High-Temperature Plasticity Enhanced by Multiple Secondary Phases in a High-Si Austenitic Stainless Steel

doi:10.1007/s40195-022-01388-5

Abstract

Abstract:

An austenitic stainless steel with 6 wt% Si and multiple secondary phases was produced with the aim to achieve enhanced plasticity during hot deformation. The microstructure of the steel after fracture was characterized via electron back-scattered diffraction, transmission Kikuchi diffraction and scanning transmission electron microscopy. From the tail of the gage to the necking region, the microstructure of the material evolved from low-angle grain boundaries (LAGBs) to mixtures of LAGBs and high-angle grain boundaries (HAGBs), and fine equiaxed recrystallized grains. The elongation to failure in the tensile test exceeds 167%. During the hot deformation, continuous dynamic recrystallization of the austenitic matrix was promoted by the multiple secondary phases. The dislocations introduced by the secondary phases were rearranged and continuously transformed into HAGBs. The initially coarse grains (30.5 μm) were refined into ultra-fine equiaxed grains (1 μm), which contributed significantly the enhanced plasticity during hot deformation of the steel. In the necking area of the sample, twins were nucleated in the stress concentration regions and accommodated the local strain by discontinuous dynamic recrystallization, which was also beneficial to improving the plasticity.

Key words: High-Si austenitic stainless steel, High-temperature deformation, Plasticity enhancement, Dynamic recrystallization, Secondary phases

Sihan Chen, Tian Liang, Guangcai Ma, Chengwu Zheng, Deli Chen, Yingche Ma, Kui Liu. High-Temperature Plasticity Enhanced by Multiple Secondary Phases in a High-Si Austenitic Stainless Steel[J]. Acta Metallurgica Sinica (English Letters), 2022, 35(9): 1519-1530.

Figures/Tables 13

Fig. 1 Schematic diagram of the thermomechanical and experimental procedures

Fig. 2 a STEM-BF image of the microstructure of the steel after aging at 750 °C for 1000 h, b SAED patterns of χ, σ and Cr3Ni5Si2, c EDS maps

Table 1 Chemical composition of the secondary phases (wt%)

Position	Phase	Si	Cr	Mn	Ni	Cu	Mo	Fe
1	Cr₃Ni₅Si₂	8.45	20.41	2.02	26.86	1.14	0.59	Bal.
2	χ	8.17	25.11	1.24	18.26	0.13	4.66	Bal.
3	σ	7.66	39.41	1.31	7.15	0.24	0.86	Bal.

Fig. 3 a, b IPF map (the inset showing the grain size distribution histogram) and phase map of the alloy microstructure after aging at 750 °C for 1000 h. The magnified c IPF map and d phase map of the area highlighted with a dashed frame in a

Fig. 4 Engineering and true stress-strain curves during tensile testing at 800 °C

Fig. 5 a Longitudinal section of the fractured specimen, b cross-sectional width, c-e IPF maps, phase maps, misorientation distribution maps and KAM maps in the highlighted areas c, d, and e in a. The black and red lines represent HAGBs (θ > 15°) and LAGBs (2° ≤ θ ≤ 15°) in γ-grains, respectively

Fig. 6 a IPF map and b phase map showing the microstructure after sufficient recrystallization obtained by means of TKD. The area for examination was sectioned near the fracture surface

Fig. 7 IPF and GB maps showing the recrystallization promoted by secondary phases

Fig. 8 STEM-BF images revealing a dislocation tangle hindered by secondary phases, b dislocation wall in the γ-matrix, c microstructure of the recrystallized grains and secondary phases (marked with yellow arrows), and d EDS map corresponding to the area in image c

Table 2 Chemical composition of secondary phases in Fig. 8 (wt%)

Position	Phase	Si	Cr	Mn	Ni	Cu	Mo	Fe
1	σ	8.92	37.98	1.17	9.02	0.52	2.67	Bal.
2	χ	7.59	24	1.68	17.7	0.77	5.75	Bal.
3	Cr₃Ni₅Si₂	8.07	17.82	1.7	20.17	1.19	0.34	Bal.

Fig. 9 Here, a1 and a2 are the magnified images of the areas highlighted with dashed rectangles in the left-side image

Fig. 10 a IPF and b grain boundary (GB) maps of the deformed microstructure in the late stage of plastic strain. The black and light blue lines indicate boundaries with misorientations of 2 ≤ θ ≤ 15° and θ > 15°, respectively, and the red lines denote twin boundaries. c STEM-BF image revealing morphology of recrystallized grains. d Twin boundaries inside the recrystallized grains. e New-born χ phase

Fig. 11 Summary diagram of the microstructural evolution of the alloy during hot deformation at 800 °C. a Initial microstructure before tensile test. Process of CDRX including dislocations inhibited by the secondary phases b, subgrain boundaries formed through the dislocation entanglement c, recrystallization occurring in the regions with high density secondary phases d and twin boundaries emerged in the late stage of plastic deformation e

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