Ultrafine niobium powder enabled synergetic liquid-phase sintering toward full densification of elemental powder metallurgy NiTi shape memory alloys

Abstract

Elemental powder metallurgy (EPM) represents a cost-effective approach for fabricating NiTi shape memory alloys. However, achieving full densification in EPM NiTi alloys remains challenging due to Kirkendall pores and the uncontrolled leakage of the Ni-Ti binary liquid phase. This study introduced fine Nb powder to generate a ternary NiTi-Nb eutectic liquid phase. This strategy effectively suppressed the anti-densification effect caused by the random binary Ni-Ti liquid phase, enabling near-full densification (relative density >99 %) and significantly enhancing the comprehensive mechanical properties (compressive strength was 3051 MPa, fracture strain was 39 %, shape recovery ratio under 8 % pre-strain was 70.0 %). Compared to coarse Nb powder, the use of refined Nb particles promoted the earlier and more complete formation of a uniformly distributed eutectic liquid phase. This enhancement stems from the synergistic interplay of thermodynamic (increased local reaction driving force via the Gibbs-Thomson effect, lowering the local melting point) and kinetic (accelerated diffusion and increased nucleation sites) factors associated with particle refinement, achieved by reducing the particle curvature radius and shortening diffusion paths. Consequently, both microstructural homogeneity and densification were improved. The controlled generation and uniform spatial distribution of the liquid phase are thus identified as critical factors determining the feasibility of achieving high densification in EPM Ni-Ti-based alloys. This study provides a novel solution for the development of low-cost, high-performance Ni-Ti-based alloys suitable for applications such as biomedical implants, sealing, and coupling fields.