α+β titanium alloys, owing to their low density, high specific strength, good fracture toughness and excellent high-temperature stability, are widely used in key load-bearing components for aerospace applications. Such alloys typically consist of a hexagonal close-packed (HCP) α phase and a body-centered cubic (BCC) β phase, and their strength–ductility balance depends not only on α phase size, lamellar morphology, β phase distribution and texture state, but also on the crystallographic structure of the α/β interface and its role in dislocation motion. Deformation of the HCP-structured α phase can proceed via prismatic (a) slip, basal (a) slip, pyramidal (a) slip, pyramidal (c+a) slip and twinning, among which prismatic (a) slip usually accommodates most of the shear deformation, but plastic shear is mainly confined to the a-axis direction and is difficult to independently satisfy three-dimensional strain coordination requirements; (c+a) slip containing a c-axis component can provide plastic deformation along the c-axis and plays a key role in alleviating local strain incompatibility and maintaining deformation continuity near interfaces.

Although existing studies have clarified the basic rules of slip transfer across α/β interfaces from the perspectives of the Burgers orientation relationship, geometric compatibility, residual Burgers vectors and β layer deformability, a systematic understanding is still lacking at the atomic scale of how interfacial coherency regulates dislocation transfer, dislocation pile-up and (c+a) slip activation. In duplex microstructures, coherent and incoherent interfaces with different lattice continuity and misfit may coexist simultaneously; the former, owing to lower lattice misfit and higher structural continuity, facilitates cross-interface slip transfer and strain coordination, whereas the latter may act as obstacles to dislocation motion, inducing local stress concentration and compensatory (c+a) slip. Therefore, describing the plastic heterogeneity near interfaces solely at the macroscopic scale in terms of α phase morphology or β phase distribution is insufficient, and the mechanistic correlation of plasticity must be established from the perspective of interfacial coherency and atomic-scale crystallographic matching.

To address the above issues, a team comprising researchers from Northeastern University and other institutions conducted a systematic study using in-situ SEM from ZEPTOOLS. Combining in-situ SEM/EBSD slip trace analysis, site-specific FIB-TEM, two-beam diffraction and aberration-corrected STEM-GPA multiscale characterization, the team systematically revealed the regulatory mechanism by which α/β interfacial coherency governs dislocation slip transfer and cooperative activation of (c+a) dislocations in a TC25 titanium alloy.

Title:Interfacial coherency-dependent slip transfer and cooperative activation of〈c+a〉dislocations at α/β interfaces in a TC25 alloy 
Journal:Advanced Light Materials 
URL:https://doi.org/10.1016/j.almate.2026.08.003

This study first focuses on the construction and crystallographic characterization of TC25 alloys containing α/β interfaces with different coherency. The research team used the high-temperature titanium alloy TC25 with a nominal composition of Ti-6.5Al-2.5Zr-1.5Mo-1.5Sn-0.25W-0.2Si (wt%) as the model material, and obtained duplex microstructures dominated by well-matched or poorly matched α/β interfaces, respectively, through different thermomechanical processing routes. The average grain sizes in the coherent and incoherent regions are approximately 9.54 μm and 8.23 μm, respectively, which are comparable in size and thus offer good comparability. Based on EBSD orientation data, the team converted the measured Euler angles into orientation matrices and traversed all symmetrically equivalent BOR variants, quantitatively describing the degree of interfacial crystallographic matching in terms of plane normal deviation and direction deviation. The results show that the coherent interfaces exhibit plane normal deviations of 3.2–9.1° and direction deviations of 3.4–9.3°, whereas the incoherent interfaces show significantly increased values of 20.4–32.1° and 12.4–32.2°, respectively, providing a clear crystallographic classification basis for the subsequent comparison of deformation behaviors.

On the above basis, the research focuses on the slip initiation and evolution near interfaces with different coherency. Using the in-situ tensile stage from ZEPTOOLS, the team applied a quantitative uniaxial tensile load to the specimens within the SEM, with the loading direction parallel to the longitudinal axis of the specimen and a loading rate of 2 μm/s, and interrupted at 0%, 1% and 10% strain to acquire SEM images and EBSD orientation maps. The results show that no obvious slip traces are present near the interfaces in the undeformed state; at 1% strain, slip traces appear in some grains accompanied by cross-slip and multiple-slip features, and combined with slip trace angle and Schmid factor analysis, the basal and prismatic slip families can be identified as dominant. At 10% strain, the number of slip traces increases significantly and exhibits a high-density multi-directional crossing distribution, with an increased proportion of pyramidal slip families; meanwhile, KAM maps indicate that the orientation gradients and dislocation pile-ups near the α/β interfaces are markedly enhanced after deformation, reflecting the direct influence of interfacial coherency on the mode of plastic transfer.

The study subsequently revealed in depth the microscopic mechanisms underlying interfacial coherency through site-specific FIB-TEM, two-beam TEM dislocation analysis and aberration-corrected STEM-GPA. At coherent interfaces, FIB lamellae show that the α/β interface is flat and continuous, dislocation activity is continuously distributed across the interface, (a), (c) and (c+a) type dislocations coexist under different diffraction vector conditions, and (c+a) dislocations are distributed on both sides of the interface while maintaining good continuity; atomic-scale characterization also confirms that the interface has good lattice continuity and crystallographic correspondence, and the GPA strain field is relatively continuously distributed along the interface without isolated sharp concentrations. In contrast, incoherent interfaces exhibit curved and irregular morphologies, with dislocation-dense regions mainly concentrated at the interface front on the α side, where numerous discrete dislocation lines pile up within the α phase while the adjacent β phase remains relatively clean; (c+a) dislocations are mainly confined to the α phase and lack effective transmission, atomic-scale observations show interface undulations, local lattice interruptions and interplanar spacing differences, and GPA reveals pronounced local shear strain concentration.

The study finally links interfacial coherency with macroscopic mechanical properties and elucidates the essential difference between the two types of interfaces. Owing to lower lattice misfit and higher structural continuity, coherent interfaces provide a structurally continuous and crystallographically favorable environment for cross-interface slip transfer, enabling (a) slip and (c+a) slip to cooperatively participate in strain coordination, constituting a cooperative activation mode; incoherent interfaces, in contrast, trigger high-critical-resolved-shear-stress (c+a) slip as a compensatory strain mechanism through dislocation pile-up and local stress concentration, readily leading to strain localization. Mechanical testing shows that the TC25 alloy in this study has a yield strength of 1039 MPa, an ultimate tensile strength of 1154 MPa and an elongation of 20.3%, exhibiting superior ductility while maintaining relatively high strength compared with conventionally forged or rolled α+β titanium alloys. The conclusions indicate that interfacial coherency is the key structural parameter determining whether an α/β interface acts as a plastic transfer channel or a dislocation obstacle source, and that increasing the proportion of coherent or near-coherent α/β interfaces is a potential strategy for improving the plastic stability of titanium alloys.

The ZEPTOOLS PicoFemto SEM in-situ high-temperature tensile stage is an advanced instrument integrating a mechanical tensile module and a high-temperature environment module, capable of performing tensile experiments while in-situ heating samples within a scanning electron microscope. This product adopts a new-generation high-temperature tensile scheme, in which the heating stage is placed in direct contact with the sample, ensuring temperature accuracy and stability.