Reasons for Melting of Chemical Elements and some Consequences

When studying the causes for melting, solid chemical elements are considered to be the simplest systems. Data of the melting enthalpy, ΔH_m, and entropy, ΔS_m, taken from the literature are investigated as well as the molar specific heat capacities immediately below and above the melting temperature, T_m. In many cases, the molar specific heat capacity at constant volume exceeds the classical limit for the lattice vibrations C_V = 3R above the Debye temperature. This excess specific heat is due to electronic transitions of bonding electrons and of the electrons near the Fermi‐edge to states with higher energy. The wave functions of the electrons in these higher states are different from those of the original lower states causing the core ions to be attracted to different new positions. In addition, the vibrations may modify the local potential of the electrons and thus their wave‐functions with similar results. Melting occurs if sufficient core ions relax to new positions within the lifetime of the excited states and if their order and arrangement is changing according to the random time series of the different occupied electronic states with their respective wave‐functions. This mechanism can also be applied to explain diffusion, thermal expansion and the glass transition temperature.     

On the solid state heat transport phenomena measurement

The paper deals with theoretical and experimental aspects of lumped capacitance model (LCM) application for the study of heat transport in different materials. Patented construction of the measuring chamber together with special software the fundamental features of which are presented here allows evaluating thermal conductivity k, specific heat capacity c_p and thermal diffusivity α. Obtained results are in relatively very good agreement with those obtained from independent measurement or table values.     

Melting during heat treatment of a sheet of a niobium alloy – Failure analysis

The niobium alloy Nb‐5.4 At% Hf‐2 At% Ti is an important structural material used for high temperature applications. It is commonly produced in the form of sheet; however, it is also available in other mill forms. The sheets are vacuum‐annealed to improve the formability prior to fabricating different components out of them through metal forming. Annealing in vacuum is adopted in view of the high proneness to oxidation of niobium‐ base materials. In the present instance vacuum annealing of a sheet of the niobium alloy was carried out at 1175 ^oC; a plate of the nickel‐base superalloy 600 was used as charge support. It was noticed that the vacuum annealing resulted in melting of the charge as well as the charge support material. The plate of superalloy 600 was then replaced by a sheet of commercially pure titanium and next trial of annealing heat treatment carried out; there was no problem of melting. Failure analysis was carried out. The analysis brings out how phase diagrams can be used to choose the right material for supporting the charge in order to prevent melting during heat treatment. The failure is very expensive, if the material to be heat treated is costly, as it happens to be in the present case.     

Specific Heat Capacity and Thermal Conductivity of Foam Glass (Type 150P) at Temperatures from 80 to 400 K1

Low-temperature specific heat capacities of foam glass (Type 150P) have been measured from 79 to 395 K by a precision automated adiabatic calorimeter. Thermal conductivities of the glass foams have been determined from 243 to 395 K with a flat steady-state heat-flow meter. Experimental results have shown that both the specific heat capacities and thermal conductivities of the 150P foam glass increased with temperature. Experimentally measured specific heat capacities have been fitted by a polynomial equation from 79 to 395 K: C _p /J · g⁻¹· K⁻¹=0.6889+0.3332x− 0.0578x ²+0.0987x ³+0.0521x ⁴− 0.0330x ⁵− 0.0629x ⁶, where x=(T/K − 273)/158. Experimental thermal conductivities as a function of temperature (T) have been fitted by another polynomial equation from 243 to 395 K: λ/ W · m⁻¹· K⁻¹=0.14433+0.00129T − 2.834 × 10⁻⁶ T ²+2.18 × 10⁻⁹ T ³. In addition, thermal diffusivities (a) of the form glass sample were calculated from the specific heat capacities and thermal conductivities and have been fitted by a polynomial equation as a function of temperature (T): a/m² · s⁻¹=−1.68285+0.01833T − 5.84891 × 10⁻⁵ T ²+8.11942 × 10⁻⁸ T ³ − 4.24975 × 10⁻¹¹ T ⁴.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China.     

Melting Enthalpy and Entropy of Binary Systems of Alloys of 3d Transition VII and VIII Metals and Carbon: Thermodynamic Calculation and Experimental Study

Thermodynamic calculations have been performed of the melting enthalpy and entropy of the Co–C, Ni–C, and Fe–C binary system alloys having eutectic compositions and of alloys whose compositions correspond to the minimum temperatures of the Mn–C, Fe–Co, Fe–Ni, and Mn–Ni melting diagrams. Heat capacities of one- and two-phase alloys of investigated systems have also been calculated as a function of temperature. In order to express a dependence of the Gibbs energy (G) of the liquid phase on the composition, the model of regular solutions has been used. The relation between the G of interstitial solid solutions and concentration was described by the two-sublattice model. The values have been experimentally verified using a high temperature calorimeter. These results have shown that the discrepancy between experimental and calculated results is within the calorimetric measurement uncertainty and does not exceed 10%.     

A quantitative metallographic assessment of the evolution of porosity during processing and creep in single crystal Ni‐base super alloys

The present work reviews previous research on the evolution of porosity. It presents new results from a detailed study on the evolution of porosity during casting, heat treatment and creep of a single crystal Ni‐base superalloy subjected to uniaxial tensile creep at 1050 °C and 160 MPa in [001] and [110] directions. A quantitative metallographic study was performed on carefully polished metallographic cross sections, monitoring sampling fields of 4500 × 1000 µm² using the back scatter contrast of an analytical scanning electron microscope; evolutions of pore sizes and pore form factors were analyzed and all important details which were previously revealed in a synchrotron study could be reproduced. In addition, it was observed that micro cracks form at larger cast pores. They interlink and thus initiate final rupture. The [110] tensile creep tests showed lower rupture strains than the [001] experiments. In agreement with earlier work, this can be rationalized on the basis of aligned porosity along primary dendrites.     

Comparison of the mechanical and microstructural properties of heat‐treated boron steel in different cooling media

This paper focuses on the effects of heat treatment parameters on the microstructural and mechanical properties of quenchable 30MnB5 steel. Heat treatment parameters, such as different cooling media and different heating times at the same temperature, were investigated and compared. Tensile and hardness tests were performed at room temperature, and then the microstructures of the specimens were studied using optical microscopy and the results were compared. The results showed that boron steel heat treated using a water quenching process exhibited the best mechanical properties because of the formation of a martensitic microstructure.     

Secondary‐Emission signal for weld formation monitoring and control in eletron beam welding (EBW)

The process of emission of term electrons from the zone of effect of the electron beam are analyzed. During the experiments, the samples of stainless steel and titanium alloy were welded. Experiments were conducted to examine the spectrum of oscillations of the secondary current at various values of the specific power of the electron beam. The conducted research showed that the signal spectrum of the secondary current in electron beam welding contains a characteristic high‐frequency (15…?25 kHz) component. It was established that the described frequency spectrum is not created by some control system in the electron beam machine and reflects the oscillations in the system – «keyhole‐plasma». Empirical density distribution of the high‐frequency signal was constructed in the amplitude range. It was shown that the parameters of the density distribution is closely linked with the nature of interaction between the beam with the metal and can be used for remote control of technology process.     

Microstructure and castability of lead‐free silicon brass alloys

Development of lead free brass alloys is an important subject due to the environmental demands and new regulations and standards that restrict lead contamination to decrease the toxicity. Pb‐free Si brasses are interesting alloys for this application, due to economical aspects and promising properties. In this work four Pb‐free brasses prepared with 1, 2, 3 and 4 wt% Si, based on Cu60/Zn40 alloy, are characterized. Their microstructure is examined and phases formed are identified. Castabilty indicated by fluidity and porosity is studied and correlated with the phases formed and their volume fraction.