Reaction sintering and joining nickel aluminide to AISI 316 stainless steel by self-propagating high temperature synthesis

Abstract

Self-propagating high temperature synthesis (SHS) is a process whereby reactants are ignited to spontaneously transform to products in an exothermic reaction. The aim of this study is to propose a method to join nickel aluminide with AISI 316 stainless steel by SHS and to study the combustion synthesis of nickel aluminide. From the heat of combustion synthesis junctions were formed between annular AISI 316 stainless steel and a powder metallurgy compact of Ni and Al blends. The Al mole ratio for testing the joining grade in the initial powder mixture varied from 25 at.-% to 40 at.-%. In order to check the sufficiency of the SHS reaction, the test temperature was compared with the thermodynamic calculation values. The metallographic analysis indicated that NiAl and Ni₃Al were formed in the joint layer.     

Synthesis and processing of nanosized sodium potassium niobate powders

Abstract

Sodium potassium niobate powders have been produced successfully using a Pechini sol–gel method modified with a novel niobium precursor. The decomposition of the gels produced by this method was observed with thermogravimetric analysis and evolved gas mass spectrometry. This showed three main weight loss stages up to 1000°C arising from the loss of water, methane and carbon dioxide. The effect of different heat treatments of the equimolar gel was studied and correlated to particle properties and phase development. The resultant calcined powders were nanosized and single phase with some residual organic material and water. The effect of varying the composition between sodium niobate and potassium niobate was also studied and correlated to particle size and phase development.     

Mechanical and chemical properties of Ni3Si and Ni3(Si,Ti) alloys multiphased by chromium addition

Abstract

The alloying behaviour and microstructure of Ni–Si–Cr ternary and Ni–Si–Ti–Cr quaternary alloys were first characterised by optical microscopy, X-ray diffraction, and scanning electron microscopy with electron probe analysis. The microstructures of the Ni–Si–Cr ternary alloys consisted of large dispersed Ni₅Si₂ phase and finely precipitated Ni₃Si phase in nickel solid solution, while the Ni–Si–Ti–Cr quaternary alloys consisted of finely precipitated Ni₃(Si,Ti) phase and nickel solid solution. Then, the high temperature mechanical properties, bend strength, and oxidation and corrosion properties of the alloys were investigated. The Ni–Si–Cr ternary alloys showed significant strengthening over a wide range of temperatures, and also large compressive plastic deformation at high temperatures. The strength and fracture toughness at ambient temperatures were correlated with the volume fraction of Ni₅Si₂ phase. The Ni–Si–Ti–Cr quaternary alloys did not show increased yield strength, but exhibited improved tensile ductility and plasticity over a wide range of temperatures. Both Ni–Si–Cr ternary and Ni–Si–Ti–Cr quaternary alloys showed substantially improved oxidation resistance in air at 1173 K, compared with Ni₃Si and Ni₃(Si,Ti) alloys. Also, the Ni–Si–Cr ternary and Ni–Si–Ti–Cr quaternary alloys showed corrosion resistance comparable to that of the Ni₃Si and Ni₃(Si,Ti) alloys.     

Implementation of omnidirectional crawl for a quadruped robot

As a reptile animal crawls in a cluttered environment, so a quadruped robot should be able to crawl on an irregular ground profile with its static stability by adopting the straightgoing and standstill-turning free gaits. The generalized and explicit formulations for the automatic generation of straight-going gaits and various standstill-turning gaits are presented in this paper. The maximized stride for the straight-going gait and the maximum turning angle for the turning gait of a quadruped robot named TITAN-VIII in a gait cycle are discussed by considering the robot's mechanism constraints and the irregularities of the ground profile. The control algorithm, including control of the joint positions of the robot, is described to implement the desired walking path of the quadruped robot. The effectiveness of the proposed method is demonstrated through experimental result.     

Dielectric behaviour and magnetoelectric effect in Ni0·25Co0·75Fe2O4-Ba0·8Pb0·2TiO3 ceramic composites

Abstract

Dielectric and magnetoelectric properties are reported in magnetoelectric (ME) composites containing lead doped barium titanate as the electrical component and a mixed Ni-Co ferrite as the magnetic component. Changes were observed in dielectric properties as well as in the ME effect as the molar ratio of the components was varied. A maximum value of ME conversion factor of 0·42 mV cm ^{- 1} Oe ^{- 1} (52·78 VA ^{- 1}) was observed in the composite containing 15 mol-% ferrite. Larger ME effects were observed in composites with higher contents of ferroelectric phase. This conclusion differs from earlier theoretical studies which predict a large ME interaction in composites with a 1:1 molar ratio of participating phases. Apart from the technological importance of the ME effect, some anomalies in magnetic phase transition induced by electric fields are also of scientific interest. The variation in dielectric properties with temperature resulted in two maxima, corresponding to the ferroelectric and ferrimagnetic Curie temperatures. This behaviour is unlike anything previously observed in other ME composites.     

An Optical Readout Scheme Providing High Spatial Resolution for the Evaluation of Combinatorial Libraries on Optical Fibers

We have developed a novel method for combinatorial chemistry that allows for fully parallel synthesis and full library analysis. The key feature is the use of linear supports for synthesis, where the position of a compound along the support encodes its synthetic history. Use of an optical fiber as the linear support allows for the optical evaluation of libraries: the location of an emitting fluorophore can be determined using fluorescent optical time domain reflectometry. We have demonstrated that limitations on the spatial resolution imposed by the fluorescence lifetimes are overcome by using a second fiber as an optical delay.     

Influence of extrusion ratio and temperature on microstructure and mechanical properties of 2024 aluminium alloy consolidated from nanocrystalline alloy powders via hot hydrostatic extrusion

Abstract

Nanocrystalline 2024 aluminium alloy powders with an average grain size less than 50 nm, prepared by a unique technique which combines rapid solidification and mechanical milling, were consolidated into bulk material under various technical conditions via hot hydrostatic extrusion and the microstructure and mechanical properties of the consolidated alloy were experimentally investigated. The influence of the two main technical parameters, extrusion ratio and temperature, on the microstructure and mechanical properties of the as extruded alloy is made clear and the reasons why these two parameters had such an influence on the microstructure and mechanical properties of the alloy are also discussed. Furthermore, suggestions are given for rationalising the extrusion ratio and temperature for the consolidation of the nanocry stalline 2024 aluminium alloy powders via hot hydro static extrusion.     

Efficiency and Optimality of Two-Period Limit Cycle Walking

This paper investigates the efficiency of a two-period gait from the kinetic energy viewpoint. First, we formulate a steady two-period gait for a compass-like bipedal robot by using a simple recurrence formula for the kinetic energy of an asymmetric rimless wheel. Then, we theoretically show that, in the case where the mean value of the hip angle is constant, the generated two-period steady gait is less efficient than a one-period symmetric gait in terms of kinetic energy. We also show that the symmetric gait is not always optimal from another viewpoint. We then extend the analysis to biped walking and investigate the validity of the derived method through numerical simulations of virtual passive dynamic walking.     

Carbothermal synthesis of zirconium diboride (ZrB2) whiskers

Abstract

In the present investigation, carbothermal synthesis technique is used for the production of ZrB₂ whiskers. The synthesis is carried out by heating the mixture of ZrB₂, H₃BO₃ and carbon black over the temperature range of 1300 to 1700°C in argon atmosphere. Different elements such as Ni, Co and Fe are tested for their role as catalysts. The synthesised powder contains ZrB₂ as the major phase with minor phases of ZrC and B₄C. Whisker yield is found to be low at lower temperatures (<1500°C) and then increases with the pyrolysis temperature. The Ni addition seems more effective as a catalyst than Co or Fe. Shorter length whiskers are found in the case of Co catalyst, whereas use of Fe catalyst shows whisker with rod shape and a special bird's nest type whiskers. The electron microscope study of whisker reveals the presence of various defects.