S-570 Scanning Electron Microscope Research Articles

Cracking analysis of HVOF coatings under Vickers indentation

The fracture strength of five HVOF coatings, which are made of hard metals, Tribaloy alloy, and superalloys, respectively, coated on 1018 low carbon steel substrate, is studied under Vickers indentation, associated with FEA stress computation. The cross sections of the coating specimens are examined on a Hitachi Model S-570 scanning electron microscope (SEM), which investigates the quality and measures the geometry of the coatings. The mechanical properties of the coatings and the substrate are determined in the cross sections using the nano-indentation technique. The cracking behavior of the coatings under different indentation loads is investigated using a Vickers hardness tester. Three-dimensional finite element analysis (FEA) simulation of the Vickers indentation test is conducted to determine the stress fields in the coating/substrate systems in order to understand the fracture mechanisms of the coatings under the indentation loads using the ABAQUS software package. The FEA stress results are in good agreement with the experimental observation of Vickers indentation.

Deformation and fracture behavior of an Al–Li alloy 8090

Deformation and fracture behavior of an Al–Li alloy 8090

Surface hardening of titanium by thermal oxidation

Titanium and titanium alloys have been widely used in industrial and medical fields due to their excellent corrosion resistance and biocompatibility. Poor wear resistance, however, is one of the major problems of these alloys and restricts them from wider application. Nitriding has been the major surface hardening method used for titanium alloys so far [1–4]. Oxygen can also strengthen titanium by solid solution, but reports of the oxidation strengthening of titanium are lacking. In the present study, the oxygen diffusion layers of titanium, which were obtained after thermal oxidation at 700– 900 ◦C for 1–4 hr, were systemically investigated in order to explore the feasibility of an oxygen diffusion hardening process as an alternative surface modification technique for titanium. Specimens of size 10 mm × 10 mm × 2 mm were cut from commercially pure titanium sheet of TA2 grade (Northwest Nonferrous Metal Institute, China). All the specimens were annealed at 800 ◦C for 1 hr, and then pickled in HF aqueous solution to remove the surface oxide layers and the underlying oxygen diffusion layers. The thermal oxidation was carried out at 700–900 ◦C for 1–4 hr in an air atmosphere. The surface hardness of the oxygen diffusion layers was measured using an HV-1000 Vickers microhardness tester (Shanghai Materials Tester Machine Company, China) with 50–1000 g indenter loads. The cross-sectional hardness profile was measured using a 50 g indenter load. The crystal structure was analyzed using a PHILIPS X-ray diffractometer with Cu-Kα radiation at a scan speed of 4 ◦ min−1. The microstructure of the diffusion layers was observed using a LEICA MPS60 optical microscope and a HITACHI S-570 scanning electron microscope (SEM). The surface oxide on all the specimens was identified as rutile TiO2, as indicated by the X-ray diffraction (XRD) pattern of the oxide film on the 700 ◦C/1 hr treated specimen shown in Fig. 1. By quenching in water following oxidation, the oxide films on all of the specimens could be removed by gently grinding with SiC paper, exposing the underlying oxygen diffusion layers. Fig. 1 shows the XRD patterns of the oxygen diffusion layers of the specimens with various treatments. It can be seen that with increasing treatment temperature or period of time, all the diffraction peaks, corresponding to α-Ti, move to lower angles, suggesting increased lattice parameters. Detailed analyses reveal that the (002) peak, representing the c axis, shifts the most, while the (100) peak, representing the a axis, moves the least. This can be attributed to the fact that the solution of oxygen atoms in octahedral interstices of α-Ti causes mainly the elongation of the c axis and hardly affects the a axis [5, 6]. Table I lists the lattice parameters that were calculated from the 2θ values of (002) and (101) diffraction peaks and the estimates of the associated oxygen concentrations [6]. The higher the treatment temperature and the longer the period of time, the higher the oxygen concentration. However, under the treatment conditions involved in the present research, the oxygen concentration did not reach the highest value of 34 at.%, as reported in Reference [6]. The micrographs in Fig. 2 show the oxygen diffusion layers of the specimens after 800 ◦C/1 hr and 900 ◦C/1 hr treatments. It can be seen that in the region near to the surface there is a black line (indicated by an arrow) that runs across the grain boundaries. The hardness indenter marks in Fig. 2 indicate an obvious hardening effect, even on the interior side of the black line, suggesting that the black line is not the boundary between the diffusion layer and the metal substrate. In

Cathodoluminescence imaging of oxidised zirconium alloys

Cathodoluminescence (CL) imaging techniques have been applied to the characterisation of the zirconium oxide films formed from binary Zr alloy matrices using a homemade CL imaging system attached to an Hitachi S-570 scanning electron microscope (SEM). Evidence from the CL images shows the involvement of the individual Zircaloy alloying elements in the luminescence process. β-annealed and slow-cooled samples which have strongly segregated alloying elements at their grain boundaries showed strong CL signals from these areas. Use of this heat treatment was necessary to compensate for the low resolution of the equipment. Strong CL signals were observed also from cracks in the oxides on some alloys and are believed to be caused by the redistribution of iron to the region adjacent to the crack, but below the original surface. Increasing redistribution of Fe with increasing oxidation temperature is seen in the CL images. The cyclic behaviour observed in the weight gain measurements is also observed in the CL intensity measurements. Experimental data from this investigation support the theory that this phenomenon is caused by the successive formation of cracks parallel and normal to the plane of the metal matrix.

Microstructure characteristics of a hypereutectic Al-Si alloy manufactured by rapid solidification/powder metallurgy process

Hypereutectic Al-Si-Fe-Cu-Mg alloys manufactured by rapid solidification and powder metallurgy (RS/PM) process contain high volume fraction and finely dispersed Si particles and Al-Si-Fe intermetallic compounds (1–10). As a result, the unfavorable effects of coarse primary Si and Al-Si-Fe intermetallic compound caused by conventional ingot metallurgy (IM) process on mechanical properties of the alloy are remarkably eliminated (2, 7). In addition, the sizes of the matrix grain and the other second phases are refined and the solid solubility limit of alloying elements is increased by this process, thus the properties of the alloy are much more improved. In this paper, the microstructure and phase morphology of a RS/PM Al-Si-Fe-Cu-Mg alloy with high silicon content were reported. The nominal composition of the alloy was designed Al-17Si-6Fe-4.5Cu-0.5Mg(wt%). The powders were prepared by means of ultrasonic N2-gas atomization process at pressure of 2 MPa and temperature of 900 ◦C. Consolidation of the alloy powders was carried out by canning, degassing and hot extrusion with a reduction ratio 14:1 after holding at 450 ◦C for 1 h. The phases of the extruded alloy were identified by a D/max-rB X-ray diffractometer (XRD) using Cu-Kα1 radiation. The microstructure was analyzed by S-570 scanning electron microscope (SEM). The SEM specimens were cut directly from the extrudate, followed by grinding, polishing and etching with Killer’s reagent. The size and distribution of the second phase particles in the extruded alloy were measured by image analysis technique. The analysis of the phase morphology was performed using Philips-EM420 transmission electron microscope (TEM) attached with energy dispersive spectroscopy (EDS), operating at an accelerating voltage of 100 KV. Thinning of TEM samples after mechanical grinding of the extrudate was performed by ion beam milling. As shown in Fig. 1, the microstructure of the alloy was primarily consisted of Al, Si, Al5FeSi, Al7Cu2Fe and Al4Cu2Mg8Si7 phases. In contrast to the alloy powder [11], new phases Al5FeSi and Al7Cu2Fe were formed in the microstructure of the extruded alloy. By comparing the microstructures between the powder and extruded alloy, it can be concluded that in the course of vacuum degassing or heating before extrusion, Al5FeSi was formed by phase transformation, regarding Al9FeSi3 as its master phase and Al7Cu2Fe was precipitated from the matrix. Fig. 2 illustrated the microstructure of the extruded alloy under SEM. A large amount of fine particle phases were uniformly distributed in aluminum matrix. The results of image analysis of the particle phases were shown in Fig. 3. It was shown that the average size of these particles was only 0.5 μm and maximum size was less than 2 μm. Because of their small size, the chemical compositions of the particle phases could not be ascertained by EDS. It was presumed that these phases were mixture of Si, Ai5FeSi, Al7Cu2Fe and Al4Cu2Mg8Si7. According to the previous research results [11], there was a great amount of fine needle-shaped intermetallic compound Al9FeSi3 in the powder alloy, yet herein we found it disappeared after hot extrusion. It was

The mechanical properties of the macro-interface in squeeze cast SiCw/AZ91 magnesium matrix composites

Squeeze casting is one of the most cost-effective methods of fabricating discontinuously reinforced metal matrix composites. One of the advantages of squeeze casting over other fabrication methods of composites, such as powder metallurgy, stir casting, etc, lies in its possibility to fabricate selectively reinforced composites, such as selectively reinforced squeeze cast pistons [1], which is low cost, lightweight and durable. In a selectively reinforced metal matrix composite, the preform is normally only used to strengthen a portion of the casting, and the rest casting is not reinforced. Then, a boundary is introduced between the composite and the matrix zones, which is commonly called the macrointerface [2, 3]. As with the interface between the matrix and the reinforcement in the composite, there also exists discontinuity of mechanical and physical properties at the interface between the matrix zone and the composite zone. Most studies in the field of squeeze cast metal matrix composites have been carried out on the region of the casting containing the preform, very few investigations have been focused on the macrointerface between the matrix zone and the composite zone [2–5]. Discontinuously reinforced magnesium matrix composites have great potential in aircraft and automotive application because of their low density and high specific properties [6]. In our previous works, the interface between SiC whisker and magnesium in squeeze cast SiCw/Mg composites has been studied in detail [7–9]. The main purpose of the study is to evaluate the mechanical properties of the macro-interface in squeeze cast SiCw/Mg composites, which play an important role in the application of selectively reinforced magnesium matrix composites. All the castings were produced by squeeze casting under CO2/SF6 atmosphere. In the matrix-composite system, the matrix alloy is commercial heat-treatable AZ91 magnesium alloy (8.5–9.5% Al, 0.45–0.90% Zn, 0.15–0.30% Mn, 0.20% Si, 0.01% Ni, balance Mg), the reinforcement is β-SiC whisker whose volume fraction in the preform is 20% with silica binder. The preforms were cylindrical with 50 mm diameter and 50 mm height. The mold and the preform are preheated to 500 ◦C. Magnesium melt overheated to 800 ◦C was poured into the preform. The casting pressure exerted was 100 Mpa. The properties of macro-interface were evaluated under tensile test on bimaterial tensile specimens containing the macro-interface at the mid-length of the reduced section. The tensile specimens were dog bone-shaped with gauge length of 15 mm and thickness of 2 mm. The specimens were machined from the casting cylinder, with the tensile axis parallel to the axis of the casting cylinder (direction of squeeze casting). For comparison, the tensile specimens with the same dimensions machined from the matrix zone and composite zone, respectively, were also tested. The fracture surfaces were examined using Hitachi S570 scanning electron microscope (SEM). Fig. 1 illustrates the typical stress-strain curves for the AZ91, SiCw/AZ91 composite and the bimaterial tensile specimen containing macro-interface between AZ91 alloy and SiCw/AZ91 composite zone, respectively. It can be seen that the ultimate tensile strength of the specimen containing the macro-interface is lower than that of the AZ91 matrix alloy. The same result was also obtained on selectively reinforced SiCw/Al composite that the ultimate tensile strength of the specimens containing the macro-interface is lower than that of the matrix alloy [3]. The yield strength, modulus and elongation of the bimaterial specimen containing macro-interface lay between AZ91 matrix alloy and SiCw/AZ91 composite. It suggests that there exists a certain interfacial bond strength between the matrix and composite region, and both the matrix zone and composite zone contribute to the deformation during the tensile test of specimen containing the macro-interface. The deformation behavior at different regions of the specimen containing macro-interface can be characterized by strain gauges pasted in different parts of the bimaterial tensile specimen, as shown in Fig. 2. Fig. 3 shows the stress-strain curves at different parts of the

Investigation of Interlaminar Defects and their Influence on Interlaminar Strength

In this paper, we directed our attention to the interlaminar defects and their influence on the interlaminar strengths. With the aid of a S-570 scanning electron microscope, the morphology and distribution of interlaminar defects were inspected and documented. According to their shape, size and cause of formation, the defects were classified into five types: flakiness void, irregular shaped debond, local imperfectly cured resin, debond in two multi-directional plies, and inhomogeneous fibers and the large scale debond by these fibers. The cause of defects formation was discussed by analyzing the manufacturing process of composites. The influence of defects on the interlaminar strength and its mechanism was analyzed experimentally and theoretically. The results indicate that these defects, with different effects, decrease the interlaminar strength because they form interlaminar cracks, and the interlaminar shear strength is less affected than interlaminar tensile strength, which is measured according to GB4944 test method. To comprehend defects distribution effect, a four-point-bending test method was introduced to measure the interlaminar peel strength, and a discussion was made on the correlation between the interlaminar tensile strength, interlaminar peel strength and in-plane transverse tensile strength. Finally the concept of interlaminar defect coefficient, which can be used to characterize the defects, was set up and the formula to calculate it was proposed.

Structural and thermal behavior of coal combustion and gasification byproducts: SEM, FTIR, DSC, and DTA measurements

Structural and thermal behavior of coal combustion and gasification byproducts: SEM, FTIR, DSC, and DTA measurements

High-resolution scanning electron microscopy (HRSEM) of mitochondria from various rat tissues

Our current understanding of mitochondrial ultrastructure has been derived primarily from thin sections using transmission electron microscopy (TEM). This information has been extrapolated into three dimensions by artist's impressions (1) or serial sectioning techniques in combination with computer processing (2). The resolution of serial reconstruction methods is limited by section thickness whereas artist's impressions have obvious disadvantages.In contrast, the new techniques of HRSEM used in this study (3) offer the opportunity to view simultaneously both the internal and external structure of mitochondria directly in three dimensions and in detail.The tridimensional ultrastructure of mitochondria from rat hepatocytes, retinal (retinal pigment epithelium), renal (proximal convoluted tubule) and adrenal cortex cells were studied by HRSEM. The specimens were prepared by aldehyde-osmium fixation in combination with freeze cleavage followed by partial extraction of cytosol with a weak solution of osmium tetroxide (4). The specimens were examined with a Hitachi S-570 scanning electron microscope, resolution better than 30 nm, where the secondary electron detector is located in the column directly above the specimen inserted within the objective lens.

High-Vacuum Evaporation and a Cryostage to Observe Fractured Yeast

Investigators have long realized the potential advantages of using a low temperature (LT) stage to examine fresh, frozen specimens in a scanning electron microscope (SEM). However, long working distances (W.D.), thick sputter coatings and surface contamination have prevented LTSEM from achieving results comparable to those from TEM freeze etch. To improve results, we recently modified techniques that involve a Hitachi S570 SEM, an Emscope SP2000 Sputter Cryo System and a Denton freeze etch unit. Because investigators have frequently utilized the fractured E face of the plasmalemma of yeast, this tissue was selected as a standard for comparison in the present study.In place of a standard specimen holder, a modified rivet was used to achieve a shorter W.D. (1 to -2 mm) and to gain access to the upper detector. However, the additional height afforded by the rivet, precluded use of the standard shroud on the Emscope specimen transfer device. Consequently, the sample became heavily contaminated (Fig. 1). A removable shroud was devised and used to reduce contamination (Fig. 2), but the specimen lacked clean fractured edges. This result suggested that low vacuum sputter coating was also limiting resolution.