U. Hecht

Heterogeneous nucleation and growth of the β(Ti) phase in the Ti–Al system—experiments and model calculations

The barrier to heterogeneous nucleation of the β(Ti) phase on TiB(2) and other borides has been evaluated using the plane to plane matching model. The results are compared to the known nucleation of the α(Ti) phase on the β(Ti) phase. According to this comparison, the barrier to heterogeneous nucleation of the β(Ti) phase on TiB(2) can be judged to be small. This is in agreement with inoculation experiments. The addition of a Ti-Al-TiB(2) master composite to a β(Ti) solidifying TiAl based alloy leads to a significantly refined microstructure. Microsegregations enable us to attribute this refinement to refined equiaxed β(Ti) dendrites. However, model calculations based on the hemispherical cap model predict that the refinement via heterogeneous β(Ti) nucleation should be more potent. First calculations indicate that structural imperfections of TiB(2) particles limit the nucleation site diameter. Thereby, the nucleation barrier is increased and the refinement is less pronounced.

Qualification of an Investment Casting Process for Production of Titanium Aluminide Components for Aerospace and Automotive Applications

Over the past years significant efforts were dedicated to developing production processes for aerospace and automotive parts from light weight titanium aluminides. Considerable progress was achieved worldwide in the field of near net shape casting. Access has developed an own production route, based on centrifugal investment casting. Various components from TiAl-based alloys, including turbocharger wheels and stator vanes were successfully cast and supplied for engine tests. A newly developed automated centrifugal casting line was recently taken into operation. This presentation will describe our activities that aimed to qualify the casting line for series production of turbocharger wheels: apart from basic issues of casting cluster design, process control and quality inspection we will discuss the casting process from the perspective of its techno-economic characteristics

Eutectic Solidification of Ternary Al-Cu-Ag Alloys: Coupled Growth of α(Al) and Al2Cu in Univariant Reaction

Coupled, regular eutectic growth of α(Al) and Al2Cu from ternary Al-Cu-Ag liquid alloys is investigated with focus on the formation and the characteristics of eutectic cells in unidirectionally solidified, polycrystalline, bulk samples. The topologic anisotropy of the lamellar eutectic leads to destabilization along the lamellae with elongated cells being intermediate to stable cells, irrespective of the crystallographic orientation of the phases. The formation of stable cellular patterns with elongated or regular cell structure is explained with reference to the crystal orientation of the phases α(Al) and Al2Cu, measured by electron backscatter diffraction (EBSD).

Time resolved X-ray imaging of eutectic cellular patterns evolving during solidification of ternary Al–Cu–Ag alloys

Abstract Synchrotron radiography has enabled time and space resolved investigation of directional solidification of ternary AlüCuüAg eutectic. Due to a high spatial resolution the lamellar spacing of the eutectic can be identified. The morphological instability of a planar eutectic solidüliquid interface towards eutectic cells observed in sheet-like samples is investigated for different solidification conditions. The results are summarized in a stability diagram and compared to findings in bulk samples and theoretical predictions based on the criterion of constitutional undercooling. During growth of eutectic cells an intriguing dual phase fragmentation in the intercolumnar/cellular regions with Al2Cu fragments settling and Al-rich fragments rising in the melt was observed, which may be unique for coupled growth of multicomponent eutectics.

Columnar to Equiaxed Transition in Peritectic TiAl Based Alloy Studied by a Power-Down Technique

Columnar to equiaxed transition (CET) was studied in a peritectic TiAl-based alloy with chemical composition Ti-45.1Al-4.9Nb-0.25C-0.2B (at.%). Solidification experiments were conducted in a Bridgman-type apparatus using cylindrical moulds made of high-purity Y2O3. The methodology containing appropriate etching and observations under flat light in stereo-microscope was used to identify the morphology of primary β phase grains and position of CET in the samples˰ All samples prepared by power down-technique showed sharp CET. The position of the CET measured from the beginning of the sample depends on the applied cooling rate and increases from approximately 65 to 115 mm by decreasing cooling rate from 50 to 15 K/min. Based on terrestrial experiments, the future work focused on microgravity and hypergravity CET experiments and numerical modeling is proposed. A Bridgman furnace front tracking method will be applied in future work to complement the experimental results here as part of the European Space Agency GRADECET programme. This modeling will input directly into planned microgravity and hypergravity CET experiments.

Solidification of Bulk Lamellar Eutectics

Eutectic alloys from the ternary system Al-Cu-Ag are excellent model alloys for the investigation of coupled eutectic growth, not only because most materials properties are well known but also because the system offers a rich variety of crystal structures and crystal orientation relationships (ORs) being associated to distinct minima of the solid-solid interface energy. This paper describes three research topics specifically related to bulk lamellar Al-Al2Cu eutectics, e.g. the maze-to-lamellar transition during early growth, the role of fault lines during lamellar spacing selection close to the pinch-off limit and the onset of eutectic cell formation above the constitutional supercooling limit. These topics are central to the microgravity experiments SETA presently being prepared for the MSL / SQF.

Phase Field Modeling of β(Ti) Solidification in Ti-45at.%Al: Columnar Dendrite Growth at Various Gravity Levels

At present, our understanding of the interaction between melt flow and solidification patterns is still incomplete. In columnar dendritic growth buoyancy driven flow may alter the dendrite tip and spacing selection and consequently the microsegregation of alloying elements. With the aim of supporting directional solidification experiments under hyper-gravity using a large diameter centrifuge (LDC), phase field simulations of β (Ti) dendrite growth have been performed under various gravity conditions for the binary alloy Ti-45at.%Al. The results show that Al segregation at the growth front causes convection rolls around the dendrite tips. The direction of the gravity vector is an essential parameter. When g is opposite to the direction of dendrite growth, increasing gravity leads to a marked decrease of the primary dendrite spacing and to a decrease of the mushy zone length. When g is aligned parallel to the direction of dendrite growth, the primary dendrite spacing and mushy zone length are almost unchanged, however the secondary dendrite arms grow more prominently as the magnitude of g increases.

Analysis of phase formation in Ni-rich alloys of the Ni–Ta–W system by calorimetry, DTA, SEM, and TEM

Abstract The partial enthalpies of dissolution of pure Ni, W and Ta in liquid ternary Ni–Ta–W alloys have been determined at (17735)K using a high temperature isoperibolic calorimeter. Measurements were performed in Ni-rich alloys (from 80 to 100 at.% Ni) along sections with constant Ta:W atomic ratios 1:0, 2:1, 1:2, and 0:1. The partial enthalpies and thereby the integral enthalpy of mixing of these ternary alloys are calculated from the partial enthalpies of dissolution using SGTE Gibbs energies for pure elements as reference. The obtained thermochemical data confirm that in the investigated Ni-rich alloys the binary interactions between Ta and W as well as the ternary Ni–Ta–W interactions are negligibly small. Due to this the variation of the integral enthalpy of mixing of the ternary alloys is well described as linear combination of the constituent Ni–Ta and Ni–W binaries. Such behaviour of the ternary liquid alloys is related to a very low probability of new ternary stable phases to occur in solid state. This prediction is confirmed by differential thermal analysis, scanning electron microscopy, and transmission electron microscopy of the as-solidified and annealed samples obtained as last alloy compositions in the series of calorimetric dissolution.

Enthalpies of formation measurements and thermodynamic description of the Ag–Cu–Zn system

Abstract The enthalpies of formation of β and γ alloys of the Ag–Cu–Zn system were determined by dissolution calorimetry. The melting and solid-state transformation temperatures as well as the enthalpies of the order/disorder and β/χ transformations were measured by differential scanning calorimetry. Thermodynamic descriptions are presented for the binary Ag–Zn system and for the ternary Ag–Cu–Zn system in the entire composition ranges. The thermodynamic model parameters of the constituent binaries Ag–Cu and Cu–Zn are taken from earlier assessments. Those for Ag–Zn and the Ag–Cu–Zn system are established based on relevant experimental data available in the literature completed with experimental data obtained in the present work. Several vertical and isothermal sections as well as the liquidus surface and thermodynamic properties are calculated using the evaluated parameters and show reasonably good agreement with experimental data available.

Structure and Properties of TiAl-Based Alloys Doped with Niobium and Chromium

Quaternary as-cast Ti96–xNb2Cr2Alxand Ti93–xNb5Cr2Alxalloys, where x = 44, 46, 48, and 50, melted from pure components (~99.9 wt.%) in a laboratory arc furnace, were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM/EDX), and differential thermal analysis (DTA). Bending testing and fracture toughness measurements were performed at room temperature, and compression testing was carried out in a range from room temperature to 750°C. The alloys were found to consist mainly of a superfine lamellar structure formed during decomposition of hightemperature phases. In addition, the alloys with 50 at.% Al contain grains of the γ phase, and all others have precipitates of the cubic β/β0phase at boundaries of the high-temperature phases because of double enrichment of the last melt drops with Cr. The tests show that the Ti96–xNb2Cr2Alxalloys with 46 and 48 at.% Al possess the most balanced properties.