Thermodynamic properties and phase equilibria in the Nickel-Lanthanum system: Transition of melts into the amorphous state

Abstract

The vapor composition and thermodynamic properties of liquid and crystalline Ni-La alloys were studied over a wide temperature range, 685–1854 K, and the whole range of compositions. Measurements were performed by the integral variant of the effusion method under superhigh oilless vacuum conditions and Knudsen mass-spectrometry. To extend the range of measurements toward lower temperatures, an approach based on initiating and studying equilibria in reactions with LiF and MgF2 admixtures, which caused the formation of volatile interaction products, was used. A representative array of data was obtained, including more than 1600 component activity values at various compositions and/or temperatures. For the first time, a complete and correct thermodynamic description of all intermediate phases in the Ni-La system was obtained. The accuracy and reliability of the thermodynamic functions found were proved by the coincidence of the results obtained using various calculation methods in studying samples of various compositions under various experimental conditions (various effusion cell and inner cavity coating materials) and agreement with independent experimental data on phase transitions. The thermodynamic functions of Ni-La melts were described with accuracy not inferior to the accuracy of measurements on the assumption of the formation of associates of two types, NiLa and Ni2La. Approaches to determining the contributions of various intercomponent chemical interaction types (covalent and metallic) to thermodynamic functions were developed. The thermodynamic data obtained and the model description of melts suggested were used to calculate phase equilibria in the Ni-La system and construct its phase diagram. The interval of the transition of Ni-La melts into the amorphous state was displaced from the compositions of maximum chemical short-range order close to Ni2La and coincided with the region of the predominance of much less stable NiLa groups characterized by a higher negative entropy of formation. It was this characteristic that determined the dynamics of changes in the structural state of melts, ΔC p and Δfus S, and could therefore be used for creating quantitative criteria of the propensity of liquid metals to form amorphous solids.