Integrated thermodynamic and experimental investigation of directional solidification in Nb-modified eutectic Al–12.6Si alloy

Control of solidification thermal parameters combined with microalloying additions is an effective strategy for tailoring the microstructure and properties of Al Si cast alloys. In this study, the solidification behavior, microstructural evolution, and hardness of a near-eutectic Al–12.6Si–0.1Nb alloy were investigated under directional solidification with transient heat extraction. The relationships among cooling rate (T R(α-Al) ), liquidus isotherm growth rate (V L(α-Al) ), secondary dendrite arm spacing (SDAS), and hardness were evaluated to assess the combined effects of Nb addition and solidification thermal conditions. Thermodynamic calculations using the TCAL8 database were performed to predict the solidification path and were validated through experimental thermal and microstructural analyses. Continuous thermal monitoring enabled the determination of V L(α-Al) and T R(α-Al) , while optical microscopy and SEM/EDS were used to characterize the resulting microstructures. Hardness measurements were correlated with SDAS and thermal parameters along the length of the directionally solidified ingot. The results showed that decreasing V L(α-Al) and T R(α-Al) with increasing distance from the water-cooled chill plate led to a systematic increase in SDAS. Thermodynamic predictions were consistent with experimental observations revealing 𝛼 − A l and eutectic Si as the predominant phases. The 0.1 wt% Nb microalloying plays a predominantly solutal and thermodynamic role in the Al–12.6Si alloy exhibiting a limited effect on dendritic refinement and eutectic Si modification. The addition of Nb introduced a solidification range ( Δ ⁢ 𝑇 ≈ 2 2 ° ⁢ C ) which extended the local solidification time ( 𝑡 𝑆 𝐿 ) and increased both the secondary dendrite arm spacing and eutectic spacing. Although S D A S displayed good correlations with growth rate and cooling rate ( 𝑅 2 > 0 . 7 0 ) this parameter proved to be more dependent on the local solidification time. Comparative analysis revealed that, despite the coarser microstructures observed in the Nb-containing alloy, hardness levels remained comparable to the base Al–12.6Si alloy and superior to those reported for the Al–12.6Si–3.2Bi alloy. This behavior demonstrates that Nb operates through a key compensatory mechanism where significant solid-solution strengthening of the 𝛼 − A l matrix preserves mechanical performance mitigating the resistance loss expected from microstructural coarsening.