni.

Peritectic titanium alloys for 3D printing

Metal-based additive manufacturing (AM) permits layer-by-layer fabrication of near net-shaped metallic components with complex geometries not achievable using the design constraints of traditional manufacturing. Production savings of titanium-based components by AM are estimated up to 50% owing to the current exorbitant loss of material during machining. Nowadays, most of the titanium alloys for AM are based on conventional compositions still tailored to conventional manufacturing not considering the directional thermal gradient that provokes epitaxial growth during AM. This results in severely textured microstructures associated with anisotropic structural properties usually remaining upon post-AM processing. The present investigations reveal a promising solidification and cooling path for α formation not yet exploited, in which α does not inherit the usual crystallographic orientation relationship with the parent β phase. The associated decrease in anisotropy, accompanied by the formation of equiaxed microstructures represents a step forward toward a next generation of titanium alloys for AM.3D printing of titanium alloys today is based on known alloy compositions that result in anisotropic structural properties. Here, the authors add lanthanum to commercially pure titanium and exploit a solidification path that reduces texture and anisotropy.

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.