Grain boundary character and stress corrosion cracking behavior of Co-Cr alloy fabricated by selective laser melting

doi:10.1016/j.jmst.2021.03.063

Abstract

Abstract:

In this work, we used the selective laser melting (SLM) fabricated Co-Cr alloy with prominent residual strain, extremely non-equilibrium microstructures, and low stacking fault energy as a precursor to fabricate materials with the optimal grain boundary character distribution. The grain boundary engineering (GBE) of the Co-Cr alloy was achieved by a simple heat treatment of the SLM-fabricated Co-Cr alloy. The obtained GBE Co-Cr alloy exhibited 81.47% of special grain boundaries (Σ3ⁿ n = 1, 2, 3)), while it substantially disrupted the connectivity of the random high-angle boundaries, successfully reducing the propensity of intergranular degradation. Slow strain rate tests (SSRTs) showed that the GBE Co-Cr alloy possessed lower stress corrosion cracking (SCC) susceptibility and higher ductility in the corrosive environment (0.9% NaCl solution) than in the air. The high fraction of special boundaries, coupled with the stress-induced martensitic transformation (SIMT) in the GBE Co-Cr alloy yielded these results, which unique and rarely simultaneously satisfied for common structural materials. The current “SLM induced GBE strategy” offers a novel approach towards customized GBE materials with high SCC resistance and ductility in the corrosive environment, shedding new light on developing high-performance structural materials.

Key words: Grain boundary engineering, Selective laser melting, Co-Cr alloy, Stress corrosion cracking, Ductility

Xin Dong, Ning Li, Yanan Zhou, Huabei Peng, Yuntao Qu, Qi Sun, Haojiang Shi, Rui Li, Sheng Xu, Jiazhen Yan. Grain boundary character and stress corrosion cracking behavior of Co-Cr alloy fabricated by selective laser melting[J]. J. Mater. Sci. Technol., 2021, 93: 244-253.

Figures/Tables 12

Fig. 1. (a) SEM image of gas atomized Co-Cr alloy powders; (b) Shape of the SSRT specimens.

Fig. 2. (a) OM and (b-d) SEM image of As-SLM Co-Cr alloy at low and high magnification. (e-f) TEM image of As-SLM Co-Cr alloy. The inset in (e) is the corresponding selected area electron diffraction (SAED) patterns

Fig. 3. EBSD results of As-SLM Co-Cr alloy: (a) Phase map, (b) Grain boundary (GB) map, (c) Kernel average misorientation (KAM) map, (d) Inverse pole figure (IPF), and (e) Pole figure (PF).

Table 1. Grain boundary character of As-SLM Co-Cr alloy. (length %).

Type	Σ1	Σ3	Σ9	Σ27	RB
Percentage	73.6	0.845	0.194	0.093	25.268

Fig. 4. (a) OM and (b) SEM image of GBE Co-Cr alloy, respectively. (c) TEM image and (d) high-resolution transmission electron microscopy (HRTEM) image of GBE Co-Cr alloy. The insets in (c) and (d) are the corresponding SAED image. The yellow circle in (c) is the corresponding selected area for SAED.

Fig. 5. EBSD results of GBE Co-Cr alloy: (a) Phase map, (b) Grain boundary (GB) map, (c) Kernel average misorientation (KAM) map, (d) Inverse pole figure (IPF) map, and (e) Pole figure (PF) map.

Table 2. Grain boundary character of GBE Co-Cr alloy. (length %).

Type	Σ1	Σ3	Σ9	Σ27	RB
Percentage	3.98	72	6.63	2.84	14.55

Fig. 6. Typical stress-strain curves of GBE Co-Cr alloy in 0.9% NaCl solution and air.

Table 3. Mean value of SSRT results of GBE Co-Cr alloy in 0.9% NaCl solution and air.

Conditions	UTS (MPa)	Elongation (%)
0.9% NaCl	1074.96(10.15)	16.51(1.97)
Air	1070.29(4.81)	10.96(0.56)

Fig. 7. Fractured surface morphology of GBE Co-Cr alloy in (a-b) 0.9% NaCl solution and (d-e) air conditions after SSRT. The side surface of GBE Co-Cr alloy in (c) 0.9% NaCl solution and (f) air conditions after SSRT. KAM maps in the near fractured surface of GBE Co-Cr alloy in (g) 0.9% NaCl solution and (h) air after SSRT. The red circles in 7a and 7d are the corresponding selected magnification area for 7b and 7e, respectively.

Fig. 8. OM image of the secondary cracks of GBE Co-Cr alloy after testing in (a) 0.9% NaCl solution and (b) air, respectively; (c) SEM image, (d) IPF maps, (e) KAM maps and (f) phase maps of typical secondary cracks area after testing in 0.9% NaCl solution. The red arrow in Fig. 8a indicates the loading direction in the SSRT. The red rectangle in the Fig. 8c is the selected test zone for EBSD.

Fig. 9. (a-d) TEM and (e-f) HRTEM image of GBE Co-Cr alloy after SSRT in 0.9% NaCl solution. The insets in 9c, 9e, and 9f are the corresponding SAED patterns, respectively. The yellow circles in 9c and 9e are the corresponding selected area for SAED.

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