Effect of upsetting force on microstructure of welds in resistance spot welding of 400 MPa ultra-fine grain steel

The ultra-fine grain （UFG） steel is welded by using resistance spot welding technique with or without requirement of upsetting force. Metallographic inspection shows that the grain size of weld nugget is larger than that of the base metal and the microstructure is altered significantly. In addition, contracting defects such as air holes can be found in the nugget center. The experiments show that the defects can be effectively avoided by the technique of adding upsetting force during the nugget cooling and crystallizing processes. In tensile shear tests, the welding joint starts to crack from the inner edge of the corona bond. The results of micro-hardness tests show that the newly born martensite structure dramatically improves the hardness of the joint. Under the interactions between residual stresses and regenerated fine grains, the micro-hardness of the heat-affected zone （ HAZ ） is lower than that of the nugget, but evidently higher than that of the base metal.

PROPERTIES AND MICROSTRUCTURE OF AROMATIC LIQUID CRYSTALLINE COPOLYESTERS

The properties and structures of thermotropical liquid crystalline copolyesters based on p-hydroxybenzoic acid (PHBA), terephthalic acid (TPR) and bisphenol A (BPA) were studied by DSC, WAXD, hot stage polarized microscopy and NMR. It was found that most of the copolyesters were soluble in many common organic solvents. The copolyesters had low T-m/T-f values and a broad range of liquid crystal phase, making the polymers readily melt-processable. The effects of annealing at different temperatures on the copolyester containing 33% PHBA were also discussed. It was noted that annealing at ca. 200 degrees C (below Tc - n) could lead to the increasing of the crystallinity of the copolyester while the microstructure and sequence structure had not changed. Annealing at ca. 280 degrees C (near Tc - n) could bring a change of crystal and sequence structure and simultaneously made the microdomains be ordered more perfectly.

Microstructures and mechanical properties of nanocrystalline NiTi intermetallics formed by mechanosynthesis

The formulation of nanocrystallinc NiTi shape memory alloys has potential effects in mechanical stimulation and medical im- plantology. The present work elucidates the effect of milling time on the product＇s structural characteristics, chemical composition, and mi- crohardness for NiTi synthesized by mechanical alloying for different milling durations. Increasing the milling duration led to the formation of a nanocrystalline NiTi intermetallic at a higher level. The formation of nanocrystalline materials was directed through cold fusion, fractur- ing, and the development of a steady state, which were influenced by the accumulation of strain energy. In the morphological study, uninter- rupted cold diffusion and fracturing were visualized using transmission electron microscopy. Particle size analysis revealed that the mean particle size was reduced to -93 μm after 20 h of milling. The mechanical strength was enhanced by the formation of a nanocrystalline in- termetallic phase at longer milling time, which was confirmed by the results of Vickers hardness analyses.

Different crystallinity of poly(d,l-lactide-co-p-dioxanone) copolymers acquired by control of chain microstructure

Poly（d,l-lactide-co-p-dioxanone） （P（LA-co-PDO）） copolymers with different chain microstructures were synthesized by onestep or two-step bulk ring-opening polymerizations of d,l-lactide （LA） and p-dioxanone （PDO） monomers using stannous octoate [Sn（Oct）2]/n-dodecanol as the initiating system. The average sequence lengths of the lactidyl （LLA） and dioxanyl （LpDo） units were calculated from the ^1H NMR spectra. It was found that both LLA and Lpoo values from the two-step syntheses were significantly longer than those from the corresponding one-step syntheses, indicating more blocky structure achieved for the twostep copolymers. Corresponding to this difference in microstructure, the two-step copolymers were semi-crystalline, while the one-step copolymers were completely amorphous. In conclusion, the crystallinity of P（LA-co-PDO） copolymers could be adjusted conveniently to meet specific applications by changing the microstructure of the copolymers via different polymerization routes.

Microstructural Evolution and Thermal Stability of Ultra-fine Grained Al-4Mg Alloy by Equal Channel Angular Pressing

Experiments were conducted to evaluate the grain refinement and thermal stability of ultra-fine grained Al-4Mgalloy introduced by equal-channel angular pressing (ECAP) at 473 K. The results show that the intensities of X-ray(111/222) and (200/400) peaks for the alloy processed by ECAP decrease significantly and the peak widths of halfheight become broadening compared with the corresponding value in the annealed alloy. The microstructure of 2passes ECAPed alloy consists of both elongated and equiaxed subgrains. The residual strain in the alloy increaseswith increasing passes numbers, that appears as increasing dislocation density and lattice constant of matrix. Anequiaxed ultra-fine grained structure of～0.2μm is obtained in the present alloy after 8 passes. The ultra-fine grainsare stable below 523 K, because the alloy retains extremely fine grain size of～1μm after static annealing at 523 Kfor 1 h.

Properties and microstructure of VC/Cr_3C_2-doped WC/Co cemented carbides

This paper deals with the effects of codoped VC/Cr3C2 and sintering temperature on the magnetic and mechanical properties of ultra-fine grained WC-12%Co alloys. Results show that the synergistic action of doped VC/Cr3C2 in optimal proportion enhances both the hardness and transverse rupture strength （TRS） of the alloys, with more homogeneous microstructtLre. When the alloy is sintered at 1430℃ and with 0.5% Cr3C2/0.2% VC, the TRS reaches 3786 MPa, the hardness is 91.7 HRA and the grain size smaller than 0.6 μm. The numerical analyses on grain growth during the sintering process show that both VC precipitating on the WC grain boundary and Cr3C2 dissolving in the Co phase decrease the solid/liquid interfacial energy γ, the process of dissolution and reprecipitation is greatly retarded and the coarsening of WC grains is inhibited.

Microstructure and magnetic properties of electrodeposited Gd-Co alloy films

Gd-Co alloy films were synthesized by potentiostatic electrolysis on Cu substrates in urea-acetamide-NaBr-KBr melt at 353 K. The electroreduction of Co^2＋ and Gd^3＋ was investigated by cyclic voltammetry. The reduction of Co^2＋ is an irreversible process. Gd^3＋ cannot be reduced alone, but it can be inductively co-deposited with Co^2＋. Both the Gd content and microstructure of the prepared Gd-Co alloy films can be controlled by the deposited potential. The content of Gd was analyzed using an inductively coupled plasma emission spectrometer （ICPES）, and the microstructure was observed by scanning electron micrograph （SEM）. The films were crystallized by heat-treatment at 823 K for 30 s in Ar atmosphere, and then were investigated by XRD. The hysteresis loops of the Gd-Co alloy films were measured by a vibrating sample magnetometer （VSM）. The experimental results reveal that the deposited Gd-Co alloy films are amorphous, while the annealing causes the samples to change from amorphous to polycrystalline, thus enhancing their magnetocrystalline anisotropy and coercivity. Moreover, the magnetic properties of the Gd-Co alloy films depend strongly on the Gd content.

Strengthening of HT250 by modified ultra-fine ceramic powders

In this study, three kinds of modified ultra-fine ceramic powders marked A, B and C, which were prepared by each of three different modifiers mixing with a commercial SiC, were added to HT250 cast iron, respectively, and the effects of the modified ultra-fine ceramic powders on microstructure, mechanical properties and wear resistance were studied. Metallographic examination, tensile test, scanning electron microscopy, and three-dimensional surface topography were applied to analyze and compare the samples containing modified powder with the original samples. The results showed that the most obvious modification effect among the powders was seen in the sample containing powder A, with the graphite and eutectic cells being refined, the tensile strength being increased by 36.9%, and the wear resistance being improved by 45.5% and 47.2% under loads of 150 N and 300 N, respectively. The improvements of mechanical properties and wear resistance in the HT250 cast iron with the modified ultra-fine ceramic powders were attributed to the synergistic effect of the grain refinement with the powder acting as a hard particle phase and the lubrication by the graphite.

Microstructure and Mechanical Properties of Ultrafine Grained Mg15Al Alloy Processed by Equal-channel Angular Pressing

An as-cast magnesium alloy with high Al content Mg15Al was subjected to equal-channel angular pressing （ECAP） through a die with an angle of φ= 90？ at 553 K following route Bc. It is found that the network β-Mg17Al12 phases in the as-cast Mg15Al alloy are broken into small blocks and dispersed uniformly with increasing numbers of pressing passes. Moreover, many nano-sized Mg17Al12 particles precipitate in the ultra-fine α-Mg matrix. The grains are obviously refined. However, the grain structure is inhomogeneous in different areas of the alloy. The average size of the primary phase α-Mg is reduced to about 1 μm while grains of around 0.1-0.2 μm are obtained in some two-phase areas. With additional ECAP passes （up to 8）, coarsening of the grains occurs by dynamic recovery. Room temperature tensile tests show that the mechanical properties of Mg15Al alloys are markedly improved after 4 ECAP passes. The ultimate tensile strength and elongation to failure increase from 150 MPa to 269.3 MPa and from 0.05% to 7.4%, respectively. Compared with that after 4 passes, the elongation to failure of the alloy increases but the strength of the alloy slightly decreases after 8 ECAP passes. Fracture morphology of the ECAP-processed alloy exhibits dimple-like fracture characteristics while the as-cast alloy shows quasi-cleavage fractures.

Diflferent Cooling Rate Dependences of Different Microstructure Units in Aluminium Glass by Molecular Dynamics Simulation

Constant-pressure molecular dynamics simulation and the pair analysis technique have been performed to study the microstructural evolution of aluminium during rapid solidification.The microstructure characteristics of icosahedral ordering increase with decrease of the cooling rate,whereas the microstructure unit characteristics of hep crystalline structure decrease.There are two kinds of microstructure units which are similar to those in the fee crystal containing interstitialcies.These two kinds of microscopic units are nearly independent of the cooling rate.The microscopic structural unit characteristics of fee crystalline structure do not depend on the cooling rate either.These results may help us understand the microstructure of glass and its stability.

Effects of different magnetic flux densities on microstructure and magnetic properties of molecular-beam-vapor-deposited nanocrystalline Fee4Ni36 thin films

The nanocrystalline Fe64Ni36 thin films were prepared by molecular-beam- vapor deposition under different magnetic flux densities. The microstructure and magnetic properties of thin films were examined by AFM, TEM, HRTEM and VSM. The results show that with the increase of magnetic flux densities, the changing trend of the average particle size is the same as the coercive force except 6 T. Under 6 T condition, the thin film became the mixture of bcc and fcc phases, which leads to slight increase of the coercive force. In addition, the HRTEM result shows the short-range ordered clusters （embryos） or nucleation rate of thin films increase with increasing magnetic flux densities.

Microstructure and Hardness of Laser Shocked Ultra-fine-grained Aluminum

Ultra-fine-grained commercial purity aluminum was produced by severe cold rolling, annealing and then strain- ing at ultra-high rate by a single pass laser shock. Resulted microstructure was investigated by transmission electron microscopy. Microhardness of annealed 0.6μm ultra-fine grained aluminum increased by 67% from 24 to 40 HV. Many 0.3 μm sub-grains appeared at the shock wave center after a single pass laser shock, while high density dislocation networks were observed in some grains at the shock wave edges. Accordingly, microhardness at the impact center increased by 37.5% from 40 to 55 HV. From the impact center to the edge, microhardness decreased by 22% from 55 to 45 HV.

FeWO_(4)结晶釉的取向生长控制及光催化活性研究

具有光催化活性的结晶釉结构对于陶瓷板材表面功能化具有重要的应用价值,但目前功能化表面结晶釉的结构控制及其生长方式对光活性的影响尚不明确。本研究以三氧化钨、硫酸亚铁、磷酸铝、高岭土等为原料,通过调控不同生长取向结构的FeWO_(4)结晶釉,在1080℃成功烧制出高亲核性晶面暴露的结晶釉。结果表明,不同取向的结晶釉釉层呈现不同的晶面暴露,取向生长的结晶釉可以提供更高的催化活性,同时可以促使高亲核性的(100)晶面暴露,从而提高陶瓷板表面的光催化性能。本研究可为光功能化结晶釉材料表面功能化的设计和调控提供理论和实践参考。

Sc对Al-Bi偏晶合金凝固组织及耐磨性影响的研究

采用微合金化的方法,研究了稀土元素Sc对Al-Bi偏晶合金凝固组织及耐磨性的影响。通过OM、SEM、TEM、XRD多功能材料表面性能测试仪等设备对试样进行检测分析。结果表明:微量Sc元素的加入降低了富Bi相的形核能,增加了形核位置,有效地改善了富Bi相的成分偏析现象,减小了Al基体的晶粒大小。同时微量Sc元素能显著提高Al-Bi合金的硬度及摩擦系数,当Sc含量在0.25 wt%时硬度与摩擦系数皆取得极大值,此时较未加Sc时,硬度提升了109%,摩擦系数提升了33%。当Sc的含量增加到1 wt%时,试样中发现了密集的Al3Sc相,具有规则的几何轮廓,并且有明显的偏聚行为。富Bi相与Al3Sc相两种相单独析出,互不干涉。

生物基环氧化天然橡胶形状记忆复合材料的制备与性能

以壳聚糖作为填料、脱蛋白环氧化天然橡胶(ENR)作为基质材料,制备了生物基形状记忆复合材料,通过傅里叶变换红外光谱、原子力显微镜、电子万能试验机等研究了该形状记忆复合材料的微观结构、力学性能及形状记忆性能。结果表明,加入壳聚糖后ENR的有序结晶相以更小、更均匀的尺寸分散在整个体系中;由于壳聚糖的增强增韧作用,该形状记忆复合材料的耐磨性能增强、力学性能逐渐提高。此外,壳聚糖的加入进一步提高了ENR的形状记忆性能。

Surface-induced Microstructure and Performance Changes in P3HT Ultrathin Films

In this work,poly(3-hexylthiophene)(P3HT)ultrathin films(P3HT-T)were prepared by spin-coating a dilute P3HT solution(in a toluene:o-dichlorobenzene(Tol:ODCB)blend with a volume ratio of 80:20)with ultrasonication and the addition of the nucleating agent bicycle[2.2.1]heptane-2,3-dicarboxylic acid disodium salt(HPN-68L)on glass,Si wafers and indium tin oxide(ITO)substrates.The electrical and mechanical properties of the P3HT-T ultrathin films were investigated,and it was found that the conductivity and crack onset strain(COS)were simultaneously improved in comparison with those of the corresponding pristine P3HT film(P3HT-0,without ultrasonication and nucleating agent)on the same substrate,regardless of what substrate was used.Moreover,the conductivity of P3HT-T ultrathin films on different substrates was similar(varying from 3.7 S·cm^(-1)to 4.4 S·cm^(-1)),yet the COS increased from 97%to 138%by varying the substrate from a Si wafer to ITO.Combining grazing-incidence wide-angle X-ray diffraction(GIXRD),UV-visible(UV-Vis)spectroscopy and atomic force microscopy(AFM),we found that the solid order and crystallinity of the P3HT-T ultrathin film on the Si wafer are highest,followed by those on glass,and much lower on ITO.Finally,the surface energy and roughness of three substrates were investigated,and it was found that the polar component of the surface energyγp plays a critical role in determining the crystalline microstructures of P3HT ultrathin films on different substrates.Our work indicates that the P3HT ultrathin film can obviously improve the stretchability and simultaneously retain similar electrical performance when a suitable substrate is chosen.These findings offer a new direction for research on stretchable CP ultrathin films to facilitate future practical applications.

晶体冰微观结构研究的最新进展

水是生命之源,对人类和整个生态系统都至关重要。作为水的固态形式,冰的结构研究对环境科学、大气科学和天体物理学等领域相关自然现象的理解有重要指导意义。然而,由于氢键的影响冰的晶体结构非常复杂(仅目前发现就有二十种晶体相和三种无定形相),且通常存在于低温和高压等极端环境,因此其结构研究一直都充满挑战。早期冰的结构研究主要通过谱学手段,如X射线衍射、拉曼光谱和中子衍射等,揭示了平均结构信息。随着显微成像技术的发展,如透射电子显微镜、扫描隧道显微镜、原子力显微镜等,人们对冰的结构研究已经进入到分子尺度。本文将简要归纳过去近20年晶体冰微观结构的研究进展,并探讨未来晶体冰结构研究的挑战与机遇。

不同酯化程度的硬脂酸对其油凝胶性质的影响

油凝胶是一种特殊的凝胶系统,被认为是食品工业中塑性脂肪的有效替代品。不同凝胶因子结晶行为的特异性导致了油凝胶的结构存在差异。该文选择4种不同酯化程度的硬脂酸[硬脂酸(stearic acid,SA)、单硬脂酸甘油酯(glyceryl monostearate,GMS)、二硬脂酸甘油酯(glyceryl distearate,GDS)和三硬脂酸甘油酯(glycerol tristearate,GTS)]研究其作为凝胶因子的结晶行为。通过流变、差示扫描量热仪(differential scanning calorimeter,DSC)、X-射线衍射仪(X-ray diffraction,XRD)和偏振光显微镜(polarized light microscope,PLM)等方法,探究酯化程度和浓度对凝胶因子结晶行为的影响。研究发现,4种油凝胶都具有类固体行为且形成强凝胶,特别是GMS基油凝胶具有较高的油结合能力。热力学性能结果显示,GMS基油凝胶更具热稳定性能,存在最稳定的β多晶型结构,表现出小针状结晶。傅立叶变换红外光谱(Fourier transform infrared spectroscopy,FTIR)显示GMS基油凝胶中存在氢键,而在SA基、GDS基、GTS基油凝胶的光谱图谱中没有发现。综上,GMS额外的2个游离羟基端基与脂肪酸羰基基团之间的氢键促进了GMS网络的形成,使GMS基油凝胶更具致密的网络结构,更能有效束缚液油。这些发现为油凝胶在塑性脂肪替代中的应用提供了强有力的理论支持。

川东北部飞仙关组白云石化成因探讨

川东北地区飞仙关组储集岩主要是白云岩。白云石化成因类型对勘探有利区块和勘探目标的选择都具有重要的意义。研究认为,区内飞仙关组岩石中的白云石主要有三种类型:第一种为半自形—他形细—中晶白云石;第二种为泥晶白云石;第三种为自形中晶-粗晶白云石。它们在有序度,C、O同位素,微量元素,包裹体,阴极发光等方面具有不同的地球化学特征。通过研究认为,这三种类型的白云石成因分别为混合水白云石化、渗透回流白云石化和埋藏白云石化。其中混合水白云石化是该区优质鲕滩储层得以发育的主要成岩因素之一,有利的混合水白云石化带主要发生在台地边缘鲕坝(滩)中,这也是鲕滩储层发育的最有利地区,渗透回流白云石化主要发生在台地边缘鲕粒坝(滩)后方的局限海环境中,其分布范围大致与石膏分布范围一致,储集物性略差。

剧塑性变形制备超细晶/纳米晶结构金属材料的研究现状和应用展望

综合目前剧塑性变形方法制备超细晶及纳米晶结构金属材料的研究现状,介绍等通道转角挤压、高压扭转、累积叠轧焊、多向锻造等剧塑性变形方法及其特点与原理;探讨剧塑性变形金属材料的组织演变和晶粒细化机制;分析金属材料经剧塑性变形后强度与延展性的变化趋势,及其对超塑性变形的影响规律;展望剧塑性变形方法对金属材料应用的前景。