Investigation of nitrogen solubility in liquid Fe-V alloys with the use oflevitation technique

A methodology and the results of experimental studies on nitrogen solubilit y in liquid Fe-V alloys are presented in the article. The investigations we re carried out using the levitation metal melting technique. Liquid Fe-V al loys (%V - 1.5; 5.8; 7.8; 12.2) were saturated with nitrogen at the tempera ture 2173 K. Nitrogen partial pressure was varied within the range of 0.1-2 .1 MPa. The experiment involved melting down a I g-mass sample in an electr omagnetic field produced by a levitation coil, filling the reaction chamber with nitrogen up to the required pressure, and heating the sample up to th e preset temperature. The sample was held in this condition for 180 s. Then, the power supply of the coil was switched off and a metal drop fell by gravity into a small cop per mould in which the metal solidified in the conditions of rapid crystall isation. Nitrogen contents were determined by the method of extraction to a neutral gas using a Leco TC-336 apparatus. High nitrogen concentrations were obtained in metal samples as the result o f high nitrogen partial pressures in the gaseous phase within the reaction chamber. In these conditions, all interactions of nitrogen in the alloy exa mined could be revealed, namely: nitrogen-vanadium, nitrogen-nitrogen, and nitrogen-nitrogen-vanadium interactions. In the case of the liquid Fe-V alloy, a solid phase of vanadium nitride sep arates from the liquid after exceeding the nitrogen solubility limit. The t ests were carried out within such a range of nitrogen concentrations where a given Fe-V alloy was still homogeneous. Investigation results have shown a nonlinear dependence of the activity coe fficient, f(N), not only on the vanadium content of alloy, but on its nitro gen content as well. Therefore, interaction between nitrogen atoms in liqui d Fe-N-V alloys has been documented. Particularly clearly visible is the strong effect of nitrogen on its own ac tivity coefficient in the case of higher vanadium contents of alloy. This i s probably connected with the sensitivity threshold (error) of the experime ntal method employed in the investigations described. Using experimental data and findings from the previous work on the liquid F e-N alloy, temperature relationships for the following interaction paramete rs were derived: e(N)((V)), r(N)((V)), r(N)((N,V)) and t(N)((N,V, V)) have been determined for the temperature 2173 K. The derived thermodynamic characteristics make a basis for conducting furth er studies on the Fe-N-V alloy, when the system in the state of equilibrium is composed of the liquid and separated vanadium nitrides. The results of such studies, with consideration being given to the effect of nitrogen part ial pressure, will be the subject. of a subsequent publication.