The microstructure analysis of the La-doped W wire
Figure 1a illustrates the schematic of the non-slip drawing mechanism. With an initial diameter of 2.0 mm, we used the non-slip equipment to draw the raw La-doped W wire under ice bath conditions. As opposed to the traditional cone trolley slipping process, by employing a rubber-wrapped capstan to provide static friction, we maintained a constant grip on the wire (no slip friction), effectively cutting heat generation to a great extent. The concurrent use of an ice bath lowers the wire temperature, which helps preserve the fine grain size, twins, and high quantities of dislocations during the dynamic drawing process. The enhanced strength of W wires allows for further reduction of wire diameter. Figure 1b-e displays the drawing length, diameter distribution uniformity, wire diameter, and mechanical strength of the La-doped W wire. Figure 1b compares a reel winding 50 km of La-doped W wire with an empty reel. Figure 1c depicts the diameter tolerance of the La-doped W wire across a length of 50 km, demonstrating exceptional uniformity with a tolerance of approximately 38.0 ± 0.1% µm. The high uniformity suggests that it is highly feasible to produce the La-doped W wire at long lengths with narrow diameter distribution, and the non-slip drawing method is also highly suitable for industrial applications. Figure 1d presents the scanning electron microscopy (SEM) image of an ultra-thin La-doped W wire alongside a human hair. With a remarkably smooth surface, the W wire’s diameter is 38.0 µm, approximately half the diameter of a human hair. Figure 1e testifies the strength of a single 38.0 µm diameter La-doped W wire, capable of lifting a 500 g weight (Supplementary Movie 1).
To explore the effects of La element and the drawing process on microstructure, we performed the focused ion beam (FIB) etching on the cross sections of La-doped and pure W wires, both with a diameter of 38.0 µm (see inset in Fig. 1f). We employed the two TEM images from La-doped and pure W wires separately to compare the microstructures of post-etching specimens. Figure 1f reveals the lath-shaped grain morphology with a grain size of approximately 21.5 nm in the La-doped W sample. Figure 1i shows the high-resolution TEM (HRTEM) image of nanoprecipitates uniformly distributed at the grain boundaries, with a diameter of about 25.8 nm. Supplementary Fig. 1 shows an enlarged view of the precipitate in the grain boundary. We further analyzed the chemical composition of precipitates by the energy dispersive X-ray spectrometer (EDS) (Fig. 1g), revealing the W-La-O composition of the precipitate. The content of La in the precipitates is as high as 30.81% (Fig. 1h), indicating that La oxides mainly distribute at the grain boundaries in the precipitate form.
Figure 1j-k depicts the average dimensions of the La oxide precipitates and the grain sizes statistically. The precipitates and the grains sizes exhibit a narrow size distribution, with the nanoprecipitates between 0.4–0.9 nm and grains between 11–23 nm. The refined and axially oriented grains, coupled with enormous amounts of dislocations from the extreme plastic deformation under the cold drawing, contribute to enhanced tensile strength39. We prepared a pure W wire under identical conditions to clarify the effect of the La element on the microstructure and grain size. The aberration-corrected scanning TEM (STEM) image of pure W wires shows no precipitates at grain boundaries (Fig. 1l). The statistical columnar graph exhibits that the average grain size of pure W wire ranges from 15 to 35 nm (Fig. 1m), which is significantly larger than the La-doped W wire but still at the nanometer size level. The above results indicate that the non-slip ice bath drawing process restrains grain growth due to the smaller heat generation during the drawing process. The La-doped and pure W wires, processed using the same non-slip ice bath drawing technique, possess the nano-crystalline structures. The nano crystallines serve the purpose of accumulating geometrically necessary dislocations (GND), increasing the total dislocations, and inducing back stress40 in the wire crystals, thus enhanced the tensile strength and augmented the elongation through strain hardening.
Nano twins and stacking fault structures in bcc metals
The SFE is an important intrinsic parameter of metal materials, which influences the deformation mechanism and mechanical properties of metal materials. The SFE is closely related to structural phase transformations, especially in severe plastic deformation. The change of SFE plays a decisive role in the metal deformation mechanism and grain refinement mechanism41. Typically, nano twins and stacking fault (SF) structures facilitate the plastic deformation in low SFE fcc metals, thus inducing high strength of metallic materials by obstructing mobile dislocations19, 42, 43, 44, 45, storing dislocations during plastic deformation46, and migrating under stress47, 48, 49. To further investigate the impact of La addition on the W crystal structure, we conducted first-principle calculations based on DFT models50 to compare the SFE of bcc pure W and W-La alloys (Fig. 2a). We reveal that the SFE of the La-W alloys is 36.2% lower than that of pure W, demonstrating that adding La oxides reduces the SFE of bcc crystal structure W greatly, thereby making the binary La-W alloys more prone to induce twins and SF structure.
To verify the results of the DFT calculation, we analyzed the FIB samples of La-doped W wire and the pure W wire by TEM. Figure 2b shows a bright-field TEM image of the La-doped W wire, revealing large quantities of nano-twinned structures exist within the bcc matrix. The TEM observation aligns with the simulation calculations on the SFE, as adding La reduces the SFE of W crystal structure in bcc metals, culminating in forming more nano-twinned structures under stress. Figure 2c presents an enlarged view of twins from Fig. 2b, and the inset in Fig. 2c displays the HRTEM image with an interlayer width of 1.41 nm. Figure 2d illustrates the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of a TB, highlighted by a yellow solid line, where the lattice spacings on either side of the line are both 0.13 nm, matching the (211) plane and correlate with the standard La2O3 (JCPDS No: 05-0602), the inset image shows the corresponding selected fast Fourier transform (FFT), taken along [315//135] zone axis (ZA) at the yellow solid line area. The above observation confirms that the twinned structure originates in the La oxide precipitates area.
Figure 2e-f shows the HRTEM images of typical dislocated SF regions within the La oxide precipitates. The red dotted lines label the phase boundary between the matrix and the La oxide precipitate, and the yellow dotted lines denote the SF structures. Labeled by white lines, we measured one SF structure having a width of 2.08 nm and a length of 12.46 nm. The SFs present within the La oxide precipitate regions suggest that adding La facilitates the SF induction in precipitates. In contrast, despite having high intrinsic strength due to a nano-crystalline structure, the pure W wire exhibits no twinned structures in the TEM images, agreeing with the simulation predictions. Based on the above analysis, we confirm that adding La lowers the SFE and introduces twin and SF structures to bolster the strength of ultra-fine La-doped W wires.
Pinning effect in the La oxide precipitated region
Labeled in a yellow solid line, Fig. 2g illustrates the HAADF-STEM image of the phase interface between the W-matrix and the La oxide precipitate. The lattice spacings on the W-matrix side are 0.08 nm and 0.11 nm, corresponding to the (400) and (220) planes, respectively, in agreement with standard W (JCPDS No: 04-0806). On the La oxide precipitate side, the lattice spacing is 0.13 nm, representing and matching the (211) plane of standard La2O3 (JCPDS No: 05-0602). The inset shows an FFT pattern along the [315] ZA within the La oxide precipitate region. We prove that the ordered La oxide precipitates pinning the phase and twin boundaries. Figure 2h-i are the HAADF-STEM images of the La oxide precipitate outlined by the red-colored dotted circles, with widths of 0.94 nm and 1.77 nm, respectively. Both have FFT patterns along the [315] ZA from the circled areas. We also captured the partial dislocations existing in precipitates, marked with red ‘T’ symbols, presenting that La oxide precipitates are prone to induce dislocations during the deformation process due to the pinning effect.
The strain field surrounding the La oxide precipitate is mapped out using the geometrical phase analysis (GPA) approach. Figure 2j shows the HRTEM image of a La oxide precipitate at a grain boundary with GPA strain maps on the εxx (Fig. 2k) and εxy (Fig. 2l), respectively. The GPA maps display the dislocations marked with yellow arrows at the La oxide precipitate, revealing the impact of the lattice distortion. The severe lattice distortion improves the stiffness and strength of the saw wires greatly, which are also beneficial to the overall tensile-bearing capability. The distortion indicates that a significant lattice strain exists near the La oxide precipitates, leading to elevated energy levels. La oxide doping stabilizes the grain boundaries by the pinning effect, thus stabilizing grain boundary segregation thermodynamically, reducing the grain boundary energy, and lessening the instability of nanocrystal metals51. The in-situ TEM movie of the La-doped W wire’s drawing fracture process (Supplementary Movie 2) and the fracture microstructure in STEM image (Supplementary Fig. 2) after drawing further confirm the La oxide precipitate’s pinning effect. The schematic illustration (Supplementary Fig. 3) shows the mechanisms of the pinning effect under stress. The pure W cracks along the nanograin under stress, while the La oxide in the grain boundary can absorb the stress energy and plays the role of pinning effect to prevent the fracture, thereby increasing the strength. The La oxide precipitates provide additional sites for dislocation nucleation simultaneously pinning the dislocation motion, thus obtaining higher strength and plasticity.
Plastic deformation mechanism
We have shown that incorporating the La oxide into the W matrix leads to the formation of nanoprecipitates at the grain boundaries, which induce an interface pinning effect during plastic deformation. Figure 3a is an HRTEM image showing the interfacial district of the bcc → hcp transition with the orientation relationship (OR) following [111] bcc // [2\(\:\stackrel{\text{-}}{\text{1}}\stackrel{\text{-}}{\text{1}}\)0] hcp. As depicted in Fig. 3a, we outline the grain boundary with white-colored dotted line, the La oxide nanoprecipitates with red-colored dotted circle, and the dual-phase structure with yellow-colored dotted circle which formed by the pinning effect. The bcc → hcp transition follows the classical Burgers mechanism52. During the phase transformation, there are six potential variants53, which are listed in Supplementary Table 1. Nevertheless, Burger-like transformations do not always produce all the predicted orientations. As the SAED pattern indicated in Fig. 3a1, the OR between the bcc and the hcp phases is [111] bcc // [2\(\:\stackrel{\text{-}}{\text{1}}\stackrel{\text{-}}{\text{1}}\)0] hcp, which is the same as the V1 phase transition in Supplementary Table 1. Figure 3a demonstrates that the lattice distortion and phase transition occur under stress because of the pinning effect by the La oxide precipitates. Atomic-scale observations reveal that plastic deformation further activate composition-segregated bcc → hcp phase transition, where the dual-phase crystalline structure with bcc and hcp phase fraction continues to accommodate the plasticity induced by sliding. The phase transition structures can be manipulated to realize the synergetic enhancement of strength and ductility54, 55. Specifically, Fig. 3b depicts the Schematic diagrams of the Burgers orientation relationships and the lattice correspondence between bcc and hcp structures, the atomic-scale schematic diagrams dissect the atomic motion mechanism of composition segregation bcc → hcp phase transition procedure. The (110) plane of W matrix transforms to the (0001) plane of La oxide. The [111] direction of W matrix is parallel to the [2\(\:\stackrel{\text{-}}{\text{1}}\stackrel{\text{-}}{\text{1}}\)0] of La oxide56.
Using two models of pure W and W-La crystal structure as depicted in Supplementary Fig. 4a, we further elucidated the strain distribution during deformation through MD simulations (Supplementary Fig. 4c). The strains in the pure W matrix are distributed at the grain boundaries, while in the W-La crystal structure, the strains exist uniformly in the dual-phase interface between W and La oxide phase during deformation. At elevated stress levels, numerous partial dislocations become active, contributing to the plasticity. Activated under high stress, abundant partial dislocations in the dual-phase play a critical role in the alloy’s deformation behavior, enhancing the strength. Additionally, Supplementary Fig. 4b demonstrates that the theoretical stress of the W-La cluster is higher than pure W, which further proves that the interface pinning effect and the dual-phase interface caused by the addition of La benefit the enhancement effect.
The mechanical properties of La-doped and pure W wire
Manufacturing W alloy wire with long length, high strength, outstanding plasticity, and superb hardness is a perennial objective. We contrasted the structural differences between the La-doped and pure W wire, here, we will assess the influence of the microstructure on the mechanical properties. Figure 4a-b depicts the engineering and true stress-strain curves for two types of W wire. The La-doped W wires exhibit an ultimate engineering tensile strength of 6.92 GPa (true tensile strength of 7.21 GPa) with an elongation of 4.2%, which is the highest strength wire publicly reported at present, whereas the pure W wires have a tensile strength of 5.3 GPa (true tensile strength of 5.47 GPa) and an elongation of 3.15%. Both Fig. 4a and Fig. 4b indicate that the bcc → hcp phase transition induced by La oxides’ pinning effect in the cold-drawing could promote the strength-ductility synergy. Figure 4c illustrates the strain hardening rates, derived from the engineering stress-strain curves, showing that the La-doped W wires are 2.35% and the pure W wires are 2.16%. The representative SEM image of La-doped W wire’s fracture morphology highlights a noticeable diameter shrinkage (Fig. 4h). Marked with a yellow line, the fracture cross section’s diameter is 30.09 µm, resulting in a calculated shrinkage of 37.27%. A magnified view of the fracture surface morphology reveals a dense, fibrous distribution within the La-doped W wire, suggesting a multitude of fiber interfaces (Fig. 4i). The interfacial interaction and plastic deformation, induced by the non-slip drawing process, create an in-situ protective layer, endowing the W wire with excellent tensile strength57. Measured by the nanoindentation technique, we compared the hardness of La-doped and pure W wires and verified the excellent beneficial effects of adding the La oxide on the tensile strength, plasticity, and hardness of W wire (Fig. 4d-e). Figure 4f shows the SEM image of the La-doped W wire’s cross-section, with the inset highlighting the nanoindentation impression. The strength and hardness of the La-doped W wire are significantly higher than that of pure W wire.
As can be found in Fig. 4g and Supplementary Table 2, the La-doped and pure W possess significantly higher ultimate tensile strength (UTS) compared with the reported saw wires, demonstrating an exceptional tensile capability. The materials used as saw wires include pure W58, 59, 60, 61, K-doped W62, 63, 64, Re-doped W65, Ti66, 67, 68, and steel69, 70. By using the same drawing method and equipment, we prepared the 25 µm diameter stainless steel (ss) wire, achieving a tensile strength of 3.00 GPa (Supplementary Fig. 5), which is much higher than many reported steel wires69, 70. We demonstrate that the ice bath assisted non-slip drawing method provides an effective fabrication strategy for enhancing the strength of ultra-thin saw alloy wires. Moreover, the UTS of La-doped W wire is 30.6% higher than the pure W wire with the same manufacturing conditions, indicating the efficacy of La oxide precipitates’ pinning effect (as proved in Fig. 2) in improving the mechanical properties. The nano twins, SF, and severe lattice distortion vastly improve the elasticity and strength of the saw wires, which are also beneficial to the overall tensile-ductility bearing capability.
In summary, coupling the ice bath assisted non-slip drawing with the La oxide addition; we prepared the ultra-thin La-doped W wires exhibiting s an exceptional strength-ductility synergy of tensile strength reaching 6.92 GPa, and fracture elongation of 4.2%, which are 30.6% and 33.3% higher than the pure W wire. Besides, we achieved a continuous drawing length of 50 km. Pinned at the grain boundaries, the hcp La oxide precipitates form a coherent interface with the bcc W matrix, inducing nano twins by lowering the SFE of W, triggering the lattice distortion and dislocations, confining the dislocation movement pathway, and altering the plastic deformation mechanism of W. The ice bath and non-slip drawing condition fabrication preserves the high-density defects and ultrafine grains, thus achieving a synergetic enhancement of strength-ductility. The nanodomain manipulation strategy opens the way of preparing high performance ultra-thin bcc metal wires with robust strength-ductility synergetic property.