Post-Dynamic Softening of Austenite in a Ni-30%Fe Model Alloy After Hot Deformation
CHEN Wenxiong1,2, HU Baojia1,2, JIA Chunni1,2, ZHENG Chengwu1,2(), LI Dianzhong1,2
1.Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2.School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China
Cite this article:
CHEN Wenxiong, HU Baojia, JIA Chunni, ZHENG Chengwu, LI Dianzhong. Post-Dynamic Softening of Austenite in a Ni-30%Fe Model Alloy After Hot Deformation. Acta Metall Sin, 2020, 56(6): 874-884.
Multi-pass processing is commonly used in hot working of steels. Dynamic recrystallization (DRX) occurs during hot deformation, while post-dynamic softening takes place during the inter-pass times and post-deformation annealing. Three different mechanisms are believed to be responsible for the post-dynamic softening stage. These are static recovery (SRV), static recrystallization (SRX), and post-dynamic recrystallization (P-DRX). Each of these mechanisms can change the microstructure of austenite (i.e. grain size and distribution). As a result, the post-dynamic softening behavior of austenite may play an important role in the microstructures and the final mechanical properties of the steel product. In this work, a Ni-30%Fe model alloy is used to study softening of austenite in post-deformation annealing after the hot deformation at 900 °C and strain rate 0.001 s-1. The microstructures in the annealed samples are carefully analyzed by EBSD in conjunction with TEM. The results show that P-DRX and sub-structural restoration are believed to be responsible for softening of the material after hot deformations. The P-DRX generally consumes deformed structures by the growth of the preformed nuclei of dynamic recrystallization. The sub-structural restoration in austenite usually takes place through the dislocation climb, leading to sub-boundary disintegrations and dislocation annihilations. When the sample is deformed to the peak strain, the deformation microstructure is composed of both recrystallized grains and deformed matrix. The large gradient of stored energy between the recrystallized grains and deformed matrix effectively promotes the strain-induced migration of the large-angle grain boundaries, which makes the P-DRX become the predominated post-dynamic softening mechanism during the post-deformation annealing. Meanwhile, the sub-boundaries within the deformed matrix gradually disintegrate through the restoration mechanism, which also contributes to the post-dynamic softening of austenite. On the other hand, the dislocation annihilation can result in a reduction of the stored energy within the deformation matrix, which inhibits the further migration of grain boundaries. In contrast, when the sample is deformed to the steady-state stage of the dynamic recrystallization, a fully recrystallized microstructure is obtained. The sub-structural restoration process of the fully recrystallized microstructure is much faster than that in the deformed matrix during the post-deformation annealing. It makes the sub-structural restoration become the predominated post-dynamic softening mechanism of this alloy in the steady-state condition. Furthermore, the disintegration of large numbers of sub-boundaries leads to an increase of the dislocation density in local region around the grain boundaries, which facilitates local migration of the high-angle grain boundaries and accelerates the softening of the material.
Fund: National Natural Science Foundation of China(51771192);National Natural Science Foundation of China(51371169);National Natural Science Foundation of China(51401214)
Fig.1 Diagram of the double-pass hot compression test (t—time, ε—strain)
Fig.2 Schematic of the post-dynamic softening fraction measurement (σm is the stress at the end of the first deformation, σ1 and σ2 are the 0.2% offset yield stresses for the first and the second deformations, respectively)
Fig.3 Flow curve of the hot deformation of the Ni-30%Fe model alloy at temperature T=900 ℃, strain rate =0.001 s-1 (dash line), and flow curves of the second-pass deformation after different annealing time at the first strains of 0.3 and 0.8, respectively (solid lines)
Fig.4 The softening fractions as a function of annealing time for the Ni-30%Fe alloy at different initial strains
Fig.5 EBSD maps (a, d), the corresponding local misorientation maps (b, e) and the angular distributions of the local misorientations (c, f) of the Ni-30%Fe alloy after the hot deformation under T=900 ℃, =0.001 s-1 with strains of 0.3 (a~c) and 0.8 (d~f) (The silver, green, blue, black and red lines in Figs.5a and d represent boundaries with 0.8°≤θ<2°, 2°≤θ<5°, 5°≤θ<15°, θ≥15° and ?3 twin boundaries, respectively. θ is misorientation angle of the boundary) Color online
Fig.6 EBSD maps of the Ni-30%Fe alloy at different annealing time of 0 s (a), 2 s (b), 10 s (c) and 60 s (d) after the hot deformation under T=900 ℃ and =0.001 s-1 at a strain of 0.3 (The silver, green, blue, black and red lines represent boundaries with 0.8°≤θ<2°, 2°≤θ<5°, 5°≤θ<15°, θ≥15° and ?3 twin boundaries, respectively) Color online
Fig.7 Misorientation profiles measured along the lines L1 (a), L2 (b), L3 (c) and L4 (d) in Fig.6
Fig.8 TEM images of the Ni-30%Fe alloy at different annealing time of 0 s (a), 2 s (b), 10 s (c) and 60 s (d) after the hot deformation under T=900 ℃ and =0.001 s-1 at a strain of 0.3
Fig.9 EBSD maps of the Ni-30%Fe alloy at different annealing time of 0 s (a), 2 s (b), 10 s (c) and 60 s (d) after the hot deformation under T=900 ℃ and =0.001 s-1 with a strain of 0.8 (The silver, green, blue, black and red lines represent boundaries with 0.8°≤θ<2°, 2°≤θ<5°, 5°≤θ<15°, θ≥15° and ?3 twin boundaries, respectively) Color online
Fig.10 Misorientation profiles measured along the lines K1 (a), K2 (b), K3 (c) and K4 (d) in Fig.9
Fig.11 TEM images of Ni-30%Fe alloy at different annealing time of 0 s (a), 2 s (b), 10 s (c) and 60 s (d) after the hot deformation under T=900 ℃ and =0.001 s-1 with a strain of 0.8 (Inset in Fig.11c shows the local enlarged image)
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