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 [7,8,10,11,39,[43], [44], [45], [46]].

Fig. 3. Comparison of fracture surfaces between the NCR-Ti5Al5 and the CR-Ti5Al5 alloys. (a, b) Fracture surface of the NCR-Ti5Al5 sample at different magnifications, exhibiting notably deep and large dimples. (c, d) Fracture surface of the CR-Ti5Al5 sample at different magnifications, showing fine and uniformly distributed dimples.

Fig. 4. 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 neighboring matrix. (h) One-dimensional (1D) concentration profile quantitatively showing the chemical composition of the Cr-rich phase and its neighboring 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. 5. Qualitative STEM-EDS results showing Cr distribution in the NCR-Ti5Al5 HEA, indicating that no Cr-rich phase formed.

Fig. 6. XRD pattern of CR-Ti5Al5 confirming the formation of the Cr-rich BCC phase.

Fig. 7. Nanoindentation hardness of the Cr-rich region, Heusler precipitate region, and matrix region containing L12 particles in CR-Ti5Al5, showing that the hardness of the Cr-rich bcc region is the smallest one among the three regions.

Fig. 8. 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.

Fig. 9. TEM-EDS images showing more information about the Cr-rich hcp phase in the CR-Ti5Al5 HEA after tensile deformation. (a) STEM image exhibiting the interdependent distribution of the Heusler and hcp phases. (b) Qualitative EDS showing the elemental distribution of Cr, Fe, Co, Al, Ti, and Ni in the CR-Ti5Al5 HEA, identifying the locations of the Cr-rich HCP phase and Heusler particles (enriched in Ni, Al, and Ti) corresponding to (a). The volume fraction of the Cr-rich hcp phase was estimated to be ～2.9 %. (c) Typical HRTEM image of the Cr-rich hcp phase and its corresponding FFT, further confirming the bcc to hcp phase transformation of the Cr-rich phase.

Fig. 10. Statistical results of the diameter and volume fraction of the L12 precipitates, Heusler precipitates, and matrix in CR-Ti5Al5 and NCR-Ti5Al5: (a) size data and (b) volume fraction data.

Fig. 11. Considerably inhomogeneous partitioning of all elements among the four phases (L12, Heusler, Cr-rich, and matrix) in CR-Ti5Al5 according to APT data, showing that there was a relatively low Cr concentration in both the L12 and Heusler precipitates.

Fig. 12. Equations of state of Cr and non-equiatomic Cr51Fe25Co18Ni6 in bcc and hcp structures at ground state. (a) The comparison of total energy as a function of volume between bcc Cr and hcp Cr, revealing that the total energy of hcp Cr is much higher than that of bcc Cr. (b) The comparison of total energy as a function of volume between bcc Cr51Fe25Co18Ni6 and hcp Cr51Fe25Co18Ni6, revealing that the total energy of hcp Cr51Fe25Co18Ni6 is higher than that of bcc Cr51Fe25Co18Ni6 at large volume conditions, but is slightly lower at small volume conditions (i.e., corresponding to higher condition pressure).

Fig. 13. Work-hardening rate as a function of true stress, showing that the estimated Cr-rich phase transformation critical stress (denoted by σtrans) was approximately 1258-1284 MPa according to the first inflection points in the two work-hardening rate curves.

Fig. 14. Yield strength contributions from different hardening mechanisms in the CR-Ti5Al5 alloy. The calculated values agree well with the experimental data.

Fig. 15. Yield strength contributions from different hardening mechanisms in the NCR-Ti5Al5 alloy. The calculated values agree well with the experimental data.

Fig. 16. MD simulations of Cr-rich phase strengthened FeCoNiCr HEA polycrystal. (a) Cross-section view of the FeCoNiCr HEA polycrystal. The embedded zone has a core-shell structure of Ni6Co18Fe25Cr51 bcc phase. The atoms with fcc and bcc local structure are colored by green and blue, receptivity. (b) The true stress-strain curve upon tensile loading. The labels of (a, b, c, d, e, f) marks the strain under which the snapshots are collected for microstructural analysis. The insert is the corresponding work-hardening rate curves as a function of true strain.

Fig. 17. Variation of grain boundary stress concentration with strain, compared to the average stress in grain interior.

Fig. 18. Comparison of time-dependent stress near crack in the presence and absence of transforming phase during deformation.

Fig. 19. Microstructural evolution of the Cr-rich phase strengthened FeCoNiCr HEA during the tensile deformation. (a-f) show the microstructures corresponding to the six points (a, b, c, d, e, f) labeled in the strain-stress curve of Fig. 16(b). The atoms with fcc, bcc and hcp local structure are colored by green, blue and orange, receptivity. Our MD simulations indicated that the occurrence of stress-induced bcc→hcp transformation within the Cr-rich phase.