• Increasing Y and decreasing Nd contents enlarge grain size from 17.2 to 29.2 μm while promoting the formation of 14H and 18R LPSO phases, which dominate mechanical properties over grain size effects.
• Higher Y and lower Nd additions enhance tensile strength, yield strength, and elongation, with further improvements after solid solution treatment.
• Fracture patterns shift from ductile to brittle to mixed with changes in Y/Nd content, and solid solution treatment transitions fracture from intergranular to a combination of ductile and deconvolutional modes.
• Deformation mode analysis shows that increased LPSO phases and twins activate pyramidal slip while suppressing prismatic slip, revealing key mechanisms for alloy design.