金属表面强化后梯度显微结构对力学性能的影响

表面强化后具有梯度显微结构的材料由于其出色的力学性能受到广泛关注。从梯度显微结构的制备、对力学性能(强度–塑性、加工硬化、摩擦磨损和疲劳性能)的影响两个方面展开综述。首先,按照工艺制备原理的不同总结了金属梯度显微结构形成机理和制备方法。其次,归纳了梯度显微结构金属力学性能(强塑性、加工硬化、摩擦磨损)研究最新进展及对增强机理进行分析,同时对出现的反常试验现象进行了解释。从表面强化后形成的梯度显微结构与残余应力等方面综合分析金属材料低周、高周疲劳性能提高的原因。最后,指出目前梯度显微结构金属面临的一些问题及未来的研究方向。

激光冲击TC17钛合金诱生的微结构的特征及热稳定性

研究激光冲击(Laser shock peening,LSP)诱生TC17钛合金的表层梯度微结构特征及其热稳定性具有重要的理论与应用价值。TEM的研究结果表明:LSP处理后,靠近基体处以较高密度形变孪晶和位错为主;越接近冲击表面应变速率越高,发生孪生的原子来不及进行位置重排、孪晶数量较少,因而以高密度位错缠结、位错胞以及位错胞转变成的亚微米级亚晶为主;最表面晶粒平均尺寸由原始~43μm瞬间细化至~396nm,表面晶粒瞬间显著细化是旋转动态再结晶的结果。LSPed试样经573K/1h退火后各深度处的位错密度降低,孪晶密度/位错胞以及最表面晶粒尺寸变化不大;而LSPed试样经673K/1h退火后其各深度处的位错密度下降更明显,孪晶数量也有所下降,而位错胞和最表面晶粒平均尺寸长大明显;673K是激光冲击TC17钛合金诱生的微结构的热稳定临界温度,维氏硬度试验结果也表明当退火温度达673K后硬度值下降明显。

液滴自发运动的微粗糙结构梯度表面参数研究

采用多体耗散粒子动力学(MDPD)方法,研究液滴在微结构粗糙梯度表面上的自发运动现象.表面微结构的构成影响液滴自发运动速度和距离.对相关的具体参数如构成微结构表面立柱的截面形状、排布、边长、面积等参数进行系统研究.研究表明,微结构表面的排布影响液固接触区域的边缘大小和形状,进而影响液滴自发运动输送效果;不对称结构立柱朝向对液滴运动也有影响,其中先宽后窄截面形状的立柱上液滴运动速度较快;对于对称的立柱组成的梯度表面,定义周长和面积的比值为长积比,该参数越大,液滴自发运动速度越快.本文可为微流控表面设计提供一定的数值模拟依据.

梯度微结构对金属玻璃拉伸增韧的影响

本文通过数值模拟分析了梯度微结构对Ti46Zr20V12Cu5Be17金属玻璃(Metallic Glass,MG)拉伸性能的影响。将自由体积理论以UMAT子程序形式导入ABAQUS中,模拟分析MG基体中剪切带的演化。在此模型中,颗粒和初始自由体积被以各种梯度方程的形式分布于MG基体中,对材料模型施加单轴拉伸荷载。结果表明:含有颗粒按凹槽状梯度分布的MG基复合材料拉伸塑性最好,并且变形呈现颈缩现象;对于初始自由体积呈凸形分布的MG塑性被很好改善;当改变自由体积梯度分布程度时,梯度越小,塑性提高越明显;而就颗粒来说,两侧密集区层数越多,变形中越不容易出现穿透试样的主剪切带。

A gradient nano/micro-structured surface layer on copper induced by severe plasticity roller burnishing

In order to investigate a gradient nano/micro-structured surface layer on pure copper produced by severe plasticity roller burnishing （SPRB） and grain refinement mechanism, the microstructure characteristics and material properties of sample at various depths from the topmost surface were investigated by SEM, TEM, XRD, OM etc. The experimental results show that the gradient nano/micro-structure was introduced into the surface layer of over 100μm in thickness. The remarkable increase in hardness near the topmost surface was mainly attributed to the reduced grain size. The equiaxed nano-sized grains were in random orientation and the most of their boundaries were low-angle grain boundaries （LAGBs）. The coarse grains are refined into the few micro-sized grains by dislocation activities;deformation twinning was found to be the primary form for the formation of submicron grains;the formation of nanostructure was dominated by dislocation activities accompanied with rotation of grains in local region.

Microstructure of TA2/TA15 graded structural material by laser additive manufacturing process

TA2/TA15 graded structural material（GSM） was fabricated by the laser additive manufacturing（LAM） process. The chemical composition, microstructure and micro-hardness of the as-deposited GSM were investigated. The results show that the TA2 part of exhibiting near-equiaxed grains was Widmanst？tten α-laths microstructure. The TA15 part containing large columnar grains was fine basket-weave microstructure. The graded zone was divided into four deposited layers with 3000 μm in thickness. As the distance from the TA2 part increases, the alloy element contents and the β phase volume fraction increase, the α phase volume fraction decreases and the microstructure shows the evolution from Widmanst？tten α-laths to basket-weave α-laths gradually. The micro-hardness increases from the TA2 part to the TA15 part due to the solid solution strengthening and grain boundary strengthening.

热处理温度及其对耐火浇注料热震参数的影响

耐火浇注料通常在现场进行热处理,由于耐火浇注料层的厚度,形成了冷热面之间急剧的温度梯度,因而这一过程会导致不均匀的显微结构。在本研究工作中,试样分别在600℃、800℃和950℃煅烧并描述其特性,评价了高铝和莫来石浇注料的热震参数——R″″和R_(st)。为了进行比较,事先在1200℃对相同材料的不同试样进行了热处理并在上述温度下描述了其特性。研究表明,对于浇注筑衬的炉壁,显微结构的梯度会导致材料的抗热震性的局部变化。可是一般来说,只有温度梯度才会对预烧砖炉壁的抗热震性能产生一定的影响。

Diversity change of microbial communities responding to zinc and arsenic pollution in a river of northeastern China

Pollution discharge disturbs the natural functions of water systems. The environmental microbial com-munity composition and diversity are sensitive key indicators to the impact of water pol utant on the microbial ecology system over time. It is meaningful to develop a way to identify the microbial diversity related to heavy metal effects in evaluating river pol ution. Water and sediment samples were col ected from eight sections along the Tiaozi River where wastewater and sewage were discharged from Siping City in northeastern China. The main pollutants contents and microbial communities were analyzed. As the primary metal pol utants, zinc （Zn） and arsenic （As） were recorded at the maximum concentrations of 420 and 5.72 μg/L in the water, and 1704 and 1.92 mg/kg in the sediment, re-spectively. These pollutants posed a threat to the microbial community diversity as only a few species of bacteria and eukaryotes with strong resistance were detected through denaturing gradient gel electrophoresis （DGGE）. Acineto-bacter johnsoni , Clostridium cel ulovorans, and Trichococcus pasteuri were the dominant bacteria in the severely pol uted areas. The massive reproduction of Limnodrilus hoffmeisteri almost depleted the dissolved oxygen （DO） and resulted in the decline of the aerobic bacteria. It was noted that the pollution reduced the microbial diversity but the L. hoffmeisteri mass increased as the dominant community, which led to the overconsuming of DO and anaerobic stinking water bodies. Water quality, concentrations of heavy metals, and the spatial distribution of microbial popula-tions have obvious consistencies, which mean that the heavy metals in the river pose a serious stress on the microorganisms.

A novel microstructural design and heat treatment technique based on gradient control of carbon partitioning between austenite and martensite for high strength steels

Based on gradient control of carbon partitioning between martensite and austenite during heat treatment of steels,a stepping-quenching-partitioning(S-Q-P) process is developed for high strength steels.The S-Q-P process involves several quenching processes at progressively lower temperatures between martensite-start(Ms) and martensite-finish(Mf) temperatures,each followed by a partitioning treatment at either the initial quenching temperature or above that temperature.A novel microstructure is designed based on the S-Q-P process.Sizes and distributions of retained austenite and high-carbon martensite surrounded by martensite laths can be manipulated by the partitioning duration and temperature,and quenching temperature of the S-Q-P process.Alloying element Si is employed in the S-Q-P steel to suppress formation of carbides and create suitable amount of retained austenite.A steel of 0.39C-1.22Mn-1.12Si-0.23Cr(wt.%) treated by the S-Q-P process is endowed with some special microstructural characteristics:some strips of retained austenite located at edges of martensite blocks with high density of dislocations and between martensite laths,some small blocks of twinned martensites distributed among bundles of the low-carbon martensite lath.The mechanical properties of the medium carbon steel after the S-Q-P process can reach ultimate tensile strength(Rm) of 1240 MPa,total elongation(EI) of 25%,and product of strength and ductility(PSD) of 31.2 GPa% that are much more improved than those after the conventional quenching-tempering(Q-T) and currently prevailing quenching-partitioning(Q-P) treatments.The PSD of the tested steel after the S-Q-P process increases by 67% and 32% as compared with those after the Q-T and Q-P processes,respectively.

Microbial Community Changes Along a Land-Use Gradient of Desert Soil Origin

Soil harbors remarkably stabilize bacterial communities at the phylum level. However, no two soils have exactly the same structure of bacterial phyla. The structure of microbial community is strongly influenced by the type of land-use through changes in soil attributes. Using high-throughput pyrosequencing and quantitative polymerase chain reaction techniques, soil microbial community structures were investigated along a land-use gradient of 100- and 27-year farmlands, a 33-year Pinus forest, a 28-year poplar forest, and a 21-year shrubland, as well as a native desert from which all cultivated systems were converted. The results revealed that the dominant phylotypes in the native soil comprised primarily of Alphaproteobacteria, Actinobacteria, Bacteroidetes, and Firmicutes, accounting for 〉 71.4% of the total bacterial 16S rRNA sequence reads. Changes in land-use led to a significant decrease in these dominant phylotypes down to 33.4%. In contrast, the phylotypes with low abundance, such as Acidobaeteria, Chloroflexi, Nitrospira, and Gammaproteobaeteria, increased sharply from 4.5%-5.9% in the native soil to 20.9% 30.2% of the total 16S rRNA gene sequences in the cultivated soils except for the soil from the shrubland. These contrasting changes in the major taxa appear to be correlated with the changes in soil attributes. For instance, bacterial and archaeal amoA genes were found to be 960- and 3 800-fold more abundant in the soil from the 100-year farmland than the native soil. The changes in numerically less dominant nitrifying phylotypes are consistent with soil inorganic nitrogen dynamics. Quantification of the 16S rRNA genes demonstrated that bacteria and archaea were about two to three orders of magnitude more abundant in the cultivated soil than in the native soil. Hence, land-use type affects the soil bacterial community structure, which has profound consequences on ecosystem function.

Vibration modes of flexoelectric circular plate

Beams,plates,and shells,as the fundamental mechanical structures,are widely used in microelectromechanical systems(MEMS)and nanoelectromechanical systems(NEMS)as sensors,actuators,energy harvesters,and among others.Deeply understand the electromechanical coupling of these dielectric structures is of crucial for designing,fabricating,and optimizing practice devices in these systems.Herein we demonstrate the electromechanical coupling in flexoelectric circular plate,in which higher-order strain gradients were considered to extend the classical electromechanical properties to isotropic materials,in which the non-uniform distribution of the electric potential along the radial direction was considered.Analytical solutions for the vibration modes of the flexoelectric circular plates showed that the dynamic modes were totally different from the piezoelectric circular plates owing to the inversion symmetry breaking by the strain gradient.The electromechanical coupling dynamic modes are sensitive to bending,twisting modes owing to the sensitivity of the flexoelectric effect to bending.This work provides a fundamental understanding of the electromechanical coupling in flexoelectric circular plate,which is helpful in designing novel flexoelectric circular plate-based devices,such as flexoelectric mirrors.