Structure features of bulk submicrocrystalline aluminum alloys at high-rate deformation

The structure formation in commercial Al alloys subjected to severe deformation by dynamic channel-angular pressing has been experimentally investigated. The dependences of the structural characteristics and hardness on the deformation rate and scheme are analyzed. The optimal deformation modes for the formation of submicrocrystalline materials with high hardness and strength are found.     

Internally resonating lattices for bandgap generation and low-frequency vibration control

The paper reports on a structural concept for high stiffness and high damping performance. A stiff external frame and an internal resonating lattice are combined in a beam-like assembly which is characterized by high frequency bandgaps and tuned vibration attenuation at low frequencies. The resonating lattice consists of an elastomeric material arranged according to a chiral topology which is designed to resonate at selected frequencies. The concept achieves high damping performance by combining the frequency-selective properties of internally resonating structures, with the energy dissipation characteristics of their constituent material. The flexible ligaments, the circular nodes and the non-central interactions of the chiral topology lead to dynamic deformation patterns which are beneficial to energy dissipation. Furthermore, tuning and grading of the elements of the lattice allows for tailoring of the resonating properties so that vibration attenuation is obtained over desired frequency ranges. Numerical and experimental results demonstrate the tuning flexibility of this concept and suggest its potential application for load-carrying structural members parts of vibration and shock prone systems.     

Dissipative processes under the shock compression of glass

New experimental data on the behavior of the K8 and TF1 glasses under shock-wave loading conditions are obtained. It is found that the propagation of shock waves is close to the self-similar one in the maximum compression stress range 4-12 GPa. Deviations from a general deformation diagram, which are related to viscous dissipation, take place when the final state of compression is approached. The parameter region in which failure waves form in glass is found not to be limited to the elastic compression stress range, as was thought earlier. The failure front velocity increases with the shock compression stress. Outside the region covered by a failure wave, the glasses demonstrate a high tensile dynamic strength (6-7 GPa) in the case of elastic compression, and this strength is still very high after transition through the elastic limit in a compression wave.     

Nonlocal superelastic model of size-dependent hardening and dissipation in single crystal Cu-Al-Ni shape memory alloys

We propose a nonlocal continuum model to describe the size-dependent superelastic effect observed in recent experiments of single crystal Cu-Al-Ni shape memory alloys. The model introduces two length scales, one in the free energy and one in the dissipation, which account for the size-dependent hardening and dissipation in the loading and unloading response of micro- and nanopillars subject to compression tests. The information provided by the model suggests that the size dependence observed in the dissipation is likely to be associated with a nonuniform evolution of the distribution of the austenitic and martensitic phases during the loading cycle.     

A study of the dynamic impact behaviour of IN 718 and ATI 718Plus® superalloys

The dynamic impact response of IN 718 and ATI 718Plus®, in both the solution heat treated and age-hardened conditions, were investigated at different deformation temperatures and strain rates using a direct impact Hopkinson pressure bar. Analyses of the results provide a vital but previously not reported information that the ATI 718Plus® offers a higher resistance to damage during high strain rate ballistic impact deformation compared to the most widely used Iron-nickel based superalloy, Inconel 718. ATI 718Plus® showed higher strain hardening and strain rate sensitivity, in both heat treatment conditions, than IN 718. The difference in the deformation behaviour of both alloys, in the annealed condition, is attributable to the compositional modification in ATI 718Plus® which has been reported to lower its stacking fault energy and increases the tendency for deformation twinning. However, in the age-hardened condition, the difference is believed to be related to the disparity in the operative strengthening mechanism, of the precipitates present in both alloys. Furthermore, a higher susceptibility to strain location and the formation of adiabatic shear band, in aged IN 718, is attributable to the stronger temperature-softening characteristics observed in the alloy and to the limited strain hardening tendency under dynamic impact loading.     

Fatigue strength of structural materials at sonic and ultrasonic loading frequencies

Experimental data on the influence of sonic and ultrasonic frequency loading on the fatigue strength of steels, and titanium, aluminium, and nickel-based alloys tested with longitudinal and transverse vibrations at room temperatures are discussed. Results of fatigue tests in liquid nitrogen at low (16 Hz) and high (3 kHz) loading frequencies are also given for a number of materials. The influence of the loading-cycle asymmetry on fatigue strength is studied for structural materials tested at 10 kHz frequency loading with a mean tensile and compressive stress. Limiting amplitude curves are plotted. Measurements of the energy dissipation in materials were carried out during fatigue tests with symmetrical and asymmetrical loading cycles at high-frequency with large amplitude longitudinal vibrations of the specimen. Measurements of the amplitude dependency of the energy dissipation and dependency of the energy dissipation obtained during continuous loading by fatigue tests were also made.     

Mössbauer study of the polymorphism in iron and iron-nickel alloys under deformation and high pressure

Structural and phase transitions in iron and binary iron-nickel alloys containing 6–32 at % nickel have been investigated by Mössbauer spectroscopy in situ under quasi-hydrostatic compression and compression shear. It is shown that the deformation decreases the onset pressure of the α-phase transition to the high-pressure ∈ and γ phases. Shear changes the transformation mechanism due to the formation of a large number of structural defects; it is an independent parameter of the phase transition kinetics. Compression shear leads to an irregular change in the hyperfine parameters of the initial and newly formed phases, which is due to the stress relaxation as in the trip effect.     

Comparing specific features of the structural formation of aluminum alloys during severe and intense plastic deformation

We compare the deformation behavior and specific features of the structural formation of two aluminum alloys, AMTs and V95, upon megaplastic (MPD) and severe plastic (SPD) deformation. It is established that upon SPD by dynamic channel angular pressing, a submicron crystalline structure is formed with grain sizes of 200 to 600 nm; and that upon MPD by high quasi-hydrostatic pressure shearing, a nanostructure is formed with grain sizes of 55 to 100 nm. The sequence of the phase and structure transition is established for an increase in the rate of deformation and velocity of the materials. Mechanisms of elastic energy relaxation are determined as a function of the extent of dislocation mobility.     

层错四面体对单晶铜层裂行为影响的分子动力学研究

层错四面体是一种典型的三维空位型缺陷,广泛存在于受辐照后的面心立方金属材料中,对材料的力学性能有显著的影响.目前,关于层错四面体对辐照材料层裂行为的影响还缺乏深入系统的研究.本文使用分子动力学方法模拟了含有层错四面体的单晶铜在不同冲击速度下的层裂行为,对整个冲击过程中的自由表面速度及微结构演化等进行了深入的分析.研究发现,层错四面体在冲击波作用下会发生坍塌,并进一步诱导材料产生位错、层错等缺陷.在中低速度加载下,层错四面体坍塌引起的缺陷快速向周围扩展,为孔洞提供了更宽的形核区域,促进了孔洞的异质成核,造成材料层裂强度大幅度减小.当冲击速度较高时,层错四面体坍塌导致的局部缺陷对材料的层裂强度不再有明显影响.     

Investigation on ultrasonic volume effects: Stress superposition,acoustic softening and dynamic impact

Conventional high power ultrasonic vibration has been widely used to improve manufacturing processes like surface treatment and metal forming. Ultrasonic vibration affects material properties, leading to a flow stress reduction, which is called ultrasonic volume effect. The volume effect contains multi-mechanisms such as stress superposition due to oscillatory stress, acoustic softening by easier dislocation motion and dynamic impact leading to extra surface plastic deformation. However, most researches ignored the stress superposition for the convenience of measurement, and few studies considered ultrasonic dynamic impact since the relatively low ultrasonic energy in macro scale. The purpose of this study is to investigate the characteristics and mechanisms of different ultrasonic volume effects in micro-forming. A 60 kHz longitudinal ultrasonic-assisted compression test system was developed and a series of ultrasonic-assisted compression tests at different amplitudes on commercially pure aluminum A1100 in micro-scale were carried out combining the surface analysis by SEM, EDX and micro-hardness test. Three different ultrasonic volume effects, stress superposition, acoustic softening and dynamic impact, were confirmed in the ultrasonic-assisted compression tests. In order to quantitatively predict stress superposition, a hybrid model for stress superposition is developed considering the elastic deformation of experimental apparatus in practice, the evolution of the modeling results fitted well with the experimental results. With low ultrasonic amplitude, stress superposition and acoustic softening occurred because vibrated punch contacted with the specimen all the time during compression. However, with higher amplitude, due to the extra surface plastic deformation by larger ultrasonic energy, forming stress was further reduced by the ultrasonic dynamic impact. A possible method to distinguish the effects of dynamic impact and acoustic softening is to analyze the waveform of the oscillatory stress in the process. In the case of ultrasonic dynamic impact effect, a higher amount of oxidation was observed on the specimen surface, which could be the result of local heating by surface plastic deformation and surface friction when the vibrated punch detached from the specimen. The findings of this study provide an instructive understanding of the underlying mechanisms of volume effects in ultrasonic-assisted micro-forming.     

内凹负泊松比蜂窝结构的面内双轴冲击响应

利用有限元模拟方法研究了内凹负泊松比蜂窝结构的面内双轴冲击响应。用节点扰动方法建立了具有不同规则度的内凹负泊松比蜂窝结构,并将其在不同冲击速度下的变形模态、应力-应变曲线和能量耗散能力与规则蜂窝进行了对比分析。结果表明,冲击速度是内凹蜂窝结构变形模态最主要的影响因素。此外,在双轴冲击下,由于不规则度的引入,延长了应力-应变曲线的平台阶段,抑制了结构的各向异性程度,从而使结构的变形特征从局部密实转向整体密实。在能量吸收能力方面,结构的不规则性导致了密实化阶段的滞后,因此在相同的压缩程度下,其塑性耗散能低于规则模型。     

Grain refinement in coarse-grained 7475 Al alloy during severe hot forging

Grain refinement taking place in a coarse-grained 7475 Al alloy was studied in multidirectional compression at 490°C and at a strain rate of 3?×?10^?4?s^?1. The integrated flow curve displays significant work softening just after yielding and an apparent steady-state plastic flow at high strains. The structural changes are characterized by the development of deformation or microshear bands in coarse-grain interiors, followed by homogeneous evolution of new grains at high strains. The new grains are considered to be developed by a kind of continuous reaction through grain fragmentation that is similar to continuous dynamic recrystallization (cDRX). The mechanism of fine grain production and the factors controlling grain refinement during hot multidirectional deformation are discussed in detail.     

金属蜂窝夹芯结构抗水下冲击性能

为揭示高强度水下爆炸冲击载荷作用下金属夹芯结构的抗冲击性能,在实验室开展小尺寸水下爆炸加载技术对金属蜂窝夹芯结构性能影响的实验研究。基于实验结果,开展了全尺寸数值模拟金属蜂窝夹芯结构在水下冲击载荷作用下的动态响应和抗冲击性能研究。结果表明,数值模拟、实验和理论模型计算的结果具有良好的一致性。由于蜂窝芯材相对密度对夹芯结构能量耗散方式和载荷传递机制的影响,结构动态响应、失效模式以及抗冲击性能随着冲击强度的变化表现出较为明显的不同。通过抗冲击参数分析,建立了反映金属蜂窝夹芯结构抗冲击性能的结构横向变形、固支反力、透射脉冲和塑性能耗随冲击强度和芯材相对密度变化的结构-载荷-性能量化关系。     

差动柔性绳传动惯性稳定平台载荷分析与结构校核

针对一种新型柔索传动差动结构稳定平台结构设计问题,开展平台载荷分析和框架结构强度以及稳定性校核研究。根据平台的实际工况和载荷特点,将载荷划分为惯性载荷、质心偏移加速度载荷、摩擦载荷和环境载荷,分类计算并进行载荷综合和功率估算;以回转架结构为例进行结构静动态校核,根据载荷分析结果,对框架结构进行有限元分析,校核结果显示回转架具有良好的静动态特性:平台框架调转及随载体机动过程中的变形在系统允许的范围之内,满足设计要求;框架结构一阶固有频率为156.04 Hz,远高于伺服系统工作带宽,不会引起谐振,具有较高的结构动态稳定性。     

Realistic problems in the physics of plasticity and hardness for single crystals and polycrystalline materials

The basic results from investigations of certain real problems in the physics of plasticity for single crystals and polycrystalline metal alloys carried out under the direction of the authors are given. The microdeformation patterns and formation of the flow limit in polycrystalline material are treated; the features of the mechanisms of deformation, deformational hardening, and the defect substructure in high-strength metal alloys are characterized. Analyses are carried out for phenomena involving activation of grain boundaries by grain boundary flows of impurity atoms, and experimentally based features of deformation on different structural levels under active extension, creep, and sign-alternating loading conditions. The main attention is given to the development of collective deformation modes. A discussion of some structural aspects of the realization of meso-level plastic flow with different deformation conditions is presented. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 5–15, August, 1998.     

Role of external impacts in nanostructured titanium alloys

Specifics of the effects of electroplastic deformation, ion implantation (II), and ultrasonic treatment (UST) on the structure and characteristics of coarse-grained (CG) and ultrafine-grained (UFG) VT1-0, VT6 and TiNi titanium alloys are investigated. The introduction of pulse current during cold rolling promotes increased deformability and causes stress jumps during tension that result from phase transformations or the electroplastic effect (EPE). It is shown that EPE is a structurally sensitive property dependent on the size of grains. Methods of surface II and UST change the phase composition and lead to additional structural refinement in layers with a thickness of 0.1–10 microns.     

On the nature of ultrahigh plastic (Megaplastic) strain

The features of structural transformations occurring at ultrahigh plastic strains, which are proposed to refer to as gigascopic, have been analyzed. It is shown that, during such deformation, additional channels of elastic energy dissipation must be effectively implemented. It is concluded that structural changes are characterized by certain cyclicity. A specific route of structural rearrangements is determined by temperature, the height of the Peierls barrier for dislocations and their ability of diffusive rearranging, and the difference in the free energies of the crystalline and amorphous states.     

Influence of the temperature and structural state on the systematic features of plastic deformation in Mo-Re-based alloys

The salient aspects of the formation of a dislocation structure during plastic deformation of the alloys Mo-47 wt. % Re and Mo-47 wt. % Re-1 vol. % ZrO₂ at T=300 K in previously recrystallized and polygonalized structural states have been studied with an electron microscope. The studies revealed the systematic features of rotational modes of plastic deformation in these alloys under conditions of substructural hardening and high-temperature grain-boundary slip. An analysis is made of the effect that the substructure and highly dispersed particles of the oxide phase have on the plastic deformation and mechanical properties of Mo-Re-based alloys at T=300-1500 K.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 96–104, December, 1994.     

Modeling creep in L12-superstructure single crystals

Creep curves for compression and extension under constant loads and constant external stress are calculated using a model of the creep of L1₂-superstructure single crystals. It is shown that features of the creep in alloys with L1₂ superstructure can be explained via the superpositioning of normal and anomalous deformation mechanisms. It is established that during creep, the localization of deformation, which can lead to inverse creep, is important.     

Structural and phase transformations in ferromanganese alloys during deformation under pressure

Using optical metallographic, TEM, Mössbauer spectroscopy, and X-ray analysis the structural and phase transformations in Fe-(3–55) wt % Mn alloys during shear deformation under pressure were investigated. It is established that a large deformation under high pressure causes the formation of a nanocrystalline structure with grain sizes of 40–60 nm. Nanostructure increases the hysteresis of inverse (hcp-fcc) transformation and stabilizes the (hcp) ? phase in alloys containing more than 40 wt % Mn, up to normal conditions. The Fe-3 wt % Mn alloy after shear under pressure treatment became nanostructured, retaining the original bcc phase state.