强流脉冲电子束辐照诱发纯钼表面的损伤效应及结构缺陷

利用强流脉冲电子束（HCPEB）装置对纯钼表面进行辐照处理,并利用X射线衍射仪,扫描电子显微镜（SEM）、透射电子显微镜（TEM）详细分析了辐照表面的微观结构和损伤效应. 1次HCPEB辐照后,纯钼表层积聚了极大的残余应力,多次辐照后表面未融化区域出现大量绝热剪切带,且局部区域发生开裂. 微观结构分析显示,辐照后材料表面形成发散状的位错组态和大量空位簇缺陷;绝热剪切带内部是尺寸为1 μm 左右等轴状的再结晶晶粒. 剪切带造成的材料表面局部软化以及间隙原子偏聚于晶界是材料发生开裂的主要原因. 另外,表面熔化区域可形成尺寸为20 nm左右的纳米晶. 关键词： 强流脉冲电子束 纯钼 绝热剪切带 空位簇缺陷     

Comparison of shear banding in BMGs due to thermal-softening and free volume creation

This paper reports a comparative study of shear banding in BMGs resulting from thermal softening and free volume creation. Firstly, the effects of thermal softening and free volume creation on shear instability are discussed. It is known that thermal softening governs thermal shear banding, hence it is essentially energy related. However, compound free volume creation is the key factor to the other instability, though void-induced softening seems to be the counterpart of thermal softening. So, the driving force for shear instability owing to free volume creation is very different from the thermally assisted one. In particular, long wave perturbations are always unstable owing to compound free volume creation. Therefore, the shear instability resulting from coupled compound free volume creation and thermal softening may start more like that due to free volume creation. Also, the compound free volume creation implies a specific and intrinsic characteristic growth time of shear instability. Finally, the mature shear band width is governed by the corresponding diffusions (thermal or void diffusion) within the band. As a rough guide, the dimensionless numbers: Thermal softening related number B, Deborah number (denoting the relation of instability growth rate owing to compound free volume and loading time) and Lewis number (denoting the competition of different diffusions) show us their relative importance of thermal softening and free volume creation in shear banding. All these results are of particular significance in understanding the mechanism of shear banding in bulk metallic glasses (BMGs). Supported by the Chinese Academy of Sciences under the project “Multi-Scale Complex System” (Grant No. KJCX-SW-L08), the National Natural Science Foundation of China (Grant Nos. 10725211 and 10721202), and the Doctorial Start-up Fund of Hunan University of Science and Technology (Grant No. E50840)     

Adiabatic shear failure and dynamic stored energy of cold work

Dynamic testing of statically predeformed specimens of magnesium (AM50) shows that the failure strain increases with the level of prestrain. Interrupted and other dynamic tests show that the temperature rise prior to localization has only a minor influence on adiabatic shear band (ASB) formation. For all the tests, the dynamic deformation energy until ASB formation is found to be relatively constant, indicating that ASB is dependent almost solely on dynamic deformation processes, with quasistatic and thermal effects prior to localization being very marginal. We suggest the concept of a constant dynamic mechanical energy (toughness) as a quantitative criterion for ASB formation--this concept being related physically to the dynamic stored energy of cold work. All in all, the tests indicate that ASB failure is more dependent on energy considerations that on strain criteria, as has been considered until now.     

平头弹丸正撞击下延性金属靶板的破坏模式

 对延性金属靶板在刚性平头弹丸正撞击下的破坏模式进行了研究。一般而言,延性金属靶板的破坏模式可以分为两种：带有总体变形的局部简单剪切破坏和局部化的绝热剪切冲塞破坏。首先基于Bai-Johnson热塑性本构关系建立了一个局部化的绝热剪切冲塞模型,然后结合描述带有总体变形的局部简单剪切破坏的Wen-Jones模型,找出了两种模型之间转化的临界条件。理论预测与文献中的实验结果吻合得很好。     

Extraordinary plasticity of ductile bulk metallic glasses

Shear bands generally initiate strain softening and result in low ductility of metallic glasses. In this Letter, we report high-resolution electron microscope observations of shear bands in a ductile metallic glass. Strain softening caused by localized shearing was found to be effectively prevented by nanocrystallization that is in situ produced by plastic flow within the shear bands, leading to large plasticity and strain hardening. These atomic-scale observations not only well explain the extraordinary plasticity that was recently observed in some bulk metallic glasses, but also reveal a novel deformation mechanism that can effectively improve the ductility of monolithic metallic glasses.     

Multiscale structural relaxation and adiabatic shear failure mechanisms

Stages of plastic shear instability and adiabatic shear failure are associated with the initiation of collective modes of defect ensembles that have a self-similar nature of autosoliton and "blow-up" structures. Experimental and structural studies have confirmed simulation results for the stages of plastic strain localization and adiabatic shear failure based on the constitutive equations that relate structural relaxation and failure mechanisms to the formation of multiscale collective modes in defect ensembles.     

Theory of the elastic anomalies in the invar alloy Fe3Pt

A quantitative theory of the elastic anomalies in the ordered Invar alloy Fe₃Pt is presented within the Stoner theory of band magnetism. It is shown that the instability of the magnetic moment on the iron atoms in the fcc lattice is directly responsible for the very marked softening of the shear elastic constants in addition to the observed anomalous volume expansion and forced volume magnetostriction. This is contrasted with the stable behaviour of the magnetic moment on an iron atom in the bcc lattice.     

TC4钛合金及40Cr钢破片中绝热剪切带的TEM分析

 对含有约热剪切带（ASB）的TC4钛合金及40Cr钢破片作了扫描电镜（SEM）和透射电镜（TEM）分析。结果表明，TC4钛合40Cr钢中的ASB是由平均直径不到0.3 μm的微晶组成，微晶列有一定择优取向，电子衍射花样表现为不连续的环状。ASB内位错密度均很低，其邻近基体区含有高密度位错，ASB内没有察到相变发生。     

How does the structural inhomogeneity influence the shear band behaviours of metallic glasses

ABSTRACT

Here we propose a model to understand the influence of structural inhomogeneity on the shear band behaviours of metallic glasses. By considering the inhomogeneous structure and stress concentration, the model predicts that the strain for shear band nucleation in metallic glasses can be variable and far below the theoretical elastic limit. During sliding, the shear band will approach a dynamic equilibrium state of balanced free volume generation and annihilation. By considering the accumulation of irreversible structure change, the shear band will finally develop into fracture. Under fluctuating load, the shear band shows an ‘activate-arrest’ behaviour, which results from a delayed response to the external load change. These results reasonably explain and correlate the physics behind the elastic limit, stick-slip shear band behaviour, implicit shear events, and shear band fracture in metallic glasses. The study can provide another perspective and platform to understand the correlations between structural inhomogeneity and shear band behaviours in metallic glasses, and further explore other shear band related phenomena not only in metallic glasses but also in the class of shear-softened materials.     

Unusual phonon softening in delta-phase plutonium

The phonon density of states and adiabatic sound velocities were measured on fcc-stabilized 242Pu0.95Al0.05. The phonon frequencies and sound velocities decrease considerably (soften) with increasing temperature despite negligible thermal expansion. The frequency softening of the transverse branch along the [111] direction is anomalously large ( approximately 30%) and is very sensitive to alloy composition. The large magnitude of the phonon softening is not observed in any other fcc metals and may arise from an unusual temperature dependence of the electronic structure in this narrow 5f-band metal.     

On the modified grain-size-distribution method to evaluate the dynamic recrystallisation fraction in AISI 304 stainless steel

The purpose of the present work was to verify the grain size distribution (GSD) method, which was recently proposed by one of the present authors as a method for evaluating the fraction of dynamic recrystallisation (DRX) in a microalloyed medium carbon steel. To verify the GSD-method, we have selected a 304 stainless steel as a model system and have measured the evolution of the overall grain size distribution (including both the recrystallised and unrecrystallised grains) during hot compression at 1,000 °C in a Gleeble machine; the DRX fraction estimated using the GSD method is compared with the experimentally measured value via EBSD. The results show that the previous GSD method tends to overestimate the DRX fraction due to the utilisation of a plain lognormal distribution function (LDF). To overcome this shortcoming, we propose a modified GSD-method wherein an area-weighted LDF, in place of a plain LDF, is employed to model the evolution of GSD during hot deformation. Direct measurement of the DRX fraction using EBSD confirms that the modified GSD-method provides a reliable method for evaluating the DRX fraction from the experimentally measured GSDs. Reasonable agreement between the DRX fraction and softening fraction suggests that the Kocks-Mecking method utilising the Voce equation can be satisfactorily used to model the work hardening and dynamic recovery behaviour of steels during hot deformation.     

100 GPa以上冲击压力区蓝宝石的近红外发光

采用三层夹心靶结构,利用铜箔与抛光蓝宝石之间良好的接触条件,使用辐射式高温计观测到c取向蓝宝石在130～172 GPa冲击高压下的红外辐射。实验信号显示,位于近红外波段的蓝宝石冲击辐射随着压力的增加而变强;强度对比显示,蓝宝石的近红外发射强于界面发射。基于冲击作用下滑移带的温度高于冲击平衡温度的概念,将蓝宝石体内滑移带温度作为蓝宝石发射温度代入线性吸收公式,从而将蓝宝石发光强度的增加和界面冷却引起的发光强度下降相结合;将得到的界面一蓝宝石发射强度数值模拟结果与实验结果相比较,发现二者能够重合。由实验信号的上升趋势得到三个压力下0.85μm近红外波长处蓝宝石的冲击吸收系数数值。     

简支圆板在平头弹丸撞击下的变形与穿透

 采用准静态近似理论方法分析简支圆板在平头弹丸正撞下的变形和穿透。首先基于虚功原理得到了静态荷载位移关系,并将其用于预测低速冲击下圆板的能量吸收能力。当考虑了材料的应变率敏感性,并且不发生绝热剪切冲塞破坏的前提下,理论预测和实验结果吻合良好。     

Ideal shear banding in metallic glass

As the most fundamental deformation mechanism in metallic glasses (MGs), the shear banding has attracted a lot of attention and interest over the years. However, the intrinsic properties of the shear band are affected and even substantially changed by the influence of non-rigid testing machine that cannot be completely removed in real compression tests. In particular, the duration of the shear banding event is prolonged due to the recovery of the stressed compliant frame of testing machine and therefore the temperature rise at the operating shear band is, more or less, underestimated in previous literatures. In this study, we propose a model for the ‘ideal’ shear banding in metallic glass. The compliance of the testing machine is eliminated, and the intrinsic shear banding process is extracted and investigated. Two important physical parameters, the sliding speed and the temperature of shear band, are calculated and analysed on the basis of the thermo-mechanical coupling. Strain-rate hardening is proposed to compensate thermal softening and stabilise the shear band. The maximum value of the sliding speed is found to be on the order of 10 m/s at least, and the critical temperature at which strain-rate hardening begins to take effect should reach as high as 0.9T_g (T_g is the glass transition temperature) for a stable shear banding event in metallic glass according to the early experimental data. This model can help to understand and control the shear banding and therefore the deformation in MGs.     

Electronic structure and bonding in metal hydrides,studied with photoelectron spectroscopy

We present photoemission spectra of various binary and ternary metal hydrides and of some intermetallic compounds. An analysis of these data with respect to the known heats of hydrogen solution in the metals demonstrates two important properties of the metal-hydrogen bond: First we find that core level, shifts in ternary systems are not simply related to those in binary ones. In contrast to a frequently used assumption, metal-hydrogen interaction in a ternary hydride cannot be a pair interaction between the atomic constituents. Secondly, we find from our studies of the valence band spectra of some intermetallic compounds an inverse correlation between the heat of hydrogen solution and the density of states at the Fermi level.We analyse the core level shift data from binary hydrides using the experimental heats of hydrogen solution. We find a very good agreement between calculated and measured core level shifts in transition metal hydrides. However, in rare earth hydrides our approach fails. The reason for this behaviour originates in the photoemission process itself. A thermochemical interpretation of core level shifts can only be successful in the adiabatic limit of core excitation. The systematic behaviour of our results can be explained, if core excitation is considered to be adiabatic in transition metal hydrides but sudden in the rare earth hydrides. We also discuss the impact of such an interpretation on the concepts of adiabatic and sudden core excitation in metals.     

Transition metal surface kink electronic structure

In the tight binding approximation applied to a non-degenerate band, we show that the kink local density of states does neither depend on the surface nor on the step type, but only on the bulk crystal structure. This result is not valid for the kink local density of states of each atomic orbital of a degenerate band like the transition metal d band. Nevertheless, the total local densities of states for different kinks are very close to each other and also only depend on the crystal structure.     

On the generation of nanograins in pure copper through uniaxial single compression

Attempts at generating nanograins through uniaxial single compression were made by deforming copper samples at 298 K and 77 K. At 298 K, dynamically-deformed samples (DDS) become softer, in contrast to quasi-statically deformed samples (QDS), which show a hardness close to the saturation value. The microstructure of DDS is characterised by deformation twins and equiaxed micron-sized grains, and the observed softening is due to the occurrence of recrystallisation (RX). At a reduced temperature of 77 K, nanograins are generated in DDS, whereas QDS show forest dislocations and twins. The generation of nanograins, which evolve through rotational DRX, is associated with the formation of shear bands with an amorphous structure. Compared with twinning, it appears that amorphisation plays a more pronounced role in high strain rate deformation at reduced temperatures (77 K). The hardness of DDS, obtained from compression at 77 K, exceeds the saturation value by 16%, whereas that of QDS corresponds approximately to saturation.     

Coherent population transfer in an atom by multiphoton adiabatic rapid passage

Coherent population transfer in an atom using a sequence of adiabatic rapid passages through single-photon resonances is well-known, but it requires that the frequency sweep match the changing frequencies of the atomic transitions. The same population transfer can be effected via a single multiphoton adiabatic rapid passage, which requires only a small frequency sweep, if it is possible to select the desired multiphoton transition from the many possible transitions. Here we report the observation of population transfer between Rydberg states by high order multiphoton adiabatic rapid passage.     

Strain incompatibility and its influence on grain coarsening during cyclic deformation of ARB copper

It is well known that the characteristic length scale in ultra-fine grained and nanocrystalline metals has a significant effect on the mechanical behaviour. The inhibited ability to accommodate imposed strain with conventional dislocation mechanism has led to the activation of unconventional deformation mechanisms. For one, grain coarsening at shear bands has been observed to occur within metals with sub-micron grain size upon cyclic deformation. Such grain coarsening is often linked to the observed cyclic softening behaviour. The purpose of this study was to investigate the relationship between strain localisation associated with shear banding and the observed deformation-induced grain coarsening in ultra-fine grained metals. The investigation was carried out using ultra-fine grained, oxygen-free high conductivity copper processed by accumulative roll-bonding. A close relationship between strain localisation and deformation-induced grain coarsening was revealed. As strain localisation is not only found at shear bands, but also at other places whereby heterogeneous microstructure or geometric discontinuity is present, hence the present study bears a general significance. Such strain localisation sites may also include a hard constituent embedded in a relatively ductile matrix, micro-crack tips and artificial notches. The stress concentration at these sites provides a high input of strain energy for grain boundary motion leading to grain coarsening. Furthermore, when the grain size is very small, the stress gradient leading away from the stress concentration sites is also believed to increase the driving force for grain boundary migration within the affected regions.     

Stability analysis of a bidirectional homogeneously broadened ring laser

We study the stability of steady state solutions of a resonantly tuned bidirectional homogeneously broadened ring laser retaining the complete longitudinal spatial dependence. We prove that in two limits, the stable steady states are only weakly stable since the relaxation rates are very large. This implies a high sensitivity to any kind of perturbation which will easily induce a transition from one direction of propagation to the other one. We also analyze a popular approximation which uses a truncated harmonic expansion and the adiabatic elimination of the polarization. When the adiabatic elimination is justified, we show that the harmonic truncation does not further limit the range of applicability of the stability analysis.