高压扭转处理超细晶纯钛微观组织演变及热稳定性研究

发布时间：2018-05-25 16:31

本文选题：微观组织 + 微压缩　；参考：《哈尔滨工业大学》2015年硕士论文

【摘要】：随着微型产品需求日益增加,微成形技术得到迅猛的发展。塑性微成形技术具有生产成本低、成形件性能好等优点,非常适合微型零件的批量化生产,在微机电系统以及微系统技术等领域具有广阔的应用前景。然而,一般金属材料的平均晶粒尺寸与微成形制件的特征尺寸相当,极大地影响微型零件的填充质量和尺寸精度。而利用剧烈塑性变形方法制备的具有超细晶粒度的材料能明显改善常规材料的变形不均匀性,在塑性微成形技术领域中具有明显的优势。本文通过高压扭转(HPT)工艺制备出不同圈数HPT纯钛试样,并研究其在高压扭转过程中微观组织演变规律以及显微硬度分布规律。通过研究超细晶纯钛的存储能、再结晶温度以及超细晶纯钛退火过程中的静态热稳定性,揭示超细晶纯钛热稳定性机理。最后研究了应变速率和晶粒尺寸对纯钛在室温微压缩过程中的变形行为、表面形貌和微观组织变化的影响。研究了纯钛在高压扭转过程中微观组织演变规律以及显微硬分布规律,结果表明:在高压扭转过程中发生α相(密排六方结构)到ω相(六方结构)转变,且随着高压扭转圈数的增加,ω相的体积分数逐渐增加。在高压扭转过程中,试样的显微硬度值显著增加,但在试样中心存在低硬度区。试样边缘处显微硬度随着扭转圈数的增加而逐渐增加,当扭转圈数大于5圈时,试样显微硬度值达到饱和。扭转圈数为1/4圈时,晶粒受到环向的纯剪切变形作用,在晶粒内部有大量的位错和孪晶,晶粒尺寸依然较大。扭转圈数为1圈时,试样中心处晶粒尺寸依然较大,而试样半径1/2处晶粒已细化到200nm。扭转圈数大于5时,组织内部已无孪晶,试样中心处同时存在已细化区和未细化区,而试样半径1/2处晶粒已细化到150nm以下。表明高压扭转工艺具有较强的晶粒细化能力。研究了高压扭转不同圈数试样的形变存储能、再结晶温度以及再结晶激活能。结果表明:高压扭转5圈试样再结晶温度在703.8℃到748.8℃之间,再结晶激活能在94.4KJ/mol到112.2KJ/mol之间,晶粒长大的激活能约为109KJ/mol。高压扭转变形超细晶纯钛在退火过程中微观组织演变可分为几个阶段,各个阶段可以相互重叠:第一阶段存在于剧烈塑性变形材料的晶粒中的位错重新分布和其数量的减少;第二阶段剧烈塑性变形时形成的不平衡晶界中位错重新分布,导致大角度晶界形成。晶界宽度较窄,约为几个原子尺寸;第三阶段不平衡晶界组织发生回复现象,导致内应力和晶格畸变同时减小。此时晶粒尺寸依然很小,无再结晶形核阶段;第四阶段发生再结晶过程,若回复后组织存在个别不平衡晶界,则在再结晶过程中出现晶粒异常长大;第五阶段加热过程中晶粒长大。纯钛室温微压缩实验表明:超细晶纯钛的应变速率敏感性高于粗晶纯钛。当晶粒尺寸低于200nm时,材料的流动应力非常平稳,这与超细晶纯钛晶粒内部较高的位错密度有关。随着微压缩试样晶粒尺寸的增大,流动应力逐渐降低,但当试样晶粒数目非常少时,材料的流动应力反而升高。通过SEM观察其形貌,结果表明随着晶粒尺寸的增大,微压缩试样侧面的凹凸不平感越来越明显,这是由于表面层晶粒所占的比重较大,使得单个晶粒变形对于试样整体变形影响较大所致。当晶粒尺寸处于超细晶范畴时,试样表面光滑、表面质量较高。通过对微压缩试样中心处的微观组织进行观察,结果表明:晶粒尺寸较大时,出现一定数量的孪晶,晶粒呈长条状并且晶粒内部存在大量的小角度晶界,表明其变形机制为位错滑移以及孪生变形。晶粒尺寸较小时,晶粒仍然近似于等轴状,晶粒内部小角度晶界数量较少。
[Abstract]:With the increasing demand for micro products, micro forming technology has developed rapidly. The plastic micro forming technology has the advantages of low production cost and good forming properties. It is very suitable for mass production of micro parts. It has a broad application prospect in microelectromechanical systems and micro system technology. However, the average metal material is average. The size of the grain is equivalent to the characteristic size of the micro forming part, which greatly affects the filling quality and the size accuracy of the micro parts. The material with superfine grain size prepared by the strenuous plastic deformation method can obviously improve the deformation nonuniformity of the conventional material, and has obvious advantages in the field of plastic micro forming. This paper is used in this paper. HPT pure titanium samples with different cycles were prepared by high pressure torsion (HPT) process, and the microstructure evolution and microhardness distribution in the process of high pressure torsion were studied. The thermal stability of ultrafine crystal pure titanium, the recrystallization temperature and the static thermal stability during the annealing process of ultrafine crystal pure titanium were studied, and the thermal stability of ultrafine crystal pure titanium was revealed. The effect of strain rate and grain size on the deformation behavior, surface morphology and microstructure changes of pure titanium in the process of chamber temperature microcompression was studied. The microstructure evolution and microhardness distribution of pure titanium during high pressure torsion were studied. The results showed that alpha phase (six square knot) occurred during the high pressure torsion process. The volume fraction of Omega phase increases with the increase of the number of high pressure torsional rings. In the process of high pressure torsion, the microhardness value of the sample increases significantly, but there is a low hardness zone at the center of the sample. The microhardness at the sample center increases with the increase of the number of torsional rings, when the number of torsional rings is more than 5 circles. The microhardness value of the sample is saturated. When the number of torsional rings is 1/4 ring, the grain is subjected to the pure shear deformation of the ring direction. There are a large number of dislocation and twin crystals in the grain. The grain size of the sample center is still larger when the number of twisting rings is 1 rings, and the grain size of the specimen has been refined to the number of 200nm. torsional rings at 1/2. At 5, there is no twin in the tissue. At the same time, there are already refined and unrefined regions at the center of the sample, and the grain size of the specimen has been refined to less than 150nm at 1/2. It shows that the high pressure torsion process has a strong grain refinement ability. The results show that the recrystallization temperature of the high pressure torsion 5 ring specimens is between 703.8 and 748.8, and the activation energy of recrystallization is between 94.4KJ/mol and 112.2KJ/mol. The activation energy of grain growth is about 109KJ/mol. high pressure torsion deformation of ultrafine crystal titanium in the annealing process, the microstructure evolution can be divided into several stages, each stage can overlap each other: first order The dislocation redistribution and the decrease in the number of dislocation in the grain of the strenuous plastic deformation material; the dislocation redistribution in the unbalanced grain boundary formed in the second stage of severe plastic deformation, resulting in the formation of a large angle grain boundary. The width of the grain boundary is narrower, about several atomic sizes, and the third phase unbalance grain boundary tissue has a recovery phenomenon. The internal stress and lattice distortion decrease at the same time. At this time, the grain size is still very small and no recrystallization nucleation stage. The fourth stage of recrystallization process, if there is an individual unbalanced grain boundary after the recovery of the tissue, the abnormal grain growth in the recrystallization process; the grain growth during the fifth stage heating process. The pure titanium room temperature micro compression experiment shows that: The strain rate sensitivity of ultrafine pure titanium is higher than that of coarse-grained pure titanium. When the grain size is lower than 200nm, the flow stress of the material is very stable, which is related to the higher dislocation density inside the ultrafine crystal pure titanium grain. The flow stress is increased. The morphology of the surface is observed by SEM. The results show that with the increase of grain size, the concave and convex side of the microcompression specimen becomes more and more obvious, which is due to the larger proportion of the grain in the surface layer, which makes the deformation of the single grain larger to the whole deformation of the sample. The surface of the specimen is smooth and the surface quality is high. By observing the microstructure at the center of the microcompression specimen, the results show that when the grain size is larger, there are a certain number of twins, the grain is long and there is a large number of small angle grain boundaries in the grain, which indicates that the deformation mechanism is dislocation slip and twin deformation. Grain size is a grain ruler. The grain size is still approximately equal to the equiaxed shape, and the small angle grain boundaries in the grains are relatively small.
【学位授予单位】：哈尔滨工业大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TG146.23

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