Fig. 1. Schematic of the novel strategy of using local stress concentration-induced phase transformation at grain boundaries (GBs). (a) GB regions with precipitates are prone to initiating cracks due to the local stress concentration induced by GB precipitates. (b) The proposed strategy of using this local stress concentration to trigger a phase transformation of the second phase preseted at GBs, providing additional transformation-induced plasticity (TRIP).

Fig. 2. Exceptional combination of strength and ductility achieved by the CR-Ti5Al5 alloy at room temperature. (a) Engineering stress–engineering strain curves of the Ti5Al5 alloy samples (NCR-Ti5Al5, CR-Ti5Al5-1, and CR-Ti5Al5-2). The insert shows the corresponding true stress and work-hardening rate curves as a function of true strain. (b) Yield strength (YS) versus the product of strength and elongation of the designed Ti5Al5 HEA compared with those of other high-performing FeCoNiCr-based HEAs.

Fig. 3. Typical microscopic structure of the undeformed CR-Ti5Al5 HEA. (a) A STEM-HAADF image showing a unique four-phase structure: bcc Heusler precipitates and Cr-rich bcc phase in the GB region, and fcc matrix and fcc L1₂ nanoprecipitates in the grain interior. (b) STEM-EDS maps revealing the qualitatively elemental distribution among the four phases in the alloy, confirming the presence of the Cr-rich phase. (c-e) SAED patterns of c the Cr-depleted bcc Heusler precipitate, (d) the disordered Cr-rich bcc phase, and e the disordered fcc matrix containing Cr-depleted L1₂ nanoprecipitates. (f) High-resolution SEM image showing the morphology of the three phases: Heusler particles (blue arrows), L12 particles (red arrows), and Cr-rich phase (green arrows) in the alloys. (g-j) Atomic-scale analyses using atom probe tomography (APT). (g) Three-dimensional (3D) reconstruction of the 45 at.% Cr and 12 at.% Ni isoconcentration surfaces presenting the morphologies of the Cr-rich phase and its neigh boring matrix. (h) One-dimensional (1D) concentration profile quantitatively showing the chemical composition of the Cr-rich phase and its neigh boring matrix. (i) 3D reconstruction of the 35 at.% Ni isoconcentration surfaces revealing the morphology of the L1₂ nanoprecipitates in the matrix. The number density and average diameter of the L1₂ nanoprecipitates were 4.79 × 10²³ m^-3 and ~5.9 nm, respectively. (j) 1D concentration profile quantitatively showing the chemical composition of the L1₂ nanoprecipitates and the matrix.

Fig. 4. Tensile deformation mechanisms in the CR-Ti5Al5 HEA at room temperature. (a) Bright-field TEM images at a tensile strain of 5% showing interaction of the Cr-rich phase and Heusler particles. A considerable number of dislocations (white arrows) pile up at the interface between the Cr-rich phase and the Heusler particles, indicating a severe local stress concentration. (b-g) Representative TEM-EDS images after tensile fracture showing the Cr-rich phase undergoing a stress-induced phase transformation from the original bcc to hcp structure. (b) STEM-HAADF image showing a typical deformed microstructure containing Heusler particles, microvoids (red circles), the Cr-rich hcp phase, and stacking faults (SFs, yellow arrows). (c) Qualitative STEM-EDS showing the elemental distribution corresponding to (b). (d), (e) Microdiffraction patterns of the Heusler and Cr-rich hcp phases, respectively. (f) Interface between the L2₁ and Cr-rich hcp phases showing a clear dislocation wall (the region outlined by red dotted lines). (g) Interface (red dotted line) between the Cr-rich hcp phase and fcc matrix.