Enhanced strength-ductility of CoCrFeMnNi high-entropy alloy with inverse gradient-grained structure prepared by laser surface heat-treatment technique

doi:10.1016/j.jmst.2021.09.043

Abstract

Abstract:

The inverse gradient-grained CoCrFeMnNi high-entropy alloy with a desirable mechanical property that evades the strength-ductility trade-off is fabricated by the process of cold rolling and subsequent laser surface heat-treatment. Due to the gradually decayed thermal effect along with the thickness, the grain size increases from the hard core to the soft surface in terms of the inverse gradient-grained sample, which is in good consistent with the microhardness profiles. The hetero-deformation induced strengthening and strain hardening caused by the inverse gradient-grained structure improve the strength-ductility combination, as well as the high-order hierarchal nanotwins due to the enhanced interaction with dislocations. For the laser surface heat-treatment technique, the strength and ductility are significantly increased by enlarging the microhardness difference and decreasing the thermal stress. Considering the high volume fraction of gradient-grained layer and a great deal of high-order hierarchal nanotwins in the central region, the laser surface heat-treatment technique is a promising way to produce the gradient-structured materials without thickness limitation.

Key words: High-entropy alloy, Laser surface heat-treatment, Cold rolling, Inverse gradient-grained structure, Strength-ductility combination

Bohong Zhang, Jie Chen, Pengfei Wang, Bingtao Sun, Yu Cao. Enhanced strength-ductility of CoCrFeMnNi high-entropy alloy with inverse gradient-grained structure prepared by laser surface heat-treatment technique[J]. J. Mater. Sci. Technol., 2022, 111: 111-119.

Figures/Tables 10

Fig. 1. Schematic diagram for the preparation of inverse gradient-grained CoCrFeMnNi high-entropy alloy.

Fig. 2. Microstructure and corresponding element distributions of the experimental CoCrFeMnNi high-entropy alloy: (a) electron channeling contrast imaging (ECCI) image; (b) EDS maps of alloying elements; (c) EDS spectroscopy.

Fig. 3. Orientation imaging maps of the experimental CoCrFeMnNi high-entropy alloy under conventional annealing and laser surface heat-treatment processes: (a) HOMO-800; (b) HOMO-610; (c) surface region of the IGGS-1000-4; (d) central region of the IGGS-1000-4.

Fig. 4. Microhardness distributions along the depth direction of the IGGS-1000-4, HOMO-610 and HOMO-800 samples.

Fig. 5. Simulated temperature curves at various depths during laser surface heat-treatment.

Fig. 6. XRD patterns of the experimental CoCrFeMnNi high-entropy alloy with inverse gradient-grained structure and homogeneous structure: (a) IGGS-1000-4; (b) HOMO-610; (c) HOMO-800.

Fig. 7. TEM images of (a) surface region and (b) central region for the IGGS-1000-4 sample.

Fig. 8. Tensile property of the cold-rolled CoCrFeMnNi high-entropy alloy treated by various heat treatment processes: (a) engineering stress-strain curves; (b) strength-ductility combination.

Fig. 9. Schematic illustrations of the microstructural evolution during uniaxial tensile deformation in the IGGS-1000-4 sample.

Fig. 10. Microhardness profiles of the samples treated by various laser surface heat-treatment parameters: (a) microhardness distributions of the IGGS-1000-4 and the gradient micron-grained TWIP steel in Ref. [17]; (b) surface heating temperature of the irradiated-region varies from 750 °C to 1100 °C along with the same scanning speed (5 mm/s) and time (one-time); (c) irradiated-surface is controlled at 950 °C with one-time laser scanning for different scanning speeds; (d) the heating temperature and scanning speed of irradiated-area are 950 °C and 5 mm/s, together with the different scanning times.

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