V.T. Witusiewicz

Computational and experimental study of phase stability, cohesive properties, magnetism and electronic structure of TiMn2

Abstract By an ab initio approach we calculated phase stability, cohesive and magnetic properties, and the electronic structure of TiMn 2 for the C14 and C15 Laves structure types. The nonmagnetic C14 phase is the ground state in accordance to experiment, whereas a metastable ferromagnetic C15 phase is predicted with a local magnetic moment of 0.78 μ β for Mn. The energy of formation was measured by a calorimetric drop experiment resulting in a value of −86.76±6.79 kJ mol −1 at 298 K being in good agreement to the ab initio result of −88.8 kJ mol −1 . Model calculations based on Miedema’s approach failed to yield reasonable results. The calculated densities of states reveal strong hybridisation between Ti-like and Mn d-like states.

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.

On the entropy of mixing

The excess entropy of formation of binary alloys has been described as the sum of configurational, vibrational, electronic and magnetic contributions. A relation for configurational and vibrational contributions is given in terms of physical properties of the components and a few empirical constants. It is demonstrated that the simple formula can be applied to calculate the excess entropy of liquid binary alloys.

Thermodynamics of liquid and undercooled liquid Al-Ni-Si alloys

Abstract The partial and the integral enthalpies of mixing of liquid Al–Ni–Si alloys have been determined by high-temperature isoperibolic calorimetry for three sections with constant concentration ratios of Ni and Si at 1575±3 K. With a view to a direct use of the thermodynamic properties, the values of the integral enthalpy of mixing of the ternary alloys together with the literature values of the constituent binaries Al–Ni, Ni–Si and Al–Si are analytically described by the thermodynamically adapted power series according to the right-hand Colinet geometry using a least-squares regression analysis. A regular association model adequately describes the thermodynamic properties of liquid and undercooled liquid Al–Ni–Si alloys. The Δ H ( x Al , x Ni , x Si ), Δ S ( x Al , x Ni , x Si ) and Δ G ( x Al , x Ni , x Si ) functions show the existence of essential contributions of ternary interactions among the alloy components. The strongest tendency toward chemical short-range ordering in the liquid state occurs near to compositions corresponding to the intermetallic compound Ni 2 AlSi.

Enthalpy of mixing of liquid Al-Cu-Si alloys

Abstract The partial and the integral enthalpies of mixing of liquid Al–Cu–Si alloys have been measured by high temperature calorimetry at 1575±3 K. Results for three sections with constant concentration ratios of Al and Si are given in tabular form. A least-square regression analysis of the integral enthalpy of mixing data results in the following relationships (in kJ mol−1; T=1575±3 K): Δ H =x Al x Cu α Al–Cu +x Al x Si α Al–Si +x Cu x Si α Cu–Si +x Al x Cu x Si α Al–Cu–Si α Al–Cu =(−75.6±6.5)+(−63.8±8.8)x Al +(232±33)x 2 Al +(−131±28)x 3 Al α Al–Si =−12.6+2.5x Si α Cu–Si =(−16.2±2.2)+(97.7±37.1)x Cu +(−1138±208)x 2 Cu +(3514±489)x 3 Cu + (−4748±515)x 4 Cu +(2216±199)x 5 Cu α Al–Cu–Si =(−241.7±8.8)+(1165±32)x Al +(891±34)x Si +(−935±30)x 2 Al +(−640±35)x 2 Si + +(−1563±52)x Al x Si . The minimum of ΔH changes with composition from −14.5 kJ mol−1 (Cu–25at.%Si) to −17.3 kJ mol−1 (Al–60at.%Cu). The presence of additional ternary interactions or ternary associates in the liquid state could not be observed.

Thermodynamics of liquid Al–Ni–Zr and Al–Cu–Ni–Zr alloys

Abstract The partial and the integral enthalpies of mixing of liquid Al–Ni–Zr alloys and liquid Al–Cu–Ni–Zr alloys have been measured by high temperature calorimetry at 1565±5 K. A least square treatment of the data results in the following relationships (kJ mol−1): for Al–Ni–Zr Δ H=α Al–Ni x Al x Ni +α Ni–Zr x Ni x Zr +α Al–Zr x Al x Zr +α Al–Ni–Zr x Al x Ni x Zr α Al–Ni =−129.6−331.6x Al +331.0x Al 2 α Ni–Zr =−83.1−217.8x Ni +419.9x Ni 2 −1195.2x Ni 3 +810.6x Ni 4 α Al–Zr =−177.1−170.0x Zr +230.0x Zr 2 α Al–Ni–Zr =−1955.9+5842.2x Zr +4620.5x Al −3760.2x Zr 2 −2399.9x Al 2 −5964.0x Zr x Al for the quasiternary cut Al–Cu0.63Ni0.37–Zr Δ H= Δ H Cu 0.63 Ni 0.37 x Al +α Cu 0.63 Ni 0.37 –Al x Al x+α Cu 0.63 Ni 0.37 –Zr x x Zr +α Al–Zr x Al x Zr +α Al–Cu 0.63 Ni 0.37 –Zr x Al x x Zr with x=x Cu 0.63 Ni 0.37 Δ H Cu 0.63 Ni 0.37 =2.7 α Cu 0.63 Ni 0.37 –Al =−132.4−132.8x Al +197.4x Al 2 α Cu 0.63 Ni 0.37 –Zr =−64.2−181.1x+539.1x 2 −575.5x 3 α Al–Zr =−177.1−170.0x Zr +230.0x Zr 2 α Al–Cu 0.63 Ni 0.37 –Zr =−531+2020.3x Zr +3079.0x Al −832.8x Zr 2 −2749.7x Al 2 −5530.1x Zr x Al The ΔH(α(x)) relations of the enthalpy show the existence of essential contributions of ternary and quaternary interactions among the alloy components. The excess entropy and Gibbs energy of mixing are estimated on the basis of the ΔH-values data using an empirical relation. The results confirm that the liquid Al–Cu–Ni–Zr alloys exhibits near the ternary composition Al0.42Ni0.42Zr0.16 the strongest tendency toward chemical short-range ordering.

Enthalpy of mixing of liquid and undercooled liquid ternary and quaternary Cu-Ni-Si-Zr alloys

Abstract The partial and the integral enthalpies of mixing of liquid Si–Zr, Cu–Si–Zr, Ni–Si–Zr and Cu–Ni–Si–Zr alloys with mole fractions of zirconium in the range from 0 up to 30 at.% have been determined by high-temperature isoperibolic calorimetry at 1575–1720 K. The data of the constituent ternaries were described using a regular association model. The enthalpy of mixing of liquid and undercooled liquid Cu–Ni–Si–Zr alloys were estimated for all compositions on the basis of the model description of the constituent binary and ternary systems.

The Influence of Fluid Flow on Intermetallic Phases in Al-cast Alloys

Technical Al-Si alloys always contain sufficient amounts of Fe and Mn, especially alloys made from scrap. During casting, Fe-containing intermetallics, such as Al-Fe, Al-Fe-Si and Al-Fe- Mn-Si phases, are formed between the aluminum dendrites. Fe and Mn-rich intermetallic phases are well known to be strongly influential on mechanical properties in Al-Si alloys. In the present work the influence of controlled fluid flow conditions on the morphology and spatial arrangement on intermetallic phases in cast Al-Si alloys is characterized. A binary Al-7wt.%Si and a ternary Al- 7wt.%Si-1wt.%Fe alloy was solidified under and without the influence of a rotating magnetic field (3mT at 50Hz) over a range of solidification velocities (0.015- 0.18mm/s) at a constant temperature gradient G of 3K/mm. The scientific results reached so far indicate a strong influence of the electromagnetic stirring on the primary dendrite and secondary dendrite arm spacings.

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).

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.