Pore-induced decoupling of crystallographic microstrain and elastic modulus in β-Ti-16Nb-4Sn alloy

This study resolves a fundamental materials physics paradox in porous biomedical alloys: how can increasing crystallographic defects lead to decreased macroscopic stiffness? A β-Ti-16Nb-4Sn alloy with controlled porosity (approximately 6% to 55%) was fabricated via powder metallurgy. X-ray diffraction analysis reveals that higher porosity amplifies microstrain (from 0.3 × 10⁻³ to 0.54 × 10⁻³, + 80%) and dislocation density (from 4.8 × 10¹⁴ m⁻² to 7.8 × 10¹⁴ m⁻², + 62%) while refining crystallites from approximately 45 nm to 34 nm. Contrary to classical strengthening expectations, compression tests show a drastic 91% reduction in elastic modulus (from 80 GPa to 7 GPa) and 93% reduction in compressive strength (from 1100 MPa to 75 MPa). Quantitative analysis using the Gibson-Ashby scaling law (C₁ = 0.383, C₂ = 0.206) demonstrates a clear decoupling mechanism: the geometric loss of load-bearing capacity due to porosity completely overrides the intrinsic stiffening effect of crystallographic defects. This allows the macroscopic modulus to be independently tuned to bone-matching levels. The efficacy of this approach is validated in vivo, where the low-modulus porous scaffold exhibits superior osseointegration and vascularization compared to its dense counterpart. Our findings establish a novel design principle for orthopedic implants, enabling independent control of microstructure and stiffness through coupled β-phase stabilization and pore-architecture engineering.