A polymer-like ultrahigh-strength metal alloy

The development of advanced materials with both high strength and high flexibility is crucial for numerous emerging technologies, including morphing aircraft and artificial muscles. Existing materials typically exhibit a trade-off between these properties: high-strength steels are stiff, while flexible polymers are weak. This inherent limitation arises from the opposing correlation between strength and flexibility with respect to material bonding strength. The challenge lies in designing a material that breaks this trade-off, achieving a combination of steel-like strength (yield strength >1 GPa) and polymer-like flexibility (Young's modulus ≈10 GPa). Previous attempts using shape memory alloys (SMAs) have shown promise, but still fall short of the ideal combination. This research aims to overcome this long-standing challenge by developing a novel metal alloy that achieves both ultrahigh strength and polymer-like flexibility simultaneously, opening up new possibilities for advanced applications.

Significant efforts have been dedicated to creating metal alloys with both high strength and low modulus. Several alloys based on shape memory alloys (SMAs) have exhibited high strength (around 1 GPa) but with moderately low moduli (around 30 GPa). A Mg-Sc strain glass alloy showed a lower modulus (around 20 GPa), but with comparatively lower strength (around 0.3 GPa). These materials, however, still fall within the established strength-flexibility trade-off, failing to reach the desired combination of ultrahigh strength and polymer-like flexibility. This work builds upon previous research on strain glass alloys and shape memory alloys, aiming to surpass the limitations of existing materials by creating a novel microstructure with superior mechanical properties.

The researchers fabricated a Ti-50.8 at.% Ni dual-seed strain glass (DS-STG) alloy using a three-step thermomechanical treatment: 1. **Severe Deformation:** The initial B2 alloy underwent severe deformation (50% elongation), resulting in a deformation-stabilized B19' martensite. This step enhances the strength through cold-working. 2. **Annealing:** The deformed alloy was annealed at 573 K for 10 minutes. This step created a unique dual-crossover strain glass (DC-STG) microstructure, a precursor to the final DS-STG state. The DC-STG, while not yet exhibiting the target low modulus, is a critical intermediate stage. 3. **Deformation:** The DC-STG alloy underwent further deformation (12% elongation). This final deformation step introduced aligned R and B19' martensite seeds within the DC-STG matrix, forming the final dual-seed strain glass (DS-STG) microstructure responsible for the observed properties. The mechanical properties, microstructure, and phase transitions of the alloy were characterized using various techniques, including tensile testing, in situ X-ray diffractometry (XRD), and high-resolution transmission electron microscopy (TEM). The XRD measurements were crucial in identifying the phase transformations under stress, revealing the nucleation-free reversible transitions between the strain glass and martensite phases. TEM provided detailed microstructural information, confirming the presence and arrangement of the R and B19' martensite seeds within the strain glass matrix. The temperature dependence of the mechanical properties was also investigated over a wide range to assess the material’s suitability for various applications.

The DS-STG alloy demonstrated an unprecedented combination of ultrahigh yield strength (1.3-1.8 GPa) and ultralow Young's modulus (10.5 GPa), exhibiting a flexibility figure of merit significantly exceeding that of existing structural materials. The alloy showed a super-large recoverable rubber-like elastic strain of approximately 8% with a narrow hysteresis (around 15%). These properties were maintained over a wide temperature range (-80°C to +80°C), encompassing the typical operating temperature range of aerospace applications. Furthermore, the alloy exhibited excellent high-strain fatigue resistance, outperforming traditional flexible materials such as TiNi SMAs and engineering polymers. In situ XRD analysis revealed a nucleation-free reversible transition between the strain glass and R and B19' martensite phases, explaining the polymer-like elastic behavior. The high yield strength was attributed to the high density of dislocations and B2 deformation twins formed during the thermomechanical processing. The three-step processing route was shown to be effective for both plate and wire samples, with the wire sample exhibiting even higher yield strength due to more significant cold deformation. Analysis of the annealing temperature in step 2 demonstrated the critical role of the DC-STG state as a necessary precursor to the formation of the DS-STG state, which is directly responsible for the exceptional combination of properties. The exceptional properties of the DS-STG arise from the interaction and phase instability of multiple phases, facilitating easy transitions and producing an ultrasoft lattice. The thermal stability of the DS-STG is linked to the persistence of the R and B19' martensite seeds over a wide temperature range. Finally, the excellent fatigue resistance of the DS-STG is due to the strengthening effect of the thermomechanical processing, preventing dislocation movement and multiplication.

This study successfully demonstrates the development of a novel metal alloy that surpasses the conventional strength-flexibility trade-off. The DS-STG alloy exhibits an exceptional combination of properties that opens up exciting possibilities for advanced applications. The unique 'dual-seed strain glass' microstructure, achieved through a readily scalable three-step thermomechanical treatment, is the key to the material's outstanding performance. The nucleation-free phase transitions contribute significantly to the polymer-like flexibility, while the deformation-strengthening mechanisms ensure high strength. The ability of the alloy to maintain its exceptional properties over a wide temperature range and to exhibit excellent fatigue resistance further enhances its practical applicability. The results have important implications for the design and development of next-generation materials for aerospace applications and flexible devices.

This research presents a groundbreaking approach to materials design, resulting in a polymer-like ultrahigh-strength metal alloy. The DS-STG alloy, fabricated using a simple and scalable three-step thermomechanical process, overcomes the long-standing strength-flexibility trade-off. Its unique properties, stemming from a novel microstructure and nucleation-free phase transitions, have broad implications for advanced technologies. Future research could explore further optimization of the processing parameters to enhance the alloy’s performance and investigate its suitability for diverse applications, such as biocompatible implants and flexible electronics.

While the study demonstrates the excellent properties of the DS-STG alloy, further research is needed to fully evaluate its long-term durability and reliability under various operational conditions. The scalability of the manufacturing process to industrial levels requires thorough investigation and optimization. Additionally, a more comprehensive understanding of the underlying mechanisms governing the alloy's behavior at extreme temperatures and under complex loading conditions would enhance its practical utilization.