超细晶纯铝、铁、钛的宽温域变形稳定性研究

Various mechanical behaviors of ultrafine-grained (UFG) materials produced by severe plastic deformation (SPD) generally exhibit distinctive characteristics from those of conventional grained materials, so that the relevant studies have received attention in recent years. In the present preject, three kinds of ultrafine-grained pure metals with different crystalline structures are selected as the target materials, such as face-centered cubic (fcc) UFG Al, hexagonal close-packed (hcp) UFG Ti, and body-centered cubic (bcc) UFG Fe, which are to be prepared by the severe plastic deformation technique. The high/low temperature tensive/compressive deformation and damage behavior, and thermal stability of these materials are studied, focusing on the joint effect of applied load and temperature on the microstructures, mechanical properties as well as deformation and damage characteristics in a wide range of temperature (from liquid nitrogen temperature to above recrystallization temperature). The macroscopic mechanical response curve in all experimental conditions will be established. In addition, the microstructural changes of these materials under uniaxial tension/compression or cyclic deformation at high/low temperatures will be examined by electron channeling contrast (ECC) technique in scanning electron microscopy (SEM) and by transmission electron microscopy (TEM). The specific role of temperature in high/low temperature deformation of UFG metals will be expounded comprehensively. This work attempts to clarify the specific impact of the crystalline structure on deformation stability of UFG materials at high/low temperature, and to explore the effect of annealing on microstructure and mechanical properties of UFG metals. Combined with related experimental results, the corresponding physical mechanism model will be established.These findings are expected to provide useful reference and theoretical basis for the development of advanced structural UFG materials with high deformation stability.

严重塑性变形(SPD)制备的超细晶材料由于具备优越的力学性能受到研究者的普遍关注。但由于材料在SPD过程中积累了大量的应变而使其处于亚稳态，服役温度对其结构及性能将会产生明显影响，且不同晶体结构的超细晶金属结构稳定性不同。本项目选择三种纯金属（fcc铝、bcc铁和hcp钛），用SPD方法制备出具有不同晶体结构的超细晶纯金属，在宽温域（从液氮温度到高于再结晶温度）研究其在不同应变速率下单向变形,以及在不同加载频率下应力幅控制的循环变形及损伤行为。构建在各种加载条件不同温度下超细晶金属的宏观力学响应曲线，并观察分析其表面变形特征、裂纹的萌生与扩展、断口形貌以及微观组织结构的演化规律。全面清晰地阐述温度在超细晶材料高、低温变形中的具体作用，并研究晶体结构对超细晶材料结构稳定性的具体影响。结合相关实验结果提出并建立相应的物理模型，为开发具有高变形稳定性的超细晶材料提供有用的参考数据和理论基础。

由严重塑性变形（SPD）方法制备的超细晶金属因其表现出优异的性能而受到研究者的普遍关注。而制备过程中积累的大量应变能导致这种材料处于亚稳态，在服役过程中存在性能衰减的风险。选择具有代表性的bcc工业纯铁、hcp工业纯钛、fcc纯铜等作为实验材料，通过ECAP和ARB方法制备出所需的超细晶金属。在宽温域环境下考察其力学性能、表面变形特征、微观结构变化，以退火处理对该类材料微观结构及力学行为的影响。发现：SPD处理使不同晶体结构的金属材料均表现出更高的应力水平，且应变硬化行为的缺失。超细晶纯金属的稳态流变应力和压缩屈服应力一般随着温度的升高而降低。而bcc超细晶铁在温度高于其再结晶温度时，其强度变化无明显规律性。温度对超细晶纯金属的表面变形特征产生不同的影响。fcc超细晶铜在低于再结晶温度下压缩后表面主要表现为损伤严重的大尺度剪切带变形；当温度高于再结晶温度，剪切带基本消失。hcp超细晶纯钛在低于再结晶温度下，以剪切带变形为主；当高于再结晶温度，表面呈现出粗化晶粒变形而形成的挤出挤入带。bcc超细晶铁在低于再结晶温度的范围内表现出较好的晶界协调变形能力；当高于再结晶温度时，晶界协调变形能力降低，有明显的裂纹。温度对微观结构变化的影响不同。fcc超细晶铜在低于再结晶温度下，大部分保持超细晶晶粒和高密度位错；在接近再结晶温度时，晶粒发生了明显的粗化；在高于再结晶温度时，晶粒发生了严重粗化，位错密度极大降低。hcp超细晶纯钛在低于再结晶温度时，位错密度有所增加；当温度升高到接近再结晶温度时，位错密度明显降低，晶界变得较为清晰；当温度高于再结晶温度，粗化的晶内存在网格位错或一些位错缠结。而bcc超细晶铁表现出独特的微观结构变化，在温度低于再结晶温度时，存在超细尺度的亚结构；在接近再结晶温度时，位错密度明显降低，流变剪切带消失，晶粒大部分为动态再结晶形成的超细晶；在高于再结晶温度情况下，发生了明显的晶粒择优生长。