Deformation Mechanism and Fracture Behavior of a Coarse-Grain Ni-Co-Based Superalloy During Superplasticity

doi:10.1007/s40195-025-01914-1

Abstract

Abstract:

The advent of coarse-grain superplasticity has provided a pathway for novel applications in material forming. This article investigated the underlying deformation mechanisms that enabled achieving superplastic elongation exceeding 230% in a coarse-grained Ni-Co-based superalloy. The deformed microstructure and fractographic characteristics of the alloy were examined utilizing optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). The results of the analysis revealed that below 1100 °C, the process of dynamic recrystallization (DRX) occurred at a sluggish rate, resulting in low plasticity and the initiation of severe cracks. Complete DRX occurred when the deformation temperature exceeded 1100 °C, leading to a more uniformly deformed microstructure, reduced crack initiation, and enhanced ductility demonstrated by elongation to failure surpassing 230%. The augmented occurrence of the DRX facilitated prolonged plastic-forming periods, which delayed fracture propagation and promoted the deformation flow within the alloy, thereby transitioning the fracture behavior from intergranular-brittle at 1050 °C to ductile intergranular at 1140 °C. At this temperature, the deformation was predominantly governed by the discontinuous-DRX (DDRX) mechanism and grain growth, facilitated by the formation of twin boundaries.

Key words: Coarse-grain superplasticity, Deformation mechanism, Ni-Co-based superalloy, Fracture mechanism, Microstructure evolution

Rashad A. Al-Hammadi, Rui Zhang, Chuanyong Cui, Xipeng Tao, Yizhou Zhou. Deformation Mechanism and Fracture Behavior of a Coarse-Grain Ni-Co-Based Superalloy During Superplasticity[J]. Acta Metallurgica Sinica (English Letters), 2025, 38(11): 2024-2034.

Figures/Tables 13

Table 1 Nominal chemical composition of the studied Ni-Co-based superalloy (wt%)

Ni	Co	Cr	Ti + Al	Mo	W	C + B + Zr
Bal.	20-26	13-15	7.72-8.40	2.4-2.8	1.1-1.3	0.04-0.11

Fig. 1 a OM, b SEM imaging of the alloy kept at 1141 °C for 2 h

Fig. 2 a Process of preparing the starting microstructures; b GOS overlapped grain boundary map

Fig. 3 OM micrographs of the post-superplastic microstructures at a-c 1050 °C, d, e 1100 °C, f 1140 °C, respectively

Fig. 4 GOS maps deformed at a 1050 °C, b 1140 °C

Table 2 Statistical analysis of all grain boundary fractions of the post-superplasticity structures of coarse-grained alloy

Condition	∑3	∑9	∑27	HAGBs	MAGBs	LAGBs
Initial microstructure	60 ± 1	1	0.1	97 ± 2	1 ± 0.2	2 ± 0.5
Deformed at 1050 °C	30 ± 1.5	1.5	2	58 ± 1	2 ± 0.3	40 ± 1.5
Deformed at 1140 °C	69 ± 1	1.7	0.6	98 ± 1	1 ± 0.1	1 ± 0.5

Fig. 5 Fracture morphologies of the alloy after superplasticity at a-c 1050 °C, d-f 1100 °C, g-i 1140 °C

Fig. 6 a, b TEM images near the fracture point, deformed at 1050 °C

Fig. 7 Relationship between the voids and elongation of the coarse-grained superplasticity

Fig. 8 Inverse pole figure of an enlarged grain deformed at 1050 °C

Fig. 9 Corresponding disorientation gradients for the deformation conditions of a, c at 1050 °C, b, d at 1140 °C

Fig. 10 Grain boundary characteristic distribution maps of a the starting microstructure, deformed at b 1050 °C and c 1140 °C

Fig. 11 Schematic demonstrating the DRX nucleation mechanism; a the initial microstructure, b-d the deformed microstructure as the temperature increased

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