Phase transition and heterogeneous strengthening mechanism in CoCrFeNiMn high-entropy alloy fabricated by laser-engineered net shaping via annealing at intermediate-temperature

doi:10.1016/j.jmst.2021.03.028

Abstract

Abstract:

High-entropy alloys (HEAs) have attracted tremendous attention owing to their controllable mechanical properties, whereas additive manufacturing (AM) is an efficient and flexible processing route for novel materials design. However, a profound appraisal of the fundamental material physics behind the strengthening of AM-printed HEAs upon low/intermediate-temperature annealing is essential. In this work, CoCrFeNiMn HEAs have been prepared using laser-engineered net shaping (LENS) and subsequently annealed at different temperatures. The CoCrFeNiMn HEA annealed at intermediate-temperature (873 K) exhibits a strong strain hardening capability, resulting in ultimate strength of 725 MPa and plasticity of 22%. A ternary heterogeneous strengthening mechanism is proposed to explain this phenomenon, in which equiaxed grains, columnar grains, and σ precipitates play different roles during tensile deformation. The resultant excellent strength and ductility can be ascribed to the heterostructure-induced mismatch. The equiaxed grains provide adequate grain boundaries (GBs), which induce dislocation plugging-up and entanglement; the columnar grains induce the onset and arrest of the dislocations for plastic deformation; and the σ precipitates hinder the movement of slip dislocations. The results provide new insights into overcoming the strength-ductility trade-off of LENS-printed HEAs with complex geometries.

Key words: High-entropy alloys, Phase transition, Heterogeneous strengthening, Intermediate-temperature

Yunjian Bai, Heng Jiang, Kuo Yan, Maohui Li, Yanpeng Wei, Kun Zhang, Bingchen Wei. Phase transition and heterogeneous strengthening mechanism in CoCrFeNiMn high-entropy alloy fabricated by laser-engineered net shaping via annealing at intermediate-temperature[J]. J. Mater. Sci. Technol., 2021, 92: 129-137.

Figures/Tables 10

Fig. 1. Schematic diagram of (a) the scanning strategy and (b) tensile specimen.

Fig. 2. (a) XRD patterns of all samples. (b) The enlarged image of the (111) diffraction peaks. (c) The (111) lattice parameters of different samples.

Fig. 3. The average grain color map of (a) the as-printed sample and that annealed at (b) low-temperature, (c) intermediate-temperature, and (d) high-temperature, respectively. The combination map of grain boundary and phase distribution of (e) the as-printed sample and that annealed at (f) low-temperature, (g) intermediate-temperature, and (h) high-temperature, respectively.

Fig. 4. The columnar grain distributions of the as-printed sample (a) and that annealed at (b) low-temperature, (c) intermediate-temperature, (d) high-temperature, respectively.

Fig. 5. KAM maps of (a) the as-printed simple and that annealed at (b) low-temperature, (c) intermediate-temperature, and (d) high-temperature, respectively. And the LM maps of (e) the as-printed simple and that annealed at (f) low-temperature, (g) intermediate-temperature, and (h) high-temperature, respectively.

Fig. 6. (a) The typical engineering tensile stress-stain curves of all samples. (b) The corresponding true stress-strain curves of (a), and the inset is the strain hardening rate.

Table 1 Tensile properties of the CoCrFeNiMn HEA in this work.

Sample	σ_y (MPa)	σ_UTS (MPa)	E_f (%)
As-printed	499	666	18
673 K	527	700	21
873 K	510	725	22
1073 K	391	637	26

Fig. 7. EBSD results of the sample annealed at intermediate-temperature after fracture: (a) grain map, (b) phase distribution map, (c) KAM map, (d) grain boundary map, and IPF of (e) equiaxed grains and (f) columnar grains.

Fig. 8. The schematic illustration of the sample annealed at intermediate-temperature: (a) before deformation, (b) during deformation, and (c) after deformation.

Fig. 9. Ultimate strength versus elongation-to-failure plot of AM-printed CoCrFeNiMn HEA.

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