Enhancement of strength-ductility balance of heavy Ti and Al alloyed FeCoNiCr high-entropy alloys via boron doping

doi:10.1016/j.jmst.2020.10.023

Abstract

Abstract:

As one of the most effective mechanisms, precipitation-hardening is widely used to strengthen high-entropy alloys. Yet, heavy precipitation-hardened high-entropy alloys usually exhibit serious embrittlement. How to effectively achieve ultra-high strength and maintain reliable ductility remains a challenge. Here, we report a study of doping extremely little boron to meet this target. We found that adding of 30 ppm boron into the heavy Ti and Al alloyed FCC FeCoNiCr high-entropy, (FeCoNiCr)₈₈Ti₆Al₆ HEA (at.%) which is strengthened mainly by both coarse BCC-based (Ni, Co)₂TiAl Heusler and fine L1₂-type FCC-based (Ni, Co)₃TiAl precipitates and shows ultrahigh strength but poor ductility, could significantly change the original microstructure and consequently improve mechanical performance, owing to the well-known effect of boron on reducing the energy of grain boundaries. The boron addition can (1) eliminate microcavities formed at Heusler precipitate-matrix interfaces; (2) suppress the formation and segregation of coarse BCC Heusler precipitates; (3) promote the formation of L1₂ nanoparticles. This changes of microstructure substantially improve the tensile ductility more than by ~86 % and retain comparable or even better ultimate tensile strength. These findings may provide a simple and costless solution to produce heavy precipitation-strengthened HEAs with ultrahigh strength and prevent accidental brittleness.

Key words: High-entropy alloy, Precipitation strengthening, Boron, Ductility

Yongliang Qi, Tinghui Cao, Hongxiang Zong, Yake Wu, Lin He, Xiangdong Ding, Feng Jiang, Shenbao Jin, Gang Sha, Jun Sun. Enhancement of strength-ductility balance of heavy Ti and Al alloyed FeCoNiCr high-entropy alloys via boron doping[J]. J. Mater. Sci. Technol., 2021, 75: 154-163.

Figures/Tables 13

Fig. 1. (a) Comparison of mechanical responses of the Ti3Al3, Ti6Al6 and Ti6Al-30B HEAs. (b) Tensile properties of the Ti6Al6 and Ti6Al6-30B compared to other FeCoNiCr-based HEAs [[14],[15],[17],[34], [35], [36], [37], [38]].

Table 1 The tensile yield strength (YS), ultimate tensile strength (UTS), and tensile elongation (EL) for the Ti3Al3, Ti6Al6 and Ti6Al6-30B samples.

Samples	YS (MPa)	UTS (MPa)	EL (%)
Ti3Al3	639 ± 8	1067 ± 11	46.5 ± 1.7
Ti6Al6	1238 ± 14	1456 ± 12	6.9 ± 0.3
Ti6Al6-30B	1119 ± 11	1476 ± 13	12.9 ± 0.5

Fig. 2. (a) XRD patterns of the three HEAs (Ti3Al3, Ti6Al6 and Ti6Al6-30B) in the as-aged state. (b) Lattice parameters of the Ti3Al3, Ti6Al6 and Ti6Al6-30B HEAs, obtained from correspondingly refined XRD patterns.

Fig. 3. (a) HAADF-STEM image shows a triplex microstructure of the Ti6Al6 HEA containing FCC matrix, FCC L12-type (Ni, Co)3TiAl particles and BCC-based (Ni, Co)2TiAl Heusler particles. (b) [113]BCC microdiffraction pattern from the Heusler particles in (a) indicated by black dashed circles. (c) A typical TEM-EDS spectrum and correspondingly chemical composition of Heusler particles. (d) [111]FCC selected area diffraction pattern from the matrix containing FCC L12-type (Ni, Co)3TiAl nanoparticles. (e) The morphology of these L12-type nanoparticles in the matrix. (f)-(k) Qualitative distribution of Ti, Al, Co, Cr, Fe and Ni, respectively.

Fig. 4. TEM results revealing different phases in the triplex microstructure of the Ti6Al6-30B HEA. (a) HAADF-STEM image from the Ti6Al6-30B HEA. (b) Qualitative boron distribution in (a). (c) [011]BCC selected area diffraction pattern from the Heusler particles in (a) indicated by black dashed circles. (d) A typical TEM-EDS spectrum and correspondingly chemical composition of Heusler particles. (e) [111]FCC selected area diffraction pattern from the matrix containing FCC L12-type (Ni, Co)3TiAl nanoparticles. (f) The morphology of these L12-type nanoparticles in the matrix.

Fig. 5. Typical SEM images at different magnifications from electrochemically polished samples. (a) and (b) Ti3Al3. (c) and (d) Ti6Al6. (e) and (f) Ti6Al6-30B. L12 nanoprecipitates are indicated by red arrows; Heusler precipitates are indicated by blue arrows; Cavities are indicated by yellow dotted circles.

Fig. 6. Statistical results on volume fraction and size of cavities, L12 and Heusler particles and matrix in both the Ti6Al6 and Ti6Al6-30B HEAs. (a) Volume fraction data. (b) Size data. Cavity, L12 and Heusler particles and matrix all are approximately treated to be spherical.

Fig. 7. Bright-field TEM micrographs of the Ti6Al6-30B HEA at the tensile strain of 3%. (a) Dislocations shearing coherent L12 nanoprecipitates in the matrix. (b) The corresponding EDS maps of (a), confirming the location of L12 nanoprecipitates in the matrix, which are enriched in Ni, Al and Ti, but depleted in Cr and Fe. (c) Dislocations locally blocked by Heusler particles mostly formed at grain boundaries, revealing the non-shearable nature of Heusler particles.

Fig. 8. Grain boundary characterization of the Ti6Al6-30B HEA before tensile deformation by EBSD technique, revealing amounts of twin boundaries uniformly formed in this alloy.

Fig. 9. The contributions of L12 and Heusler precipitates to yield strength in the Ti6Al6 and Ti6Al6-30B HEAs.

Fig. 10. Work-hardening responses of the Ti3Al3, Ti6Al6 and Ti6Al6-30B HEAs.

Fig. 11. Room-temperature tensile fracture surfaces of the Ti6Al6 and Ti6Al6-30B HEAs. (a) Ti6Al6. (b) Ti6Al6-30B.

Fig. 12. Schematic sketch of microstructure changes influencing the overall mechanical properties of heavy precipitation-strengthened HEAs. (a) A detrimental microstructure containing segregated precipitates and microcavities at precipitate-matrix interfaces, resulting in high tensile strength but little ductility. (b) An optimized microstructure with homogeneously dispersed precipitates and without microcavities, achieving a good combination of tensile strength and ductility.

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