Whether a material is ductile or brittle is determined by its mechanism for releasing strain energy, either through dislocation motion and interaction or crack nucleation and propagation. Body-centered cubic (BCC) metallic materials, known for their high strength, often exhibit limited ductility due to insufficient dislocation interactions and multiplication1–3. Commercial-purity molybdenum (Mo) metal, for example, typically displays only a small percentage of tensile elongation at room temperature4. Ductility decreases further as temperature drops, severely limiting its engineering applications5,6. Clearly, if dislocation plasticity can be promoted and/or the crack nucleation can be hindered, the materials can deformed by dislocation activities, leading to the increase of ductility1.
Rare earth elements, as vital strategic resources, exert a transformative influence on material properties7–11. Although their presence may not significantly contribute to weight, value, or volume, it is often indispensable for optimal material performance. In less-ductile BCC structured metals and alloys, adding rare earth oxides like CeO212, La2O313–15, and Y2O316–18 may show promise in simultaneously enhancing strength and ductility. Prior researches hypothesize that rare earth oxides not only promote grain nucleation, but the oxygen vacancies at the surface may adsorb detrimental impurities such as O, N atoms et. al., thereby reducing grain boundary-induced crack nucleation15,19. However, this hypothesis lacks empirical evidence and fails to account for the increased intergranular dislocation activity. Recently, we realize that rare earth elements, owing to the strong correlation between electrons in f-orbitals and their multivalence nature, exhibit a range of exotic electronic states20. In the field of chemistry, during the complex redox reactions21 or thermal processes involved in rare earth oxide synthesis22, oxygen partial pressure undergoes specific changes near the growth interface where diffusion is facilitated, allowing for highly tunable oxygen vacancy concentrations. The combined impact of these alterations in electronic and geometrical structures may lead to crystal structure collapse and ordered-to-disordered structural transitions23,24. This insight prompts us to consider whether the introduction of rare earth elements in alloys results in unique precipitate-matrix interfaces that influence dislocation behavior in distinctive ways.
In this study, through integrated Differential Phase Contrast-Scanning Transmission Electron Microscopy (iDPC-STEM) and Electron Energy Loss Spectroscopy-STEM (EELS-STEM) analysis, we first ascertain that precise control of the interfacial reaction during even conventional liquid-solid mixing effectively facilitates the formation of an ultrathin amorphous interfacial layer between La2O3 particles and the Mo matrix, which exhibits a propensity for vacancy absorption and modification of dislocation configurations. This phenomenon is truly closely linked to the alterations in the electronic density of states within La. To further harness this potent effect, we adopt rotary swaging and produced a Mo alloy containing irregular-shaped La2O3 (0.6 wt.%) nanoparticles with amorphous interface. Combining three-dimensional tomography and in situ TEM mechanical testing, we have identified that this distinctive interface plays a crucial role in enhancing dislocation activities. The irregular-shaped amorphous interface is capable of promoting the formation of numerous and diverse dislocation sources at the particle-matrix interface, significantly bolstering dislocation plasticity. Consequently, the resulting alloy demonstrates strength of approximately 783 MPa and an elongation of about 37.5% at room temperature. At -50°C, it even demonstrates an elongation at fracture of approximately 37% and a yield strength of approximately 1014 MPa, surpassing the performance of other Mo alloys.
The sintered Mo-La2O3 (0.6 wt. %) alloy was produced using power metallurgy, with Mo-La2O3 alloy powders obtained through a liquid-solid mixing method. Subsequently, the powders were squeezed into a cylindrical compact (90 mm in diameter), and then sintered at 1850°C for 4 h in a dry hydrogen atmosphere. Figure 1a displays a typical High-Angle Annular Dark-Field STEM (HAADF-STEM) image of the as-prepared alloy. It is observed that the nearly spherical La2O3 nanoparticles are mostly dispersed within grains. To reveal the atomistic interfacial structure, we employed HAADF-STEM along with the advanced iDPC-STEM technique, which enables sub-angstrom resolution imaging of both light and heavy atoms. Figure 1b and Extended data Fig. 1 present the atomic-resolution HAADF-STEM and iDPC-STEM images viewed along the [101] zone axis of La2O3. Of particular significance is the discovery of an ultrathin amorphous layer at the particle-matrix interface, with a measured thickness of approximately 1.5 nanometers. This is further confirmed by the fast Fourier Transform image in the upper right corner of Fig. 1b, where a typical amorphous ring characteristic indicative of its amorphous nature is clearly discerned.
To investigate the potential alterations in electronic structures, we then employed EELS- STEM technique, which possesses the ability to obtain the density of electronic state and valence state information at atomic resolution. Figure 1c shows the O K edges and La M edges recorded from two region. Region I is in the internal La2O3 particle (as marked by blue rectangle in Extended data Fig. 2), and Region II (green rectangle) is in proximity to the amorphous layer. We observe substantial shifts in both the O K edge and La M edge from Region I to Region II. Specifically, O K edge appears a 2.2 eV shift to low energy from Region I to Region II, indicating the existence and considerable concentration of oxygen vacancies25,26. To maintain the electron balance, the electron structure of La changes accordingly. In the magnified images of the edge energy-loss near-edge structures (ELNES) of La M5,4 edge (3d◊4f) excitation edge in Fig. 1d, La M5 edge (the former) appears a 0.3 eV shift to high energy from Region I to Region II, while La M4 edge (the latter) appears a 0.5 eV shift to low energy, which reflects the changes of the 4f unoccupied density of state of La atom27,28.
Meanwhile, oxygen vacancies are clearly observed in close proximity to the amorphous interfacial layer within the nanoparticles. The iDPC-STEM image of the perfect lattice of La2O3 is shown in the enlarged image at the upper right corner of Fig. 1e for comparison. It becomes apparent that oxygen atoms are locally absent in the oxides near the amorphous interface as highlighted by the pale-yellow open circles in the iDPC-STEM image (acquired from the area demarcated by the orange square, region III in Extended Data Fig. 1). Additionally, as shown in the magnified iDPC-STEM image in Fig. 1f of Region IV in Extended Data Fig. 3 (all viewed along the zoon axis [100] of the Mo matrix), oxygen atoms tend to aggregate into nano-sized clusters near the particle-matrix interface in the Mo matrix. The oxygen atoms occupy octahedral interstitial sites within the Mo lattice, indicated by pink spheres.
The above results demonstrate that the significant change of electronic density of states truly exists from the La2O3 oxide interior to the interface, which is primarily responsible for the interface amorphization. While other oxides, for instant Al2O3, usually display crystalized interface. It is also noted that the amorphization is quite sensitive to the control of interfacial reaction. Long-time annealing or hot isostatic pressing sintering may eventually lead to crystallization of this ultrathin layer. Our in situ TEM mechanical testing further elucidates the pivotal role of such an amorphous interface in enhancing dislocation interactions and multiplication. As shown in Fig. 2a and Supplementary video 1, when an edge dislocation (With a Burgers vector [111]) encounters two nearby particles, instead of looping around29,30, a portion of the dislocation is absorbed, leaving three dislocation segments. Dislocation segments I and III (marked by violet dashed lines) are anchored at one end of the interface, while segment II (marked by yellow dashed lines) is pinned at both particles. Dislocation segment II continues to advance, giving rise to two screw dislocations firmly anchored at the interface. These screw dislocations exhibit limited mobility, significantly impeding the movement of the connected edge dislocation (segment II). Consequently, this interaction triggers the cooperation of different dislocation components, promoting various dislocation interactions and the operation of single-arm dislocation sources, as illustrated in Extended data Fig. 4.
This dislocation-particle interaction is then computationally verified via Molecular Dynamics (MD) simulations at room temperature. The detailed information of simulation is descripted in Method. A BCC Mo configuration with a 20 nm amorphous particle (simulated using amorphous MoPd alloy for modelling the effect of amorphous interface) was employed (Fig. 2b). Under applied loading, an edge dislocation was inserted and glided on the (\(\text{1}\stackrel{\text{-}}{\text{1}}\text{1}\)) plane. Snapshots in Fig. 2b illustrate the configuration during increasing shear strain (Supplementary video 2). When the dislocation meets the amorphous particle, the interacting segment dissolve into the amorphous interface, a known high-capacity dislocation sink31,32. Despite this interaction, the dislocation continues to glide, with one end remaining anchored to the amorphous interface. This simulation corroborates experimental observations, confirming the distinct behavior of dislocations near amorphous interfaces.
Furthermore, it is noteworthy that the La2O3 nanoparticles exhibit no specific orientation relationships with the Mo matrix. The absence of a preferred orientation allows the particles to assume various shapes without inducing interfacial cracks. One might question whether, in the case of a corrugated interface, dislocation segments would become “float” and immobilized at the interface's ridges instead of being absorbed. This hypothesis was initially proved by our computational simulation. We replaced the spherical particles in the previous simulation with particles containing notches at the interface, as shown in Fig. 2c and Supplementary movie 3. As the gliding dislocation interact with the particle, it leaves behind two dislocation segments at the notch. If more irregular shapes (e.g., holes) exist on the interface, more dislocation segments would be created after the gliding of dislocations. These dislocation segments, with both ends pinned at the edge of the notch, function as typical Frank-Read dislocation sources upon loading. In this way, one dislocation can be divided into many segments during the dislocation-particle interactions.
To harness this mechanism, we employ the process of rotary swaging to treat the sintered Mo-La2O3 alloy, subjecting the material to compression in all directions, inducing substantial deformation in both the particle and the Mo matrix. Extended data Fig. 5 provides a picture of Mo-La2O3 alloy bar after the process of rotary swaging. Given the relative forces acting on specific crystallographic planes may differ, the particles are deformed into irregular shapes. As shown in the HAADF-STEM image in Fig. 2d, the La2O3 nanoparticles exhibit ellipsoid shapes in the projected view, rather than being spherical. Concurrently, there is slight variability in the thickness of the amorphous layer (see Extended data Fig. 6). Furthermore, as shown in Fig. 2d, dislocations curled up and tangled around the particles severely in the initial microstructure of this alloy processed by rotary swaging.
Since the TEM images only provide two-dimensional projections of the actual structure, we conducted 3D tomography analysis33 to characterize the morphology of these La2O3 nanoparticles. This involved capturing a series of HAADF images, with each image taken at one-degree intervals while tilting the x-axis of the sample holder under a constant two-beam condition34–36. All the images were processed using Tomviz software to generate a 3D tomographic reconstruction of the particles. As depicted in Fig. 2e, Extended data Fig. 7 and Supplementary videos 4, 5, the results of the 3D tomography unequivocally clarify that the particles exhibit irregular shapes irrespective of their sizes. More precisely, these particles are neither spherical nor faceted, instead, they feature conspicuous ridges and valleys that extend widely around the particle-matrix interface.
The newly produced Mo-La2O3 alloy demonstrates superior tensile properties in comparison to other Mo metal and alloys. We first compared the tensile properties of pure Mo and the Mo-La2O3 alloy processed via rotary swaging. The tensile samples are dog-bone shaped with gauge sizes of 25 mm in length and 5 mm in diameter. Figures 3a and b show the tensile engineering and true stress-strain curves of the tested alloys at different temperatures, respectively. At ambient temperature (approximately 20°C), the tensile yield strength (\({\sigma }_{y}\)) of the Mo-La2O3 alloy processed by rotary swaging measures approximately 783 MPa, with a corresponding elongation at fracture of approximately 37.5%. These values are notably superior to those of commercial-purity Mo metal37. Obvious work hardening is also obtained at both room and low temperature. We further directly compared the properties of the Mo-La2O3 alloy processed by rotary swaging to other previously reported high-performance Mo alloys containing different oxides and grain sizes. As illustrated in Fig. 3c, the strength-ductility trade-off is obvious in those alloys at even room temperature, while our Mo-La2O3 alloy processed by rotary swaging displays high strength and significant tensile elongation, distinguishing it from the aforementioned alloys. Remarkably, this excellent combination of strength-ductility remains even at -50°C (Fig. 3a). The material’s yield strength reaches 1014 MPa, and the elongation at fracture reaches approximately 37% with obvious work hardening. This low temperature ductility surpasses that of previously reported Mo and Mo alloys (Fig. 3d).
The large elongation achieved here was found to be highly related to the enhanced dislocation activities. We employed dislocation tomography techniques to reconstruct the actual dislocation structure around the La2O3 nanoparticles in this Mo-La2O3 alloy after fracture. The series of HAADF-STEM images was captured by using the same method as previously described. We maintained a two-beam condition with \(\overrightarrow{g}\)= [002] to ensure the visibility of dislocations during sample tilting. Supplementary movie 6 presents a representative 3D dislocation tomographic reconstruction, while Fig. 4a displays three images extracted from the reconstruction at different tilting angles. It reveals the formation of complex dislocation structure around the La2O3 nanoparticles. Numerous dislocations are observed with one end pinned to the interface. Dislocation segments with both ends pinned at the interface are also found, primarily at the interfacial ridges (marked by orange arrows). Furthermore, substantial dislocation entanglements are formed on or around the particles (as marked by blue arrows).
To enhance the clarity of the visual representation, we employed Chimera software to simulate the three-dimensional dislocation structure based on the experimental images. Figure 4b presents images extracted from the simulated 3D dislocation structure. The intricate interactions between dislocations and particles initiate interactions among dislocations in different slip direction and planes are enhanced. In Fig. 4c and Extended data Fig. 8, the index of dislocation Burgers vectors (\(\overrightarrow{b}\)) obtained by \(\overrightarrow{g}\bullet \overrightarrow{b}\) analysis (\(\overrightarrow{g}\), vector diffraction) provides further insight into the dislocations pinned at specific particles. These dislocations exhibit a range of \(\overrightarrow{b}\) values, including 1/2\(\left[\text{111}\right]\), 1/2\(\text{[1}\stackrel{\text{-}}{\text{1}}\text{1]}\), 1/2\(\left[\text{1}\stackrel{\text{-}}{\text{1}}\stackrel{\text{-}}{\text{1}}\right]\) and [001]. Additionally, the dislocations predominantly consist of mixed dislocations containing both edge and screw components.
The TEM characterization mentioned above has confirmed that the advantageous role of irregularly shaped La2O3 nanoparticle with amorphous layer in promoting dislocation interactions and dislocation multiplication. To reveal the dynamic of the complex dislocation-particle interactions, in situ TEM straining experiments were conducted on the Mo-La2O3 alloy, processed by rotary swaging, from ambience to low temperature to directly observe dislocation-particle interactions. It is observed that when an incoming dislocation interacts with the nanoparticle, multiple dislocation segments are generated. Those with one end pinned at the interface function as single arm sources as shown in Extended data Fig. 9. While, the dislocation segments with both ends pinned at the interface operate as Frank-Read dislocation sources. Snapshots in Fig. 4d (from Supplementary video 7) demonstrated a typical example showing the formation of Frank-Read dislocation sources and the subsequent generation of new dislocations at room temperature. The Frank-Read dislocation segment pinned at the surface of particle 1 bowed out and divided into two dislocation segments, denoted as d1’ and d1’’, marked by yellow dashed curves. While d1’ detached from the particle later, the original Frank-Read dislocation source continued to generate new dislocations, as indicated by blue dashed lines. At room temperature, the operation of these Frank-Read dislocation sources can persist through several cycles until the dislocation segments eventually detach from the particle-matrix interface. While such interfacial Frank-Read dislocation sources are recreated as subsequent dislocations gliding towards and interacting with the particles.
The operation of these dislocation sources becomes more stable at low temperature due to the increased difficulty of detachment. It is detected that during deformation at -50°C, an abundance of short Frank-Read dislocation sources forms as incoming dislocations looped around the nanoparticles, as marked by the pink dashed squares in Fig. 4e. Figure 4f show the operation of these Frank-Read dislocation sources. (the series of snapshots is captured from Supplementary video 8). It is observed that when the dislocation (marked by blue arrow) interacted with a particle (labeled as p2), one single-arm dislocation source (S1, blue arrows) and one Frank-Read dislocation source (F-R, pink arrows) formed. Subsequently, these sources commenced operation under applied stress, continuously generating new dislocations. The operation of these dislocation sources proved to be highly durable. Even as the applied load increased significantly, these dislocation sources continued to operate stably, offering great benefits to the low temperature dislocation plasticity of the material.