Papers by Author: Andreas Stark

Abstract: A matrix type high speed steel YXR3 designed for a combination of wear resistance and toughness is investigated for its mechanical properties after hardening by deep cryogenic treatment follow by tempering. The deep cryogenic quenching carried out at -200 °C for 36 hours and the single step tempering results in an obvious improvement in wear resistance while balancing the toughness, comparing with the conventional quenching followed by a double tempering treatment. The quantitative image analysis reveals little difference in the MC carbide size distribution between tempering at different temperatures. The synchrotron high energy XRD confirms the MC type carbide with some evolution in its orientation together with tempered martensite approaching the BCC structure at higher temperatures. In contrary to the conventional quenching and tempering, the lowest tempering temperature at 200 °C yields a moderate drop in hardness with increase in surface toughness proportionally while exhibiting exceptional wear resistance. Such thermal cycle can be recommended for the industry both for the practicality and improved tool life.

Abstract: Either at higher temperatures or when a certain alloying element content is exceeded, γ-TiAl alloys contain the β phase (bcc) or its ordered derivate β_o (B2). The relatively soft β phase can facilitate hot deformation, but β_o is detrimental for creep strength and ductility. Thus, knowledge about β_o→β phase transformation is desirable. Surprisingly, even for the binary Ti-Al system it is under discussion whether the ordered β_o phase exists. Also, the effect of alloying elements on the β phase ordering is still unclear. In the present work the ordering of the β phase in binary Ti-(39,42,45)Al and ternary Ti-42Al-2X alloys (X=Fe, Cr, Nb, Ta, Mo) which was experimentally investigated by neutron and high energy X-ray diffraction is compared with the results of first principles calculations using density functional theory. Except for Cr the experimentally determined and the predicted behavior correspond.

Abstract: TiAl alloys are increasingly used as a lightweight material, for example in aero engines, which also leads to the requirement for suitable repair techniques. Transient liquid phase bonding is a promising method for the closure of cracks (in non-critical or non-highly loaded areas). The brazing solder Ti-24Ni was investigated for brazing the alloy Ti-45Al-5Nb-0.2B-0.2C (in at. %). After brazing, the joint exhibits different microstructures and phase compositions. The transient liquid phase bonding process was investigated in the middle of the joint region in situ to acquire time resolved information of the phases, their development, and thus the brazing process. These investigations were performed using high-energy X-ray diffraction at the “High-Energy Materials Science” beamline HEMS, located at the synchrotron radiation facility PETRA III at DESY in Hamburg, Germany. For this, we used an induction furnace, which is briefly described here. During the analysis of the diffraction data with Rietveld refinement, the amount of liquid was refined with Gaussian peaks and thus could be quantified. Furthermore, while brazing four different phases occurred in the middle of the joint region over time. Additionally, the degree of ordering of the β_o phase was determined with two ideal stoichiometric phases (completely ordered and disordered). Altogether, the phase composition changed clearly over the first six hours of the brazing process.

Abstract: Titanium aluminides based on the L1₀ ordered g-phase are promising structural light-weight materials for applications in aircraft engines. Typical compositions for γ-TiAl alloys lie in the range Ti-(44-48)Al (at.-%). For high creep resistance, a two-phase microstructure based on lamellar (α₂+γ)-colonies is desirable that may be tuned towards better ductility by introducing pure γ-grains (near lamellar or duplex microstructure).γ-TiAl alloys are often alloyed with niobium for increased oxidation resistance and improved mechanical properties. HEXRD and TEM studies of the alloy Ti-42Al-8.5Nb revealed that the orthorhombic O-phase forms during annealing at 500-650°C. This orthorhombic phase has been known in Nb-rich, Al-lean, α₂-based Ti-aluminides since the late 1980ies (Nb> 12.5 at.-%, Al< 31 at.-%) but the finding in γ-based alloys is new.TEM imaging showed that the O-phase is located within α₂ lamellae of lamellar (α₂+γ)-colonies. O-phase domains and α₂ phase form small columnar crystallites based in the α₂/γ interface. The columnar crystallites grow parallel to the [0001] direction of the α₂ phase and appear as facets when observed along this direction. The evolution of domains and facets with annealing time and the chemical homogeneity of the phases are investigated.The results of STEM imaging show that O-phase domains form during annealing at 550 °C for 8hours or 168 hours. After 168 hours of annealing Nb segregations are observed by EDX mapping within O-phase domains. In comparison, no segregation of niobium is detected after 8 hours of annealing.

Abstract: Gas-atomised spherical powders of Ti-45Al-5Nb and Ti-45Al-10Nb alloys were produced using the plasma melting induction guided gas atomisation (PIGA) technique. The phase composition was determined by X-ray diffraction at the synchrotron beamline HEMS at PETRA III (DESY), as well as by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), two dimensional and focused ion beam (FIB) based three dimensional electron back scatter diffraction (EBSD) measurements. Due to the high cooling rates the alloy composition of both alloys consists of hexagonal-close-packed α- and body-centred-cubic β-phase. The α-phase is dominant in the larger powder size fractions. Considerable amounts of the β phase were only found in the powder particle size fractions smaller than 32 μm for the Ti-45Al-5Nballoy and smaller than 45 μm for the Ti-45Al-10Nb. A pronounced dendritic cauliflower-like structure was observed in bigger powder particles of the Ti-45Al-10Nb alloy. This gives proof that diffusion took place during the initial β-grain formation, even though there is no orientation relation between the final grain and the dendrite structure in the powder particles. The presence of dendritic structures showed that the cooling rate during powder atomization was still too low to reach the critical growth rate for a planar solidification. The absence of preferred misorientation angles between α-grains indicates that α-grains are not formed out of already solidified β-grains by a solid state phase transformation.

Abstract: In recent years intermetallic γ-TiAl based alloys with additional amounts of the ternary bcc β Ti(Al,Nb) phase attracted increasing attention due to their improved workability at elevated temperatures. Depending on alloy composition and heat treatment the ductile high-temperature β phase can transform to several ordered phases at lower temperatures. However, currently available phase diagrams of these multiphase alloys are quite uncertain and the precipitation kinetics of some metastable phases is far from understood. In the present study various transformation pathways of the third phase were observed in situ by means of high-energy X-ray diffraction using synchrotron radiation. A Ti-45Al-10Nb (at.%) specimen was subjected to a temperature ramp of repeated heating cycles (700 °C - 1100 °C) with subsequent quenching at different rates. Depending on the quenching rate reversible transformations of the B2-ordered β_o phase to different ω-related phases were observed. The results indicate that the complete transformation from β_o to hexagonal B8₂-ordered ω_o consists of two steps which are both diffusion controlled but proceed with different velocities.

Abstract: 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.

Abstract: β solidifying γ titanium aluminide alloys exhibit fine and homogeneous microstructures in the cast condition, in particular if the alloys contain B as a grain refining agent. Following work in the literature [1], it is demonstrated that the grain refining effect due to B is solely attributed in such alloys to its effect on the solid-state β/α transformation. The results further show that grain refinement in these materials can be obtained by a simple heat treatment, even if the microstructure has been extensively coarsened through prior heat-treatment.

Abstract: 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.

Abstract: Innovative steel materials of the third generation of advanced high-strength steel (AHSS) are based on complex multiphase microstructures on a submicron scale, which are adjusted in a heat treatment procedure. Established methods for microstructural characterization are usually applied after the heat treatment process (ex-situ) at room temperature and comprise amongst others X-ray analysis based on laboratory tubes with photon energies of several keV. The corresponding penetration depths are on the micron scale. Additionally, the results may be affected by the metallographic preparation process. Using very hard synchrotron X-ray radiation with photon energies of up to 100 keV, penetration depths in the millimetre range are realized and macroscopic volumes (mm³) can be investigated. Furthermore the photon flux of synchrotron sources is several orders of magnitude higher compared to laboratory tubes. Consequently in-situ measurements during a heat treatment process can be performed. Using the example of the standardized multiphase TRIP steel HCT690T, a microstructural investigation with high energy synchrotron X-ray radiation is discussed and compared to established diffraction methods using Co-and Cu-K_α-radiation. In-situ diffraction measurements during a heat treatment are exemplarily shown.