## 热变形后Ni-30%Fe模型合金中奥氏体的亚动态软化行为

1.中国科学院金属研究所沈阳材料科学国家研究中心 沈阳 110016

2.中国科学技术大学材料科学与工程学院 沈阳 110016

## 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 Chengwu,1,2, 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

 基金资助: 国家自然科学基金项目.  51771192国家自然科学基金项目.  51371169国家自然科学基金项目.  51401214

Corresponding authors: ZHENG Chengwu, associate professor, Tel: (024)83970106, E-mail:cwzheng@imr.ac.cn

Received: 2019-09-20   Revised: 2019-12-29   Online: 2020-05-25

 Fund supported: National Natural Science Foundation of China.  51771192National Natural Science Foundation of China.  51371169National Natural Science Foundation of China.  51401214

Abstract

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.

Keywords： Ni-30%Fe austenitic model alloy ; hot deformation ; sub-structure restoration ; post-dynamic recrystallization ; austenite

Wenxiong CHEN, Baojia HU, Chunni JIA, Chengwu ZHENG, Dianzhong LI. Post-Dynamic Softening of Austenite in a Ni-30%Fe Model Alloy After Hot Deformation. Acta Metallurgica Sinica[J], 2020, 56(6): 874-884 doi:10.11900/0412.1961.2019.00310

## 1 实验方法

### 图1

Fig.1   Diagram of the double-pass hot compression test (t—time, ε—strain)

$X=σm-σ2σm-σ1×100%$

### 图2

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)

## 2 实验结果

### 2.1 应力-应变曲线及软化速率

Ni-30%Fe合金在$ε˙$=0.001 s-1、温度T=900 ℃条件下进行等温热压缩变形时的应力-应变曲线如图3中的虚线所示。在热变形初期，随着塑性变形的发生位错会大量增殖，由于奥氏体中位错交滑移的速率较低，材料内部因动态回复所导致的软化并不足以抵消位错增殖所带来的硬化作用，流动应力迅速升高，材料内部的应变储能也逐渐增加。随着变形的继续进行，当材料内局部应变储能达到形核临界值时，DRX开始发生，流动应力增速随之减缓。当应变达到峰值应变时(本工作中将流动应力达到峰值时的应变称为峰值应变，本实验条件对应的峰值应变约为ε=0.3)，DRX晶粒大量出现，材料内部因DRX导致的软化速率开始超过加工硬化速率，此时，流动应力达到峰值，随后逐渐降低[18,19,20,21]。随着热变形的进一步进行，材料内部DRX的发生程度不断增大直至完全取代原始变形晶粒，此时，流动应力及平均晶粒尺寸均达到动态平衡，不再随应变而发生变化，材料达到动态再结晶稳态阶段[22,23,24] (本实验条件下，ε=0.6时即达到了稳态阶段)。

### 图3

Fig.3   Flow curve of the hot deformation of the Ni-30%Fe model alloy at temperature T=900 ℃, strain rate $ε˙$=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)

### 图4

Fig.4   The softening fractions as a function of annealing time for the Ni-30%Fe alloy at different initial strains

### 图5

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 ℃, $ε˙$=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 Ʃ3 twin boundaries, respectively. θ is misorientation angle of the boundary)

Color online

### 2.3 变形至峰值应变后保温过程中的奥氏体软化行为

Ni-30%Fe合金奥氏体在$ε˙$=0.001 s-1T=900 ℃条件下变形至ε=0.3时，流动应力达到峰值，此时，动态再结晶晶核已开始大量生成，如图6a所示。此时材料内部的动态再结晶程度较低，其微观组织由未长大的再结晶晶核及大尺寸原始变形晶粒组成。由于变形的持续累积，变形态原始晶粒内部形成了极为丰富的亚结构，这些亚结构的存在为大角度晶界迁移提供了驱动力，因此，变形后的保温过程中再结晶晶核能快速向原始变形基体晶粒内部生长(图6b~d)，此过程是典型的P-DRX过程。同时，在已长大的再结晶晶粒内部生成了大量Ʃ3孪晶界。通常认为，奥氏体中退火孪晶的形成与大角度晶界迁移是直接相关的[27,28,29]，因此，奥氏体晶粒内部大量平直孪晶界的出现可认为是再结晶晶界快速迁移的结果，这进一步证明在保温阶段材料内的再结晶晶核发生了明显的长大，即发生了P-DRX过程。

### 图6

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 $ε˙$=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 Ʃ3 twin boundaries, respectively)

Color online

### 图7

Fig.7   Misorientation profiles measured along the lines L1 (a), L2 (b), L3 (c) and L4 (d) in Fig.6

### 图8

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 $ε˙$=0.001 s-1 at a strain of 0.3

### 图9

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 $ε˙$=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 Ʃ3 twin boundaries, respectively)

Color online

### 图10

Fig.10   Misorientation profiles measured along the lines K1 (a), K2 (b), K3 (c) and K4 (d) in Fig.9

### 图11

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 $ε˙$=0.001 s-1 with a strain of 0.8 (Inset in Fig.11c shows the local enlarged image)

## 4 结论

(1) 在变形后保温过程中，Ni-30%Fe合金奥氏体亚动态再结晶和亚结构恢复2种不同的机制均可能成为其主要软化方式。亚结构恢复的速率与变形过程中形成的亚晶界特征相关，亚晶界取向差角度越小，保温过程中亚结构恢复的速率就越快。

(2) 当热变形中奥氏体内发生部分动态再结晶时，再结晶晶粒与变形基体储能差较大，亚动态软化首先通过亚动态再结晶进行；同时，亚结构的恢复也在不断发生，随着保温时间的延长，亚结构的恢复逐渐占据主导，成为主要的软化机制。

(3) 当热变形中奥氏体内动态再结晶达到稳态时，变形亚结构取向差角度较小，亚结构恢复会优先发生；随着保温时间的延长，亚结构的快速恢复引起的亚晶界大量分解所产生的高密度位错会促进大角度晶界的局部迁移，大大促进晶粒的粗化，进而引起材料的加速软化。

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