M. Rösner-Kuhn

Enthalpy measurements of undercooled melts by levitation calorimetry: the pure metals nickel, iron, vanadium and niobium

A circulating gas cooling (CGC) system is added to a combination of an electromagnetic levitation apparatus and a drop calorimeter to measure the enthalpies of the pure metals nickel, iron, vanadium and niobium in the undercooled temperature range. The CGC system extends the experimental temperature range to lower temperatures. The effect of the CGC system on the heat losses during the drop of the sample is discussed and the evaluation procedure presented. The measured enthalpies of the metals in the undercooled range confirm the temperature dependences of the enthalpies of the liquid phase above their melting points.

Temperature dependence of the mixing enthalpy of liquid Ti–Ni and Fe–Ti–Ni alloys

Abstract Using electromagnetic levitation alloying calorimetry, the concentration and temperature dependence of the mixing enthalpy of liquid Ti–Ni and Fe–Ti–Ni alloys has been determined. For the Ti–Ni alloys, the measurements have been carried out at 1800 and 1980 K. On base of the experimental data, a description of the mixing enthalpy has been calculated with a regular associate model. For Fe–Ti–Ni alloys, measurements have been performed along the concentration sections Fe84Ti16–Ni at 1897 K, Fe2Ti–Ni at 1916 K and NiTi–Fe at 1918 K. Based on previous results for the constituent binary alloy systems Ti–Fe and Fe–Ni and the corresponding solution model descriptions the mixing enthalpy of liquid ternary Fe–Ti–Ni alloys is calculated with a regular associate model considering the binary associates, TiNi3 and TiFe. The model calculation for the ternary alloy system is in excellent agreement with the data. This shows that the mixing enthalpy of liquid Fe–Ti–Ni alloys is entirely determined by the interaction parameters of the base binary systems.

Measurements of dendritic growth and recalescence rates in undercooled melts of cobalt

Abstract Electromagnetic levitated liquid cobalt was undercooled with the purpose to trigger solidification at desired temperatures. Triggering was performed at the underside of the sample. The solidification velocity was determined by detecting the trigger signal and the temperature rise at the top of the sample by a pyrometer. Measurements were carried out up to 102 K below the melting point. The values of the dendritic growth rate can be well described by the theory of Lipton, Kurz and Trivedi (LKT theory) and follow v = A ·Δ T α , where Δ T is the undercooling, A is a pre-exponential factor which was found to be 1.146×10 −4 m s −1 K − α and α =2.48. The recalescence rate, T , is generally given by T =B· Δ T β . The result obtained for cobalt is B =2.79×10 2 K 1− β s −1 and β =1.066.

Enthalpy measurements of the solid high-temperature β-phase of titanium and zirconium by levitation drop calorimetry

Abstract The molar enthalpy, Δ H , of β-titanium and β-zirconium has been measured by levitation drop calorimetry in a temperature range of T =1637–1908 K for Ti and T =1821–2105 K for Zr. The melting temperatures are 1939 K for Ti and 2125 K for Zr. The values obtained are represented by the following enthalpy–temperature functions in J·mol −1 : β-titanium: Δ H =−348+21.47· T +3.91·10 −3 · T 2 ; β-zirconium: Δ H =24 556−2.63· T +9.17·10 −3 · T 2 . The experimental results of the enthalpy are tainted with a relative error of ±2.0% for Ti and ±3.1% for Zr. The average deviation between the experimental values and the curves given by the two functions is ±0.53% for Ti and ±0.25% for Zr. In combination with enthalpy data of the liquid phases the values for the enthalpy of fusion can be obtained: Δ H f =13 017±649 J·mol −1 for Ti; Δ H f =17 282±695 J·mol −1 for Zr. The resulting values for the entropy of fusion are Δ S f =6.71 J·mol −1 ·K −1 for Ti and Δ S f =8.13 J·mol −1 ·K −1 for Zr. The results are compared with those provided by the literature where just one experimental enthalpy study is available for β-titanium in the temperature range investigated. The available literature data for the β-zirconium are derived from measurements of the enthalpy at lower temperatures and heat capacity studies.

Concentration dependence of the spectral emissivity of liquid binary metallic alloys

A review is given on the literature data on the concentration dependence or the emissivity of liquid binary metallic alloys (Ni-Fe, Ce-Cu, Ni-Cr, Ti-Al). Our measurements at the liquidus temperatures are presented for the systems Ni-V, Fe-V, and Fe-Nb and the pure components atλ=547 nm andλ=650 nm. All available results are interpolated in comparison to the phase diagrams of the systems. This comparison indicates that the nonvariant liquidus temperatures have the highest deviation from a linear interpolation between the emissivity of the pure components. The corresponding concentration ranges are characterized by stronger atomic interactions in the melt. Therefore errors in noncontact temperature measurements occur if the concentration dependence is neglected or estimated from the pure components.

Description of the concentration and temperature-dependent mixing enthalpy of binary metallic melts

Abstract An introduction is given to different thermodynamic models for representing mixing enthalpy measurements. The TAP series and the associate model are applied and compared on the basis of the concentration and temperature-dependent mixing enthalpy of the system MgPb. A new appoximation method is introduced to calculate the parameters of the associate model. The number of available enthalpy values for the approximation process is systematically varied in order to simulate the flexibility of the models. From this point of view the associate model is a useful model for the representation of the mixing enthalpy and the prediction of the excess heat capacity.

Enthalpy measurements of the solid high-temperature β-phase and the liquid phase of hafnium (3 wt.% Zr) by levitation drop calorimetry

Abstract The molar enthalpy, Δ H , of hafnium (3 wt.% Zr) has been measured by levitation drop calorimetry for the solid β-phase in a temperature range from T =2096 to 2455 K and for the liquid phase from T =2339 to 2988 K. The temperatures of the α to β and the β to liquid transition of this material are 2012 and 2471 K, respectively. The enthalpies obtained are represented by the following enthalpy-temperature functions: Solid β-Hf Δ H /J mol −1 =−8299+31.79 T /K +1.42×10 −3 T 2 /K 2 . Liquid Hf Δ H /J mol −1 =99171+37.51(T−2471) /K . The experimental values scatter within a relative error of ±4.8%. The average deviation between the values and the curves given by the two functions is ±1.06% for the measurements on the solid phase and ±1.27% for the liquid phase. The resulting values for the enthalpy of fusion, Δ fus H , and the entropy of fusion, Δ fus S , are 20.247±2.1 kJ mol −1 and 8.19 J mol −1 K −1 , respectively. The results are compared with enthalpy estimates given in the literature where only two experimental data points are available for the solid β-phase.