【作者】 李建国；

【导师】 李玉龙； 索涛；

【作者基本信息】 西北工业大学 ， 固体力学， 2018， 博士

【摘要】 传统工艺所制备金属的性能已逐渐难以满足当今快速发展的工程需求,而纳米晶金属材料所表现出高强度和改良后可兼具有较好塑性的优异性能,令其具有很好的应用前景。目前,通常采用大塑性变形法制备块体纳米结构金属材料,但该过程中高密度位错的引入使其塑性较低。为了获取强塑性匹配的高性能金属,学者们提出了很多改良方法,其中就有通过降低金属堆垛层错能后在塑性变形的纳米晶粒内部引入大量共格孪晶,从而有效地提高晶粒的位错存储能力来改善其塑性。但是,针对低层错能纳米结构金属材料的研究工作目前仅限于准静态条件下的拉伸加载,发现其均匀伸长率的确随着堆垛层错能的降低逐渐增大,而其变形机制、断裂机理等尚不清楚,特别是不同加载条件下力学行为（例如高应变率、低温等极限条件下）依然未曾涉及。全面地研究低层错能纳米结构金属的各项力学性能及其变形行为不仅有利于深入地理解该类材料的变形机制和失效机理,而且对于进一步改善纳米金属材料的综合性能具有指导意义。本文选用FCC结构固溶态铜铝合金作为研究对象,铝原子的添加可有效降低合金的堆垛层错能,采用等径通道挤压法获取最小平均晶粒尺寸约为90nm的纳米结构Cu-Al合金（NS Cu-Al）。使用电子万能材料试验机和分离式霍布金森压杆在宽应变率范围和不同环境温度下系统地研究不同层错能NS Cu-Al的力学行为,利用透射电子显微镜（TEM）观察对比变形前后内部微观结构特征的演化,用以具体地分析其对应的变形机制。在获取压缩实验数据后,分析其变形动力学因子（包括率敏感性和温度敏感性）发生的变化,同时采用应变率跳跃实验进行对比验证,并基于热激活理论计算对应的热激活体积,由于不同的变形机制对应不同的激活体积,因此激活体积的计算可从另一角度定量地分析出不同材料变形过程中的主控机制。值得注意的是,高应变率下,随着层错能的降低,NS Cu-Al压缩变形时逐渐由均匀变形过渡到局部剪切变形,显微观察验证了绝热剪切带（adiabatic shear band,ASB）的出现,热塑性失稳导致了“应力-应变”曲线上发生了突然的应力跌落现象,这是首次报道FCC结构纳米晶金属在单轴压缩时发生绝热剪切变形。采用光学显微镜,扫描电子显微镜（SEM）观察了ASB的微观形貌,利用TEM仔细观察对比了ASB内外的微观结构特征,并使用纳米压痕法表征了ASB内外的力学性能。随后,分别从力学和材料学角度出发阐释了FCC结构低层错能NS Cu-Al中ASB的形成原因和主导机制。实验揭示了大塑性变形后晶体材料内织构是引起局部塑性变形的主要因素。因此,基于晶体塑性理论同时考虑了绝热温升对于晶体滑移的影响,本文建立了“力-热”耦合作用晶体塑性有限元多晶模型,用以验证Cu-Al合金中不同织构组分对于ASB形成的影响。最后,针对大塑性变形制备纳米结构金属“低塑性”的问题研究了不同加载方向上低层错能NS Cu-Al的拉伸断裂行为,明确了其过早出现软化甚至失效的控制机制。当前材料表现的拉伸塑性并不令人十分满意,但本文的工作明确了织构及微孔洞对拉伸破坏的影响,因而在材料制时弱化织构强度以及提高晶粒尺寸均匀性可有效改善金属拉伸塑性。总结起来,本文取得的主要结论有:（1）晶粒尺寸的减小大大增加了NS Cu-Al的强度,准静态下,最低层错能NS Cu-Al的屈服应力可达750MPa以上,高应变率下其强度已超过1.1GPa。值得注意的是,大塑性变形后,中/高层错能金属通常表现出理想“弹-塑性”行为,但随着层错能的降低,NS Cu-Al晶粒内部出现大量的孪晶及层错等面缺陷,其力学曲线上表现为:准静态条件下,Cu-6.87 at.%Al(?_{SF E}（28）21m J/m²)开始表现出分段的硬化行为;Cu-11.14 at.%Al更表现出连续的硬化行为,直到很大的应变。对比压缩变形前后微观结构的演化,对于中/高层错能合金而言,晶界及其附近位错发射和相互作用是控制变形的主要机制。层错能降低后孪晶及层错有一定的调节变形能力,同时孪晶和层错等面缺陷对垂直于界面的移动位错的运动有阻碍作用,这有助于提高晶粒内部位错的储存能力,从而改善材料的应变硬化能力,但其主要的变形依然是通过晶界上部分位错的发射及其移动来进行调节的。（2）高应变率下,层错能的降低导致NS Cu-Al由均匀的塑性变形过渡到局部剪切失稳。从力学角度出发,高应变率加载时,升高的应力强度和温度敏感性,以及增强但依然相对较弱的应变率敏感性是导致低层错能纳米结构FCC金属中出现ASB的主要原因;从材料学角度出发,高应变率单轴压缩时,各向异性的绝热剪切行为表明强织构是ASB起始的重要原因。XRD测试结果也显示,大塑性变形后其内部织构组分包括:Goss织构{110}<001>,黄铜型织构{110}<112>和?带（{110}<uvw>）连接由黄铜型织构过渡到Goss织构时生成的旋转Goss型织构{110}<011>,这些织构均有利于局部塑性变形的开始。（3）采用晶体塑性有限元模型验证了低层错能金属中主要组分Goss织构对于ASB形成的影响机制。有限元模拟获取力学曲线与实验结果十分吻合,根据宏观力学曲线的变化可分阶段描述ASB的形成。另外,研究FCC结构金属中常见的几种典型织构对ASB形成的影响发现,立方织构可有效抑制ASB的形成,其他织构模型中ASB的出现跟变形时可被激活滑移系的数目有关,数目越少则越有利于局部塑性变形的起始。（4）低层错能纳米结构合金中存在的微观织构会显著影响拉伸断裂行为。实验结果表明,材料拉伸变形时存在剪切失效和拉伸断裂两种失效行为,且其断裂机制强烈地依赖于加载应变率。准静态条件下,微孔洞的形成与生长是NS Cu-Al过早出现软化和失效的直接原因;而高应变率拉伸时,局部剪切区内大量变形热的集聚造成了绝热剪切变形的出现,会加速失效的发生。更多还原

【Abstract】 The properties of metals produced with the traditional methods have gradually not met the requirements of the quickly developing engineerings.However,the nanocrystalline（NC）metals present some excellent mechanical properties,such as high strength,improved good ductility et.al.Generally,severe plastic deformation（SPD）methods are used to fabricate the bulk nanostructured（NS）metals or alloys,but the NS materials always show little or even no ability to deform plastically due to the introduction of high density of dislocations during the SPD process.In order to produce the high-performance NS methals with strength-plasticity matching,scholars propose a number of improving methods,one of them is tailoring the stacking fault energy（SFE）of metals to creat numerous deformation twins in the nanocrystalline grains by SPD.The enhancement of dislocation storage ability can effectively increase the plasticity of NS metals.Nevertheless,the mechanical properties of NS alloys with low SFE were seldom systematically investigated except for the tensile properties under quasi-static loading.The results show that the uniform elongation rate definitely increases with the decreasing SFE of metals,but due to the lack of experimental exploration the deformation and fracture mechanisms still is unknown,especially no reports on the mechanical behaviors under different loading conditions,for example,the extreme conditions like high strain rate and low temperature et.al.To systematically investigate the mechanical properties and deformation behavior not only helps us interpret the deformation and failure mechanisms in depth but also guide us to further improve the comprehensive performance of NS metals。In the present work,the solid solution copper aluminum（Cu-Al）alloy with face-centered cubic（FCC）structure is used.The addition of Al atoms can effectively reduce the SFE of alloys.The equal channel angular pressing（ECAP）method is employed to produce the NS Cu-Al alloy,the minimum average grain size can reach to about 90nm.Then mechanical behaviors of the different SFE NS Cu-Al alloys are comprehensively studied at a wide range of strain rates and temperatures by using electric universal materials tester and split Hopkinson pressure bar.Transmitted electron microscope（TEM）is adopted to observe the microstructure feature before and after deformation and make a comparison to thoroughly understand the deformation mechanisms.On the basic of compressive experiment results the deformation kinematics factors including strain rate sensitivity（SRS）and temperature sensitivity are analysed and the strain rate jump tests are also conducted under quasi-static loading to calculate the SRS fator.Meanwhile,based on the thermal activity theory the corresponding thermal active volume is measured.Because different deformation mechanisms correspond to a different thermal active volume we can use the calculation results to qualitatively discuss the controlling mechanisms during the deformation.It is worthy noting that under high strain rate loading a transition from uniform deformation to localized shearing deformation occurs in the NS Cu-Al alloys when the SFE becomes lower.The microscopy observations verify the formation of adiabatic shear bands（ASB）.The thermal plastic instability results in the abrupt stress collapse on the stress-strain curves.This is the first report on the ASB in nanocrystalline metals with FCC structure under dynamic uni-axial compression.The micro morphology of the ASB is observed by using optical microscope（OM）and scanning electron microscope（SEM）.After that,the microstructure characteristics inside and outside the ASB are compared through TEM observations and nano-indentation tester is operated to explore the different mechanical properties caused by the microstructural differences.In order to better understand the formation process of the ASBs in the NS Cu-Al with the lower SFE,this work firstly explains the formation reasons of the ASBs from the mechanical side,and then in the perspective of materials science the underline mechanisms are discussed.The experimental results indicate that in the crystalline materials after SPD texture should be the main factor which contributes to the localized shearing deformation.Therefore,based on the crystalline placticity finite element method（CPFEM）and the adiabatic temperature rise during the high strain rate loading is considered at the same time,a themo-mechanics coupling CPFEM polycrystalline model is built to verify the influence of texture components on the formation of ASBs in the NS Cu-Al alloys with the low SFE.Finally,with regard to the problem of low ductility in the NS metals or alloys after SPD,the tension fracture behaviors of the NS Cu-Al alloy with the lowest SFE are studied in different loading directions to consider the effects of texture.The mechanical curves and microstructural observations tell us the controlling mechanisms which can lead to the early softening behavior and even failure.Though the plastic performance still is not satisfied now,this work has already studied the effects of texture and micro-voids on the fracture under tension tests.Weakening the intensity of texture and avoiding the emergence of micro-voids during materials processing may be effective to improve the plastic ductility of low SFE NS alloys.To sum up,the conclusions can be shown as follows:（1）The refinement of grain size can significantly enhance the strength of NS Cu-Al alloy.Under quasi-static loading,the yield strength of the NS Cu-Al alloy with the lowest SFE can arrive at higher than 750MPa.At a high strain rate the flow strength is over 1.1GPa.It is interesting to note that after SPD the medium/high SFE metals always present nearly perfectly elastic-plastic deformation behavior,but in this work,a large number of plain defects like deformation twins and satcking faults appear in the grains of NS Cu-Al alloys when the SFE value becomes lower.The microstructural evolution makes the NS alloys show different behaviors observed from the stress-strain curves:under quasi-static laoding,NS Cu-6.87at.%Al(?_SFE=21mJ/m²)strats to display an apparent hardening behavior at the early stage of plastic deformation and then it disappears when the true strain is larger than a certain strain.For NS Cu-11.14 at.%Al alloy,a continuous strain hardening behavior appears even at a large deformation.Compare the before-and-after microstructural features,it can be observed that in medium/high SFE alloys the dislocations emission and interactions at or near the grain boundaries（GBs）should be the main controlling deformation mechanisms.However,when the SFE decreases the appearance of deformation twins and stacking faults can contribute to accomadate the plastic deformation.It is clear that the interfaces between the matrix and deformation twins or stacking faults can inhibit the movement of dislocations to enhance the storage ability in the grains.As such it improves the strain hardening ability during the plasticity.But TEM observations and active volume calculations reveal that the emission and movement of partial dislocations at the GBs accomadate the plastic deformation.（2）Under high strain rate loading,the decrease of SFE causes the occurrence of a transition from uniform plastic deformation to localized shearing instability.The microstructural evolution makes the mechanical properties change apparently.From the point of mechanis,at a high strain rate the enhanced strength and temperature sensitivity of NS Cu-Al alloys,the increased but still relatively low SRS together lead to the formation of ASBs in NS FCC alloys with low SFE.In the perspectives of materials science,the anisotropic adiabatic shearing behaviors under dynamic uni-axial compression indicate that micro-texture plays an important role during the formation of ASBs.X-ray diffraction measurement shows that after SPD the texture components in the NS Cu-Al alloys mainly includes Goss texture（{110}<001>）,Brass texture（{110}<112>）and the rotated Goss texture results from the transition from{110}<112>to{110}<001>through the connection of fiber（{110}<uvw>）.The pre-existence of strong textures is favor for the beginning of localized plastic deformation.（3）The CPFEM models verify the contribution of Goss texture which dominates in the NS Cu-Al alloy with low SFE to the formation of ASBs.The simulation stress-strain curves show good agreement with the experimental results.Based on the varation of macroscopic mechanical responses the formation of ASBs can be described in several stages.Moreover,in terms of effects of several typical textures on the formation of ASBs for FCC metals,the Cubic texture（{100}<001>）is effective to restrict the occurrence of ASBs,in the other textures,the appearance of ASBs is related to the number of slip systems that can be actived easily,the less can more easily lead to the initiation of localized deformation.（4）The strong texture after SPD in the NS Cu-Al alloy can significantly affect the tension fracture behaviors,including shearing failure and typical tension fracture.Meanwhile,the fracture mechanisms are strongly dependent on the loading rate.Under quasi-static tension,the initiation and growth of micro-voids are the main reason for the premature softening and failure in the NS Cu-Al alloy.However,at a high strain rate the adiabatic shearing deformation which is caused by the heat generation inside a localized shearing zone is responsible for the sudden failure.更多还原