Effect of temperature on the anisotropy of AZ31 magnesium alloy rolling sheet under high strain rate deformation

In order to investigate the effect of temperature on the anisotropic behaviour of AZ31 magnesium alloy rolling sheet under high strain rate deformation, the Split Hopkinson Pressure Bar was used to analyse the dynamic mechanical properties of AZ31 magnesium alloy rolling sheet in three directions, rolling direction(RD), transverse direction (TD) and normal direction (ND). The texture of the rolling sheet was characterised by X-ray analysis and the microstructure prior and after high strain rate deformation was observed by optical microscope (OM). The results demonstrated that AZ31magnesium alloy rolling sheet has strong initial {0?0?0?2} texture, which resulted at the obvious anisotropy in high strain rate deformation at 20 °C. The anisotropy reflected in stress–strain curve, yield stress, peak stress and microstructure. The anisotropy became much weaker when the deformation temperature increased up to 250 °C. Continuing to increase the deformation temperature to 350 °C the anisotropy of AZ31 rolling sheet essentially disappeared. The decreasing tendency of anisotropy with increasing temperature was due to the fact that when the deformation temperature increased, the critical resolved shear stress (CRSS) for pyramidal 〈c + a〉 slip, which was the predominant slip mechanism for ND, decreased close to that of twinning, which was the predominant deformation mechanism for RD and TD. The deformation mechanism at different directions and temperatures and the Schmid factor (SF) at different directions were discussed in the present paper.     

Effect of thermomechanical processing on the creep behaviour of Udimet alloy 188

Udimet alloy 188 was subjected to grain-boundary engineering involving thermomechanical processing in an attempt to improve the creep performance and determine the effects on creep deformation processes. The as-received sheet was cold-rolled to either 10, 25 or 35% reduction per pass followed by a solution treatment at 1191°C for 1 h plus air cooling. This sequence was repeated four times and the resultant microstructure and grain-boundary character distribution were described using electron backscatter diffraction. The fraction of general high-angle grain boundaries tended to increase with increased cold rolling. The 10 and 25% cold-rolled materials exhibited lower creep rates than the 35% cold-rolled material. The measured creep stress exponents and activation energies suggested that dislocation creep with lattice self-diffusion was dominant at 760°C for stresses ranging between 100 and 220 MPa. A transition in the creep exponent below the applied stresses of 100 MPa indicated that a different secondary creep mechanism was rate-controlling at low stresses. A significant amount of grain-boundary cracking was observed both on the surface and subsurface of deformed samples, but surface cracks were greater in number and size than those within the bulk. The cracking behaviour was similar in both vacuum and air environments, indicating that grain-boundary cracking was not caused by environment. To assess the mechanisms of crack nucleation, in situ scanning electron microscopy was performed during elevated-temperature (T ≤ 760°C) tensile-creep deformation. Sequential secondary electron imaging and electron backscatter diffraction orientation mapping were performed in situ to allow the evolution of crack nucleation and linkage to be followed. Cracking occurred preferentially along general high-angle grain boundaries and less than 15% of the cracks were found on low-angle grain boundaries and coincident site lattice boundaries. A fracture initiation parameter analysis was performed to identify the role of slip system interactions at the boundaries and their impact on crack nucleation. The parameter was successful in separating the population of intact and cracked general high-angle boundaries at lower levels of strain, but not after crack coalescence dominated the fracture process. The findings of this work have significant implications regarding grain-boundary engineering of this alloy and potentially for other alloy systems.     

Slip band formation and mobile dislocation density generation in high rate deformation of single fcc crystals

The mechanisms for the nucleation, thickening, and growth of crystallographic slip bands from the sub-nanoscale to the microscale are studied using three-dimensional dislocation dynamics. In the simulations, a single fcc crystal is strained along the [111] direction at three different high strain rates: 10⁴, 10⁵, and 10⁶?s^??1. Dislocation inertia and drag are included and the simulations were conducted with and without cross-slip. With cross-slip, slip bands form parallel to active (111) planes as a result of double cross-slip onto fresh glide planes within localized regions of the crystal. In this manner, fine nanoscale slip bands nucleate throughout the crystal, and, with further straining, build up to larger bands by a proposed self-replicating mechanism. It is shown that slip bands are regions of concentrated glide, high dislocation multiplication rates, and high dislocation velocities. Cross-slip increases in activity proportionally with the product of the total dislocation density and the square root of the applied stress. Effects of cross-slip on work hardening are attributed to the role of cross-slip on mobile dislocation generation, rather than slip band formation. A new dislocation density evolution law is presented for high rates, which introduces the mobile density, a state variable that is missing in most constitutive laws.     

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.     

基于MOSFET的高重复频率高压脉冲源设计

设计了一种基于功率金属氧化物半导体场效应晶体管(MOSFET)的高压脉冲电源。该发生器采用多只MOSFET的串联技术,形成高压、高重复频率开关组件。用高压开关组件开展脉冲发生器设计,搭建了一个15只1 kV的高速MOSFET串联的脉冲发生器实验装置,在500 Ω负载上获得前沿小于5 ns、幅度大于10 kV、脉宽约100 ns,瞬态频率达400 kHz的高压脉冲。设计的高压开关组件结构紧凑,可靠性高,可应用于多种脉冲发生器。     

Atomistic study of temperature and strain rate-dependent phase transformation behaviour of NiTi shape memory alloy under uniaxial compression

Molecular dynamics simulation was conducted to investigate the phase transformation behaviour of nickel–titanium (NiTi, 50%-50% at.%) nanopillar under uniaxial compression at loading rates varying from 3.30 × 10⁷ to 3.30 × 10⁹ s^?1 and at temperatures varying from 325 to 600 K. The phase transformation of NiTi was observed to be sensitive to loading rates and temperatures. The phase transformation stress of B2 → B19 increased with increasing temperature while it was insensitive to loading rate. The phase transformation stress of B19 → B19′ → BCO increased with increasing strain rate and decreasing temperature. In addition, reverse phase transformation was observed during compression due to the interaction between the phase transformation of B19 → B19′ → BCO and the deformation twinning/dislocation slide-induced plasticity of the BCO phase, leading to different residual crystal structures after loading. Moreover, a diagram for the phase transformation behaviour of NiTi in the simulated ranges of strain rate and temperature was obtained, from which the contrary experimental observations on the phase transformation behaviour of NiTi from the studies of Nemat-Nasser et al. (Mech. Mater. 37 (2005) p.287) and Liao et al. (J. Appl. Phys. 112 (2012) p.033515) at various strain rates could be well explained.     

Influence of the total gas flow rate on high rate growth microcrystalline silicon films and solar cells

This paper reports that high-rate-deposition of microcrystalline silicon solar cells was performed by very-high-frequency plasma-enhanced chemical vapor deposition. These solar cells, whose intrinsic μ c-Si:H layers were prepared by using a different total gas flow rate (F_total), behave much differently in performance, although their intrinsic layers have similar crystalline volume fraction, opto-electronic properties and a deposition rate of ～ 1.0~nm/s. The influence of F_total on the micro-structural properties was analyzed by Raman and Fourier transformed infrared measurements. The results showed that the vertical uniformity and the compact degree of μ c-Si:H thin films were improved with increasing F_total. The variation of the microstructure was regarded as the main reason for the difference of the J--V parameters. Combined with optical emission spectroscopy, we found that the gas temperature plays an important role in determining the microstructure of thin films. With F_total of 300~sccm, a conversion efficiency of 8.11% has been obtained for the intrinsic layer deposited at 8.5~\AA/s (1~\AA=0.1\,nm).     

A combination of a Drickamer anvil apparatus and monochromatic X‐rays for stress and strain measurements under high pressure

A modified Drickamer anvil apparatus has been developed to combine with monochromatic synchrotron radiation for high‐pressure X‐ray diffraction and radiography in the GSECARS bending‐magnet station, 13‐BM‐D, at the Advanced Photon Source, Argonne, USA. Using this experimental set‐up, deformation experiments can be carried out at pressures in excess of 30 GPa at high temperatures. Differential stresses and total axial strains of polycrystalline platinum and Mg₂SiO₄ ringwoodite have been measured up to 32 GPa at room temperature using tungsten carbide anvils. The total axial strain of the platinum increases with pressure and reaches about 55% at the highest pressure. A test run using a composite sintered diamond anvil system was performed. The use of X‐ray‐tranparent anvils enables the entire Debye rings to be observed up to 10° 2θ. With high‐energy photons (65–70 keV), this allows a coverage in Q (= 2π sinθ/λ) to about 3 Å^?1, thus making it possible to evaluate hydrostatic pressure and differential stress in crystalline minerals using diffraction. This, coupled with the ability to determine axial strain, allows deformation studies to be performed to pressures above 30 GPa.     

温度、应变率和空位缺陷对氮化硼纳米管压缩性能的影响

采用分子动力学方法,分别模拟了完好的和含有缺陷的氮化硼纳米管的轴向压缩过程。原子间的相互作用采用Tersoff多体势函数来描述。结果表明,同尺寸的锯齿型氮化硼纳米管的临界轴向压缩强度高于扶手型氮化硼纳米管,这与碳纳米管的研究结果一致。发现纳米管的压缩强度,如临界轴向内力在低温下受温度影响明显,并且和应变率的大小有关。然而,应变率对纳米管的弹性变形没有影响。另外,还发现空位缺陷降低了纳米管的力学性能。与完好的纳米管相比,含有缺陷的纳米管轴向压缩强度对于温度的影响并不敏感。     

Effect of elastic strain energy on grain growth and texture in AZ31 magnesium alloy by phase-field simulation

A phase-field model is modified to investigate the grain growth and texture evolution in AZ31 magnesium alloy during stressing at elevated temperatures. The order parameters are defined to represent a physical variable of grain orientation in terms of three angles in spatial coordinates so that the grain volume of different order parameters can be used to indicate the texture of the alloy. The stiffness tensors for different grains are different because of elastic anisotropy of the magnesium lattice. The tensor is defined by transforming the standard stiffness tensor according to the angle between the (0001) plane of a grain and the direction of applied stress. Therefore, different grains contribute to different amounts of work under applied stress. The simulation results are well-explained by using the limited experimental data available, and the texture results are in good agreement with the experimental observations. The simulation results reveal that the applied stress strongly influences AZ31 alloy grain growth and that the grain-growth rate increases with the applied stress increasing, particularly when the stress is less than 400 MPa. A parameter (△d) is introduced to characterize the degree of grain-size variation due to abnormal grain growth; the △d increases with applied stress increasing and becomes considerably large only when the stress is greater than 800 MPa. Moreover, the applied stress also results in an intensive texture of the 〈0001〉 axis parallel to the direction of compressive stress in AZ31 alloy after growing at elevated temperatures, only when the applied stress is greater than 500 MPa.     

Effect of high temperature annealing on strain and band gap of GaN nanoparticles

Black-coloured GaN nanoparticles with an average grain size of 50 nm have been obtained by annealing GaN nanoparticles under flowing nitrogen at 1200 oC for 30 min. XRD measurement result indicates an increase in the lattice parameter of the GaN nanoparticles annealed at 1200 oC, and HRTEM image shows that the increase cannot be ascribed to other ions in the interstitial positions. If the as-synthesised GaN nanoparticles at 950 oC are regarded as standard, the thermal expansion changes nonlinearly with temperature and is anisotropic; the expansion below 1000oC is smaller than that above 1000 oC. This study provides an experimental demonstration for selecting the proper annealing temperature of GaN. In addition, a large blueshift in optical bandgap of the annealed GaN nanoparticles at 1200 oC is observed, which can be ascribed to the dominant transitions from the C(Γ₇) with the peak energy at 3.532 eV.     

Effect of current density on the microstructure and corrosion behaviour of plasma electrolytic oxidation treated AM50 magnesium alloy

Plasma electrolytic oxidation (PEO) of an AM50 magnesium alloy was accomplished in a silicate-based electrolyte using a DC power source. Coatings were produced at three current densities, i.e. 15 mA cm⁻², 75 mA cm⁻², and 150 mA cm⁻² and were characterised for thickness, roughness, microstructural morphology, phase composition, and corrosion resistance. Even though the 15 min treated coatings produced at higher current density levels were thicker, they showed poor corrosion resistance when compared to that of the coatings obtained at 15 mA cm⁻². Short-term treatments (2 min and 5 min) at 150 mA cm⁻² yielded coatings of thickness and corrosion resistance comparable to that of the low current density coatings. The superior corrosion resistance of the low thickness coatings is attributed to the better pore morphology and compactness of the layer.     

Effect of high temperature band gap of GaN annealing on strain and nanoparticles

<正>Black-coloured GaN nanoparticles with an average grain size of 50 nm have been obtained by annealing GaN nanoparticles under flowing nitrogen at 1200℃for 30 min.XRD measurement result indicates an increase in the lattice parameter of the GaN nanoparticles annealed at 1200℃,and HRTEM image shows that the increase cannot be ascribed to other ions in the interstitial positions.If the as-synthesised GaN nanoparticles at 950℃are regarded as standard,the thermal expansion changes nonlinearly with temperature and is anisotropic;the expansion below 1000℃is smaller than that above 1000℃.This study provides an experimental demonstration for selecting the proper annealing temperature of GaN.In addition,a large blueshift in optical bandgap of the annealed GaN nanoparticles at 1200℃is observed,which can be ascribed to the dominant transitions from the C（Γ₇） with the peak energy at 3.532 eV.     

延性金属层裂强度对温度、晶粒尺寸和加载应变率的依赖特性及其物理建模

层裂强度表征了材料内部最大动态抗拉能力,并与材料本身的力学性质以及损伤早期演化相关.建立层裂强度计算的解析表达式,深入认识层裂强度所包含的微细观物理涵义,有利于更好地优化延性金属材料的层裂强度.目前大量的实验表明:延性金属材料的层裂强度对加载拉伸应变率、温度效应以及材料初始微细观结构具有很强的依赖关系.本文基于对孔洞成核与增长的损伤早期演化特性的分析,以及对温度效应和晶粒尺寸与材料本身力学性质之间关系的分析,给出了简单、实用的层裂强度的解析物理模型,物理模型的计算结果与典型延性金属高纯铝、铜和钽的层裂强度实验结果基本符合,从而验证了我们给出的层裂强度模型具有较好的适用性和预测性.     

High pressure torsion to refine grains in pure aluminum up to saturation: mechanisms of structure evolution and their dependence on strain

High-pressure torsion was used for the deformation processing of high-purity aluminum (4N-Al), while high-resolution electron-backscatter diffraction was used for the analysis of evolution of qualitative and quantitative microstructural characteristics. This study reveals a rather full picture of microstructure evolution in the high stacking fault energy fcc material and makes a continuous link between deformation microstructures at low, high and very high strains. Three stages of the microstructure evolution in 4N-Al at ambient temperature have been found: (i) the first stage in the range ε_eq ?≤?1; (ii) a transition stage in the range 1?<?ε_eq ?≤?8; and (iii) a saturation stage in the range ε_eq ?≥?8. In stages (i) and (ii), grain subdivision and typical features of deformation microstructures are found. Starting from stage (ii), formation of small equiaxed (sub)grains surrounded by high-angle boundaries (HABs) is found together with minor increase in the average subgrain size. At stage (iii), recrystallized-like microstructure mostly consisting of the dynamically stable equiaxed subgrains surrounded by HABs dominates the microstructure.     

Effects of pressure rise rate on laminar flame speed under normal and engine-relevant conditions

Laminar flame speed (LFS) is one of the most important physicochemical properties of a combustible mixture. At normal and elevated temperatures and pressures, LFS can be measured using propagating spherical flames in a closed chamber. LFS is also used in certain turbulent premixed flame modelling for combustion in spark ignition engines. Inside the closed chamber or engine, transient pressure rise occurs during the premixed flame propagation. The effects of pressure rise rate (PRR) on LFS are examined numerically in this study. One-dimensional simulations are conducted for spherical flame propagation in a closed chamber. Detailed chemistry and transport are considered. Different values of PRR at the same temperature and pressure are achieved through changing the spherical chamber size. It is found that the effect of PRR on LFS is negligible under the normal and engine-relevant conditions considered in this study. This observation is then explained through the comparison between the unsteady and convection terms in the energy equation for a premixed flame.     

高压熔渗生长法制备金刚石聚晶中碳的转化机制研究

在6 GPa和1500 ℃的压力和温度范围内, 利用高压熔渗生长法制备了纯金刚石聚晶, 深入研究了高温高压下金刚石聚晶生长过程中碳的转化机制. 利用光学显微镜、X-射线衍射、场发射扫描电子显微镜检测, 发现在熔渗过程中金刚石层出现了石墨化现象, 在烧结过程中金刚石颗粒表面形貌发生了变化. 根据实验现象分析, 在制备过程中存在三种碳的转化机制: 1)金属熔渗阶段金刚石颗粒表面石墨化产生石墨; 2)产生的石墨在烧结阶段很快转变为填充空隙的金刚石碳; 3)金刚石直接溶解在金属溶液中, 以金刚石形式在颗粒间析出, 填充空隙. 本文研究碳的转化机制为在高温高压金属溶剂法合成金刚石的条件下(6 GPa和1500 ℃的压力和温度范围内)工业批量化制备无添加剂、无空隙的纯金刚石聚晶提供了重要的理论指导.     

Effect of the potential function and strain rate on mechanical behavior of the single crystal Ni-based alloys: A molecular dynamics study

Molecular dynamics has been widely used to study the fundamental mechanism of Ni-based superalloys. However, the effect of the potential function and strain rate on mechanical behavior has rarely been mentioned in the previous molecular dynamics studies. In the present work, we show that the potential function of molecular dynamics can dramatically influence the simulation results of single crystal Ni-based superalloys. The microstructure and mechanical behavior of single crystal Ni-based superalloys under four commonly used potential functions are systematically compared. A most suitable potential function for the mechanical deformation is critically selected, and based on it, the role of strain rate on the mechanical deformation is investigated.     

强磁场复合交变电流作用下Zn-30wt％Bi偏晶合金的凝固

研究了强磁场复合不同强度和不同频率交变电流对Zn-30wt%Bi合金凝固组织的影响规律.结果表明：在25℃/min的冷却速率下,只施加交变电流无法抑制该合金的分层比重偏析;而单独施加纵向强磁场对合金的偏析有一定的改善作用;如果同时施加纵向强磁场和工频交变电流,在产生的交变洛仑兹力的作用下,合金的分层比重偏析基本得到抑制;增加电流密度、磁场强度和交流电流频率均有利于合金凝固组织的改善.但对上述三个参数而言,均存在一个最佳值,偏离该最佳值时,均难以获得均质的偏晶合金组织.从电磁场动力学角度探讨了复合场影响偏晶 关键词： 强磁场 偏晶合金 重力偏析 低冷却速度     

Effect of high pressure processing on textural and microbiological quality of pink perch (Nemipterus japonicus) sausage during chilled storage

The non-thermal high pressure (HP) processing was studied on fish sausage to enhance the quality during chilled storage. Pink perch (Nemipterus japonicus) sausages, packed in poly amide casing under vacuum were subjected to 400, 500 and 600?MPa pressures (dwell time: 10?min and ramp rate: 300?MPa/min) and compared with heat-set samples for physico-chemical and microbial quality parameters. Pressurized samples formed softer and glossier gels with a slight reduction in water-holding capacity. HP made the texture of sausage softer, cohesive and less chewy and gummier than heat-treated ones. Folding test seen higher acceptance values in samples treated at 500 and 600?MPa, during storage. Maximum log reduction in microbial count was observed in 600?MPa immediately, and significant difference in cooked and pressurized sausages was seen only up to 7th day. This revealed the potential application of HP in replacing conventional heat treatment for sausages preparation with enhanced shelf-life.