Materials Science Forum Vols. 449-452

Abstract: Nanostructured WC-Co coatings have been manufactured by high velocity oxyfuel (HVOF) spraying using commercially available nanocomposite powders. The effect of feedstock powder characteristics on coating microstructure has been investigated. A significant loss of carbon occurred during HVOF spraying, which was much greater for a porous, irregular powder than for a dense, spherical powder. The decarburization promoted the formation of several additional phases in the coating, including W2C, W and CoxWxC. The microstructural differences in each coating arose from the different extent of feedstock powder melting because of initial powder morphology.

Abstract: Surface modification effects have been investigated for perovskite-type mixed ionic-electronic conductors. As the mixed conducting oxides show both ionic and electronic conductivity, these can be applied as oxygen permeable membranes. We have coated surfaces of the perovskite-type mixed conductors, LaSrCoFeO3 and LaSrGaFeO3, with LaSrCoO3 and investigated the effects on oxygen permeability. Enhanced oxygen permeability was achieved when the LaSrGaFeO3 membrane was surface-modified with LaSrCoO3. However, there was no effect on oxygen permeability of LaSrCoFeO3 even when the surface of which was modified. The morphological factors related with electrochemical reactions have also been discussed

Abstract: Copper, iron and a nickel-chromium alloy were thermal sprayed onto a mild steel substrate in air and the structures of the sprayed particles that were collected during flying and those of the coatings are examined. The coatings consist of metal layers and metal oxide layers, and the minor phase layers become disintegrated and granulated on annealing. This morphology change is caused by the balance of the boundary energies.

Abstract: In Cold Spray method, a coating is formed by exposing a substrate to high velocity solid-phase particles, which have been accelerated by supersonic gas flow at a temperature much lower than the melting or softening temperature of the feedstock. Therefore, the nozzle geometry is important with regard to the cold spray method. This Cold Spray process is an exciting new spray technology that has the potential to overcome limitations of more traditional thermal spray processes for some important commercial applications. With this emerging technology, it is possible for the first time to rapidly deposit thin or very thick layers (mm to cm+) of a wide range of metals, and even some composite materials, without melting or vaporization, at or near room temperature, in an ambient air environment. Some potential areas of interest include:  corrosion protection,  wear reduction, highly conductive coatings (electrical or thermal), metal/glass or metal/ceramic joints (with less residual stress), ceramic/metal or graded metal/metal composites,  thick deposits (mm to several cm range), reclamation of worn or mis-machined parts, metallization of glass or ceramics. The applications of Cold Spray can be examined in the field of an automobile and a rocket engine.

Abstract: Effect of both substrate temperature and ambient pressure on a flattening behavior of thermal sprayed particles was systematically investigated. In the flattening of the sprayed particles onto the flat substrate surface, critical conditions were recognized both in the substrate temperature and the ambient pressure. That is, the flattening behavior changed transitionally on the critical temperature and pressure range, respectively. A transition temperature, Tt, and a transition pressure, Pt, for those critical conditions were defined and introduced, respectively. Equivalence in the dependence both of transition temperature and transition pressure on the sprayed material’s order indicated that the wetting of the substrate by the molten particle is a common domination in the flattening. Three dimensional transition map by combining both transition behavior in the flattening was proposed as a controlling principle for the thermal spray process

Abstract: Ni-Mn-Ga ferromagnetic shape memory alloys (FSMA) are a potential new class of actuator materials able to respond at higher frequencies (at least 300 Hz) with comparable strains (up to 6 %) in a moderate field (below 1 T)[1]. Magnitude of the strain depends on the values of several critical material parameters, most importantly the martensitic transformation temperature (TMart), Curie temperature (TC) and saturation magnetization (MS)[2]. It is well known that these parameters are strongly dependent on the composition of the alloy. Composition dependence of TMart, TC and MS have been experimentaly explored [3,4]. Therefore, it is possible to compile a more complete, and hence more useful composition map for designing Ni-Mn-Ga FSMAs. Ageing behavior is important in these newly developed FSMAs because ageing can affect the reliability of devices using the alloys. Ni-Mn-Ga FSMAs and Au-Cd[5] alloys have several important common characteristics, including off-stoichiometry alloy composition (designed for operation at ambient temperature) and easy twin boundary motion in the martensite state, thus similar ageing behavior is expected in Ni-Mn-Ga alloys. Ni-Mn-Ga alloys have also demonstrated strong damping due to the motion of twin boundaries[6]. Low-frequency mechanical properties are typically measured using the technique of dynamical mechanical analysis (DMA)[7]. In this paper, we present studies of composition design, subtle structure changes associated with ageing, and the temperature dependence of the low-frequency mechanical properties of several Ni-Mn-Ga single crystal alloys.

Abstract: Titanium-Nickel shape memory alloy (TiNi SMA) has great potential as a biomaterial in orthopaedic applications due to its unique thermal shape memory effects, superelasticity and high damping properties. We designed and manufactured bone fasteners using newly developed TiNi SMA wire (Af, 35􀀀 2.C). Several types of bone fastener designs were prepared for the application of orthopedic treatment of bone fracture. We applied this fastener to 82 fracture patients. Fracture types according to the anatomic location included distal fibular (21), femur shaft (periprosthetic fracture, 17), distal tibia (15), distal femur (10), metacarpal bone (9), and subtrochanter of femur (5), distal clavicle (5). Serial radiographs, complete blood count (CBC) and urine analysis were performed postoperatively. Radiological union was achieved without complications at due time after operation. There were no abnormal findings on follow-up CBC or urine analysis. On a subjective level, use and application of the TiNi SMA fastener was not as demanding as conventional fixation methods, such as circumferential wiring (cerclage) or the Dall-Miles technique. The efficacy of SMA bone fastener in this study is very excellent as demonstrated in this clinical study. It gives the new armament to orthopedic surgeon.

Abstract: Using the ultrafine laminate method, thin foils (50 µm) of Ni-rich TiNi shape memory alloys were produced. Overall composition of the Ti/Ni laminate is Ti-50.7%Ni. TiNi (B2) phase was obtained after different diffusion treatments at 1073 K for 36 ks. Aging treatment at 773 K for 3.6, 18, 36, 72 and 144 ks were also performed. Phase transformation behavior of aged foils changed from two-step, to three-step and then to two-step transformation, which is similar to the case of bulk TiNi alloys. Uniform distribution of Ti3Ni4 phase was observed for aged samples by transmission electron microscopy. Two-way shape memory effect of the aged TiNi foil was also demonstrated.

Abstract: This paper is concerned with the application of fundamental thermodynamic theories in the analysis of thermoelastic martensitic transformations in shape memory alloys, with a particular reference to polycrystalline NiTi. The discussion is delivered in two parts. The first part presents a concise overview of the fundamental theories of thermodynamics of thermoelastic martensitic transformations established in the past 30 years. The second part focuses on the principles governing the application of the theories, interpretation of the thermodynamic parameters defined in the theories, experimental determination of the parameters, and some common misperceptions and unjustified assumptions in practice concerning these parameters.