The enhancement of room-temperature plasticity in bulk metallic glasses (BMGs) through rejuvenation treatments like deep cryogenic cycling (DCT) is a topic of significant interest, yet the underlying nanoscale mechanisms linking structural evolution to mechanical behavior remain incompletely understood. This study systematically investigates the effects of DCT on the nanoindentation behavior, including creep and stress relaxation, of a model Vit-1 (Zr41.2Ti13.8Cu12.5Ni10Be22.5) BMG. We demonstrate that DCT with an upper-limit temperature of 0.5 Tg (315 K) and 35 cycles yields a remarkable increase in macroscopic compressive plasticity to 3.5%, compared to 0.6% for the as-cast state. Crucially, nanoindentation reveals that this plasticity enhancement correlates with a higher frequency and larger magnitude of displacement pop-in events, indicative of prolific shear band nucleation. Analysis of the load-displacement curves shows that DCT increases the elastic deformation work while decreasing the plastic deformation work. Kelvin-Voigt model fitting of creep curves and derived stress relaxation time spectra provide direct evidence of DCT-induced structural evolution. The treatment suppresses the activation of large-scale defects (soft spots/liquid-like regions) while promoting a more homogeneous distribution of smaller, easily activated defects. Our findings establish a clear micro-to-macro correlation, elucidating that the improved plasticity originates from DCT-tailored heterogeneous structures that facilitate shear band multiplication rather than catastrophic propagation. This work provides fundamental insights into the structural origin of property enhancement via cryogenic cycling in BMGs.
Achieving a uniform and refined duplex microstructure that offers an exceptional balance between strength and ductility remains a major challenge in the Ti-44.3Al-4.01Nb-0.96Mo-0.12B alloy, particularly when relying solely on heat treatment rather than thermomechanical processing. In this study, three distinct microstructures (DP-1, DP-2, and DP-3) were obtained through a designed multi-step heat treatment, which involved β-phase solution annealing, cyclic treatments between 1150 °C and 1230 °C, and additional annealing steps at 1220 °C and 850 °C for the optimized DP-3 condition. The DP-3 microstructure that characterized by refined γm grains (15.9 μm) and α2/γ lamellar colonies (30.8 μm) with a small amount of β0 phase content (3.34%), and the formation of γ/β0 structures, which exhibited a remarkable simultaneous improvement in room-temperature tensile properties, achieving a yield strength of 590 MPa, an ultimate tensile strength of 773 MPa, and an elongation of 1.57%, significantly outperforming DP-1 and DP-2. This microstructural optimization activates multiple mutually reinforcing deformation mechanisms, including deformation twinning, stacking faults, LPSO structures, dislocation multiplication, and ω-phase precipitation. Notably, while grain refinement contributes to strengthening, our findings reveal that the increased volume fraction of γm phases plays a more decisive role in enhancing elongation. This work provides a viable, scalable, and deformation-free heat treatment strategy for developing high-performance TiAl alloys with combined high strength and ductility.
Simultaneously improving strength and ductility at cryogenic temperatures remains a key challenge for structural alloys, particularly in the development of cost-effective alloys with superior cryogenic mechanical properties. Here, Fe2.5Ni0.9Cr0.5Al0.4 alloy was 80% cold-rolled then short-annealed to form a hierarchical heterogeneous microstructure. The alloy annealed at 900 °C for 10 min exhibited an FCC matrix containing BCC/B2 precipitates and partially recrystallized/unrecrystallized regions, resulting in yield strength (YS) of 920 MPa, ultimate tensile strength (UTS) of 1413 MPa, and uniform elongation (UE) of 38.4% at room temperature. At liquid nitrogen temperature (77 K), the alloy showed a simultaneous enhancement in strength and ductility, with YS, UTS, and UE reaching 1286 MPa, 1753 MPa, and 41.0%, respectively. This exceptional response is associated with the increase in BCC phase fraction from 6.7% to 46.1%, substantial dislocation accumulation, stacking faults, and deformation twins during cryogenic deformation. Load-unload-reload analysis revealed a high heterogeneous deformation-induced (HDI) stress of 910 MPa at a true strain of 36%, confirming the dominant role of HDI strengthening. This work demonstrates a practical short-time annealing route for developing low-cost Fe-rich alloys with superior cryogenic strength-ductility synergy.
The contradiction between ultra-high strength and high ductility remains a critical challenge in structural materials. Here, a design strategy based on the electronic density of states is proposed to intrinsically toughen the brittle long-range ordered phases that inevitably precipitate during fabrication of large-sized metallic glass composites. Using the Cu-Zr system as a model, Nb is introduced based on the valence electron concentration compensation principle. Surplus Nb-4d electrons precisely fill the pseudogap at the Fermi level, transforming rigid covalent bonds into flexible metallic bonds. This electronic-state softening not only promotes the formation of a fully coherent nanoscale structure made up of B2 and L21 phases, but also lowers the generalized stacking fault energy by 64% while preserving the ideal work of separation. As a result, the Rice-Thomson ductility index rises from approximately 2.1 to 6.2, creating a pathway for macroscopic coherent slip transmission. Molecular dynamics simulations further show that L21 ordering takes place through diffusionless rearrangement within a single lattice constant. Overall, this work converts the toughening challenge into a predictable electron-transfer strategy, offering new templates for strengthening advanced materials that are limited by brittle phases.
In the present study, the detailed effects of pulsed laser surface processing on the structural, mechanical, and wear behavior of zirconium-based bulk metallic glasses (BMG) have been investigated. In this regard, Zr65Cu15Ti13Ni7 sheets with 2 mm in thickness and length of 50 mm were prepared by injection casting in a water-cooled copper mold. Laser surface processing was performed using a pulsed Nd:YAG laser at different powers of 70-100 W. The results illustrated that laser surface treatment has significant effects on the mechanical properties of Zr-based BMGs. At powers less than 80 W, laser treatment only leads to an increase in compressive residual stress and free volume at the surface, which increases strength and ductility. In contrast, at powers higher than 80 W, a brittle crystalline layer consisting of CuZr and Cu10Zr7 intermetallic compounds forms in the heat-affected zone, which leads to a severe reduction in strength and toughness. The maximum strength and elongation values of about 1700 ± 35 MPa and 2.1 ± 0.2% were achieved at a laser power of 70 W, respectively. Furthermore, it was found that laser surface processing did not affect the wear behavior of the studied sample.
Nb-Si based composites are attractive candidates for ultrahigh-temperature structural applications, but their widespread use is restricted by poor room-temperature fracture resistance. In this work, an Nb solid solution (Nbss)/γ-Nb5Si3 in-situ composite with a nominal composition of Nb-16Si-24Ti-6Zr-2Hf-2Cr-2Al-0.4La (at.%) was fabricated by laser directed energy deposition (LDED), and its microstructure and room-temperature mechanical behavior were directly compared with those of the as-cast counterpart. The LDED process produced a refined microstructure relative to casting, reducing the average size of the γ-Nb5Si3 phase from 41.1 μm to 9.7 μm and that of the Nbss phase from 183.5 μm to 112.8 μm. A pronounced <0001> texture also developed in the γ-Nb5Si3 phase along the build direction, giving rise to evident microstructural anisotropy. Room-temperature mechanical tests showed that the LDED composite exhibited higher fracture toughness KQ and compressive strength than the as-cast counterpart. In particular, the build-direction specimen reached a KQ value of 21.92 MPa m1/2 and a compressive strength of 2058.88 MPa, compared with 14.75 MPa m1/2 and 1741.10 MPa for the as-cast sample, respectively. Fractography and crack-path observations suggest that the improved room-temperature fracture resistance of the LDED composite is associated with microstructure refinement, the reduction of casting-related defects, and texture-related crack deflection together with Nbss ligament bridging. These results show that LDED is an effective route for tailoring the microstructure and crystallographic texture of Nbss/γ-Nb5Si3 in-situ composites and improving their room-temperature mechanical performance.
