Microstructure of electromagnetic stir cast grain refined iron base alloy

Abstract

Electromagnetic stirring using multiple induction coils has been investigated as a method of preparing semisolid FC20 alloy. The effects of additions of titanium, zirconium, and boron on the microstructure of the FC20 alloy have also been studied. The electromagnetic flux density increased with input voltage and was higher at the wall of the container than near the centre of the coil. The microstructure of the alloys varied with cooling rate: the higher the cooling rate, the smaller the particle size and the greater the degree of sphericity. The additions of titanium, zirconium, and boron also improved the alloy microstructure.     

Kinetic equations for non-equilibrium grain boundary segregation induced by applied tensile stress

Abstract

Based on an earlier model, a set of kinetic equations is derived to formulate the process of non-equilibrium grain boundary segregation induced by a low tensile stress. These kinetic equations allow excellent simulation of the grain boundary segregation of phosphorus and sulphur observed in steels subjected to low tensile stresses. The simulation results justify both the earlier model and the present kinetic equations. They also show that an applied tensile stress can increase the diffusion rate of solute-vacancy complexes and decrease that of isolated solute atoms significantly, and can also bring forward the critical time of non-equilibrium segregation for phosphorus and delay that for sulphur.     

Microstructure and wear characteristics of spray formed and hot extruded Al–Si alloys

Abstract

The microstructural and wear properties of spray formed Al–6.5Si, Al–18Si and Al–18Si–5Fe–1.5Cu (wt-%) alloys have been investigated. The microstructure of the Al–6.5Si alloy exhibits the equiaxed grain morphology of the primary α-Al phase with eutectic Si at the grain boundaries. The size of the primary Si particulates in the Al–18Si alloy varied from 3 to 8 μm embedded in the eutectic matrix. Complex intermetallic phases such as β-Al₅ SiFe and δAl₄ Si₂ Fe are observed to co-exist with primary Si in the spray formed Al–18Si–5Fe–1.5Cu alloy system. The periphery of the preforms invariably showed pre-solidified particles with a large amount of interstitial pores. An extrusion ratio of 6 : 1 for these alloys led to drastic porosity reduction and extensive breaking of second phase particles. These microstructural features showed distinct variation in the wear behaviour and the coefficient of friction of the alloys. The Al–18Si–5Fe–1.5Cu alloy shows better wear resistance compared with the other two alloys, particularly at higher loads. The coefficient of friction shows a dependence upon the applied load. However, this becomes steady at higher loads. The wear behaviour of these alloys is discussed in light of the morphology of debris particles as well as that of the worn surfaces.     

Impedance spectroscopic studies of ferroelectric Pb2Sb3DyTi5O18 ceramic

Abstract

The electrical properties of polycrystalline Pb₂Sb₃DyTi₅O₁₈ were investigated by complex impedance spectroscopy. The dielectric relaxation was found to be of polydispersive non-Debye type, and relaxation frequency shifts to higher frequencies with increasing temperature. Evidence of temperature dependent electrical relaxation phenomena as well as the negative temperature coefficient of resistance (NTCR) character of the sample have also been observed. The ac conductivity obeys the universal power law and dispersion in conductivity was observed in the lower frequency region. Also, the frequency dependent ac conductivity at different temperatures indicated that the conduction process is thermally activated. The activation energy was estimated to be 0·62 eV from the temperature variation of dc conductivity.     

Heat resistance and thermomechanical behaviour of ultralow and zero cement castables

Abstract

Knowledge of relative heat resistance as well as thermo-mechanical behaviour of refractory castables is very important for their use as linings in high temperature furnaces and refining vessels in the metallurgical, cement, and petrochemical industries. The present work aims at studying these properties for different types of refractory castable. Two classes of castable were prepared, namely ultralow and zero cement, containing either high alumina cement or hydratable alumina as bonding agent. For each class, two different castable systems were prepared, one containing an alumina-silica mixture in its matrix and the other containing magnesia-alumina. In all castables studied, calcined alumina was used as aggregate. The prepared castable samples were subjected to firing temperatures up to 1500°C. Relative heat resistance, bending strength before and after thermal cycling, hot modulus of rupture, and creep deformation were measured according to international standard specifications. It was concluded that a limited content of cement (ultralow cement castables) is beneficial with the magnesia-alumina mix in the matrix owing to the formation of calcium hexaluminate-magnesium aluminate-corundum (matrix advantage system) that results in excellent relative heat resistance as well as thermome-chanical properties. Zero cement castables on the other hand are recommended for use with the alumina-silica mixture, since the absence of cement improves the chances of mullite formation without glassy phases, thereby enhancing the properties of such refractory castables.     

Wear behaviour of in situ Cu–Al2O3 composites produced by internal oxidation of as cast alloys

Abstract

In the present study, the wear behaviour of Cu–Al₂O₃ composites and Cu–Al alloys has been investigated. The experiment involved casting of Cu–Al alloys with 0·37, 1, 2 and 3 wt-% of aluminium under inert gas atmosphere. The composites were produced by internal oxidation of alloys at 950°C for 10 h in presence of Fe₂O₃ and Al₂O₃ powders mixture. The microstructures of composites were studied using SEM and atomic force microscopy. To identify wear behaviour of specimens, dry sliding pin-on-disk wear tests were conducted according to ASTM G99-95a standard. The normal loads of 20, 30, and 40 N were applied on specimens during wear tests. The sliding speed and distances were selected as 0·5 m s^–1 and 500, 1000 and 1500 m respectively. To specify the wear mechanisms, the worn surfaces of composites were examined by SEM equipped with EDX. According to wear test results, increasing applied load and sliding distance leads to more volume loss in all specimens. Composites represent better wear resistance in comparison to alloys. Additionally, increasing the volume fraction of alumina particles in composites enhances the wear resistance, especially under high applied load. The wear mechanisms are mainly abrasion, oxidation and delamination.     

Tribological behaviour of glass fibre bundles reinforced epoxy gradient composites

Abstract

The short fibre bundles separated from the machining waste of a printed circuit board manufacturing plant were used in preparing functionally graded composites using polysulphide modified epoxy resin. Glass fibre bundles were thouroughly mixed with epoxy, which is getting polymerised with time and centrifugal force was applied to achieve graded dispersion of glass fibre bundles. The centrifugation time was varied to obtain different gradient profiles. Optical microstructures confirmed the graded dispersion of glass fibres bundles in the epoxy matrix. Increase in distance towards the centrifugation force direction increases the glass fibre concentration. Gradient characteristics in the composite have been observed in wear and friction measurements, which were conducted using a pin-on-disc machine. Worn surfaces of samples were analysed with the help of SEM. Both sliding (adhesive) and abrasive wear rates of glass fibre reinforced epoxy gradient composites reduced with increasing centrifugation time. Reduction in wear rate in glass fibre epoxy gradient composites has been attributed to the better interface bonding between epoxy coated fibre bundles and the epoxy matrix and hardening property of glass fibre. It has been found that capability to sustain pressure limit increased from 0·59 to 0·79 MPa on centrifuging the sample upto 2 min and reached to 1·19 MPa with increasing the centrifugation time to 30 min.     

Sliding wear characteristics of grey cast iron as influenced by sliding speed,load and environment

Abstract

The present investigation pertains to the observations made during sliding of a grey cast iron against a steel counterface over a range of sliding speeds, applied loads and test environments. The nature of the environment was altered through the presence of oil and suspended graphite particles therein. The presence of oil improved the wear characteristics of the samples in terms of lower wear rate and decreased frictional heating in general. An additional presence of suspended graphite particles in the oil lubricant brought about a further improvement in the wear response of the samples in all the test conditions except at the highest speed at high applied loads; the trend reversed in the latter case. Increasing speed and load led to deterioration in the wear behaviour. The behaviour of the material has been explained in terms of specific response of different microconstituents such as pearlite, ferrite and graphite and corroborated with the observed features of wear surfaces, subsurface regions and debris particles.     

Wear mechanisms of diamond coated dental burs

Abstract

Wear mechanisms of diamond burs consist of diamond wear-out, diamond pull-out, clogging by debris and degradation of the diamond binder material. These have been reported in the scientific literature and several discrepancies were found by the authors, which in itself, justifies an independent study. Diamond coated dental burs before and after use on human teeth were, therefore, compared in order to identify the predominant cause of wear. Fifteen new diamond coated burs were characterised using scanning electron microscopy before and after use on human teeth. The study focused on the condition of the same diamond particles before and after use. Clear evidence of diamond particle wear was detected rather than evidence of diamond pullout, clogging by debris or degradation of the diamond binder material.     

Biopolymer wear screening rig validated to ASTM F732-00 and against clinical data

Abstract

The ASTM standard F732-00, 'standard test method for wear testing of polymeric materials used in total joint prostheses' offers several pieces of data against which a test rig for such biopolymers can be validated. These conditions include a linear wear in the range from 0˙07 to 0˙2 mm/million cycles for ultrahigh molecular weight polyethylene (UHMWPE), and a wear rate ranking of poly tetra fluoro ethylene (PTFE)>>polyacetal≥UHMWPE. Using a modified pin on plate wear test rig, these three biopolymers were tested under a 40 N load against polished stainless steel plates while using dilute bovine serum heated to 37°C as the lubricant. When subject to multidirectional motion, mean wear factors of 37×10^–6 mm³ Nm^–1 for PTFE, 3˙8×10^–6 mm³ Nm^–1 for polyacetal and 1˙1×10^–6 mm³ Nm^–1 for UHMWPE were obtained. Therefore the wear ranking recommended by the ASTM standard was achieved. When the corrected mean weight loss of the UHMWPE test pins was converted to a linear wear rate, a value of 0˙12 mm/million cycles was obtained. Therefore the ASTM recommended target for linear wear rate was met. When the wear factors from the test materials were compared with clinical wear factors of 37×10^–6 mm³ Nm^–1 for PTFE, 3˙5×10^–6 mm³ Nm^–1 for polyacetal and 0˙95– 1˙45×10^{– 6} mm³ Nm^–1 for UHMWPE, good correspondence between in vivo and in vitro wear factors was obtained, further validating the modified pin on plate wear test rig.