The Development of Vacancies during Severe Plastic Deformation
The Development of Vacancies during Severe Plastic Deformation
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DOI：
10.2320/matertrans.mf201937
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发表时间：
2019-01-01
影响因子：
1.2
通讯作者：
Dobatkin, S. V.
中科院分区：
文献类型：
--
作者：
Cizek, J.;Janecek, M.;Dobatkin, S. V.
严重塑性变形（SPD）会在材料中引入高密度的晶格缺陷。许多实验技术，特别是电子显微镜，X射线，电子和中子衍射等，被用来表征的微观结构和缺陷的演变与应变引入材料的SPD。这些技术主要集中在平面（晶界）和线缺陷（位错）的研究。另一方面，点缺陷，即空位和它们的团聚体的研究不太详细。正电子湮没谱（PAS）是研究超细晶材料中点缺陷和位错的有效方法。本文总结了用高压扭转（HPT）方法制备的fcc（Al，Ni，Cu），bcc（Fe，Nb，W）和hcp（Mg，Ti）结构的超细晶金属中晶格缺陷的研究结果。PAS的两种技术被采用（i）正电子寿命谱（LT），允许在严重变形的材料中的晶格缺陷的类型和浓度比的特征和（ii）多普勒展宽（DB）的湮没辐射提供的均匀性的UFG结构和缺陷的空间分布的分析。后一种技术的补充映射的显微硬度分布在整个表面的HPT tests.LT的研究表明，在室温下的HPT应变不仅引入位错，但也是一个高浓度的空位。一个显着的比例变形引起的空位消失扩散到汇在晶界。剩余的空缺聚集成空缺组。不同金属中空位团的平均尺寸不同，并受空位迁移激活能的影响，正电子湮没辐射DB分析及其与显微硬度分布的相关性表明，位错密度随应变的增加趋于饱和。另一方面，空位簇的空间（横向）分布保持不均匀，即使在样品中进行了大量的HPT旋转。由于空位产生率的增加，空位团的平均尺寸随着离样品中心的径向距离而增加。
A high density of lattice defects is introduced to materials by severe plastic deformation (SPD). Numerous experimental techniques, in particular electron microscopy, X-ray, electron and neutron diffraction, etc. are employed to characterize the evolution of microstructure and defects with strain introduced to the material by SPD. These techniques concentrate mainly on the investigation of planar (grain boundaries) and line defects (dislocations). On the other hand, point defects, namely vacancies and their agglomerates are investigated in less detail. Positron annihilation spectroscopy (PAS) proved to be an effective method for the investigation of point defects and dislocations in ultra-fine grained (UFG) materials. This study summarizes the results of the investigation of lattice defects in UFG metals with fcc (Al, Ni, Cu), bcc (Fe, Nb, W) and hcp (Mg, Ti) structure prepared by high pressure torsion (HPT). Two techniques of PAS were employed (i) positron lifetime spectroscopy (LT) allowing to characterize the type and concentration ratio of lattice defects in the severely deformed material and (ii) Doppler broadening (DB) of annihilation radiation providing analysis of the homogeneity of the UFG structure and spatial distribution of defects. The latter technique was complemented by mapping of microhardness distribution throughout the surface of the HPT specimens.The LT studies revealed that HPT straining at room temperature introduced not only dislocations but also a high concentration of vacancies. A significant fraction of deformation-induced vacancies disappeared by diffusion to sinks at grain boundaries. Remaining vacancies agglomerated into vacancy clusters. The average size of vacancy clusters differs in various metals and is affected by the activation energy for migration of vacancies in the given material.The analysis of DB of positron annihilation radiation and its correlation with microhardness distribution indicated that dislocation density tends to saturate with strain. On the other hand, the spatial (lateral) distribution of vacancy clusters remains non-uniform even in samples subjected to a high number of HPT revolutions. The average size of vacancy clusters increases with radial distance from the centre of the sample due to the increasing production rate of vacancies.