Effect of Cu and CuP Additions on the Microstructure and Nanoindentation‐Based Fracture Behavior of CoNiAlSi Ferromagnetic Shape Memory Alloys

In this study, the microstructural and mechanical performance of CoNiAlSi‐based ferromagnetic shape memory alloys was comparatively investigated using a base CoNiAlSi alloy and Cu‐ and CuP‐modified derivatives produced by vacuum induction melting under identical heat treatment conditions. SEM, EDS, XRD, and DSC analyses were performed to evaluate the microstructure, phase constitution, and martensitic transformation behavior, while nanoindentation and scratch tests were used to assess local mechanical performance. The alloying additions improved microstructural homogeneity without introducing new brittle phases or suppressing the thermally induced martensitic transformation. Fracture toughness was determined using an energy‐based nanoindentation approach based on load–displacement ( P – h ) curves. The calculated fracture toughness increased from 10.53 MPa√m for the base CoNiAlSi alloy to 14.99 MPa√m with Cu addition and 21.08 MPa√m with CuP addition, corresponding to improvements of approximately 42% and 100%, respectively, relative to the base alloy. Scratch tests further showed that the base alloy exhibited higher surface rigidity, whereas Cu and CuP additions promoted greater plastic deformation capability. Overall, CuP microalloying provided the most balanced combination of retained martensitic transformation, improved microstructural homogeneity, enhanced elastic stiffness, and superior local fracture resistance, indicating its potential for engineering applications requiring improved damage tolerance.