Enhancing microsegregation during rapid directional solidification through ternary microalloying

• Demonstrate a method to control the composition and width of the dendrite matrix and interdendritic zone in laser powder bed fusion through ternary microalloying. • CALPHAD-based approach used to select ternary element based on modification of the element on the solidus and liquidus boundary demonstrated in a Cu-Cr binary system. • Time-of-flight Secondary Ion Mass Spectrometry (ToF-SIMs) used for the first time to measure the very small elemental concentration differences in the dendrite and interdendritic region. • Developed microsegregation theory to explain significant widening of the interdendritic region without changes in the primary dendrite arm spacing observed with ternary Zr addition to the Cu-Cr system. The nano-cellular dendritic microstructure formed during rapid directional solidification in powder bed fusion additive manufacturing creates unique properties such as simultaneous improvement in strength and ductility. However, process control of microsegregation features remains challenging due to low sensitivity of critical solidification mechanisms to process parameters. This study leverages microalloying to achieve large changes in dendrite composition, microstructure, and interdendritic zone width during laser powder bed fusion without modifying process parameters. CALPHAD simulations predict that the addition of Zr significantly steepens the solidus line of the dilute Cu-Cr alloy system, leading to enhanced Cr rejection into the melt and greater than 95% reduction of solubility of Cr in the solidified Cu matrix. Experimental validation using time-of-flight secondary ion mass spectrometry and Kelvin probe force microscopy reveals that the ternary alloy containing 0.01 wt% Zr exhibited wider interdendritic regions compared to the binary, a significantly higher number of Cr-rich particles within interdendritic regions, near complete ejection of oxygen impurity from the matrix, and greater nanoscale work function contrast. These features indicate more aggressive Cr segregation in the presence of Zr and a purer Cu matrix and provide a potentially robust method for engineering the nano-cellular dendritic solidification microstructure.