【作者】 郭伟国；

【导师】 李玉龙；

【作者基本信息】 西北工业大学 ， 固体力学， 2007， 博士

【摘要】 二战以来，特别是二十世纪九十年代以来，地域局部战争的爆发，加剧了对各种攻击型武器及其防护结构更高性能的要求。各种武器的战斗部以及战舰、飞行器、地下掩体等材料的性能研究更引起材料、冶金和力学工作者极大的重视。随着人类对太空的不断探索，不仅在太空材料方面，同样要求在国防、航空航天和民用等方面广泛被应用的结构材料能够在各种极端的环境，例如，高速碰撞、高温变形、大的塑性变形等条件下具有良好的性能。这样，一方面要求不断开发性能优异的新材料，另一方面需要深入理解和了解在极端环境下材料的性能，为实际工程应用提供不可缺少的基础。BCC多晶体金属和主要呈现BCC结构特征的合金，由于它们通常具有较高的熔点、良好的热传导性、高的断裂韧性、相对好的抗腐蚀和可焊接性等特性，因而一直得到研究者的关注。但BCC金属的性能强烈的依赖于温度、变形率和自身的微观结构。在本文中通过大量系统的试验研究和分析，意在揭示BCC金属材料在很宽温度范围、很宽应变率范围和大变形下的塑性流动行为。主要内容有：材料动态性能的测试技术、材料在不同温度不同应变率下的塑性流动特征和变形机理、本构关系的研究。其具体研究方法和内容如下：（1）材料动态性能的测试技术。基于近年开展的三种多晶体钽（Ta）金属材料、船用Nitronic-50结构钢和三种钨合金动能弹材料的性能研究，和以往积累的大量试验研究，重点开展用分离式Hopkinson压杆实现高温高应变率的耦合试验技术，对低应变率不同温度下的试验是借助电子万能试验机或常规的液压伺服试验机，通过这些综合的试验技术可实现应变率从10^-4／s到10⁴／s、温度从77K到1100K，应变超过70％的金属材料塑性流动性能的试验研究。（2）材料在不同温度不同应变率下的塑性流动特征和变形机理研究。利用（1）中的试验技术，测试三种不同工艺的Ta金属材料、船用Nitronic-50结构钢和三种钨合金动能弹材料在单轴压缩下的性能，然后结合积累的大量试验结果，分析BCC金属材料在应变率从10^-4／s到10⁴／s、温度从77K到1100K，应变超过50％的塑性流动的规律、所呈现的变形特征。通过金相分析探索BCC金属塑性流动的变形机理。（3）本构关系的研究。结合BCC金属材料在不同温度不同应变率下的塑性流动性能，分析塑性流动的规律和所呈现的变形特征，基于位错克服各种障碍和势垒的热激活滑移机制，以及基于位错运动学和动力学概念，建立一个具有物理意义的统一热粘塑性本构关系。介绍这些材料参数的意义，给出BCC金属塑性流动的具体本构模型，介绍试验确定本构关系中参数的方法，通过试验结果与本构关系的对照验证本构关系的准确性。通过本文的研究，主要结论和成果为：（1）本文所给出具有二次应力波抑制技术的分离式Hopkinson压杆连同本文所建立的气动同步机构，可以测试金属材料应变率最高到约30，000／s、温度可达到1，100K、塑性应变超过70％的材料动态应力应变曲线，应变率和高温的耦合试验测试。（2）经对BCC金属大量试验结果分析，本文发现这些BCC金属的塑性流变应力对应变率和温度非常敏感，但加载历史对流变应力的影响较小；在低的应变率下，出现第三类动态应变时效现象，随应变率的增加，动态应变时效应力峰值将移至更高温度区或消失。（3）通过分析发现在BCC多晶体及其主要呈现BCC结构特征的合金材料中几乎都发生第三类动态应变时效（DSA）现象，且DSA启动并不需要预先的应变积累。DAS出现的区域取决于温度和变形率两者的共同因素，当应变率增加时，DSA出现的区域会移向更高的区域。这个DSA的产生机理为：当温度和应变率共同达到某一临界条件时，溶质原子沿着大量聚集的林位错通过管扩散在运动位错核迅速的和连续不断的形成溶质气团，对运动位错构成连续不断的牵制或拘留，为了克服这类障碍，导致流动应力曲线随应变增加连续的上升。（4）BCC金属在塑性流变中具有的机制和特征有：低应变率下的剪切、高应变率下绝热剪切、变形孪晶的产生，当温度较高温时，BCC材料会呈现粘塑性拖-曳现象。（5）推导出适合金属材料塑性流动的统一本构模型，模型的推导结合了固体热力学、材料学、冶金学和力学等学科的知识，物理意义明确。通过试验结果与模型结果的比较，所建立的BCC本构模型与试验结果吻合较好，证明本文所给出的本构模型是合理、可信的。更多还原

【Abstract】 Since the second World War, especially since 1990’s, because the region warshave ever broken out, a variety of high performance attacking weapons andcorresponding shield structures are urgently needed. The material property research ofthe projectile bodies, battle ships, aircrafts, and beneath shield structures have beenattracting the material scientist, metallurgist and mechanical researcher interests. Nowhuman being is increasingly exploring outer space, these structural materials of thespace application, and other materials using in the defence, aviation and astronautictechnique, and civil fields are in extreme work environment, e.g., the impact in highvelocity, the deformation in high temperature, and the larger plastic flow. Thesestructural materials are required to have good performance. Such, while new materials isbeing explore, the material behavior in extreme work environment must also be highlyunderstood in order to construct the base for practice engineering application. BCC（body-centered cubic） polycrystalline metals and those alloys mainly possessing BCCstructure often exhibit higher melting point, favorable heat conductivity, well fracturetoughness, considerably better corrosion resistance and weldability, these metals alwaysget scientist comprehensive interests. But the properties of the BCC metals stronglydepend on the temperatures, the deforming rate and the material microstructures.In the present paper, through systematically large amounts of the experimentalstudies and analyses, main purpose is to detect the plastic flow behavior of the BCCmetals and alloys over a wide range of temperatures and strain rates, and largerdeformation. Main content includes: the dynamic measuring technique for the materialproperty; the plastic flow characteristic and deformation mechanism of the materialsunder different temperatures and strain rates; and the constitutive relation study. Thestudy methods and contents are briefly described as following:（1） The dynamic measuring technique for the material property. Based on a lot ofexperimental property studies of the metal materials in the foretime years, especially inrecent three years, the present issue mainly emphasizes the split Hopkinson compressive bar technique, and how to achieve the couple methods of testing temperatures and strainrates. These metals include three kinds of tantalum （Ta）, vessel steel Nitronic-50, andthree kinds of tungsten （W）. The material property under low strain-rate tests arecarried out through electronic-mechanical driving testing machine, and conventionalservohydraulic machine. Combing the spilt Hopkinson bar with quasi-staticservohydraulic machine, Strains over 70% are achieved in these tests over a temperaturerange of 77-1,100K and strain rate of 10^-4-10⁴/s, these material plastic flow propertyare studied.（2） The plastic flow characteristic and deformation mechanism of the materialsunder different temperatures and strain rates. Based on the section （1） techniques, theuniaxial compression mechanical properties of three kinds of tantalum （Ta）, vessel steelNitronic-50, and three kinds of tungsten （W） are measured, then relatinmg these resultswith accumulating other materials data of foretime years, the plastic flow characteristicand plastic flow mechanism of BCC metals are systematically analysed over atemperature range of 77-1,100K and strain rate of 10^-4-10⁴/s.（3） The constitutive relation study. In terms of plastic flow characteristic of theseBCC metals under different temperatures and strain rates, and their plastic flowprinciple and deformation behavior, based on the thermal activation glide mechanismthat dislocation overcome various obstacles and potential barriers, a unified constitutivemodel with physical concept are developed. The parameter of the model are defined andexplained. A practice constitutive model describing plastic flow stresses for BCC metalsare presented, the parameters of BCC model are obtained with the help of experiments,To verify the model, the experimental results are compared with the model predictions.Based on these systematically studies, main conclusions and achievement are asfollowing:1) In the present paper, a special split Hopkinson compressive bar withsynchronization mechanism are developed, this equipment have a trap function ofsecond stress loading pulse, with the help of this Hopkinson technique, the strain rateover 30,000/s and temperature over 1,100K can be obtained to measure plastic flow stress of materials at strain exceeding 70%. A couple testing of temperatures and strainrates can be achieved in this Hopkinson bar technique.2) Through a lot of analyses, it is found that, the plastic flow stresses of BCCmetels are very sensitive on strain rates and temperatures. But the loading history hasweak effect on plastic flow stress. The dynamic strain aging occurs at lower strain rates,the peaks in the region of dynamic strain aging will shift higher temperature, ordisappear with increasing strain rates.3) It is found that, the third dynamic strain aging happens （DSA） in all BCC（body-centered cubic） polycrystalline metals and these alloys mainly possessing BCCstructure. BCC metals do not have an initial pre-straining strain as the onset of the thirddynamic strain aging. The third DSA occurs in a matching coincidence of thetemperature and strain rate, its temperature region will shift to higher region withincreasing strain rates. Such, the mechanism of third DSA is thought as therapid/continuous formation of the solute atmospheres at the mobile dislocation core bythe pipe diffusion along vast collective forest dislocations to result in a continuous risecurve of flow stress.4) The plastic flow characteristic and mechanism of these BCC metals include that,the shear deformation at lower strain rates, adiabatic shear band occurs in higher strainrates, deformation twinning producing at low temperatures. There is the plasticviscous-drag mechanism in higher temperatures and higher strain rates.5) A unified constitutive model with physical concept are developed, this model issuitable to predict the plastic flow of metals. During deducing this model, theknowledge of solid thermal mechanics, material science, metallurgy and mechanics areused, such that, this model has remarkable physical concepts. Through comparing thetesting results with model predictions, the good agreement is obtained. These haveshown that the present model is reasonable and reliable.更多还原