疲劳Cu单晶驻留滑移带的演化及其内应力场

利用扫描电镜电子通道衬度(SEM-ECC)技术,对单滑移取向疲劳Cu单晶从基体脉络位错结构到驻留滑移带(PSBs)位错结构的演化进行了观察．且对这个演化过程中典型的位错结构进行了模拟计算,给出了PSBs演化过程中典型位错结构内应力场的分布．结果表明：在从基体脉络位错结构到PSBs位错结构的演化过程中,内应力的分布是不均匀的,位错密集区域(基体脉络和PSBs墙中)比位错贫乏区域(通道中)平均内应力分布相对集中,PSBs夹层与基体相比平均内应力的分布相对较弱,PSBs与基体边界处存在很大的应力差由观察和计算结果对PSBs演化给出了一个新的可能的演化机制．     

利用局域取向差衡量变形金属中的位错密度

利用局域取向差衡量变形金属中的位错密度,对不同变形程度的退火铁素体钢样品做电子背散射衍射(EBSD)面扫描,取得取向数据进行局域取向差分析。结果表明,在相同的测量和计算条件下,局域取向差可以表征出塑性变形的金属中位错墙和显微带等亚结构。局域取向差还可以用来比较不同样品的位错密度平均水平,特别是能够区分状态接近的微区中位错密度的相对大小。利用局域取向差分析证实了具有相同成分的铁素体+马氏体(F-M)双相钢与铁素体+珠光体(F-P)钢中,前者铁素体的位错密度高于后者。局域取向差分布图表明在冷轧板中部组织中,位错墙集中分布于晶界附近;而在边部组织中,位错墙则是分散分布并相互缠结。边部组织中的位错密度显著高于中部组织,是导致局部塑性差容易开裂的一个原因。     

高应变速率对纯钛塑性变形的影响

利用动态塑性变形(DPD)和准静态压缩变形(QSC)技术对纯钛圆柱样品进行对比压缩试验,研究了不同应变速率下纯钛形变孪晶和微结构演变。结果发现:2种变形方式的变形机制相似,低应变时以形变孪生为主,孪生饱和后转变为位错滑移主导;高应变速率促进了形变孪晶的产生,激发{4211}压缩孪晶的形成,同时使变形机制转变临界应变提前至0.2;纯钛在高应变速率和高应变(ε≥0.6)下出现绝热剪切带(ASB)。     

MICROMECHANICAL STUDY ON YIELD STRESS AND THE EFFECTS OF TWINNING FOR γ-TiAl-BASED PST CRYSTALS

基于γ-TiAl基PST晶体的微结构及滑移系和形变孪晶的空间晶体学位向分布的变形机制,提出了分析PST晶体 屈服应力对外载轴和片层界面夹角θ间的依赖关系的细观力学解析模型,重点考察了PST晶体的微结构和普通位错与形变孪晶 的临界分切应力(CRSS)之闻的差别对晶体屈服的影响．结果表明: PST晶体屈服应力σy和外载轴与片层界面之间的夹角θ 存在着强烈依赖关系．在软取向,变形由普通位错和超位错控制,变形平行于片层界面;而在硬取向,变形由形变孪晶控制,PST 晶体的变形为穿过片层界面形式．在加载角θ分别为45°,0°和90°时,屈服应力间的关系为: σy(45°)<σy(0°)<σy(90°)． 同时,分析了滑移系和形变孪晶的具体开动情况．     

激光冲击铝合金微结构演化及力学行为的分子动力学模拟

目的 揭示激光冲击铝合金的微结构演化过程及塑性变形机制,探究残余应力产生的机理,为激光冲击提升铝合金力学性能提供理论参考。方法 基于分子动力学模拟,采用活塞冲击法实现多晶铝合金(Al-Mg-Zn-Cu)在不同加载速度下的冲击强化。利用共邻分析法和位错提取法,研究铝合金的微结构演化过程、位错分布以及激光冲击影响铝合金力学性能的内在机理。结果 在冲击波加载阶段,当高速冲击波作用时,铝合金出现大量滑移系,产生高密度位错。在保载阶段,位错集中在晶界附近,导致多晶铝合金发生晶界塑性变形。在卸载阶段,不同类型位错之间进行了相互转化。铝合金两端晶粒和晶界的塑性变形,导致了残余压应力的产生。对完全卸载后的铝合金进行单轴拉伸模拟,发现0.7 km/s和1.0 km/s的冲击速度下,残余压应力抵消了部分拉伸应力,变形晶界附近产生新的位错,且晶界发生迁移和合并,导致极限应力分别提升15%和22%。结论 激光冲击对Al-Mg-Zn-Cu铝合金的微结构及力学性能影响显著,在高速冲击波作用下,铝合金两端发生剧烈的塑性变形,导致残余压应力的产生。单轴拉伸时,残余压应力抵消了部分拉伸应力,且铝合金晶粒内发生原子变形产生新的位错,同时晶界发生运动,最终使得极限应力增大,铝合金的力学性能得到提升。     

初始微结构对多晶金属Be宏观力学性能的影响

通过室温准静态(应变率10-3s-1)、高温准静态(600℃,应变率10-3s-1)和室温动态(应变率103s-1)预压缩变形分别实现金属Be内部3种不同微观结构的形成,进而实现多晶金属Be宏观压缩力学性能的调节,研究了初始微观结构对多晶金属Be压缩力学性能的影响及作用机理。结果表明,3种不同初始微结构样品中,室温准静态预压样品压缩力学响应最硬,而室温动态预压样品最软。显微组织、宏观织构测试以及原位中子衍射力学实验测试表明,室温准静态预压样品形成"弱织构"型初始微结构,微观力学响应上表现为(00.2)晶面优先受力,且由于引入一定位错使形变硬化效应相较于其它试样更为明显;而室温动态预压样品形成"强织构"型初始微结构且存在一些微孔洞,微观力学响应上表现为(00.2)晶面主要受力,微孔洞参与部分应力配分抑制了形变硬化效应;高温准静态预压样品形成的"随机取向"型初始微结构,微观力学响应上表现为初期各晶面均等受力、(11.0)晶面逐渐受力增加,且位错密度降低使内部协调变形相对容易。通过不同尺度上微观结构的协同配合可以实现宏观力学性能的调节,基于此可定制满足特定服役场景需求的性能。     

7075铝合金模锻件淬火残余内应力的消除

为了消除７０７５铝合金模锻件淬火残余内应力,采用模压冷变形方法并利用Ｘ射线衍射技术测试了淬火态及经过不同程度的模压冷变形后锻件的残余内应力。结果表明,模压冷变形是消除７０７５模锻件淬火残余内应力的一种有效方法;淬火态锻件表面为压应力,模压冷变形后的表面为拉应力;４．６％的模压冷变形是模拟消除残余内应力研究的最佳值。     

低碳微合金钢微细板条状组织在单向拉伸中的反常转动

采用光学与电子显微术研究了一种低碳微合金钢中充分细化的板条贝氏体组织的室温变形行为，分析了拉伸变形样品中未变形区、均匀变形区与缩颈区的组织形态差别．实验表明：未变形区与均匀变形区中各束板条的空间取向(板条长轴的指向)基本是随机的；但在缩颈区，所有板条接近平行于拉伸轴向，说明某些板条在拉伸过程中发生了大角度的转动，这一现象不能单由晶体学得到解释．通过与另外两组不同组织形态样品的对照比较，发现板条转动程度与板条长度以及长宽比密切相关,据此提出板条界面阻碍位错运动导致可动滑移系的自然选择与板条连续转动的机制．     

Calculation of the internal stresses at the γ/γ’ interface of DD8 single crystal nickel base superalloy after thermo-mechanical fatigue

计算了DD8单晶镍基高温合金在同相(IP)和反相(OP)热机械疲劳(TMF)后γ／γ′相界面上产生的位错网的内应力．结果表明：IPTMF条件下，γ／γ′相界面上产生的位错网可以释放掉大部分错配应力，同时因位错网的存在导致了γ′沉淀相发生了明显的筏化现象．OP条件下产生的层错未造成基体内应力分布的不同，因此未发生γ′沉淀相的筏化。     

孪晶距对纳米钨力学性能影响的分子动力学模拟EI北大核心CSCD

为了研究孪晶间距的大小对纳米钨力学性能及变形机理的影响,利用分子动力学对不同孪晶间距的孪晶钨进行了单轴拉伸模拟。使用近邻列表技术(CNA)和位错分析方法(DXA)对拉伸过程中纳米钨的变形失效过程和微结构演化进行了表征分析,从而揭示孪晶间距对纳米钨力学性能影响微观机理。结果表明:孪晶钨变形过程中出现的相变、孪晶界的变形以及去孪晶化的现象会改变孪晶钨中裂纹的扩展方式,提高孪晶界的变形能力;而随着孪晶间距的减小即孪晶密度的增加,可变形的孪晶界增多,导致纳米孪晶钨的断裂应变增加。由于孪晶界中存在能量较高的相互作用的特殊三原子结构使纳米钨中更容易出现晶体缺陷,缺陷会在拉伸载荷作用下快速形成裂纹,导致晶体断裂失效,严重降低了纳米钨的屈服强度。此外,孪晶界的存在显著降低了几何必须位错的数量同时阻碍了位错的滑移运动,位错难以发射和运动,从而导致塑性变差。     

Cyclic deformation behavior of non-isoaxial copper tricrystals and bicrystals

The cyclic hardening and saturation behaviors of copper tricrystals and bicrystals were investigated in strain-controlled multiple step tests. The results show that, for the inclined grain boundary (GB) bicrystal with single slip components, the cyclic stress strain (CSS) curve exhibits a plateau or quasi-plateau region, while the CSS curve of tricrystals shows no plateau. Observations of surface morphologies indicated that owing to the strain incompatibility of three grains, at lower strain amplitude the triple junction (TJ) retards obviously the primary slip in grains and makes deformation near it smaller than that near the bicrystal GB, while at higher strain amplitude slip can be distributed near the TJ homogeneously. The probability of crack initiation at the same TJ is closely related to the loading direction. The saturation dislocation structures of tricrystal specimens under the strain amplitude of the last step were explored by the electron channeling contrast technique in SEM (ECC-SEM). Loop patches with persistent slip band (PSB) ladders embedded were found even in the TJ vicinity for all grains. Dislocation-free zones (DFZ) occurred in the vicinity of TJ and GB, and the difference in shapes between them is due to the difference in internal stress field. Misoriented cell structure and dislocation wall structure were found near the crack tip, and the formation of them is associated with the cooperative action of crack tip, GB, grain orientation and the applied strain amplitude.     

Mechanical spectroscopy of thermal stress relaxation at metal–ceramic interfaces in Aluminium-based composites

The micromechanisms of thermal stress relaxation in aluminum-based metal-matrix composites (MMCs) have been investigated by mechanical loss and dynamic shear modulus measurements during thermal cycling between 100 and 500 K. A transient mechanical loss maximum, which is absent in the monolithic material, appears during cooling. This damping maximum is strongly dependent on the measurement parameters: oscillation frequency, oscillation amplitude and cooling rate. In addition, it increases with the volumetric reinforcement content and decreases if the matrix strength is improved. The shear modulus evolution during thermal cycling shows that no detectable interfacial debonding occurs. Compared with alloyed MMCs, Al4N-based MMCs show the highest damping maximum simultaneously with a plateau in the elastic shear modulus. The mechanical loss maximum is attributed to dislocation generation and motion near the interfaces, resulting from the differential thermal contraction of matrix and reinforcement. A new model is proposed which describes this specific mechanical behavior of MMCs in terms of the development of microplastic zones in the matrix near the metal–ceramic interfaces.     

γ-TiAl中<110>倾斜晶界断裂行为的分子动力学模拟(英文)

采用分子动力学(MD)方法研究γ-Ti Al合金中<110>对称倾斜界面的断裂行为,模拟在不同温度与应变速率下垂直界面方向的拉伸变形。结果表明:晶粒的相对取向及晶界特定的原子结构是影响位错形核临界应力的两个主要因素。取向差角度大于90°的Σ3(111)109.5°、Σ9(221)141.1°和Σ27(552)148.4°界面,位错在晶界处形核和扩展;取向差角度小于90°的Σ27(115)31.6°和Σ11(113)50.5°界面,无位错在晶界处形核,当应力达到峰值后界面直接断裂。γ-Ti Al双晶的断裂机制为微裂纹在界面处的形核及沿界面扩展;不同取向差界面的区别在于裂纹前端有无塑性区增韧。     

Fe颗粒密度对Cu-Fe-P合金残余应力影响的数值模拟

采用弹塑性有限元方法建立力学模型，模拟分析微观状态下的引线框架材料Cu-Fe-P合金局部铁颗粒密集区的残余应力分布，重点分析了颗粒密度对村料残余应力的影响.结果表明，颗粒密度越大别界面附近Cu基体和铁颗粒的残余应力越大，并且在x方向Fe颗粒处于压缩．Cu基体主要处于拉伸状态；y方向则是Fe颗粒受拉伸，Cu基体由拉伸变化到压缩状态；因此在两种反向残余应力怍用下，界面两侧的应力差电越大，甚至导致界面处被撕裂，材料表面起皮，影响带材性能的发挥．     

EFFECT OF REINFORCEMENT VOLUME FRACTION ON PRECIPITATION IN MATRIX OF SiC_p/AI COMPOSITE

The differential scanning calorimetric(DSC) and transmission electron microanalysis(TEM) techniques were used to study the kinetic process of precipitation in matrix ofcast SiC_p/2024 composites.The results showed that precipitation reactions of GPzone and intermediate phase S'(Al_2CuMg)in the composites were accelerated comparedwith SiC-free material,the peak temperatures of both reactions were decreased.Thereaction enthalpies of both the GP zone and intermediate phase S' formation in thematrix were substantially decreased after the addition of SiC.TEM analysis found thatthe alloying element Mg segregated at the SiC/Al interfaces,and was depleted in thematrix near the interface.The precipitation-free zones(PFZ) and precipitation-sparsezones(PSZ) formed near the interfaces,as a results,the volume fraction of precipitatesin matrix was reduced.     

Effect of crystallographic orientation and grain boundary character on fatigue cracking behaviors of coaxial copper bicrystals

Four coaxial copper bicrystals were employed to study the slip morphologies and fatigue cracking behaviors during cyclic deformation. Three of them had high-angle grain boundaries (GBs) with nearly the same misorientation and one bicrystal had a twin boundary (TB). Different slip bands (SBs) operated near the GBs and TB, generating different dislocation arrangements, which are mainly determined by the crystallographic orientations of the component grains. The GBs suffered impingement or shear damage caused by slip difference from both sides. It is suggested that there is an energy increase in the interfaces between matrix and persistent slip bands (PSBs), GBs and TBs per cycle during cyclic deformation due to the accumulation of lattice defects, which would make the interface unstable. After a certain number of cycles, fatigue cracks initiated firstly at GBs for some bicrystals while fatigue cracking occurred preferentially at PSBs for the others. It is confirmed that the energy growth rate is an increasing function of the shear stress, strain amplitude and strain incompatibility, which results from slip differences on both sides of the interfaces. Interfaces with different energies and strain incompatibilities have different fatigue cracking resistance. It is found that GBs with defective and complex structure, and hence high interfacial energy accompanied by high modulus of the residual GB dislocation (GBD), are preferential sites for fatigue cracking, while the fatigue cracking appeared predominantly at PSBs when the modulus of the residual GBD is lower than that of a perfect dislocation with simple GB structure and low interfacial energy. The present model for the energy can predict well which kind of interface would form cracks preferentially during cyclic deformation in one coaxial bicrystal and which GB would need more cycles to initiate fatigue cracking between coaxial bicrystals with different GB characters.     

FATIGUE BEHAVIOUR OF SiC_p/6061Al COMPOSITE

The fatigue of SiC_p/6061Al composite containing 15 v.-% SiC particles has been compared with 6061Al alloy.Dislocation structure and microprocess of fatigue crack initiation and propagation in the composite have been investigated by using SEM and TEM.The results in- dicate that the fatigue strength at 10~7 cycles of the composite is 196 MPa,i.e.about 25% higher than matrix alloy.The voids and microcracks initiated at and near the interface be- tween SiC_p and matrix,where has a higher density of dislocations,will propagate and link up to form the fatigue crack.It is an important evidence to note that the dislocation channels where screw dislocation can travel are formed near interface and corner region of SiC_o in the composite subjected to fatigue stress(σ_(max)=274 MPa N=2.4×10~5 cycles),demonstrating the relationship between fatigue crack initiation and dislocation movement in the SiC particles reinforced 6061 Al alloy composite.     

On the presence of work-hardened zones around fibers in a short-fiber-reinforced Al metal matrix composite

Dislocation densities are investigated in a short-fiber-reinforced Al–11 wt.% Zn–0.2 wt.% Mg metal matrix composite (MMC) with a special focus on regions near the fiber–matrix interfaces. Clear microstructural evidence is provided for the formation of work-hardened zones (WHZs) around fibers during creep using transmission electron microscopy (TEM). The dislocation densities in the WHZs are higher after creep than after squeeze casting, where the plastic strains associated with the thermal stresses that build up during solidification also result in an increased dislocation density close to fibers. The effect of heating and cooling on the dislocation substructure is also considered. The results are discussed in light of previous findings and provide microstructural evidence for the presence of WHZs as predicted by the Dlouhy model of MMC creep.     

Superplastic behavior of PM SiCp/6061 aluminum alloy composites at high strain rates

The superplastic behavior of a powder-metallurgy processed 6061 Al composite was investigated as a function of SiC content increasing from 0% to 30% at 10% increments over a wide temperature range from 430°C to 610°C. The materials were found to be high-strain-rate superplastic. In the temperature range where grain boundary sliding (GBS) controlled the plastic flow, the strength of the composite was lower than that of the unreinforced matrix alloy even after compensating for grain size and threshold stress. This “particle weakening” was in contrast with the particle strengthening observed in the low temperature range where dislocation climb creep was found to control the plastic flow. In the GBS regime, the strength differential between the materials was a function of SiC content and temperature, which increased with the increase in SiC content and temperature. Strong Mg segregation was detected at interfaces between SiC and Al phases in the composites. Evidence for interfacial reaction reported in the Si₃N₄ reinforced 6061 Al composites could not be detected in the current composites. Extensive formation of whisker-like fibers was observed at the fractured surface of the tensile samples above the critical temperature where particle weakening begins to be exhibited. This result suggests the possibility that partial melting in the solute-enriched region near SiC interfaces is responsible for the particle weakening in the SiC reinforced 6061 Al composite.