Rugged dislocation slip energy landscape and coordinated activation of multi-slip systems enable superior strength-ductility synergy in TiVZrNbAl(Mo) lightweight multi-principal element alloys

Refractory multi-principal element alloys (RMPEAs) possess exceptional yield strengths in the gigapascal range, positioning them as promising candidates for extreme service environments. However, their high density and limited tensile ductility at room temperature restrict their processability and industrial applicability. This study designed a series of lightweight RMPEAs with nominal compositions of (Ti₅₀V₂₀Zr₁₂Nb₁₂Al₆)_100-xMo_x (x = 0, 2.5, 5). Microstructural analysis revealed that the Mo-free M0 and 2.5 at.% Mo-doped M2.5 alloys develop a BCC1/BCC2 dual-phase modulated structure through spinodal decomposition (SD), achieving an exceptional strength-ductility synergy. Their yield strengths of 952 MPa and 1056 MPa and fracture elongations of 28.8 % and 22.6 %, respectively, surpass most reported RMPEAs and commercial titanium alloys. In contrast, the 5 at.% Mo-doped M5 alloy experiences severe ductility loss due to excessive C-14 Laves phase precipitation and restricted dislocation slip. The superior yield strength originates from significant atomic-size and modulus mismatches, while the enhanced ductility results from a deformation mechanism dominated by non-screw dislocations with edge character and the activation of multiple slip systems. This behavior is attributed to the rugged energy landscape induced by SD and lattice distortion, which increases dislocation glide resistance and shifts deformation from screw-to non-screw-type dislocation dominance. Additionally, the minimal unstable stacking fault energy differences (γ_USF < 5 %) between {110} <111> and {112} <111> slip systems promote sequential multi-slip activation. This enhances strain hardening, delays plastic instability, and broadens the plastic deformation regime. This study provides a new paradigm for designing lightweight, strong RMPEAs, advancing their potential in structural applications.