磁致塑性效应下的位错动力学机制

基于磁致塑性效应探讨了磁场作用下位错受力和运动机制, 对磁场下的位错动力学机制进行了定性和定量分析. 选择氧化铝纳米颗粒强化铝基复合材料为实验对象, 在不同磁感应强度下(0–3 T范围)对试样进行磁场处理. 结果表明, 随着磁感应强度增加, 位错密度提高, 表现出塑性变形特征. 分析认为, 磁场力不足以驱动位错运动, 位错增殖诱因在于磁致塑性效应, 即磁场改变了顺磁性位错芯与障碍间自由基对中的电子自旋状态, 促使自由基对从强键结合单线态向弱键结合三重态转化, 位错穿越障碍时所需能量减小, 退钉扎趋势明显; 位错运动中的限速环节是位错在障碍处的停留, 磁场诱发的电子激发和原子重排速度很快, 表现出磁场作用的高效性. 磁场起作用的临界磁感应强度约为3 T, 低于3 T时磁场作用随磁场强度增加而变得明显, 高于3 T 后磁场效果会减小. 计算得到3 T 时位错运动速度是10^-3 m/s, 位错线长度比未加磁场时增加两个数量级, 位移与磁感应强度平方和磁场作用时间成正比. 实验和理论研究表明磁场具有改善材料塑性变形能力的显著作用.     

强磁与应力场耦合作用下AZ31镁合金塑性变形行为

研究强磁场对AZ31镁合金塑变能力和微观组织的作用,在3 T脉冲强磁场条件下对合金进行磁场耦合应力时的拉伸实验.采用电子背散射衍射、Ⅹ射线衍射和透射电镜分析等方法研究材料的微观组织.结果表明:与0 T拉伸试样相比,3 T拉伸试样抗拉强度和延伸率分别提高了2.2%和28.7%,说明将强磁场耦合作用于材料塑性变形过程时,能在不降低材料强度的同时提高镁合金的塑性变形能力,有助于同步改善材料强韧性.磁场作用机理主要表现为磁致塑性效应,计算表明主要合金相β(Mg_(17)Al_(12))为顺磁性,有助于发挥磁场作用效果.磁场提高了位错运动灵活性并促使位错增殖,晶界处位错堆积和应力集中促进了再结晶形成,晶粒发生细化,发挥细晶强韧化效果;同时磁场诱发塑性变形时的晶粒转动,新生成非基面取向的晶粒弱化了镁合金(0001)基面织构,该组织特征有助于提高材料的塑变能力.     

石墨烯/铝基复合材料在纳米压痕过程中位错与石墨烯相互作用机制的模拟研究

石墨烯因其优异的力学性能已成为增强金属基复合材料的理想增强体.然而,目前对石墨烯/金属基复合材料在纳米压痕过程中嵌入石墨烯与位错之间的相互作用仍不清晰.本文采用分子动力学模拟方法,对90°,45°和0°位向的石墨烯/铝基复合材料进行了纳米压痕模拟,研究了压痕加载和卸载过程中石墨烯/铝基复合材料的位错形核及演化,以获取不同位向的石墨烯与位错的相互作用机制,并分析其对塑性区的影响.研究发现,石墨烯可以有效阻碍位错运动,并且石墨烯会沿着位错滑移方向发生弹性变形.在纳米压痕过程中,位错与不同位向石墨烯之间的相互作用差异导致塑性区的变化趋势不同.研究结果表明,在石墨烯/铝基复合材料中,位向不同的石墨烯对位错阻碍强度和方式不同,且石墨烯位向为45°的复合材料的硬度高于其他模型.此外,石墨烯/铝基复合材料的位错线总长度的演化规律与石墨烯位向紧密相关.本文研究可为设计和制备高性能石墨烯/金属基复合材料提供一定的理论指导.     

激光冲击强化“残余应力洞”的形成机制

利用ABAQUS有限元软件进行了单个圆形高斯光斑的激光冲击强化数值模拟,分析材料表面光斑中心区域形成的"残余应力洞"现象,并通过分析材料的动态力学响应特征揭示了"残余应力洞"的形成机制。结果表明:在冲击波加载时,光斑边界处会产生很强的剪切应力,形成向四周传播的表面稀疏波和向材料内部传播的剪切波。当稀疏波同时传播到光斑中心,发生相遇、汇聚,使材料产生急剧的上下位移过程,造成冲击波加载塑性变形后的二次塑性变形。二次塑性变形中形成了较大的剪切塑性应变,并降低了冲击波加载阶段产生的轴向和径向塑性应变,使残余压应力降低,从而形成"残余应力洞"。     

磁致伸缩材料弱磁场响应特性的实验研究

针对磁致伸缩材料在弱磁场传感器领域的应用需要,采用迈克耳逊干涉原理实验测量了零应力条件下Tb-Dy-Fe材料和Fe-Ga合金的磁场响应灵敏度,以及不同应力下Fe-Ga合金的磁场响应特性和温度响应特性.实验结果表明：在零应力,外加磁场16 mT条件下,Fe-Ga合金的磁场响应灵敏度远高于Tb-Dy-Fe材料,更合适作为弱磁场传感器敏感材料;同时,在1.2 MPa预应力和26 mT偏置磁场下,Fe-Ga合金材料具有较好的磁场响应灵敏度和较大的饱和磁致伸缩系数,因而处在最佳工作状态.所得到的材料的磁场和温度响应曲线可作为弱磁场传感器参量设计的参考依据.     

单晶Cu(001)薄膜塑性变形的分子动力学模拟

使用分子动力学方法,模拟研究了单晶Cu(001)薄膜在双向等轴拉伸应变下的塑性变形行为.当应变超过一定值时,样品通过产生位错、层错及孪晶而发生塑性变形.当应变相对较低时,不全位错首先在薄膜表面形核并在密排面上滑移,留下堆积层错;当应变增加时,位错在表面与内部同时成核生长,层错数量也随之增加.分析了相邻滑移面上的位错之间相互作用形成孪晶的微观过程.材料内部形成大量堆积层错及孪晶后,较大孪晶的密排面上的原子也会发生滑移,形成孪晶内部的层错结构以释放残余应力.     

基于力磁耦合效应的铁磁材料修正磁化模型

Jiles-Atherton (J-A)模型和Zheng Xiao-Jing-Liu Xing-En (Z-L)模型在分析应力对铁磁材料磁化的影响方面应用十分广泛.目前, J-A模型中的磁致伸缩应变与应力和磁化强度的关系式采用Jiles给出的经典拟合公式,该拟合公式中磁化强度的二次项和四次项系数与应力均为线性关系,不能准确描述铁磁材料磁致伸缩系数随应力、磁化强度的非线性变化规律; Z-L模型中磁致伸缩应变与应力和磁化强度的关系式采用了双曲正切函数tanh(x),更好地描述了铁磁材料磁致伸缩应变和磁化强度随应力的非线性变化规律,但Z-L模型却没有考虑Weiss分子场、钉扎效应的作用,且由于采用了基于弹性能的接近定理,只能描述弹性应力对磁化过程的影响.针对上述问题,本文结合Z-L模型中的非线性磁致伸缩应变关系式以及J-A模型中的磁滞理论,考虑弹性应力、塑性变形对模型参数的影响,建立了能够反映弹-塑性阶段应力与塑性变形对铁磁材料磁化曲线影响的修正磁化模型,分析了弹性拉、压应力及塑性拉、压变形对磁化曲线、矫顽力和剩余磁化强度的影响规律.通过与试验结果及原有模型的计算结果进行对比,发现修正模型能够更好地反映单次磁化、循环磁化过程中应力、塑性变形对磁化曲线的影响规律,理论预测结果与试验结果之间的相关系数均在0.98以上,可为分析力磁耦合效应对铁磁材料磁化影响规律提供更准确的理论模型.     

强激光冲击铝合金改性处理研究

利用新型聚偏1.1-二氟乙烯（PVDF）压电传感器,实现了对激光引发的冲击波压力的实时测量,得到激光引发的冲击波峰压在铝中成指数型的衰减规律;观测了不同约束层材料在铝靶表面产生的激光冲击波,研究了不同约束层对冲击效果的影响;最后用激光冲击强化装置对7050-T7451航空铝合金结构材料进行了冲击强化处理,对试件激光冲击区存在的残余压应力及位错密度进行了测量。结果显示经激光冲击处理的试件表面具有极高的残余压应力,可达-200MPa以上。激光冲击处理后铝合金的位错密度得到显著的提高,疲劳寿命提高到175％～428％。这些重要结果对激光冲击改性处理技术的实际应用具有指导性作用。     

冲击加载下铝中氦泡和孔洞的塑性变形特征研究

通过分子动力学模拟研究了在相同冲击加载强度下单晶铝中氦泡和孔洞的塑性变形特征,结果发现氦泡和孔洞的塌缩是由发射剪切型位错环引起的,而没有观测到棱锥型位错环发射. 氦泡和孔洞周围的位错优先成核位置基本一致,但是氦泡周围发射的位错环数目比孔洞多,位错环发射速度明显比孔洞快. 且氦泡和孔洞被冲击波先扫过部分比后扫过部分发射位错困难. 通过滑移面上的分解应力分析发现,氦泡和孔洞周围塑性特征的差别是由于氦泡内压引起最大分解应力分布改变造成的. 氦泡和孔洞被冲击波先后扫过部分塑性不对称是因为冲击波扫过时引起形状变化, 关键词： 分子动力学 冲击波 氦泡 孔洞     

异型磁体/铁电复合材料中的电致变磁导及电致变阻抗效应

利用应力变磁导与电致伸缩效应的乘积效应,设计和制备了一种新型的异型磁体/铁电复合材料,并研究了该材料的电磁耦合特性.在5 kV/cm的恒定电场下,观察到接近40%的电致变磁导（电感）及相应的电致变阻抗效应存在于一个较宽的频域（50—1×10⁵Hz）上.该电致变磁导（电感）及电致变阻抗显示了铁磁弛豫和铁电弛豫的联合效应.研究认为,该异型复合材料所显示的所有电磁耦合行为均可归因于电场诱导的材料中磁体内部应力场的变化. 关键词： 电磁耦合 异型复合材料 应力变磁导 电致伸缩     

Tensile properties and microstructure of 2024 aluminum alloy subjected to the high magnetic field and external stress

In order to explore the dependence of plasticity of metallic material on a high magnetic held,the effects of the different magnetic induction intensities(H = 0 T,0.5 T,1 T,3 T,and 5 T) and pulses number(N = 0,10,20,30,40,and 50) on tensile strength(σ_b) and elongation(δ) of 2024 aluminum alloy are investigated in the synchronous presences of a high magnetic held and external stress.The results show that the magnetic held exerts apparent and positive effects on the tensile properties of the alloy.Especially under the optimized condition of H~*=1 T and N~*=30,the σ_b and 8 are 410 MPa and 17% that are enhanced by 9.3% and 30.8% respectively in comparison to those of the untreated sample.The synchronous increases of tensile properties are attributed to the magneto-plasticity effect on a quantum scale.That is,the magnetic held will accelerate the state conversion of radical pair generated between the dislocation and obstacles from singlet to the triplet state.The bonding energy between them is meanwhile lowered and the moving flexibility of dislocations will be enhanced.At H~* = 1 T and N~* = 30,the dislocation density is enhanced by 1.28 times.The relevant minimum grain size is 266.1 nm,which is reduced by 35.2%.The grain rehning is attributed to the dislocation accumulation and subsequent dynamic recrystallization.The(211) and(220) peak intensities are weakened.It is deduced that together with the recrystallization,the hne grains will transfer towards the slip plane and contribute to the slipping deformation.     

Low field RYDMR: effects of orthogonal static and oscillating magnetic fields on radical recombination reactions

Reactions involving spin correlated radical pairs as intermediates are known to be sensitive to applied static and/or oscillating magnetic fields. In the reaction yield detected magnetic resonance (RYDMR) technique, an electromagnetic field in resonance with the electron Zeeman splitting produced by a strong static field is used to perturb the singlet ? triplet interconversion of the radical pair and so to affect the yield of geminate recombination. New experiments are described in which weak radiofrequency fields (? 300μT) in the frequency range 1–80 MHz are applied to radical ion pairs derived from pyrene and 1,3-dicyanobenzene, in the presence of a weak (? 3.0 mT) static magnetic field. Such experiments test the viability of RYDMR in low fields, provide insight into the crossover region between the zero-field and high field cases, and may give information on the distribution of radical pair lifetimes.     

Novel intramolecular electron transfer in axial bis(terpyridoxy)phosphorus(V) porphyrin studied by time-resolved EPR spectroscopy

Laser flash-induced spin-polarized transient electron paramagnetic resonance (TREPR) spectra for bis(terpyridoxy)phosphorus(V) porphyrin in a nematic liquid crystal isotropic and in frozen solution are presented. At room temperature, two sequential spin-polarized TREPR spectra are observed. The first is consistent with the triplet state of a radical pair, while the later is assigned to the triplet state of the porphyrin formed by charge recombination. On the basis of the spectroscopic and redox properties of the terpyridine and porphyrin moieties it is proposed that electron transfer from the terpyridine to the excited phosphorus(V) porphyrin occurs. The lifetime of the radical pair is estimated to be of about 175 ns. At low temperature, the radical pair spectrumis no longer observed and the spin polarization pattern of the porphyrin triplet is dramatically different. This behavior is explained by postulating that the electron transfer is inhibited at low temperature because molecular motion is required to stabilize the radical pair. It is proposed that in the absence of this stabilization, the porphyrin triplet state is populated via spin-orbit coupling-mediated intersystem crossing from the excited singlet state.     

Photocleavage reaction of bromine substituted aromatic acyl compounds studied by CIDEP and transient absorption spectroscopy

The photocleavage of the CBr bond in bromoacetylnaphthalene is investigated by transient absorption and time resolved EPR spectroscopy. In the transient absorption of 2-bromo-2′-acetylnaphthalene, the absorption band observed at λ_max ~440 nm is assigned to the triplet state of the parent molecule. After decay of the triplet absorption, a long lived absorption band is observed at λ_max ~380 nm, which is assigned to naphthoylmethyl radical. The yield of this radical is not dependent on the concentration of oxygen even though the absorption band of the triplet state was quenched by addition of oxygen. Thus we conclude that the spin multiplicity of the precursor molecule is singlet. The CW time resolved EPR spectrum shows a typical E?/A CIDEP pattern of three hyperfine lines of the naphthoylmethyl radical. This result suggests some contribution from triplet precursor molecules. However, a careful analysis of the time profile of the CIDEP intensity observed by FT-EPR revealed that the polarization is generated from the radical pair mechanism (RPM) from the encountered pair of two free naphthoylmethyl radicals and the radical-triplet pair mechanism. RPM polarization by the geminate radical pair, formed by the Br atom and the naphthoylmethyl radical, is not observed. This fact indicates that large spin-orbit coupling (Δg and/or fast spin relaxation by g anisotropy) spoils the RPM polarization. The finding is in contrast to the recent observation of RPM polarization in the Cl cleavage reaction of 1-(chloromethyl)naphthalene.     

Superconductivity of compressed platinum powder at very low temperatures

Superconductivity of compressed, high-purity platinum powder (average grain size 2–3 μm) was found by measurements of resistivity, AC susceptibility and magnetization. The transition temperature into the superconducting state T_c and the critical magnetic field B_c strongly depend on the packing fraction f of the samples: we found 0.62T_c(0)1.38 mK and 6.6B_c(0)67 μT for 0.8f0.5, respectively. The temperature dependence of the critical magnetic fields can be described by B_c(T)=B_c(0)(1−(T/T_c)²). The discussion of these results includes possible explanations for the origin of superconductivity in this new superconducting material.     

Electric stimulation of magnetoplasticity and magnetic hardening in crystals

The effect of electric field E on the magnetoplastic effect (MPE) has been investigated in NaCl crystals with different impurities, which provide either the plasticization of the samples in the magnetic field (positive MPE) or their magnetic hardening (negative MPE). The mobility of individual dislocations under the joint action of the magnetic and electric fields and the mechanical load on the crystals has been studied. The sharp electric stimulation of the MPE of both signs has been revealed, i.e., an increase or a decrease in the mean free path of dislocations that is roughly proportional to exp(±E/E ₀) at E ? E ₀ ～ 1–10 kV/m. In particular, in the negative-MPE NaCl(Pb) crystals, the accompanying electric field enhances the magnetic suppression of plasticity. The results are attributed to the electrically induced transformation of the additional part of the pinning impurity ions Me⁺⁺ to the magnetically active state of Me⁺ on the dislocations. The subsequent magnetic transformation of the structure of these pinning centers should lead to a sharper variation of the dislocation pinning force (either an increase or a decrease, depending on the MPE sign).     

EPR studies of photoinduced electron transfer in triad model compounds of photosynthesis

Time-resolved EPR spectra are reported for porphyrin-quinone-quinone and porphyrin-porphyrin-quinone triads obtained after photoexcitation in the nematic and soft glass phase of liquid crystals. Spin-polarized EPR spectra were observed for the triplet states of the porphyrin created by spin-selective intersystem crossing (ISC) from the excited singlet state and those of the charge-separated radical pair states (RP) generated by electron transfer (ET) processes. The EPR polarization patterns of the RP are discussed in terms of the favored decay channel of the photoexcited singlet state of the porphyrin donor. The decay pathway may either be singlet ET to the quinone(s) followed by singlet/triplet mixing to yield RPs with triplet character or triplet ET after ISC from the porphyrin singlet to the triplet state, or a superposition of both pathways. It is demonstrated that the nature of the linking bridge between donor and acceptor, i.e., aliphatic cyclohexylene or aromatic phenylene, significantly influences the ET mechanism and thus the polarization patterns of the RP spectra. Using liquid crystals, information about the orientation of the guest molecules in the liquid crystal matrix with respect to the long axes of the liquid crystal molecules can be obtained. In the porphyrin-porphyrin-quinone triads the energy and ET processes strongly depend on the type of metallation of the porphyrins, specifically, whether the distal, the vicinal or both porphyrins bear a zinc atom.     

Observation and reaction control of a singlet born radical pair formed photochemically in an SDS micelle by pulsed microwave irradiation

The photochemical reaction of tetraphenylhydrazine in an SDS micellar solution is studied using a transient absorption detected magnetic resonance (ADMR) method. This system is photo-dissociated via the singlet excited state and forms a transient radical pair. Strong microwave irradiation of this system under an × band EPR magnetic field provides an ADMR spectrum of the singlet born radical pair as the optical absorbance change of the diphenylaminyl radical. The employment of a short duration microwave pulse that efficiently flips the electron spin quantum (Δm _s = 1) controls the reaction of the radical pair, and the reaction rate constant of this system is determined. Furthermore, changing the microwave duration shows the quantum beat of this system induced by the electromagnetic field. This beat signal carries both a single beat that corresponds to the Rabi frequency and a double-frequency beat that is due to simultaneous two-spin controlling.