Thermo‐mechanical methods for improving fatigue performance of wrought magnesium alloys

Wrought magnesium alloys AZ80 and ZK60 were extruded at 300 °C with extrusion ratios of ER = 12 and 44. Resulting microstructures, crystallographic textures and mechanical properties were investigated. Extruding led to profound reduction in grain size, which drastically improved yield stress, tensile elongation and HCF performance. Strength differentials in ZK60 after extruding at ER = 12 were more pronounced than after extruding at ER = 44, whereas no such effect of ER was observed in AZ80. Swaging after extruding further increased yield stress and endurance limit, while strength differential increased and ductility was lowered.     

挤出共混中的高剪切应力对HDPE/滑石粉共混材料力学性能的影响

采用在材料熔融挤出共混过程中提高双螺杆挤出机螺杆转速的方法,研究了较高螺杆转速条件下双螺杆挤出机的高剪切应力对HDPE和HDPE/滑石粉材料的熔体流动速率、界面结合状况及力学性能的影响。结果表明:双螺杆挤出机的高剪切应力可促进超细滑石粉颗粒聚集体的分散、引发HDPE分子链的断链反应、引起共混材料界面结合力的加强和拉伸强度及弯曲模量的明显增大;一定量的极性烯类单体的加入有利于所形成的HDPE大分子自由基与极性烯类单体的接枝(嵌段)反应和原位增粘作用,能明显改善HDPE/滑石粉共混材料的力学性能。     

Processing conditions, morphology and properties of a polycarbonate + a thermotropic polymer liquid crystal blend

We have investigated phase structure – properties relationships of polycarbonate (PC) + a polymer liquid crystal (PLC) blends processed in a twin-screw extruder at several conditions. The PLC is PET/0.82 PHB – a copolyester of poly(ethylene terephtalate) and p -hydroxybenzoic acid. For comparison the blend was additionally extruded in a wide range of shear rates in a capillary rheometer at two different spinning rates and compression-molded. The blend processed in the rheometer exhibits lower values of modulus and tensile strength than the blend extruded due to destruction of the initial orientation and dispersion level gained during extrusion. The orientation of PLC-rich islands increases up to the shear rate of 50–100 s^–1, whereas deformation at higher shear rates exhibits a droplet–breakup phenomenon, confirmed by SEM micrographs. The rheological measurements (oscillation mode) evidence a high shear thinning of the PLC. By contrast, the influence of the deformation rate on the viscosity for PC and the blend is negligible, suggesting also a low interaction level in the interfacial area. This conclusion was confirmed by dynamic mechanical measurements. As expected, our experiments prove that structure and properties of the blend are affected by processing (shear and elongation) conditions. Increasing shear rate leads to elongation of dispersed domains but exceeding critical values can lead to droplet breakup and destruction of created structure. The unique morphology created during extrusion can be destroyed during additional processing (in rheometer). Formation of fibrils is also dependent on additional treatment – spinning speed. Optimized spinning speed can lead to 50% increase in stiffness of the blend. Electronic Publication     

Extrusion processing of Al–Li–Mg–Zr alloy

Abstract

The hot deformation behaviour of an Al–Li–Mg–Zr alloy was characterised in hot torsion and extrusion. The alloy was found to have similar hot ductility to existing high strength aluminium alloys, but this could be maintained at higher temperatures. Billets were extruded over a range of process conditions and a limit diagram was constructed for surface cracking. All the extrusions were found to be partially recrystallised after deformation, but the volume fraction of recrystallisation was a strong function of billet temperature and extrusion ratio. In addition, the unrecrystallised areas contained a recovered substructure where the subgrain size was inversely proportional to the temperature compensated strain rate. The as extruded structure was retained during solution treatment and as a result final mechanical properties were strongly dependent on the extrusion conditions. The use of high billet temperatures and low extrusion ratios gave the best combination of strength and toughness.

MST/839     

高剪切应力对PS/SEBS/CaCO3材料力学性能影响

采用在材料熔融挤出共混过程中提高双螺杆挤出机螺杆转速的方法,研究了较高螺杆转速条件下双螺杆挤出机的高剪切应力对PS/SEBS/CaCO3共混材料力学性能的影响。结果表明,双螺杆挤出机的高剪切应力可促进SEBS橡胶颗粒和碳酸钙颗粒聚集体的分散、引发聚苯乙烯(PS)、SEBS分子链的断链反应、形成大分子自由基、产生原位增容作用,并引起共混材料力学性能的明显改善。在220℃的挤出温度下,当螺杆转速由120 r/min提高至960 r/min时,其PS/SEBS/CaCO3共混材料的缺口冲击强度将由5.7 kJ/m2提高至11.0 kJ/m2,同时共混材料的拉伸强度和弯曲强度也获得一定的增加。双螺杆挤出机的熔融挤出温度对PS/SEBS/CaCO3共混材料力学性能的影响存在最佳值。     

Effects of ECAE temperature and billet orientation on the microstructure,texture evolution and mechanical properties of a Mg–Zn–Y–Zr alloy

Single-pass equal channel angular extrusion (ECAE) experiments of an extruded Mg–Zn–Y–Zr alloy with an intense initial basal texture were performed in two inter-perpendicular billet orientations and at 473 and 623 K. The study was aimed to determine the effects of ECAE temperature and billet orientation on the microstructure, texture evolution and mechanical properties of the ECAEed alloy. It was found that the grain refinement achieved through the single-pass ECAE in the Orient-I billet orientation (the normal direction (ND) of the extruded plate parallel with the ECAE exit direction) was more effective than that in the Orient-II billet orientation (the ND of the extruded plate perpendicular to the ECAE exit direction). The average grain sizes after ECAE at 473 K were much smaller than those after ECAE at 623 K. The pole figures of the alloy ECAEed at 473 K showed that most of the basal planes in the Orient-I and Orient-II samples were inclined about 40° and 35°, respectively, with respect to the longitudinal direction of the ECAE extrudate. However, for the alloy ECAEed at 623 K, most of the basal planes were parallel with the longitudinal direction of the ECAE extrudate. It was remarkable that the yield strengths of the alloy ECAEed at 473 K were lower than those at 623 K. The peculiar relationship between ECAE temperature and the mechanical properties of the alloy was ascribed to the texture evolution during ECAE.     

Glass-ceramics with random and oriented microstructures

The technique of hot extruding a glass-ceramic to produce a material with an aligned crystal microstructure is described. The results of a statistical investigation to analyse the degree of morphological orientation of an extruded glass-ceramic based on the Li₂O-SiO₂ system are reported. Details are also presented of the determination of the crystallographic orientation in the aligned glass-ceramic.     

Anisotropic glass-ceramics produced by extrusion through opposed dies

Oriented glass-ceramics have been produced in two morphologically different systems by extruding the green glasses through opposed dies at temperatures near their respective crystallization temperatures. The principal crystalline phase is a mica for one system and an asbestos for the other and, by using the crystals as markers, it has been possible to explore the pattern of flow and its effect on crystal orientation at various stages of the extrusion process. It is demonstrated that crystal alignment occurs during extensional flow between the dies and is modified by plug flow through the dies. Oriented glass-ceramics are expected to exhibit marked anisotropy in properties and, as far as mechanical properties are concerned, this has been confirmed by measurements of tensional modulus, fracture stress and indentation strength. The oriented crystals remain aligned during subsequent drawing down to fibres and by suitable heat-treatment can be complemented by a population of randomly oriented crystals.     

Reinforcing polyamide 1212 nanocomposites with aligned carbon nanofibers

In this study, the mechanical properties and structure orientation of pure polyamide 1212 (PA1212) were compared with those of PA1212–carbon nanofibers (CNFs) nanocomposites. The tensile strength of the composite containing 0.3 wt.% modified CNFs was improved from 328 MPa (pure PA1212) to 373 MPa after drawing. The reinforcing effect was investigated in terms of crystallization behavior, crystal morphology, alignment of CNFs, and crystal orientation degree. Spherulites developed into oriented crystals after drawing, and the CNFs aligned along the drawing direction. The heterogeneous nucleation effect of the aligned CNFs improved the crystal orientation degree, which produced the reinforcing effect. The oriented fibril structures with rigid nanofibers acting as nuclei reinforced the entire oriented crystals in the composites.     

热挤压AZ91D镁合金边角料组织和性能的研究

采用热挤压工艺直接热挤出AZ91D镁合金边角料,研究挤压温度对挤压成形镁合金组织和性能的影响,并讨论其断裂行为.结果表明:在450℃热挤压时,晶粒尺寸均匀,组织中已不存在原始边角料之间未打碎的结合面,边角料之间结合较好;在350～450℃之间热挤出时,AZ91D镁合金随挤压温度的升高,抗拉强度和延伸率均增加,当挤压温度...     

Influence of undissolved second-phase particles on dynamic recrystallization behavior of Mg-7Sn-1Al-1Zn alloy during low-and high-temperature extrusions

This study investigates the effects of fine and coarse undissolved particles in a billet of the Mg-7Sn-1Al-1Zn (TAZ711) alloy on the dynamic recrystallization (DRX) behavior during hot extrusion at low and high temperatures and the resultant microstructure and mechanical properties of the alloy.To this end,partially homogenized (PH) and fully homogenized (FH) billets are extruded at temperatures of 250 and 450 ℃.The PH billet contains fine and coarse undissolved Mg2Sn particles in the interdendritic region and along the grain boundaries,respectively.The fine particles (＜1 μm in size) retard DRX during extrusion at 250 ℃ via the Zener pinning effect,and this retardation causes a decrease in the area fraction of dynamically recrystallized (DRXed) grains of the extruded alloy.In addition,the inhomogeneous distribution of fine particles in the PH billet leads to the formation of a bimodal DRXed grain structure with excessively grown grains in particle-scarce regions.In contrast,in the FH billet,numerous nanosized Mg2Sn precipitates are formed throughout the material during extrusion at 250 ℃,which,in turn,leads to the formation of small,uniform DRXed grains by the grain-boundary pinning effect of the precipitates.When the PH billet is extruded at the high temperature of 450 ℃,the retardation effect of the fine particles on DRX is weakened by their dissolution in the α-Mg matrix and the increased extent of thermally activated grain-boundary migration.In contrast,the coarse Mg2Sn particles in the billet promote DRX during extrusion through the particle-stimulated nucleation phenomenon,which results in an increase in the area fraction of DRXed grains.At both low and high extrusion temperatures,the extruded material fabricated using the PH billet,which contains both fine and coarse undissolved particles,has a lower tensile strength than that fabricated using the FH billet,which is virtually devoid of second-phase particles.This lower strength of the former is attributed mainly to the larger grains and/or absence of nanosized M2Sn precipitates in it.     

注塑充模聚合物流动形态与力学行为分子机制研究

以聚甲基丙烯酸甲酯(Polymeric methyl methaerylate,PMMA)为实验材料,基于分子动力学模拟实验研究了注塑成型聚合物充模流动与力学行为的分子机制.构建包含10条聚合度为20的无规PMMA分子链所构成的链团模型,基于能量最小化与SA算法实现了体系能量初始化;基于周期性边界,引入COMPASS从头算分子力场及Velocity-Verlet算法,实现了PMMA胞元在恒温平面流场中的流态与力学行为模拟实验.结果表明,PMMA充模与形变过程首先需要克服包含体系内能、分子链松弛与解缠在内的“活化能”,且存在时间和应力阈值,前者体现了瞬时加载内能协调效应,后者对应于高剪切力作用下分子松弛与解缠现象.体系C原子回转半径分布表明剪切力的作用使得高分子沿流场方向取向排布,剪切力越大则取向越明显,剪切力过大则分子链将断裂而弹性恢复.MSD结果揭示了熔态聚合物充模流动的实质是大分子链定向迁移和取向排布协调运动的结果,且进一步验证了“活化能”的存在,克服这一制约之后大分子链的迁移效应才变得明显,且迁移速率随剪切应力的增大呈非线性增大变化.     

Semi–solid thixo-extrusion of AlSi7MgBe alloy

对采用近液相线半连续铸造技术制备的AlSi7MgBe合金坯料进行半固态挤压成形, 通过组织与性能的分析, 研究了AlSi7MgBe合金的半固态挤压成形性. 结果表明: 用近液相线半连续铸造技术制备的AlSi7MgBe合金坯料具有均匀、细小的蔷薇状组织, 在575℃对其二次加热可获得稳定的、适合于半固态触变成形的球化组织, 进行半固态挤压成形可获得表面光洁, 组织细小、分布均匀的成形件, 在540℃固溶5 h然后175℃时效10 h处理, 其抗拉强度为325 MPa, 伸长率为14.6%, 表明具有良好的半固态挤压成形性.     

Fabrication of Mg alloy tubes for biodegradable stent application

Though Mg alloys are promising candidates for biodegradable stents, it is very difficult to fabricate stent tubes with high dimensional accuracy using Mg alloys because of their low deformability. This study aimed to develop thin-walled, high-quality Mg alloy tubes with good performance in stent applications. Cold drawing with a fixed mandrel was carried out for extruded Mg-0.8%Ca and AZ61 alloy tubes using optimized drawing parameters and lubrication, and stent tubes with 1.5–1.8 mm outer diameter and 150 μm thickness were fabricated. A dimensional evaluation showed that the tube dimensional errors were within 0.02–2.5%. Also, an immersion test of pure Mg with different crystal orientations showed that the crystal orientation affected the corrosion properties, results that are the same with other Mg alloys. The crystal orientation of the stent tube could be controlled by changing the deformation amount and direction in the drawing, showing that it is possible to further improve the biodegradability of stents by approaching their fabrication from a processing aspect.     

Deformation mechanisms during uniaxial drawing of melt-crystallized ultra-high molecular weight polyethylene

The crystal deformation mechanisms during solid-state uniaxial drawing of melt-crystallized ultra-high molecular weight polyethylene (UHMW-PE) film have been studied as a function of draw ratio. At higher draw ratios (3) the fine slip processes during uniaxial drawing of melt-crystallized UHMW-PE result in a single-erystal-like (1 0 0) [0 0 1] texture, whereas the normals to the lamellae are inclined by more than 45° with respect to the applied force. It is postulated that in melt-crystallized UHMW-PE the coarse slip process is predominantly restricted due to the fold plane restraints, preventing lamellae from breaking up and rotating with their normals towards the draw direction. The inclination of lamellar normals with respect to the draw direction prohibits further drawing because shear stresses act perpendicular instead of parallel to the lamellar normals.     

Self-reinforcement and hydrolytic degradation of amorphous lactic acid based poly(ester-amide), and of its composite with sol-gel derived fibers

The self-reinforcing and hydrolytic degradation of an amorphous poly(ester-amide) (PEA) based on lactic acid have been studied and compared with those of poly-L-lactide (PLLA). The studied PEA-rods were self-reinforced (SR) by solid-state die drawing resulting double shear strength. The hydrolytic degradation of PEA was studied during exposure to phosphate buffered saline at pH 7.4 and at 37 °C for 18 weeks. The degradation and mechanical properties of PEA were also followed in a self-reinforced composite structure consisting of PEA and sol-gel derived SiO₂-fibers (SGF, 8 wt %). The hydrolytic degradation of the SR-PEA-rods with and without SG-fibers was significantly faster than that of SR-PLLA-rods. The weight average molecular weight (M _w) of PEA decreased by 90% from the initial M _w during the first 6 weeks in hydrolysis, when the M _w of the PLLA decreased by 10%.     

X-ray diffraction investigations of α-polyamide 6 films: orientation and structural changes upon uni- and biaxial drawing

We have investigated the influence of drawing on orientation, crystallinity, and structural properties of polyamide 6 films using X-ray diffraction. The samples were uniaxially and biaxially stretched resulting in the formation of monoclinic crystallites (α-form) in the size range of 8–10 nm. Depending on the drawing ratio, a degree of crystallinity of up to 60% is obtained. The average orientation of the crystallite axes was evaluated using the pole figure technique. The b*-axis, which corresponds to the chain direction of the polyamide molecules, lies in the film plane and shows a preferred orientation upon drawing. For uniaxial drawing, b* aligns with the drawing direction. For biaxially drawn films, which were prepared using the sequential stretching method, the second drawing determines the orientation of b*, at least at the center of the films. At the sides, b* is located between the two drawing directions reflecting the inhomogeneous distribution of mechanical stress during stretching.     

A Dual Modulated Homochiral Helical Nanofilament Phase with Local Columnar Ordering Formed by Bent Core Liquid Crystals: Effects of Molecular Chirality

Helical nanofilament (HNF) phases form as a result of an intralayer mismatch between top and bottom molecular halves in bent‐core liquid crystals (BC‐LCs) that is relieved by local saddle‐splay geometry. HNFs are immensely attractive for photovoltaic and chiral separation applications and as templates for the chiral spatial assembly of guest molecules. Here, the synthesis and characterization of two unichiral BC‐LCs and one racemic mixture with tris‐biphenyl‐diester cores featuring chiral (R,R) and (S,S) or racemic 2‐octyloxy aliphatic side chains are presented. In comparison to the achiral compound with linear side chains forming an intralayer modulated HNF phase (HNF_mod), synchrotron small angle X‐ray diffraction indicates that the unichiral derivatives form a dual modulated HNF phase with intra‐ as well as interlayer modulations (HNF_mod2) suggesting a columnar local structure of the nanofilaments. Transmission electron microscopy and circular dichroism spectropolarimetry confirm that the unichiral materials exclusively form homochiral HNFs with a twist sense‐matching secondary twist. A contact preparation provides the first example of two identical chiral liquid crystal phases only differing in their handedness that do not mix and form an achiral liquid crystal phase with an entirely different structure in the contact zone.     

Consolidation of machined magnesium alloy chips by hot extrusion utilizing superplastic flow

An examination of consolidation conditions by hot extrusion of AZ31 magnesium alloy machined chips was conducted to enhance the bonding of individual chips, in order to improve the mechanical properties. Hot extrusions were carried out in the superplastic and non-superplastic region. Microstructural observations revealed that grain refinement was attained by extruding machined chips, and the grain sizes of the chip-extruded materials were smaller than 5 m. The interfaces of individual chips of extruded materials were not identified when the chips were extruded in the superplastic region. The ultimate tensile strength was about 300 MPa and elongation-to-failure was about 10% for chip-extruded materials that were extruded in the superplastic region. These materials were comparable with the as-received alloy with respect to the room temperature strength, although the ductility was reduced to half. It was confirmed that chip consolidation utilizing superplastic flow is useful to enhance the bonding of individual grains.     

Crystal orientation and some properties of solid-state extrudate of linear polyethylene

Transparent and highly oriented samples of high dimensional stability were obtained by extruding crystallised linear polyethylene through a tapered die at temperatures higher than 80° C and below the melting point. The crystal orientation of the extrudate was examined by X-ray techniques, and it was found that there was a preferential a-axes orientation parallel to the radial direction of the rod-shaped extrudate. Electron micrographs of the fractured surface of the extrudate revealed closely packed thin fibrils of diameters of about 300 Å. Small angle X-ray scattering showed a long period of 200 to 300 Å along the longitudinal direction of fibrils, depending on the extruding conditions. This fact suggests the existence of the lamellar structure in the fibrils.