Microstructural Development of an AlNiBi Alloy and Influence of the Transient Horizontal Solidification Parameters on Microhardness

Abstract

The combination of the structural and tribological properties presented by AlNiBi alloys has motivated us to establish, as the main objective of this study, the investigation of the microstructural evolution and its influence on the microhardness (HV) of Al-3wt pct Ni-1wt pct Bi alloy horizontally solidified via a water-cooled directional solidification device. Temperature mapping by thermocouples inserted in the metal has been performed for experimental determination of the solidification thermal parameters, such as the growth rate and cooling rate (VL and TR, respectively). The microstructure has been characterized by optical and scanning electron microscopy and by microanalysis of the composition via dispersive energy spectroscopy (EDS composition). The macrostructure of the as-solidified ingot is characterized by columnar grains, and the final microstructure consists of an Al-rich primary phase (α-Al) and a eutectic mixture composed of two phases: α-Al + Al3Ni intermetallic (β) with Bi particles anchored on the β phase. The Bi droplet scale is affected by the thermal parameters. The primary phase (α-Al) is characterized by a reverse cellular-to-dendritic microstructural transition. Cellular and dendritic microstructures have been quantified by the cell, primary dendrite arm, secondary dendrite arm, and tertiary dendrite arm spacings (λC, λ1, λ2, and λ3, respectively). The relationships of λC, λ1, λ2, and λ3 with VL and TR have been established via power-type mathematical expressions. The HV dependence on λC, λ1, λ2, and λ3 has been analyzed in both cellular and dendritic microstructural zones. It has been observed that the HV values do not vary in the dendritic zone; however, Hall–Petch’s mathematical equations characterize the HV variation with these thermal and microstructural parameters in the cellular zone.