Mechanical, Morphological, Corrosion, and Thermally Activated Dimensional Recovery Behavior of Epoxy Composites Reinforced with Kraft Lignin/Fe–Mn–Si Alloy Hybrid Fillers

In this study, epoxy composites reinforced with kraft lignin, a promising green biofiller, and Fe–Mn–Si alloy particles as metallic functional fillers were developed, and their morphological properties, elemental distribution, mechanical properties, corrosion behavior, and thermally activated dimensional recovery behavior were investigated. Epoxy resin was used as the matrix, while kraft lignin and Fe–Mn–Si particles were incorporated as hybrid fillers. The composites were fabricated by casting with kraft lignin loadings of 1, 3, and 5 wt.% and a fixed Fe–Mn–Si alloy content of 3 wt.%. Neat epoxy was also prepared as a control sample. The specimens were characterized using scanning electron microscopy, energy-dispersive spectroscopy with elemental mapping, tensile testing, Shore D hardness measurements, electrochemical corrosion testing, and dimensional recovery tests. SEM–EDS observations showed that the composite containing 1 wt.% lignin exhibited a relatively uniform fracture morphology and more locally dispersed filler-related elemental signals, whereas higher lignin contents promoted particle-rich regions, microvoid-like features, and increased microstructural heterogeneity. The composite containing 1 wt.% lignin exhibited the highest tensile strength and elongation at break, with values of 39.09 MPa and 2.11%, respectively, and also showed the highest dimensional recovery ratio of 2.5%. The composite containing 3 wt.% lignin exhibited the lowest measured corrosion rate of 0.08 µm/year, while the composite containing 5 wt.% lignin showed the highest elastic modulus and Shore D hardness, with values of 5.80 GPa and 79, respectively. Overall, low lignin loading provided the most balanced mechanical and recovery-related performance, whereas higher lignin contents increased stiffness and hardness but also promoted greater microstructural heterogeneity.