Advanced Materials Research Vol. 939

Abstract: A structure of nanocomposite materials consisting of carbon nanotubes with a varying fraction of platinum nanoparticles (5, 10 and 20 wt %) is compared in the paper. High-quality CNTs obtained in the CVD process, 100-200 mm long with a standard deviation of below 20% and with a diameter of 10-20 nm, with a standard deviation of below 30%, were used in the research. Raw CNTs did not contain metallic impurities or amorphous carbon deposits. An indirect method of bonding the earlier produced platinum nanoparticles to the surface of functionalised carbon nanotubes was employed to deposit platinum nanoparticles onto the surface of carbon nanotubes. A full array of changes in the loading of carbon nanotubes’ surface with platinum nanoparticles was achieved as a result of the experiments performed, starting with homogenous deposition to the clearly developed large agglomerations of platinum nanoparticles. The studies carried out using scanning electron microscopy, transmission electron microscopy, scanning transmission electron microscopy and X-ray structural analysis have confirmed differences in the morphology, homogeneity and density of coating the carbon nanotubes’ surface with variedly concentrated platinum nanoparticles. Differences were also revealed in the structure of the newly formed nanocomposites. A nanocomposite with a 5% fraction of platinum nanoparticles demonstrates the best structure-related properties for the materials obtained.

Abstract: In this study, we predicted the maximum nonmetallic inclusion size of bearing steel using statistics of extreme values (SEV) approach, and discussed that the influence of aluminum content on the nonmetallic inclusion characteristic and fatigue life of bearing steel. The inclusion size and type was measured and identified by a scanning electron microscopy (SEM) and an energy dispersive spectrometer (EDS) respectively. The result shows that most inclusions in steels are oxysulfide and alumina cluster. A tendency was also found that the alumina size increases with increasing aluminum content, but the oxysulfide size almost remains unchanged. Furthermore, alumina clusters were formed in steel if more deoxidizer aluminum was added to the molten steel during metallurgical process. In addition, the fatigue life of steel is inversely proportional to alumina size, but it seems independent of oxysulfide size. Therefore, this study suggests that the dominant factor of influence on the fatigue life of bearing steel is alumina instead of oxysulfide. The results reported here would be beneficial to steel manufacturers attempting to improve the fatigue resistance of bearing steels.

Abstract: In this paper, the deformation behavior of adhesive layer in stretch-bending/unbending for adhesively bonded sheet metals was investigated by experiments and finite element method (FEM). We paid special attention to the cyclic shear deformation of the adhesive layers during the plastic working. Major results obtained are summarized as follows: (1) When the adhesively bonded sheet metals is bent and pulled out at a 90° angle, shear deformation due to bending of the adhesive layer starts shortly before reaching the die corner and unbending starts at the middle of the corner. (2) The die radius has a large influence on the bending behavior. (3) It is possible to suppress shear deformation of the adhesive layer by using a material with small tensile strength as one of the two adherends.

Abstract: The paper presents results of in-situ neutron diffraction experiments aimed on monitoring the phase evolution and load distribution in transformation induced plasticity (TRIP) steel when subjected to tensile loading. Tensile deformation behaviour of two TRIP-assisted multiphase steel with slightly different microstructures resulted from different thermo-mechanical treatments applied was investigated by in-situ neutron diffraction. The steel with lower retained austenite volume fraction (f^γ=0.04) and higher volume fraction of needle-like bainite in the α-matrix exhibits higher yield stress (sample B, 600MPa) but considerably lower elongation in comparison to the steel with higher austenite volume fraction (f^γ=0.08), granular bainite and ferrite matrix (sample A, 500 MPa). The neutron diffraction results showed that the applied tensile load is redistributed at the yielding point in a way that the retained austenite bears a significantly larger load than the α-matrix during the TRIP steel deformation. Steel sample with higher volume fraction of retained austenite and less strong ferrite matrix proved to be a better TRIP steel with respect to strength, ductility and the side effect of the strain induced austenite-martensite transformation. The transforming retained austenite in time of loading provides potential for higher ductility of experimental TRIP steel but at the same time acts as a reinforcement phase during the further plastic deformation.TRIP steel, austenite conditioning, austenite transformation, structure, retained austenite, tensile deformation, neutron diffraction, load partitioning, mechanical properties.

Abstract: Bending tests have been used extensively to assess the plastic deformation behavior of bulk metallic glasses (BMGs), however, a detailed experimental investigation of the deformation evolution of BMGs during such bending tests have been rarely reported. In the present work, the deformation evolution of a Zr-based BMG during three-point bending has been studied, and four distinct deformation evolution stages have been observed. After the elastic stage (stage I), the specimen starts to undergo some local plastic deformation while the bending curve still manifests an "elastic" state (stage II). In stage III, the specimen undergoes relatively stable plastic deformation. Finally, in stage IV, more plastic deformation occurs on the tensile side of the bending specimen resulting in the final fracture of the specimen. The current findings provide a fundamental understanding of the deformation mechanisms of BMGs under bending tests, which lays down a good foundation for further investigation of the deformation behavior of BMGs under more complex stress states.

Abstract: The aim of this study is to examine the effect of the clearance and interference-fit on the fatigue life of composite lap joints in double shear, 3D finite element simulations have been performed to obtain stress (or strain) distributions around the hole due to interference fit using FEM package, Non-linear contact analyses are performed to examine the effects of the clearance and interference for titanium and composite lap joint. Fatigue tests were conducted for the titanium and composite lap joints with clearance fit and interference fit with 0.5, 1, and 1.5% nominal interference fit levels at different cyclic loads. The results shows that interference fit increases fatigue life compared to clearance fit specimens, the titanium and composite lap joint with 1% interference fit level has the better fatigue life.

Abstract: Magnesium alloy sheets have a potential to be widely used in many fields of industry due to their excellent lightweight property. Although magnesium alloys have low ductility at the room temperature due to their hexagonal close-packed structure, their formability can be improved at elevated temperatures. Therefore, warm press-forming of magnesium alloy sheets is an attractive technology. The objective of the present work is to investigate the cyclic plasticity behavior of an AZ31 sheet at elevated temperatures by performing cyclic tension-compression experiments. The cyclic deformation mechanism is examined by measuring the crystallographic orientation distributions by means of X-ray diffraction method at each stage of the cyclic deformation. The present findings are summarized as follows: (1) Stress-strain responses of an AZ31 sheet were investigated at various temperatures (R.T, 100, 150 and 200°C) at strain rates of 0.001, 0.01 and 0.05 s^-1. The flow stresses were insensitive to the strain rate at the room temperature, however the strain rate dependency of the flow stress becomes dominant at elevated temperatures of over 100 °C.(2) Cyclic plasticity behavior of the sheet at various elevated temperatures (R.T, 100, 150 and 200 °C) at strain rates of 0.001, 0.01 and 0.05 s^-1 were investigated by performing warm in-plane cyclic compression-tension test. Similarly to the uniaxial tension test, apparent temperature and strain rate dependencies of the flow stress were observed at temperatures of over 100 °C. (3) At the room temperature an unusual cyclic stress-strain curve, which is very different from that of bcc and fcc metals, was observed. From the texture measurement it was found that such a specific stress-strain characteristic is due to its twinning and detwinning deformation mechanism.(4) In contrast, at an elevated temperature of 200 °C, the usual cyclic stress-strain response, which is similar to one appearing in most of metallic materials, was observed. This is because the major deformation mechanism at an elevated temperature is the slip, rather than twinning/detwinning, since the CRSS decreases drastically with increasing temperature.

Abstract: This work deals with the time-dependent creasing characteristics of coated paperboard. The correlation between the bending strength (resistance) and time-dependent problems on the actual processing phenomenon has not been sufficiently discussed in the past. It is difficult to estimate various time-dependent responses from the initial strength of the creased part, such as the maximum bending moment and the initial gradient of bending moment. In this study, therefore, a prototype testing apparatus has been applied to investigate about the dynamic relaxation of bending moment. In order to reveal the relaxation characteristics of the bending resistance during the folding motion from an initial position up to various tracking angles, a white-coated paperboard of 0.3 mm thickness was scored with a creasing rule and a grooved counter face plate under a specified feed velocity, and then the bending test was carried out by varying the tracking angle. Through this work, the followings were obtained: (1) The relaxation of the bending moment was characterized by an exponential coefficient which was independent to the nominal shear strain and the tracking angle. (2) The relaxation depends on the rubbering condition.

Abstract: Co-Cr-Mo alloy powders were subjected to controlled mechanical milling at room temperature under Ar atmosphere to fabricate bimodal microstructure in the MM powders, having nanosized grains in the surface region and micron-sized coarse grains in the center of the milled powders. Subsequently, the MM powder was compacted by spark-plasma sintering (SPS) process. The sintered compacts indicated two structure areas: (i) ultra-fine grained (UFG) regions, called shell, and (ii) the coarse grained regions called core. The shell and the core correspond to the surface and center of the MM powders, respectively. The shell regions established a continuous three dimensional network of high strength ultra-fine grained regions, which surrounded the discrete coarse grained ductile regions. Such a microstructure is referred as Harmonic Structure. The sintered Co-Cr-Mo alloy compacts exhibited outstanding mechanical properties. The yield strength increased from 605 to 635 MPa, and ultimate tensile strength increased from 1201 to 1283 MPa. Moreover, the elongation was maintained more or less same as that of coarse grained compacts. Therefore, the harmonic structure design leads to the new generation microstructure of Co-Cr-Mo alloy, which demonstrates outstanding mechanical properties, i.e. superior strength and excellent ductility as compared to conventional materials. Keywords: mechanical milling, Co-Cr-Mo alloys, mechanical properties, harmonic structure.