Materials Science Forum Vols. 539-543

Abstract: Warm compaction is a low cost process to make high density and high performance iron base powder metallurgy parts. Based on results obtained from the dynamic compacting curve, ejection force curve, X-ray diffraction, micro-hardness of iron powder, friction condition and lubricant properties, densification mechanism of warm compaction can be drawn. In the initial stage, the rearrangement of powder particles is the main factor. It contributes more in the densification of warm compaction than that in cold compaction. However, in the later stage, the plastic deformation of powder particles is the primary factor. The increase in plasticity at high temperature can harmonize the secondary rearrangement of powder particles. During the compaction, the polymer lubricant has great contribution to the densification of the powder, since it improves the lubricating condition and effectively decreases the friction in the forming process and thus enhances the compact density. The dynamic compacting curve of warm compaction can be divided into three phases. The first is the particle rearrangement dominant phase; the percentage of particle rearrangement in warm compaction is higher than that in cold compaction by 15-31%. The second is the elastic deformation and plastic deformation dominant phase. The third is the plastic deformation dominant phase. The study of the powder densification mechanism can direct engineers in designing and producing warm compaction powders for high density parts.

Abstract: The molybdenum alloy TZM (Mo-0.5wt%Ti-0.08wt%Zr) is a commonly used structural material for high temperature applications. For these purposes a high strength at elevated temperatures and also a sufficient ductility at room temperature are being aimed. Preceding investigations revealed the existence of subgrains in hot deformed TZM. It was observed that with proceeding primary recrystallization and therefore with disappearance of subgrains the yield strength drops almost to a level of pure molybdenum. It is being assumed that the existence of a dislocation substructure has a pronounced effect on the yield strength of TZM. The aim of the present study was to evaluate the subgrain and texture formation and also to estimate the dislocation arrangement within subgrains during hot deformation. Hence, TZM rods were rolled to different degrees of deformation at a temperature above 0.5 Tm. The microstructure of the initial material was fully recrystallized. Texture formation, misorientation distributions and subgrain sizes were analyzed by electron backscattering diffraction (EBSD). Mechanical properties were characterized by tensile tests at room temperature and up to 1200°C. It was revealed, that with increasing degree of deformation a distinct substructure forms and therefore yield strength rises. Consequently, the misorientation between adjacent subgrains increases, their size decreases and a <110> fibre texture develops. To estimate the influence of texture on strength of TZM the Taylor factors are calculated from EBSD data.

Abstract: The main purpose of this work was to produce nanotubes using a two step method: mechanical alloying and heat treatment. Mechanical Alloying (MA) was used to prepare the monometallic Co3C carnide and the bimetallic C-25%at Ni-25%at Mo carbide with Ti impuririties by milling of pure elemental powders of cobalt, molibdenum, titanium and carbon, in a high-energy rotatory mill under an Ar atmosphere. The nanocrystalline carbides were used to produce metal filled nanotubes and nanoparticles, by means of precipitation after heating for 15 minutes at 800°C. Microstructural characterisation of the as-milled and heat-treated powders was performed using Transmisssion Electron Microscopy (TEM) techniques. It was possible to obtain filled nanotubes, carbide nanorods, and observe a nucleation phenomena inside carbon cavities.

Abstract: Two types of titanium oxide were used, rutile (>99.9% TiO2) and anatase (>99% TiO2). Both samples were mixed with graphite in accordance with the required stoichiometry, and then milled in a tumbling mill for 50 hours in an argon atmosphere to ensure thorough mixing. The mill vial was loaded with five 25.4 mm diameter stainless steel balls giving a powder to ball mass ratio of 1:43. After milling, samples were heated to 1400°C in an alumina crucible at 20°C min-1 under a flowing nitrogen (100 mL min-1) atmosphere in a thermogravimetric analyser (TGA). Srilankite was detected in the as-milled anatase sample but the anatase to rutile transformation was not completed during milling. After heating to 800°C most of the anatase had transformed to rutile. Reduction of anatase started just below 900°C whilst rutile underwent reduction below 800°C. TGA results showed that the anatase reduction was more complex than the rutile reduction with several stages evident between 880 and 1000°C in the anatase sample whilst only two steps were observed for rutile. The initial identified products were Ti5O9 and Ti4O7 prior to TiN in anatase sample but in rutile sample only Ti4O7 was detected. Reduction was completed in rutile sample before 1180°C whilst in anatase completed at 1230°C. TiN was the final product in both systems after heating to 1400°C. These results are discussed in light of recent work demonstrating the different reductions paths of rutile and anatase.