Evolution of morphology in high molecular weight polyethylene during die drawing

Deformed high molecular weight polyethylene (HMWPE) rod, formed by die drawing at 115C, was cleaved longitudinally at liquid nitrogen temperature and the cleaved surface was etched by the permanganic etching technique, A series of etched surfaces of HMWPE sections of variable draw ratio (1–13) was analysed by scanning electron microscopy (SEM), The evolution of crystalline structure in HMWPE during die drawing was observed directly. In undrawn HMWPE, the spherulites were made up of sheaf-like lamellae and scattered within an amorphous phase. During die drawing, first, microscopically inhomogeneous deformation occurred and the spherulites aligned along the drawing direction; then at a draw ratio of about 7, local melting occurred, the spherulites disintegrated and the sheaf-like lamellae oriented, followed by strain-induced recrystallization and the growth of the lamellae; finally, at a draw ratio of about 12, plastic deformation of the lamellae occurred and microfibrils were formed by drawing the lamellae.     

Correlation of hardness and microstructure in unoriented lamellar polyethylene

The local deformation of the lamellar microstructure of isothermally melted crystallized unoriented polyethylene has been investigated using microindentation hardness (MH). The polymer can be visualized as a composite material consisting of hard and weak elements. The former, the lamellae, are considered to consist of mosaic blocks with liquid-like lattice distortions (paracrystallites). The latter are the interlamellar amorphous regions and the mosaic block lateral grain boundaries. The deformation mechanisms beneath the indenter are discussed in the light of current models of plastic deformation. MH is shown to depend on the packing density of the macromolecules in both phases and, as a result, it can be clearly correlated with the macroscopic density of the material. The unit cell expansion and lattice distortions increase in parallel as a consequence of increasing incorporation of chain defects within the lattice. This provokes a conspicuous decrease in the microhardness of the crystals. The increase in lattice distortions is consistent with the concurrent decrease of lamellar thickness and, hence, of the coherently diffracting lattice volume. These results unambiguously emphasize the physical significance of the mosaic block character of the lamellae in determining the micromechanical properties of the material. Finally it is shown that the strain boundary which defines the zone of crystal destruction under the indenter also depends on the average volume of the paracrystallites and on the volume fraction of crystalline material.     

Micromechanical properties in lamellar heterophase polymer systems

Several nanoanalytical techniques based on electron and atomic force microscopy were used to analyse the micromechanical deformation mechanisms in different nanostructured lamellae forming heterogeneous polymers: in (semicrystalline) -modified isotactic polypropylenes and (amorphous) lamellar styrene/butadiene block copolymers. It was found that the deformation processes in these two entirely different classes of materials are governed by fundamentally similar mechanisms due to similar dimension and arrangement of the nanostructures. The basic mechanism shows two steps: The initial step is determined by a plastic deformation of the soft (amorphous or rubbery) phase with a reorganisation of the hard (crystalline or glassy) lamellae and their orientation towards the deformation direction. The second step is characterised by the intense plastic deformation (yielding) of the hard lamellae up to elongations of several 100%.     

Crack growth in lamellar titanium aluminide

In-situ compact tension tests on binary lamellar titanium aluminide (TiAl) possessing the colony ``polycrystalline' microstructure illustrate a range of damage phenomena and toughening mechanisms including crack nucleation across colony boundaries, plastic deformation of bridging ligaments, and multiple cracking within colonies. Here, the effects of relative lamellae misorientation and offsets between neighboring colonies on crack growth are investigated computationally through an idealized microstructure of multiple colonies. Within each colony, the brittle Ti₃Al lamellae are represented as parallel planes of comparatively low toughness embedded in a matrix of ductile TiAl lamellae that are collectively modeled as an elastic-viscoplastic solid with higher fracture toughness. Plane strain calculations of crack growth are carried out on a compact tension geometry. The calculations are in good qualitative agreement with the in-situ observations, capturing many features of crack growth such as multiple microcrack nucleation and plastic deformation of residual ligaments. Experiments and numerical analyses show that changes in lamellar orientation and alignment across a colony boundary can contribute significantly to the fracture resistance. The numerical results demonstrate that the fracture resistance of these alloys is determined by an intricate interplay between matrix ductility, Ti₃Al and TiAl fracture toughnesses, and colony boundary toughness. This suggests the possibility of computationally-guided material optimization through microstructural control of these material properties.     

Work-hardening and susceptibility to plastic flow in metallic glasses (rolling deformation)

Fe-Ni-base metallic glass ribbons were rolled directly and indirectly by changing the rolling direction with respect to the ribbon axis. In the case of direct rolling, no visible deformation bands appeared on the rolling surfaces (except at the edges) but wavy deformation markings appeared on the side surfaces. In the case of indirect rolling, however, the deformation markings developed on the entire surface; they were wavy and straight in appearance on the rolling and side surfaces, respectively. Tensile tests, performed on the samples rolled directly and indirectly show little and much change in fracture stresses, respectively. As a result of the intersecting of plastic flow systems, fracture shear stresses increase by 7% compared with those of as-quenched samples. Fracture shear stresses on a predeformed area were, however, found to be 3% lower than those for an undeformed area. These results are discussed in terms of both work-hardening and work-softening in metallic glasses.     

Thermal stability of aluminum cold rolled to large strain

Common to metals deformed to high strains is a very fine microstructure, high strength, and limited ductility. Structure and property optimization by annealing after deformation must, therefore, be explored. In the present study, commercial purity aluminum has been annealed after cold rolling to ultrahigh strains up to and annealing processes have been studied in terms of recovery and conventional recrystallization. These processes have been analyzed by isochronal and isothermal annealing in the temperature range 140–420 °C. It has been found that the recrystallization temperature is a little affected by the rolling strain, whereas the rate of recovery and the temperature range over which recovery takes place increase significantly as the strain is increased. These observations are discussed as to how they can guide studies of nanostructured metals processed by plastic deformation.     

Direct visualization of microstructural deformation processes in polyethylene

The deformation morphology of high density polyethylene was investigated by electron microscopy of oriented thin films. A melt-drawing process was used to produce chain axis oriented and planar textured thin film, which was subsequently deformedin situ at room temperature in a scanning transmission electron microscope. Both as-drawn and annealed films were studied. Deformation along the orientation direction is initially accomodated by the interlamellar regions, which cavitate and form microfibres. For annealed films it is possible to directly observe strain-induced crystallization at about 300% strain in the fibrils. With increased deformation, suitably oriented lamellar crystals deform by two clearly visualized chain slip systems: {1 0 0}, 0 0 1 and {0 1 0}, 0 0 1. Thesec axis shear processes could be further distinguished as fine slip or as block shear. Still higher deformation causes more breakup of blocks by shear; when the block size is less than some critical size, the blocks decrystallize. The deformation leads toward a fibrillar morphology consisting of oriented crystals from crystallized amorphous material at high elongations, crystal blocks broken out of lamellae, and chains drawn out of lamellae and recrystallized. The deformation behaviour of the as-drawn films is somewhat different from that of the annealed films. Initially as for the annealed films the lamellae are separated with increasing strain as the interlamellar regions are deformed but there is less voiding. Higher deformation causes the lamellae in the as-drawn films to shear apart at significantly lower strain levels (50% as opposed to 300%) than in the annealed films. At about 100% deformation, the as-drawn film no longer has a recognizable lamellar structure. Although generalization is tempered by the simplicity of this model texture, these deformation results are highly relevant to the current microstructural understanding of lamellar deformation in different regions of a spherulite, to the morphology of commercial extruded and blown films, and to specially prepared textured polymers, such as rolled and annealed films or capillary melt flow and solidification methods which can produce texture approaching that of a single crystal.     

Time-temperature-transformation diagram of polyethylene: origin of the double population of lamellae

The factors leading to the appearance of two lamellar populations in linear polyethylene were investigated using differential scanning calorimetry and mechanical spectrometry. Two systems which underwent identical thermal treatments but which differed by their polydispersity degree show similar lamellar thickness distribution curves. It was shown that the primary factor governing the breadth of the crystallite size distribution is not the dispersity degree of the molecular weight of polyethylene. A comparison of the dynamic mechanical behaviour and the lamellar thickness distribution curves of two systems which have different sheet thicknesses indicates the existence of a single population of lamellae for the thinner specimen and two populations of crystallites for the thicker film. Thus, the appearance of a broad distribution size of crystallites depends on the existence of a distribution of the cooling rate within the sample which induces a morphological gradient because of the well known low thermal conductivity of polyethylene. In addition, based on analysis of the isothermal crystallization behaviour of linear polyethylene carried out by means of a mechanical spectrometer, a time-temperature-transformation (T.T.T diagram) was established. Thus, by drawing on physical metallurgy concepts, an original interpretation for the appearance of two populations of lamellae based on different cooling rates within the sample is found.     

Lamellar separation during the deformation of high-density polyethylene

Upon tensile straining at low deformation rates (=2×10^–5 sec^–1), spherulitic linear polyethylene behaves reversibly for extensions up to 40%. In stress relaxation experiments on unloaded specimens the stress increases with time. Samples kept under constant strain ( = 40%) over four months show macroscopic cracking. Microstructural investigation was performed using low- and wide-angle X-ray diffraction and transmission electron microscopy. These investigations reveal a very inhomogeneous deformation within those lamellar stacks for which the crystalline lamellae lie normal to the tensile axis. The deformation in that case is similar to what has been observed for elastic hard fibres. A two-mechanism model to explain the macroscopic observation on the basis of the microscopic observations is developed.     

Superplastic deformation of strongly textured Ti-6Al-4V

Changes in microstructure and texture during superplastic deformation of strongly textured Ti-6Al-4V bar have been determined in order to establish the cause of stress and strain anisotropy. The effect of strain on the microstructure of the alloy was to cause a progressive break-up, due to grain-boundary sliding, of an initially directional microstructure containing contiguous-phase. The texture of the-phase, however, changed very little with superplastic strain but that of the-phase was randomized. Shape changes predicted by permitted deformation modes in the-phase did not correlate with the observed shape changes, whereas the observed anisotropy could be explained by the break-up of the contiguous-phase. A model to account for this anisotropy is described briefly, together with a typical microstructure which should exhibit isotropic superplastic deformation.     

行星轧制变形程度对TP2铜管材组织及性能的影响

目的研究行星轧制变形程度对TP2铜管材在轧制和联拉时的组织和性能的影响。方法采用金相显微分析和拉伸实验,研究行星轧制变形程度对TP2铜管铸坯轧拉态以及拉拔态组织及性能的影响规律。结果经连续铸造的TP2铸坯为柱状晶,且由外向内成长。经行星轧制、联拉后的管材晶粒纤维流线,其晶粒显著拉长,随着轧制变形程度的增加,流线减弱,晶粒更加细化。轧制变形程度为93%与90%的轧管屈服强度、抗拉强度分别降低了22.83%和7.59%,伸长率提升了4.44%,塑性变形能力增加。结论随着轧制变形程度的增加,联拉管抗拉强度略有提高,而伸长率得到了保持。     

High temperature strength at 1773 K and room temperature fracture toughness of Nbss/Nb5Si3 in situ composites alloyed with Mo

High temperature compressive strength at 1773 K and room temperature fracture toughness have been studied in terms of microstructure, phase stability and solid solution hardening in Nb-Si-Mo in situ composites consisting of niobium solid solution and Nb₅Si₃. Molybdenum addition stabilizes the -Nb₅Si₃ phase and makes unstable Nb₃Si phase in the in situ composite. It is found that molybdenum has a strong effect to increase the yield stress of the present in situ composite at 1773 K due to solid solution hardening. Yield strength depends not only on chemical composition and volume fraction but also the Nb₅Si₃ phase itself. Room temperature fracture toughness is very sensitive to microstructure and the content of ternary alloying element, but not to the volume fraction of constituent phases within the composition ranges investigated. It is suggested that plastic deformation of Nb solid solution and interface decohesion is responsible for high fracture toughness in this alloy system. Details are discussed in relation to microstructural features and Molybdenum alloying.     

The relationship between the mechanism of formation,microstructure and properties of plasma-sprayed coatings

The mechanism of formation of plasma-sprayed coatings was examined and related to the microstructure produced. The evidence suggests that the real area of contact between individual lamellae within the coating and between lamellae and substrate is much less than the apparent area because of adsorbed and entrapped gas, oxide films or other contamination. The measured fracture toughness parameters for cohesive failure of coatings are generally much lower than would be expected for complete wetting of previously solidified material by impinging droplets, reflecting the imperfect contact between lamellae. Similar considerations apply to the lamellae-substrate interface at which the contact angle would generally be greater than for lamellae-lamellae interfaces. The difference between the fracture toughness values for ceramic and metallic coatings and the role of a metallic subcoat under ceramic coatings can be explained in terms of plastic deformation of metallic lamellae. The very high adhesive fracture toughness of NiAl coatings on steel implies more effective contact rather than inherently stronger bonding between contact points. This may be due to aluminothermic reduction of the oxide film on steel. Improvement of the mechanical properties of plasma-sprayed coatings requires methods for increasing the real area of contact between lamellae and between lamellae and substrate.     

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.     

Microstructure and scaling effects in the damage of WC-Co alloys by single impacts of hard particles

The size and nature of the craters produced by single impacts of angular Al₂O₃ particles on WC-Co alloys (6 to 50 wt% Co) were investigated as a function of microstructure, impacting particle size (63 to 405 m), velocity (35 to 93 m sec^–1) and angle of impact (30 to 90°). In general, the size of the craters increased with increase in particle size and velocity and with cobalt content of the target. Two distinct types of behaviour occurred depending on the number of WC grains encompassed by the projected area of the crater. When the number was less than 10, the crater was formed mainly by cracking of WC grains (brittle regime); when it was more than 100, the crater resulted mainly from the plastic deformation of the binder phase (ductile regime). The cracking of the WC grains in the brittle regime did not have the typical appearance of either cone or lateral cracks. The behaviour in the ductile regime correlated reasonably well with the model for plastic indentation; the derived work of plastic deformation was, however, only about 10% of the initial kinetic energy of the impacting particle. Besides the usual elastic effects, fragmentation of particles was considered to be responsible for a significant fraction of the kinetic energy loss, The effects of microstructure of the target on the crater size in the ductile regime was deduced to be mainly through its effect on the hardness, which was given reasonably well by the Lee and Gurland relation.     

Analysis of deformed YBa2Cu3O7−δ

In this study, polycrystalline YBa₂Cu₃O_7– (123) was deformed under controlled conditions with a confining pressure of 1.0GPa, temperatures of 25, 500 and 800° C, and a strain rate of 10^–4 sec^–1 in order to ascertain the micromechanisms of deformation that give rise to the macroscopic plastic behaviour. The deformed material was analysed using optical microscopy, transmission electron microscopy (TEM), and a SQUID magnetometer to study the effects of deformation on the microstructure of YBa₂Cu₃O_7– and how changes in the microstructure affected the superconducting properties. The results of these preliminary experiments suggest that the 123 material will be very difficult to deform plastically as slip occurs only on the (001) plane. The lack of multiple slip systems implies that this material will show some brittle behaviour up to a very high homologous temperature. Even when plastic behaviour can be sustained for high strains it may require high annealing temperatures to remove lattice imperfections which impede the superconducting currents. Densification by high pressure deformation may make reoxygenation difficult due to the reduced diffusion rates between the grains. These factors combined suggest that traditional fabrication techniques are not applicable to the 123 material. More work needs to be carried out to determine how annealing affects the microstructures of deformed materials and how these changes in microstructure affect the superconducting properties of these materials.     

Influence of Skin Pass Design and Aging Phenomena on Steel Sheets Surface Characteristics

This paper studies the behavior of non-alloy sheet steel in very low plastic deformation. The steel was subjected to temper rolling (skin pass) with different ratios till 5%. The sheet steel also was subjected to aging after temper rolling. The tensile curves were plotted and their characteristics were determined. The work-hardening coefficient (n) was determined after temper rolling directly and after aging to judge the general behavior of sheets. The microstructure observation was carried out o notice the surface stretcher strains (kinks). Lüder’s bands were observed in aged steel sheets leading to high surface roughness. The surface roughness was measured after temper rolling or aging to judge the surface quality.     

Deformation of polyoxymethylene by rolling

The deformation due to rolling of polyoxymethylene (POM) was investigated by using wide and small angle X-ray techniques and electron microscopy. Tensile tests of rolled POM indicate that the yield stress increases along the roll direction. This is accompanied by a decrease in the yield stress perpendicular to the roll direction. Wide angle X-ray data from uniaxially rolled POM, obtained by means of pole figures, indicate that molecular chains tilt preferentially at approximately 30° to the roll direction at low rolling deformation, and align in the roll direction when the sample is rolled to its fullest extent. A lamellar tilt of of about 30° is also observed. Thus, the chains must tilt within the lamellae. When samples are fully rolled, small angle patterns indicate at least partial breakup of lamellae. Biaxial rolling produces no such breakup, but a uniform tilting of lamellae through the entire range of deformation.     

On the plastic deformation of chain extended polyethylene

Deformation of the lamellar microstructure (average lamellar thickness 500 nm) of a high molecular weight chain-extended polyethylene after drawing five-fold has been investigated using a variety of microscopic and other techniques. As the interlammelar regions in this material are expected to be qualitatively similar to those in chain-folded polyethylene, the former may be considered as a large scale copy of the latter. It is shown that: the great majority of lamellae survive the draw with their thickness along c only slightly reduced although 20% appear to become disrupted; many lamellae undergo a high shear parallel to c, although not always a uniform one; lamellar rotation and interlammelar shear probably also occur as may the formation of voids and/or crazes; drawing does not involve melting.     

Deformation mechanism and texture and microstructure evolution during high-speed rolling of AZ31B Mg sheets

High-speed rolling of AZ31B was carried out under various preheating temperatures from RT to 350 °C. The evolution of texture, grain sizes, and dislocation density distribution (Kernel average misorientation distributions) in the mid-thickness and surface layer were investigated. Computer simulations of deformation textures were also performed in order to understand deformation mechanisms. It is concluded that the temperature increase due to the plastic and frictional working during high-speed rolling makes the <c+a> slip system more active and, hence, improves the ductility. The surface layer of the specimen has higher temperature and experiences severe shear stress; therefore the texture, microstructure, and dislocation density distribution are different from those of the mid-thickness of the specimen. Both mid-thickness and surface layer are dynamically recrystallized during the high-speed rolling.