Critical Inclusion Size and Void Growth in Dual-Phase Ferrite-Bainite Steel During Ductile Fracture
ZHAO Yafeng1,2, LIU Sujie1, CHEN Yun1, MA Hui1, MA Guangcai3, GUO Yi1()
1Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China 3Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Cite this article:
ZHAO Yafeng, LIU Sujie, CHEN Yun, MA Hui, MA Guangcai, GUO Yi. Critical Inclusion Size and Void Growth in Dual-Phase Ferrite-Bainite Steel During Ductile Fracture. Acta Metall Sin, 2023, 59(5): 611-622.
Ferrite-bainite dual-phase steel is widely used in the automotive industry owing to its high strength and excellent ductility. The impact of inclusions and void growth behavior in dual-phase steel is a major concern among researchers seeking to achieve better mechanical properties. To investigate this, a cross-length-scale multimodal method was employed to study the influence of local microstructures on void growth during ductile fracture of a dual-phase ferrite-bainite steel. During tensile testing, laboratory X-ray computed tomography (XCT) was used to measure the evolution of void volume. 3D-electron back scatter diffraction (3D-EBSD) provided information about the voids nucleated at both inclusion particles and bainite phases or their boundaries. Carefully controlled, broad-focused ion beam excavation was performed to reveal a new interface at a specific depth of the voids. Results showed that voids resulting from large inclusions are significantly bigger than either small inclusions or the bainite phase. Large inclusions lead to large voids even when the strain correlated with the growth of those voids is lower. An investigation of the dislocation densities surrounding the voids suggested that they may be related to the strain gradient around the different inclusion sizes. A critical inclusion size estimated to be around 1.85-2.86 μm was found below which nucleation occurs but with limited growth. The elevated rate of local dislocation multiplication due to local deformation gradient effects can impede the growth of smaller voids. The growth of voids is heterogeneous, and their shape correlates well with the deformability of the surrounding grains, as indicated by a Schmid factor weighted using the grain size. This weighted Schmid factor explains not only the shape of the voids but also sheds light on the ease of void coalescence based on the microstructures separating the voids.
Fund: National Natural Science Foundation of China(52201149);National Science and Technology Major Project(J2019-VI-0019-0134);Strategic Priority Research Program of Chinese Academy of Sciences(XDC04000000);ERC CORREL-CT Project(695638)
Fig.1 2D slice images of time-lap CT scans at different strain (ε) levels post ultimate tensile strength (UTS) (a-h) and stress-strain curve (i) (The rate of void nucleation increases rapidly between nominal strain of 0.05 and 0.053, which quickly coalesced and led to fracture) Color online (a) ε = 0.038 (b) ε = 0.047 (c) ε = 0.049 (d) ε = 0.05 (e) ε = 0.053 (f) fracture (g) slice at the fracture surface before fracture in Fig.1e (h) slice at the region of interest shown in Fig.1e
Fig.2 Equivalent diameters of two voids in Fig.1a as a function of true strain at the corresponding cross section of the sample (Each data point corresponds to a scan point marked in Fig.1i) Color online
Fig.3 Multimodal correlative approach to study void growth Color online (a) XCT morphology of the fractured sample, voids coloured in red (b) bulk cut out by PFIB at the corresponding location indicated by circle in Fig.3a (c) serial sectioning 3D-EBSD reconstruction (~98 μm × 98 μm × 80 μm) of the cutted bulk shown in Fig.3b (d) comparisons between 2D and 3D grain size distributions
Fig.4 High resolution XCT morphology of the two voids shown in Fig.1a together with the grains surrounding the two voids (a), illustration demonstrating void expansion caused by glide of dislocation loops (b—Burgers vector module) (b) Color online
Fig.5 EBSD band contrast maps (a-c) and EBSD maps colored with Euler angles (d-f) in low strained region showing the un-deformed images (a, d) and high strained regions featuring the mid cross section of void 1 (b, e) and void 2 (c, f) Color online
Fig.6 Sizes and types of void in the PFIB-SEM sampled volume (~98 µm × 98 µm × 80 µm) (a) and SEM images of voids type 1 (b), type 2 (c), and type 3 (d) Color online
Fig.7 Void size as a function of inclusion size Color online
Fig.8 Average geometrically necessary dislocation (GND) density in the vicinity of voids as a function of associated inclusion size
Fig.9 GND densities (ρGND) (a1, b1) and Schmid factors (a2, b2) of the left and right parts around the voids in Figs.9a3 and b3; and EBSD band contrast maps (a3, b3), Schmid factor distributions (a4, b4), weighted Schmid factor distributions (a5, b5), and GND density distributions (a6, b6) of void 1 (a1-a6) and void 2 (b1-b6) Color online
Material
μ / GPa
σy / MPa
/ MPa
rc / µm
Low-carbon steel
80
295
200
2.06
Ferrite in ferrite-bainite dual-phase steel[45]
72
370
106
2.51
Bainite in ferrite-bainite dual-phase steel[45]
76
830
258
0.51
Ferrite in 600 MPa ferrite-martensite dual-phase steel[46]
78
313
160
2.30
Martensite in 600 MPa ferrite-martensite dual-phase steel[46]
79
1014
167
0.32
Ferrite in 1000 MPa ferrite-martensite dual-phase steel[46]
78
443
141
1.85
Martensite in 1000 MPa ferrite-martensite dual-phase steel[46]
79
813
280
0.52
Ferrite in ferrite-martensite dual-phase steel[47]
79
260
159
2.86
Martensite in ferrite-martensite dual-phase steel[47]
79
1000
460
0.26
Ferrite in ferrite-martensite dual-phase steel[48]
77
430
117
2.24
Martensite in ferrite-martensite dual-phase steel[48]
77
1450
431
0.18
Table 1 Critical particle sizes across various types of steels estimated according to Equation (6)
Material
Matrix
Inclusion type
Critical size of inclusion / µm
Ref.
Steel
Ferrite
Silicate, TiN
2-4
[49]
A508 steel
Bainite
MnS
17 × 10 × 3
[50]
SAE52100
Martensite
Ca-Al-O, Ti(N, C)
21
[51]
600 MPa high-strength steel
Martensite
Inclusion
2
[52]
Ferrite-martensite dual-phase steel
Ferrite + martensite
Inclusion
Correlated with martensite phase size
[48]
High-strength steel
Martensite
Al2O3
16
[53]
High-strength steel
Martensite
TiN
11
[53]
Bearing steel
Martensite
Mg-Al-O
8.5
[54]
Bearing steel
Martensite
Calcium aluminate
13.5
[54]
Pipeline steel
Ferrite + pearlite
MnS
2.52-2.6
[55]
SLM IN718
Ni suppralloy
Void
20
[56]
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