Materials Science Forum Vols. 636-637

Abstract: The influence of Sn addition on the amorphization of CuTiZrNi alloys processed by mechanical alloying of a mixture of pure elemental powder was studied. The thermal stability and crystallization behaviour of the amorphous mechanically alloyed powders was determined and compared with rapidly quenched ribbons with the same nominal chemical compositions. X-ray diffraction and differential scanning calorimetry were employed as the experimental techniques for samples characterization. Both applied samples preparation techniques resulted in the formation of fully amorphous Cu47Ti34Zr11Ni8 and Cu37Sn10Ti34Zr11Ni8 alloys. However, significant differences in thermal stability and crystallization behaviour have been found, depending not only on the alloy composition, but on the fabrication method as well. The observed influence of Sn addition was more evident for the ribbons, resulting in the change of the number of crystallization effects, their temperatures and activation energy of crystallization. For mechanically alloyed powders these changes were not so dramatic.

Abstract: The use of ultra-bright, highly directional, high and variable energy associated to synchrotron radiation beams at SPring-8 when compared with conventional X-Ray diffraction analysis applied to slate powders heat treated at temperatures in the range of 600°C up to 1000°C is reported. Fe K-edge XAFS measurements of the products formed upon heating slate showed that in the temperature range 600-800°C, conversion of FeO present in the as-received slate powder into a mixed -Fe2O3/-Fe2O3 has occurred providing useful information on the short-range order atomic structure of the slate. Above 800°C, mainly -Fe2O3 was identified to be present in the slate. The change of the Fe valences within the slate network is most likely responsible for the detected colour change with the increasing temperature.

Abstract: Two promising powder metallurgy (PM) processes were used for the fabrication of NiTi shape memory alloys (SMA): Mechanically Activated Reactive FOrging Synthesis (MARFOS) and Mechanically Activated Reactive Extrusion Synthesis (MARES). In these two processes, equimolar powder mixtures of elemental Ni and Ti are first mechanically activated and then forged/extruded at relatively low temperature. Afterwards, heat treatments are used to promote homogenization and to adjust the composition of the NiTi matrix. When MARFOS and MARES processes are compared some differences have been observed but only in relation to the extent of phase transformation and to the degree of densification. The crystallite size was less than 100 nm for all the phases which indicates nanostructured materials and multi-step martensitic transformations could be observed in heat treated materials.

Abstract: Depending on the energy level used during mechanical alloying, the constitution of the resulting products can vary extensively. With high energy input, full transformation to the equilibrium phase, FeTi, is achieved. In contrast, for low levels of energy input, the process is akin to mixing without any phase transformation even for extended milling periods. In the present work, nanostructured FeTi powders were produced by mechanical alloying, avoiding the unfavourable agglomeration problem, by using a relatively low level of energy (e.g. 300 rpm) to mill the pure metallic constituents, Fe and Ti, followed by subsequent heat treatment at 800°C. A major achievement of this research was to show that, by modulating the milling intensity and total milling time, the high temperature synthesis reaction of FeTi (1100°C) can be partially or totally suppressed, reverting instead to a metastable reaction path at low temperature (650°C). The mechanical “activation” modifies the reactivity of the system, producing a very thin Ti /Fe layers. That in conjunction with a high level of defects induced mechanically may be responsible for the metastability. Partial substitution of Fe with Ni (10%) resulted essentially in the same phase constitution, indicating solid solution of Ni in FeTi replacing partially Fe lattice positions.

Abstract: A coarse-grained tetragonal sigma phase Fe47.4V52.6 at% alloy was ground in vacuum in a vibratory mill. The sigma phase transforms into a bcc alpha phase. A concomitant partial amorphisation of the bcc phase occurs. The near-equiatomic FeV alloys are concluded to behave in the same way as the related FeCr alloys when they are ball-milled in vacuum in similar milling conditions. From Mössbauer spectra, it is concluded that about half of the iron atoms are contained in the amorphous phase in the stationary state. From X-ray diffraction patterns there is also a clear evidence of amorphisation, not seen in previously published studies.

Abstract: The present paper focus on preliminary work carried out at INETI concerning the use of microwave radiation applied to sintering of both ceramic and metal powders. Due to the characteristics of materials-radiation interaction, microwaves can become an interesting power source in powder technology and other processing routes, since it is possible to lower the sintering temperature and shorten the sintering cycles, leading to time and energy savings. Alumina, hydroxyapatite, titanium and stainless steel powder compacts were sintered in a modified commercial oven of 2.45GHz and 1000W nominal power. Microwave susceptors were used to enable temperature rise during the initial stage of the sintering cycles. Results on densification and microstructural evaluation of microwave sintered samples are reported and compared to conventionally sintered ones, when available. For similar porosity levels upon sintering, microwave radiation generally reduces sintering times from several hours to minutes. The results obtained so far are quite encouraging since in the case of alumina and stainless steel compacts, a decrease of about 200°C in the sintering temperature was achieved. It was also found that the green density plays a key role in the densification of both metallic and ceramic powders.

Abstract: Ternary Ni-W-C cemented carbides were synthesized directly from mixture powder of NiO-WO3 by simultaneous reduction-carburization in mixed H2-CH4 gas environment in a thin bed reactor in the temperature range 973-1273K. The kinetics of the reaction was closely followed by monitoring the mass change using thermogravimetric method (TGA). The nascent particles of the metals formed by reduction could react with the gas mixture with well-defined carbon potential to form a uniform product of Ni-W-C. The gas mixture ratio was adjusted in such a way that the Ni-W-C formed was close to the two phase tie line. In view of the fact that each particle was in direct contact with the gas mixture, the reaction rate could be conceived as being controlled by the combined reduction-carburization reaction. From the reaction rate, the Arrhenius activation energies were evaluated. Characterization of the carbides produced was carried out by using X-ray diffraction, SEM-EDS as well as high resolution electron microscope (HREM). The grain sizes were also determined. Correlations were found between the carbide composition as well as grain size and the process parameters such as temperature of the reduction-carburization reaction as well as the composition of the gas mixture. The results are discussed in the light of the kinetics of the reduction of oxides and the thermodynamic constraints.

Abstract: In the present study, titanium nitride coatings on tool steel were deposited using cathodic arc physical vapour deposition technique. We studied and discussed the effect of various nitrogen gas flow rate on the surface properties of TiN-coated steel. The coating properties investigated in this work include the surface morphology, surface roughness, line profile and fractal dimension analyses using atomic force microscope. Minimum values for surface roughness, line profile and fractal dimension analyses were recorded at nitrogen gas flow rate of 200 sccm. This is mainly because of the reduction in macro-droplets and minimization of the growth defects, usually produced during etching and deposition stages. Critical limit of nitrogen gas flow rate in TiN coatings were identified and considered an important aspect to understand the performance of TiN PVD-coated steel.

Abstract: Sterilization of the NiTi alloy in boiling water or steam causes passivation, which results in an amorphous 3.5 nm thick TiO2 layer on the surface. Between the surface and the matrix a transition layer of Ni2O3 and NiO was observed, using the X-ray photoelectron spectroscopy. Differences in sterilization conditions affect the amount of metallic nickel on the surface.

Abstract: This document analyses the importance of topography characterization of coated papers, using precipitated calcium carbonates as pigments, on print quality evaluation. For topographic characterization, a laser profilometry was used as well as scanning electron microscope and a mechanical profilometry. Print quality for inkjet printing was assessed using mottle, gain, optical density and Gamut Area. Surface energy of coated papers and printing inks was evaluated using OWRK method. The results obtained showed that good correlations between print quality characteristics and surface energy as well as topographic properties can be obtained.