Thermophysical properties of metallic systems determined by pulse-heating calorimetry.

摘要

The temperature dependence of the isobaric specific heat (Cp) and electrical resistivity (ρ) were simultaneously determined for several metallic alloy systems using a pulse-heating calorimeter (PHC). Data were obtained at relatively high heating rates (typically between 25 and 350 K/s) in various temperature ranges from 300 to 1500 K. The theoretical basis of operation and some advantages and limitations of the PHC were discussed. The kinetics of the disorder/order (SRO (α)/LRO (β)) transformation in Ni₄Mo was investigated, and ΔGα→β was determined between 300 and 1400 K. For Ni₄Mo, a TTT start-curve was determined for the α to β transformation, along with LRO parameter data between 923 and 1113 K. For Fe-30 at% Al and Fe-43 at% Al binary alloys, data were contrasted between step-cooled and naturally-cooled initial conditions. Vacancy and triple-defect concentrations, and triple-defect formation enthalpy were determined for the Fe-43 at% Al alloy. In the Fe-30 at% Al alloy, the DO₃ to B2 phase transformation was detected. The allotropic (Tt) and magnetic (Curie) transformation temperatures were determined for pure Co. Isothermal annealing below Tt had the effect of raising Tt on a subsequent pulse-heating test for pure Co. In the Co-Cr-based superalloy ULTIMET™, the Cp data obtained by DSC exhibit deviation from smooth behavior between 825 and 975 K, in contrast to data obtained with the PHC. In the Ni-Cr-based alloy Hastelloy™ G-30, data indicate a structural change between approximately 875 and 1100 K, the cause of which is unexplained. For Hastelloy™ G-30, p-time data were obtained isothermally between 775 and 1475 K. The glass transition and crystallization behavior of a Zr_52.5Ti₅Cu_17.9Ni_14.6 Al₁₀ bulk amorphous alloy (BAA) were studied. Cp and ρ data were obtained in both the as-cast (amorphous) and crystalline initial conditions. The melting, glass transition, crystallization, and reduced glass transition temperatures were determined. Features of ρ-temperature data obtained at higher heating rates are interpreted as being due to some type of (unidentified) precursor structure that formed prior to crystallization. Microstructure (SEM) and microhardness data were obtained subsequent to pulse-heating and cooling on one specimen.