Deformation and annealing behaviour of a low carbon high Mn TWIP steel microalloyed with Ti

ABSTRACT

A low carbon high Mn, Ti microalloyed dual phase TWIP steel has been processed through cold rolling and annealing. X-ray diffraction reveals the maximum austenite (≈92%) in HRACST sample whereas, the 50CD sample shows 29% ferrite. The microstructure of HRAC and HRACST samples reveal austenite grains with annealing twins and deformation induced ferrite (DIF). The higher amount of DIF along with deformation twins form during cold deformation. Annealing at 500°C shows recovery, whereas at 700°C shows partial recrystallisation and at 900°C reveals almost full recrystallisation. TEM microstructures of the 900°C for 30?min samples reveal annealing twins with TiC particle. Strong Brass {110}<112> and Goss{110}<001> texture components are observed in HRAC, HRACST and 50CD samples. Goss Twin (GT) {113}<332> and Copper Twin (Cu-T) {552}<115> components are observed in 50CD sample. Addition of Ti results in an average grain size of 20?μm. Maximum YS (1176?MPa) and UTS (1283?MPa) values with the lowest ductility of 11% have been obtained for the 50CD sample which is related to the formation of extensive deformation twin and a higher fraction of DIF. 700°C-30?min and 700°C-60?min samples show an increase in ductility (23% and 34%, respectively) with a marginal decrease in tensile strength (1054?MPa). Annealing at 900°C shows ductility restoration up to 60% with higher tensile strength compared to HRACST sample. Ductile fracture of HRAC and HRACST samples transform to brittle fracture in the 50CD sample. Annealing at 900°C for 30?min shows ductile fracture with some (Fe, Mn)S and TiC particles.     

快速凝固Cu-Sn亚包晶合金的电阻率及力学性能

定量表征了快速凝固Cu-xwt%Sn(x=7, 13.5, 20)亚包晶合金的电阻率和力学性能，理论分析了冷却速率与合金性能之间的关系. 研究结果表明，在急冷快速凝固条件下，随着冷却速率的增大，合金组织显著细化、晶界增多，对自由电子的散射作用增强，Cu-Sn亚包晶合金的电阻率升高. 当晶界散射系数取r=0.992时，可用M-S模型分析其电阻率.同时，细晶强化作用增强，合金的显微硬度和抗拉强度呈线性增大，并且细晶区显微硬度略大于粗晶区显微硬度. 冷却速率的增大使合金的伸长率减小，其值在1.0%—4.6%范围.     

Effects of crystallinity on the space charge properties of polyethylene

In this paper, three kinds of annealing methods with different cooling rates are used to prepare low-density polyethylene (LDPE) samples with different microstructure in crystallinity. The effects of crystallinity on space charge properties are investigated. It was found that homo-charges are easier to accumulate in high rate cooling sample and hetero-charges are easier to accumulate near cathode in low rate cooling sample. It was also found that charge movement and neutralization occurs with the increases of the applied field and the voltage applying duration. Based on these results, the influence of crystallinity on space charge accumulation is emphasized.     

Effects of thermal and mechanical history on the viscoelastic properties of rigid poly(vinyl Chloride)

The effects of processing variables on the solid state properties of rigid PVC were studied by evaluating dynamic mechanical and tensile properties for thin film specimens of two different resins. The dynamic measurements were performed over the temperature range ?1]60 to 85°C, encompassing both the low temperature β transition and above ambient a transition (T_g). Engineering tensile strengths and energies to fracture were obtained at ambient conditions for several rates of elongation. Test specimens were prepared by solvent casting and compression molding techniques and subsequently were subjected to various thermal-mechanical histories. The results obtained were similar for both types of specimens and are described below. The various thermal histories considered include: (1) quick quenching from 225°C (samples referred to as “untreated”); (2) very slow (equilibrium) cooling after annealing at T_g; (3) quick quenching from T_g. In addition, the effects of frozen stresses were examined by systematically varying the stresses imposed on samples during the cooling processes 2 and 3. Increasing the load level imposed on specimens during equilibrium cooling resulted in enhancements of the β transition loss dispersion and tensile yield strength. Changes in loading during process 3, however, had little effect on the cooled specimens. But process 3 does alter the relaxation spectrum below T_g so that additional molecular relaxation is induced between T_β and T_α as much as 45°C below the a transition. The anomalous tan δ dispersions thus produced are accompanied by diminished tensile yield strengths and greatly increased energies to fracture. The most extreme case was encountered for the “untreated” specimens which were rapidly quenched from 225°C. The loss tangent data indicate remarkable differences in the region between T_β and T_α. When comparing the dynamic mechanical data with the fracture energy results for the same samples we note that increases in the intensity of the T < T_g anomalous dispersion correlate with increasing energies to fracture. On the other hand, the β transition intensity does not directly correlate. One molecular model which is consistent with these observations assumes that elongation induces a dilation of the polymer. Since most polymers possess Poisson ratios less than 0.5, the dilation will create extra internal volume (including free volume) in the polymer network. The increase in internal volume as elongation proceeds has the net effect of shifting the conditions of testing toward higher temperatures on a molecular relaxation scale permitting a higher level of molecular mobility at ambient conditions. As a sample continues to elongate one of two consequences is encountered: the imposed deformation cannot be accommodated by the available molecular mobility and the specimen fractures; or the deformation results in dilation to the extent that the response properties are shifted into a region of the relaxation spectrum where molecular mobility is sufficient for the specimen to accommodate the imposed deformation and yielding occurs. Yielding is expected if the effective temperature shifts as far as T_g before the sample fractures. In a case where there are additional molecular relaxation possibilities prior to the a transition, such as those in the anomalous dispersion region between T_β and T_α, sufficient dilation for yielding will be encountered before the normal T_g is reached. The anomalous T < T_g relaxation process thus tends to promote increased elongation and higher energies to fracture in PVC.     

Effect of initial orientation on the tensile properties of commercially pure titanium

Effect of crystallographic texture on uniaxial tensile deformation of commercially pure titanium was studied using in situ as well as post-mortem electron backscatter diffraction and elastoplastic self-consistent simulations. Correlation of mechanical properties and strain hardening response with deformation micromechanisms like different modes of slip and twinning was established. Tensile specimens were machined along rolling direction in the plane perpendicular to normal and transverse direction (sample A and C, respectively) as well as along transverse direction in the plane normal to rolling direction (sample B) to obtain different initial texture from cold rolled and annealed plate of commercially pure titanium. Sample B showed higher strength but lower strain hardening rate and ductility than the orientations A and C. It showed extension twinning with lateral thickening while the other samples showed coexistence of extension and contraction twinning. Schmid factor accounted for most of the observed twinning although some contraction twinning in sample A is attributed to the effect of internal stresses. A combination of in situ tensile test in a field emission gun scanning electron microscope with electron backscatter diffraction facility and elastoplastic self-consistent simulations aid in obtaining high-fidelity Voce hardening parameters for different slip and twinning systems in commercially pure titanium. The variation in tensile properties can be explained on the basis of propensity of twinning which tends to provide strain hardening at lower strain but contributes to failure at higher strain.     

纳米孪晶纯铜的强度和导电性

强度和导电性是金属材料的两个至关重要的性能．常用的金属材料强化方式往往是在提高强度的同时使材料的导电性能明显损失，文章介绍了采用脉冲电解沉积技术制备出具有高密度孪晶片层结构的纯铜薄膜．这种具有纳米尺度的孪晶片层结构的纯铜材料不仅具有非常高的拉伸强度，同时还具有非常高的导电性．拉伸实验表明，当孪晶片层平均厚度小到15nm时，样品的拉伸屈服强度可达900MPa，断裂强度高达1068MPa(约为普通纯铜的10倍以上)，并具有与无氧高导铜相当(97％IACS)的室温电导率．     

Morphologies and Mechanical Properties of High-Density Polyethylene Induced by the Addition of Small Amounts of Both Low- and High-Molecular-Weight Polyolefin under Shear Stress Applied by Dynamic Packing Injection Molding

This paper focuses on the mechanical properties and crystal morphology of a self-reinforced high-density polyethylene 5000S (HDPE 5000S) by simultaneously blending with 9 wt% high-molecular-weight polyethylene (HMWPE) and 9 wt% low-molecular-weight polyethylene (LMWPE) (A9) under the shear stress field which was engendered by a self-made dynamic packing injection molding (DPIM) machine. The results of mechanical properties, differential scanning calorimetry, and scanning electron microscopy characterization were as follows: (1) The tensile strength of the dynamic samples increased to 112.1 MPa, 4.85 times as much as that of static packing injection molding (SPIM) samples (23.1 MPa), as a result of realizing polyethylene's self-enhancement; (2) Shish-kebab structure was found in the dynamic samples; (3) The crystallinity of the DPIM A9 sample reached 68.6%, on increase by 18.7% compared with that of the SPIM sample. The formation of the shish-kebab structure and enhancement of mechanical properties are explained.     

Influence of structure on the dynamic mechanical properties of crystalline polymers

A dynamk mechanical theory is presented whose structural elements correspond to such physically measurable material parameters as the compliance of the phases, the percent crystallinity, and the orientation of the crystalline and noncrystalline regions. The theory is tested by using both structural and dynamk mechanical data from samples of isotactic polyproylene films of varying crystallinity and orientation. For the specific case of isotactic polypropylene the general theory is shown to reduce to a simple two-parameter model. It is further shown that the two-parameter model not only identifies the unique properties of each of the phases separately, but also predicts the dynamic mechanical properties of the polymer over a temperature range of—130°C to 120°C, for samples ranging from unoriented to highly oriented, and varying in crystallinity from 40 to 70%. The model can also predict the dynamic mechanical properties over the same temperature range for strips of these samples cut at varying angles to the machine direction.     

Thermal and Structural Behaviors of Polypropylene Nanocomposites Reinforced with Single-Walled Carbon Nanotubes by Melt Processing Method

In this work, polypropylene (PP) matrix reinforced with several single-walled carbon nanotubes (SWNTs) concentrations were prepared by a melt-mixing method. The effect of SWNTs on the thermal degradation behavior of polypropylene was studied by thermal gravimetric analysis. The results revealed that adding the SWNTs into the PP can increase the decomposition temperature. The results obtained from differential scanning calorimetry showed that incorporating SWNTs reduced the crystallinity but increased the crystallization temperature of the PP. The mechanical measurements showed that the tensile modulus of the nanocomposite was greatly enhanced to 882 MPa, compared to 485 MPa for pristine PP. For wide-angle X-ray diffraction tests, two cooling methods were used. The addition of SWNTs to the polymer in slow-cooled samples resulted in partial crystallization in the γ -form, while SWNTs had no effect in water-cooled samples, the sample crystallizing in the α -form. Scanning electron microscopy observations on the fracture surface of the nanocomposites showed the dispersion of the SWNTs in the nanocomposites.     

THERMAL BEHAVIOR OF PP/PA6 BLENDS UNDER DYNAMIC CONDITIONS. THE EFFECT OF DIFFERENT INTERFACIAL AGENTS BASED ON CHEMICALLY MODIFIED ATACTIC POLYPROPYLENE

The thermal behavior in dynamic conditions of polypropylene/polyamide 6 (PP/PA6) blends with a modified interphase is discussed in terms of the crystallinities of the polypropylene and polyamide components imposed by the processing step conditions and after removal of those constraints by holding the blends 5 min in the molten state in the calorimeter. As interfacial agents, two based on succinic anhydride or succinil-fluoresceine grafted atactic polypropylenes were used. The experimental program was run following the Box-Wilson experimental design methodology. Thermal scans were made using round samples (5 mm diameter and 100 μm thick) cut from compression-molded sheets with morphologies and macroscopic behavior studied previously. Changes of the amount of crystallinity of each polymer in the modified blends are contrasted with the tensile strength values of the heterogeneous materials as a whole; evidence of the different roles played by each interfacial agent acting at the interface among blend components is shown.     

On mechanical stability of molybdenum

Investigations of the strength properties of materials under different loading conditions are of practical importance in many engineering applications. The knowledge of elastic moduli as a function of strain is required for determination of strength properties. In the present work, we have determined the elastic moduli of molybdenum through first principles study of the energy changes under three different loading conditions namely ‘uni-axial tensile deformation’ along [0 0 1] direction, ‘uni-axial tensile loading’ along [0 0 1] direction and ‘hydrostatic tensile loading’. The stability conditions for the system are expressed in terms of the elastic moduli and analyzed along the deformation paths corresponding to these three loading modes. The theoretical spall strength (σ_S), tensile strength (σ_T) along [0 0 1] direction and hydrostatic tensile strength (σ_H), are evaluated as a stress at the first onset of the instability for three loading conditions, respectively. The calculated equilibrium volume and elastic moduli are compared with that reported from experimental and other theoretical works.     

Crystallization Behavior and Mechanical Properties of Microinjection Molded High Density Polyethylene Parts

Ultra-thin high density polyethylene (HDPE) parts with two different molecular weights were prepared by microinjection molding (MIM). The dependence of crystalline morphology and orientation, as well as the resulting mechanical properties of the samples, on molecular weight is described. The toughness of the high-molecular-weight (HMW) sample was over 2 times that of the low-molecular-weight (LMW) one, in parallel with a significant increase of tensile strength. Microstructure characterizations, including differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS), were performed to investigate the variations of the microstructure. It is suggested that the increased crystallinity and higher degree of both molecular and lamellar orientation were beneficial to the enhancement of strength of the HMW sample. SAXS results showed that a highly oriented crystalline structure, i.e. shish-kebabs, were formed in parts of both of the two HDPE. Furthermore, a larger number of shish and kebab structure or lamellae was formed in the HMW sample due to the fact that the crystallinity was increased and the lamellar thickness and lateral crystallite size was reduced. Therefore, a stronger physical cross-linking network was formed in the HMW sample because of the increased connection points, which was in favor of the notable improvement of toughness. We suggest this issue is of great significance for achieving materials with high performance by tailoring the microstructure.     

Effects of vacancy and deformation on an Al atom adsorbed on graphene

First-principles calculations based on density functional theory are carried out to study the adsorption energy of monovacancy and deformation on an Al atom adsorbed on graphene. The bond length and Mulliken charge of an Al atom adsorbed on intrinsic and defected graphene systems are also analyzed. We find that an Al atom, sitting above the H site of intrinsic graphene, is in the most stable location. And the adsorption energy increases with increasing graphene coverage. In 1/32 Al/VC-gra and 1/8 Al/VC-gra Al—C covalent bonds are formed, and the Al—C ionic bonds are enhanced by the vacancy. For our calculations, vacancy and deformation both enhance the adsorption energy of an Al atom adsorbed on a graphene system, but vacancy is more effective. In a tensile system, a geometric distortion is induced in the adsorption structure when the tensile deformation is greater than 15%; in a compression system, the adsorption structure begins to distort from 5%. When the tensile and compressive deformations are greater than 10%, the compressive deformation is more effective than the tension deformation on an Al atom adsorbed on the graphene system. Especially, when the deformation is relatively small, a vacancy has a greater effect on the adsorption energy of an Al atom adsorbed on graphene.     

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.     

Influence of laser irradiation on the optical and structural properties of poly(ethylene terephthalate) fibres

Laser irradiation has been previously investigated for achieving uniform heating of polyethylene terephthalate (PET) fibres in the hot-drawing stage of the production process, so as to obtain better fibre mechanical properties. The optical properties and dye uptake of PET fibres also depend on the polymer chain orientation and crystallinity within the fibre structure. This paper reports an investigation of a concept whereby laser irradiation and interferometry could be used to modify and trace a small change in the optical properties of a PET monofilament fibre, but the corresponding change in the dye uptake would not be detected visually. A copper vapour laser (550-580 nm wavelengths) was used to expose consecutive 4 mm lengths along a running length of monofilament to 39.8 W cm⁻², at a pulse rate of 9.89 kHz in order to modify, in a controlled way, the polymer crystallinity and orientation. A 3D finite element simulation, based on uncoupled heat-transfer analysis, indicated that rapid heating and cooling could be obtained with the laser to give the small changes required. Irradiated and untreated samples were analysed by interferometry and a 0.16% change was detected in the birefringence profiles, corresponding to a small reduction in the degree of orientation and crystallinity of the irradiated samples. Density measurements and wide-angle X-ray scattering (WAXS) analysis confirmed the change in crystallinity. Tests conducted for dye adsorption and tensile strength showed a small increase in the former and only a very small decrease in the latter. It was concluded that these changes in property provide the opportunity for a laser-irradiated PET monofilament fibre to be used as a subtle tracer element in brand labels for textile garments as an anti-counterfeit measure.     

High‐energy transmission Laue micro‐beam X‐ray diffraction: a probe for intra‐granular lattice orientation and elastic strain in thicker samples

An understanding of the mechanical response of modern engineering alloys to complex loading conditions is essential for the design of load‐bearing components in high‐performance safety‐critical aerospace applications. A detailed knowledge of how material behaviour is modified by fatigue and the ability to predict failure reliably are vital for enhanced component performance. Unlike macroscopic bulk properties (e.g. stiffness, yield stress, etc.) that depend on the average behaviour of many grains, material failure is governed by `weakest link'‐type mechanisms. It is strongly dependent on the anisotropic single‐crystal elastic–plastic behaviour, local morphology and microstructure, and grain‐to‐grain interactions. For the development and validation of models that capture these complex phenomena, the ability to probe deformation behaviour at the micro‐scale is key. The diffraction of highly penetrating synchrotron X‐rays is well suited to this purpose and micro‐beam Laue diffraction is a particularly powerful tool that has emerged in recent years. Typically it uses photon energies of 5–25 keV, limiting penetration into the material, so that only thin samples or near‐surface regions can be studied. In this paper the development of high‐energy transmission Laue (HETL) micro‐beam X‐ray diffraction is described, extending the micro‐beam Laue technique to significantly higher photon energies (50–150 keV). It allows the probing of thicker sample sections, with the potential for grain‐level characterization of real engineering components. The new HETL technique is used to study the deformation behaviour of individual grains in a large‐grained polycrystalline nickel sample during in situ tensile loading. Refinement of the Laue diffraction patterns yields lattice orientations and qualitative information about elastic strains. After deformation, bands of high lattice misorientation can be identified in the sample. Orientation spread within individual scattering volumes is studied using a pattern‐matching approach. The results highlight the inability of a simple Schmid‐factor model to capture the behaviour of individual grains and illustrate the need for complementary mechanical modelling.     

Effect of annealing on the microstructure and mechanical properties of ultrafine-grained commercially pure Al

The influence of annealing on the microstructure and mechanical properties of ultrafine-grained (UFG) commercially pure aluminum preliminarily subjected to severe plastic deformation by high pressure torsion has been studied. It is found that annealing of the UFG samples in the temperature range 363–473 K for 1 h leads to increases in the conventional yield strength and ultimate tensile strength, which attained maximum values (50 and 30%, respectively) after annealing at 423 K. A key role of nonequilibrium high-angle grain boundaries in the strengthening effect of UFG-Al due to annealing is discussed. The increase in the strength of UFG-Al is accompanied by a significant decrease in its ductility. A new approach of increasing the ductility of UFG-Al with retaining a high strength is proposed. It is an introduction of additional dislocation density to a UFG structure relaxed by annealing.     

Tensile properties of individual multicellular Bacillus subtilis fibers

Microfibers formed by Bacillus subtilis(B. subtilis) have attracted interest because of their potential for use as biodegradable fibers. In this work, an efficient method based on the micro-liquid bridge method(LBM) is proposed to investigate the mechanical properties and the deformation evolution in individual fibers. For the first time, tensile testing of fibers of this type containing several cells is conducted in a scanning electron microscope(SEM) chamber and the in situ deformation evolution of the fibers and the septa is observed. Experimental results show that these fibers are almost broken at the positions of the septa at low humidity, but also show that their fracture morphologies are different. At high humidity, local necking deformation occurs at the septum position. To explore the deformation mechanism of an individual bacterial fiber with a diameter of several hundred nanometers under different humidity conditions, we use the finite element method(FEM) to analyze the tensile deformation behavior of these fibers when their septa are at various separation levels. The numerical results indicate that weak interactions among the septa lead to the dispersion of both the fibrous tensile strength and the modulus. These results may be helpful in understanding the deformation mechanism, thus leading to further improvements in the mechanical performance of these fibers.     

POST-CRYSTALLIZATION AND PHYSICAL AGING OF POLYPROPYLENE: MATERIAL AND PROCESSING EFFECTS

The application properties of plastic products are significantly influenced by the structural changes after conversion, resulting from a combination of post-crystallization and physical aging for semicrystalline materials. For three different types of polypropylene—homopolymer, ethylene/propylene random- and heterophasic copolymer—and two types of conversion—injection molding and cast film extrusion—the aging processes were studied extensively. The temperature level of the aging process plays a decisive role in determining the effects; atleast two important transitions can be identified with increasing temperature. Apart from that, the nature of the polymer and the type and conditions of conversion are also important. The cumulative effects of crystallization behavior of the polymer and cooling history determine the crystallinity and superstructure of the formed article, which in turn determines the aging behavior.     

电感耦合等离子体-原子发射光谱法测定钴基钎料中的主量元素铬和镍

研究了样品溶解和仪器测定的最佳工作条件,通过选择铬、镍元素的灵敏线为分析线,在高低标溶液中加入与样品同量的基体元素消除基体的影响,实现了用电感耦合等离子体-原子发射光谱法测定钴基钎料中铬和镍.样品加标回收率在97.5%-103.5%,用于标准样品中镍、铬的测定,标准样品测定结果与标准值相符,标准样品测定值的相对偏差在±...