S. Rex

Partial and integral enthalpies of mixing of liquid Ag-Al-Cu and Ag-Cu-Zn alloys

Abstract The partial enthalpies of mixing of the components of liquid ternary Ag–Al–Cu ( T =1252±5 K) and Ag–Cu–Zn ( T =1068–1113 K) alloys have been determined using a high-temperature isoperibolic calorimeter. Measurements were performed starting from both pure Al and Zn and from binary liquid Ag–Cu alloys along sections with constant Ag:Cu ratios 1:3, 1:1, 3:1 (Ag–Al–Cu system) and 2:3, 3:2 (Ag–Cu–Zn system). The integral enthalpies of mixing of these ternary alloys are calculated from the partial enthalpies of mixing using different methods. The composition dependences of the partial and integral enthalpies were simultaneously analytically described according to a Redlich–Kister–Mugianu equation using a least-squares fit by Gauss–Newton method. In case of Ag–Al–Cu alloys square- and higher order terms in excess ternary part are needed to adequately describe the surfaces of enthalpies of mixing. Enthalpy data for the constituent binaries were adopted from latest calorimetric measurements and thermodynamic assessments of the phase diagrams. The evaluated integral enthalpy of mixing demonstrates that the minima for the Ag–Al–Cu (−17.1 kJ mol −1 ) and Ag–Cu–Zn (−10.0 kJ mol −1 ) correspond to binary compositions Al 0.40 Cu 0.60 and Cu 0.50 Zn 0.50 , respectively.

3-D CONJUGATE ANALYSIS OF COOLED COATED PLATES AND HOMOGENIZATION OF THEIR THERMAL PROPERTIES

To predict the aerothermal behavior of a transpiration cooled plate, a multiscale approach based on the homogenization method of periodic material structures is presented here. This method allows calculation of effective equivalent thermophysical properties either for each layer or for the multilayer of superalloy, bondcoat, and thermal barrier coating (TBC). From the 3-D conjugate flow and heat transfer analysis, the stationary state is extracted and transferred to the microscale unit cell discretized by finite elements. The analysis proves for different cooling configurations a significant decrease in the amount of cooling fluid to obtain a desired superalloy temperature. Beyond, the hole outlet shaping leads to a reduction of the thermal gradients on the multilayer. The effect of the different cooling designs on the effective conductivities are discussed then. Finally, the influence of the selection of the unit cell position on these effective thermal properties is investigated.

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).

Three-dimensional numerical analysis of curved transpiration cooled plates and homogenization of their aerothermal properties

Three different designs of a transpiration cooled multilayer plate-plane, convex, and concave-are analyzed numerically by application of a 3D conjugate fluid flow and heat transfer solver. The geometrical setup and the fluid flow conditions are derived from modern gas turbine components. The conjugate analysis of these designs focus on the influence of the surface curvature, the cooling film development on the plate surface, the fluid structure in the cooling channels, and on the cooling efficiency of the plate. Moreover, to predict the effective thermal properties and the permeability of these multilayer plates, a multiscale approach based on the homogenization technique is employed. This method allows the calculation of effective equivalent properties either for each layer or for the multilayer of superalloy, bondcoat, and thermal barrier coating (TBC). Permeabilities of the different designs are presented in detail for the TBC layer. The influence of the plate curvature and the blowing ratio on the effective orthotropic thermal conductivities is finally outlined.

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.

Heat-Mass Transfer and Related Microstructures in TiAl-Based Alloys Directionally Solidifying in the Earth Gravity and Microgravity

The first results are presented of ESA MAXUS 8 sounding rocket benchmark experiment on γ-TiAl (Nb,B) intermetallics solidification, that was a part of the IMPRESS EU FP6 project. Having the aim to investigate the columnar and equiaxed primary microstructures formation at diffusion controlled melt growth, an experiment was designed applying the power-down directional solidification technique with fixing quench in automatic spaceborne furnace module TEM03-5M. Two related alloys were tested: one inoculated with boron grain refiner Ti-44Al-7.5Nb-2.7B (at.%); and the other Ti-45.5Al-8Nb (at.%) without grain refinement. The series of terrestrial reference processes has been performed accordingly in counter-gravity solidification direction. The numerical studies of heat-mass transfer, melt hydrodynamics and temporal solidification dynamics of these alloys have been implemented under the earth-and zero-gravity approximations using the GIGAN software package (IPPE). The comparison is performed of numerical model findings with the real microstructure and composition of samples, whose analyses were carried out by backscattered scanning electron microscopy (SEM) and X-ray microprobe (EDX) techniques. The convection-induced peritectic macrosegregation effect appearance and development in ground-based reference ingots is analytically observed and numerically studied. The achieved microstructure formation in space benchmarks is discussed.

Effective Thermoelastic Properties of Flat and Curved Transpiration Cooled Multilayer Plates via Homogenization

A multiscale approach based on the homogenization method of periodic material structures is developed here to predict the effective thermoelastic properties of heterogeneous porous materials. This method allows the calculation of effective equivalent properties either for each layer separatlely or for the multilayer of transpiration cooled multilayer plates. As gas turbine components are usually not flat, three different cooled plate designs — plane, convex and concave — are analyzed numerically. Effective Young and shear moduli as well as Poisson and thermal expansion coefficients are presented in detail for the reference flat plate. Then for the curved plates, the most sensitive effective Young and shear moduli of the TBC layer are compared with the reference ones. The effect of the blowing ratio on the effective orthotropic stiffness properties is outlined. The influence of local geometrical imperfections of the cooling channel shapes on the effective thermoelastic properties of the TBC layer of flat plates is also discussed in detail.Copyright

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.

Experimental Performance within the SCA Prototype Test 1 Using ESA`s Solidification and Quench Furnace (MSL-SQF) on Ground

The preparation of the solidification experiments planned onboard the ISS within the SETA research program was supported by a first full test run within the SQF- Solidification and Quench Furnace of the Materials Science Lab (MSL) using an eutectic alloy of the Al-Cu-Ag system as test sample. This first SCA prototype test in the SQF comprises a directional melting, solidification and subsequent quenching of the sample. The test evaluation presented in this paper is limited mainly to the evaluation of the recorded sample temperatures and focuses on the experimental performance in the view of a scientific user of the SQF. The thermal gradient achieved in the sample reaches about 10.6 K/mm and showed a good stability over the pulling length. The position of the solidification interface relative to the middle of the adiabatic zone, however, is shifted considerably towards the cooling zone. The quenching performance reached a starting value of 27 K/s.

3-D Numerical Analysis of Curved Transpiration Cooled Plates and Homogenization of Their Aerothermal Properties

Three different designs of a transpiration cooled multilayer plate — plane, convex and concave — are analysed numerically by application of a 3-D conjugate fluid flow and heat transfer solver. The geometrical setup and the fluid flow conditions are derived from modern gas turbine components. The conjugate analysis of these designs focus on the influence of the surface curvature, the cooling film development on the plate surface, the fluid structure in the cooling channels and on the cooling efficiency of the plate. Moreover, to predict the effective thermal properties and the permeability of these multilayer plates, a multiscale approach based on the homogenization technique is employed. This method allows the calculation of effective equivalent properties either for each layer or for the multilayer of superalloy, bondcoat and thermal barrier coating (TBC). Permeabilities of the different designs are presented in detail for the TBC layer. The influence of the plate curvature and the blowing ratio on the effective orthotropic thermal conductivities is finally outlined.Copyright