|Alternative Title||Study on strength and toughness in heterogeneously-grained medium- and high- entropy alloys|
|Place of Conferral||北京|
|Keyword||塑性 强度 加工硬化 塑性变形机理 背应力硬化 中熵合金 高熵合金 冲击韧性|
金属结构材料的强度高则塑性、韧性低，反之亦然。高强度纳米结构金属材料具有低拉伸塑性的瓶颈。最近，提出了异构 (Heterogeneous grain structure, HGS) 的微观构筑设计概念，典型的 HGS 包括非均匀层片结构、晶粒尺寸多级构筑结构和晶粒尺寸呈梯度分布的结构等。HGS 内部的相邻区域间具有显著的力学响应差异、包括强度、塑性、加工硬化等，因而在拉伸变形时，相邻区域间的界面处就会形成应变梯度和背应力导致的加工硬化，在拉伸变形时弥补林位错硬化的不足，特别在高强度水平下，可以获得强度与韧/塑性的良好协调。中熵与高熵合金 (Medium- and High-Entropy Alloy, MEA/HEA) 是近期发展起来的一种新型结构材料体系，在低屈服强度范围 (<500 MPa) 克服了强度与韧性之间传统此消彼长的关系。本文尝试在单相面心立方 (FCC) 结构的五元 FeCoCrNiMn HEA 与三元 FeCoNi MEA 中，分别制备出不同类型的 HGS，研究其高强度下的拉伸力学响应和夏比冲击韧性，并阐明塑性变形和加工硬化的微观机理。
本文主要包括两部分研究结果。首先，针对 FeCoCrNiMn HEA，其化学成分组成为：Fe20.15Co20.07Cr19.91Ni19.23Mn19.36 (Wt.%)。利用大应变冷轧及不同温度的部分再结晶退火，获得非均匀层片状的 HGS，研究了拉伸性能、塑性变形行为及微结构的影响机理。其次，针对 FeCoNi MEA，大晶粒化学成分组成为：Fe35.35Co33.08Ni30.98 (Wt.%)；中等晶粒化学成分组成为：Fe36.37Co32.74Ni30.41 (Wt.%)；小晶粒化学成分组成为：Fe34.40Co32.39Ni32.53 (Wt.%)。通过热锻及高温再结晶退火得到了三种晶粒尺寸多级构筑结构的 HGS，在 77 K 至 373 K 范围内进行了夏比冲击韧性测试，研究并分析了冲击韧性 (AK)。得出的主要结果如下：
(1) 针对 FeCoCrNiMn HEA，通过冷轧并控制随后的部分再结晶退火工艺，获得了不同非均匀层片的 HGS。进行了准静态室温拉伸测试，并选取典型 HGS，进行了加卸载测试，应力松弛测试, 以及三维数字图像相关法 (Digital image correla-tion, DIC) 全场应变演化测试。首先，在准静态拉伸变形时，发现 HGS 屈服后产生瞬态硬化现象。其次，在加卸载变形过程中，观察到 HGS 的背应力呈现出两段上升的演化特点；相应地，背应力加工硬化率从下降转为升高，即出现了反转现象 (up-turn)。进而，在一定范围内，已发生再结晶层片的比例越大，则拉伸时产生的背应力越小，而背应力硬化能力越强。第三，在应力松弛测试过程中，发现 HGS 中由于背应力的作用，相对位错密度表现为下降、上升、然后再次下降和上升、直至达到饱和的特点。最后，在 DIC 全场应变测试中，发现 HGS 塑性开始阶段出现应变局部化特征， HGS 表现出抑制应变局部化的特点，阻止提前的颈缩断裂，且随着再结晶引入量在一定范围内增大，力学不相容界面密度也相应增加，背应力硬化能力增加，材料性能得到提高。
(2) 利用 XRD，DSC，SEM，EBSD 和 TEM 等微观结构表征技术，针对典型的非均匀层片的 HGS，观察并研究了 FeCoCrNiMn HEA 样品拉伸前后的微结构演化。首先， XRD 衍射谱表明 HGS 拉伸前后均为单相的 FCC 晶体结构。DSC 测试表明，大应变冷轧后 HGS 的回复温度为 554 °C，再结晶温度为 757 °C，据此调控了 HGS 的部分再结晶工艺和力学性能。EBSD 微结构观察表明， HGS 随退火温度的升高，硬的超细晶层片仅发生了回复；软层片结构则发生再结晶过程，晶粒内部位错湮灭。HGS 内部由于软硬层片的交替分布，变形产生载荷的再分配，再结晶软层片承受更大的变形，变形的不协调又在软硬层片界面处产生背应力引起背应力硬化，从而强度与塑性匹配最优。其中 600 °C 再结晶时，小晶粒相比硬基体在变形期间发生显著的协调变形。利用 TEM 进一步观察了晶粒内部位错的演化过程，相比拉伸变形之前的低位错密度，拉伸后再结晶晶粒内部的位错密度显著提高，并塞积、缠结于晶界附近；同时，位错的主要组态是位错胞状。还利用 SEM 观察了拉伸断口形貌， HGS 主要表现为韧性断裂特征，出现大量尺寸不等的韧窝，观察到少量的解离面。
(3) 在 FeCoNi 中熵合金中，基于变形加工与退火工艺控制，分别获得大晶粒 (平均晶粒尺寸 d = 1968 μm)、中等晶粒 (d = 226 μm) 和小晶粒 (d = 107 μm) 三类微观组织结构。随拉伸温度 (77-373 K) 降低，三类晶粒尺寸微结构样品的屈服强度、抗拉强度和均匀拉伸塑性均提高；同时，晶粒尺寸越小，相同温度下的强度越高。有趣的是，随着测试温度 (77-373 K) 的降低，V-型缺口夏比冲击韧性 (AK) 则基本保持不变 ，或略微降低，其中小晶粒试样的 AK 最高，中等晶粒和大晶粒试样 AK 相差不大。沿断裂面、特别是裂纹尖端，进行了光学显微观察和显微硬度测试，利用扫描电子显微镜进行了断口观察。分析表明，大晶粒试样在裂纹萌生前，V-型槽根部附近发生大量塑性变形、导致加工硬化，在裂纹扩展路径上观察到大的“凸起”现象，对应于断口形貌的层裂特征；裂纹的失稳扩展阶段，断口形貌为大量尺寸不等的韧窝及一定程度的准解离。在裂纹尖端，观察到最显著的加工硬化特征。
Structural metallic materials are either with high ductility low strength, or high strength low ductility, especially for the nano-structured. Recently, heterogeneous grain structure (HGS) has been widely proposed. Typical HGS includes heterogeneous lamella structure, multi-level grain size structure, and the structure with grain size gradient. There are significant differences in mechanical response between adjacent areas inside HGS, including strength, plasticity, work hardening, etc. Therefore, when tensile deformation occurs, strain gradient and work-hardening caused by back stress are formed at the interfaces between adjacent regions, which makes up for the insufficiency of hardening of forest dislocations, and in turn leads to the improved synergy of strength and toughness/plasticity at high strength level. Medium- and high- entropy alloys (MEA/HEA) are recently developed as a new type of structural material system, which overcomes the traditional trade-off between strength and toughness, however, in the low yield strength range (<500 MPa). This paper attempts to prepare different types of HGS at high strength level in quinary FeCoCrNiMn HEA and ternary FeCoNi MEA with single-phase face-centered cubic (FCC) structure, and investigates their tensile and Charpy impact behavior to elucidate the plastic deformation and work hardening microcosmic mechanisms there.
This work is composed by two parts. First, for the FeCoCrNiMn HEA, the large-strain cold-rolling and partial recrystallization at different temperatures were applied to obtain the HGS with different heterogeneous lamella structure. The tensile property, plastic deformation behavior and micromechanical mechanism were studied. Second, for the FeCoNi MEA, the hot-forging and high-temperature recrystallization were used to obtain the HGS with multi-level grain size structure. The impact toughness (AK) was comprehensively studied and analyzed in the range of 77 K to 373 K. Main conclusions are as follows:
(1) For FeCoCrNiMn HEA, different heterogeneous lamella HGS is obtained by cold rolling and the subsequent controlled partial recrystallization annealing process. Quasi-static room-temperature tensile tests are performed, and some typical HGSs are selected for loading and unloading test, stress relaxation test, and three-dimensional digital image correlation (DIC) full strain test. First, in the quasi-static tensile deformation, it is found that HGS yields a transient hardening phenomenon after yielding. Then, during the loading and unloading deformation process, it is found that the back stress of HGS shows a two-stage rise, accordingly, the work hardening rate of the back stress goes from up-turn. At the same time, when in a certain range, the greater the proportion of recrystallized lamella, the smaller the back stress generated during tensile and in turn the stronger the back stress hardening ability. Furthermore, during the relaxation process, it is found that the relative dislocation density of HGS, due to the back stress hardening, first decreases, and then follows an increase, decrease, and increase again until saturation. In addition, in the DIC full-field strain mapping, it is found that the strain localization appears at the beginning of yielding. At the same time, HGS can significantly suppress the strain localisation and prevent premature shrinkage of the ultrafine grain. Within a certain range, as the amount of recrystallization increases, the density of mechanically incompatible interfaces increases, which leads to the increase of back stress hardening ability, and in turn the improved mechanical properties.
(2) By using XRD, DSC, EBSD, TEM and SEM characterization techniques, the microstructure evolution of FeCoCrNiMn HEA samples before and after tensile is characterized and analyzed for typical HGS. First, through XRD test, it is found that the HGS is single-phase FCC crystal structure after tensile, and no phase changes. Then through the DSC test, it is found that the cold rolled sheet has a recovery temperature of 554 °C and a recrystallization temperature of 757 °C. Then, through the characterization of EBSD microstructures, it is observed that with an increase of annealing temperature, hard ultrafine-grains undergo recovery and recrystallization, dislocations are quenched and recrystallized soft lamellas are generated. Due to the presence of alternating soft and hard lamellas, re-distribution of both the applied load and plastic strain occurs during tensile deformation and the recrystallized soft lamella undergoes large deformation. The strain incompatibility between hard and soft grains will cause the introduction of back stresses at their interface to provide compatible deformation, leading to back stress hardening. It is found that the small grains at 600 °C undergo significant coordinated deformation compared to the hard matrix during deformation. Then the evolution of dislocations is characterized by TEM. It is observed that the dislocations inside the HGS recrystallized grains are very few before the deformation and the dislocations after the deformation are multiplied, packed, and entangled near the grain boundary. At the same time, dislocations recombine to form dislocation cell structures. Finally, the morphologies of the fractures are characterized by SEM. It is observed that the HGS is a ductile fracture with a large number of dimples of varying sizes and a certain number of cleavage facets.
(3) In FeCoNi MEA, large grains (average grain size d = 1968 μm), medium grains ( d = 226 μm), and small grains ( d = 107 μm) are obtained based on deformation processing and annealing process, respectively. With the decrease of the tensile temperature (77-373 K), the yield strength, tensile strength and uniform tensile plasticity of the three types of grain size microstructure samples are all improved; at the same time, the smaller the grain size, the higher the strength at the same temperature. Interestingly, as the test temperature (77-373 K) decreases, the V-notch charpy impact toughness (AK) remains essentially the same, or slightly lower, where the AK of the small-grained sample is the highest, the AK of the medium-grained sample and the large-grained sample are not much different. Along the fracture surface, especially the crack tip, optical microscopy and microhardness tests are performed, and fracture surfaces are observed by scanning electron microscopy. The analysis shows that prior to the formation of a crack during charpy V-notch impact loading, work hardening occurs near the V-notch root in the sample of large grains in the FeCoNi MEA due to plastic deformation. Then, a large “bump” phenomenon first appears on the crack propagation path, corresponding to the splitting characteristics of the fracture surface, and work hardening occurs. Then the crack enters into the stage of instability expansion. The fracture surface has a large number of dimples with different sizes and a certain number of quasi-cleavage surfaces, and the work hardening is not significant. Then work hardening occurs again in the crack tip path.
|杨晓强. 异构中/高熵合金的强韧化研究[D]. 北京. 中国科学院大学,2018.|
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