Experimental measurements of partition coefficients in Fe-36mass%Ni alloy

Invar alloy (Fe-36mass%Ni) is widely used in precision applications due to its low thermal expansion, where ultra-high cleanliness is required. Achieving this cleanliness requires control of secondary inclusions, which is closely influenced by solute microsegregation. One of the important parameters to predict microsegregation and the subsequent evolution of secondary inclusions is determined by the partition of solute elements during solidification. However, reported studies on the partition of solute elements in Invar alloy remains limited. This study determined partition coefficient values of Ni, Si, Mn, and Al in Fe-36mass%Ni alloy using two experimental approaches: the conventional equilibrium quenching and a random sampling method. In the quenching method, alloys with varied Si and Al contents were employed for partition coefficient determination. In the random sampling method, solidification alloys prepared under cooling rates 0.65 and 5.27 K/min were employed. Multiple sorting techniques were applied, including single-element rank sorting, average rank sorting and weighted interval rank sorting. Partition coefficients were derived by fitting concentration profiles to Scheil's equation within the solid fraction range of 0.1 to 0.8. Additionally, effect of sampling size was systematically evaluated. For Si and Mn, a standard deviation below 0.01 required more than 5,000 sampling points. In contrast, Ni remains stable even at smaller sample sizes. It was found that partition coefficients obtained from different two experimental techniques, and calculated segregation profiles satisfactorily reproduce experimental data.