George Arghir

Research on the Intermetallic Compounds Formation in a Ni-Ti Alloy Obtained through Powder Metallurgy Specific Methods

NiTi alloys, due to the special properties they posses, good corrosion resistance, biocompatibility, and shape memory, are used successfully in the medical field. The paper presents research concerning the elaboration of the NiTi alloy in the form of spherical shape powder with hollow particles. This type of powder would be the raw material for fabricating light weight products like prosthesis or surgical implants. The aim of the research was to elaborate this type of powder and determine the alloy’s phases in correlation with the need of obtaining a specific particle shape. Along with these aspects it was attempted to form different testing samples through sintering operations.

Atmospheric induced nanoparticles due to the urban street dust

ABSTRACT Urban street dust (SD) represents a natural source of mineral floating particles (FP) in the atmosphere. The FP particles have usually a wide range of sizes. These particles were monitored and collected by automatic stations in Cluj-Napoca, Romania. The SD and FP samples were investigated by XRD and SEM-EDX analysis. The obtained results prove that both, SD and FP, have a similar composition, namely quartz, clay particles (e.g., muscovite, and kaolinite), calcite, and trace of lepidocrocite. Nanostructural features and fine micro-scaled particles are observed. Quartz and clay nanoparticles were found in SD samples as well as in FP samples. The AFM investigation reveals a nano-size range for quartz particles between 80–90 nm, and 40–60 nm diameter for clay particles. Similar values were obtained by TEM microscopy. The high resolution microscopy results were confirmed by the values obtained by Scherrer formula applied to the XRD patterns. Composition and dimensional similarities found for the nanoparticles in SD and FP samples prove that mineral nanofractions in atmosphere were induced by the urban environmental interactions with the SD. Results provide valuable information on the size, shape, and composition of nanoparticles induced in atmosphere by winds and by environmental interactions with the SD. This work contributes to the evaluation of the air pollution and simultaneously it offers a basis for an improved life quality for Cluj-Napoca population.

Elaboration of Titanium-Nickel Alloy with Special Properties through Mechanical Milling

Shape memory Titanium-Nickel alloys, also known as Nitinol, are amongst the most utilized materials with special properties in the medical field. Together with the properties of shape memory and superelasticity, these alloys have a very good biocompatibility. In this study, the equiatomic Ti-Ni alloy was obtained in the form of alloyed powder, starting from elemental high purity powders, through mechanical alloying. Specimens for testing the mechanical characteristics of the material, as well as smaller samples for biocompatibility tests were manufactured. The latter ones were prepared for implantation on live tissue, on Wistar rats and Guinea pigs. The structure of samples was studied by microscopy and X-Ray diffraction analysis. All the results have demonstrated the presence of the TiNi intermetallic compound as the quantitative dominant phase. After applying an adequate thermo-mechanical treatment, the tested samples displayed measurable shape memory effect and superelasticity. The in vivo biocompatibility tests, done according to international standards, demonstrated the material’s bio-inertness in relation with living tissue. The obtained results have shown the possibility to elaborate Ti-Ni biocompatible alloys by mechanical milling and sintering.

Influence of Ion Nitriding Process Parameters on the Microstructure, Hardness and Thermal Shock Behaviour of X3CrNiMo17-13-3 Stainless Steel

The experimental researches was made on X3CrNiMo17-13-3 stainless steel samples ion nitrided at different temperatures, duration and work atmospheres. The analysis of superficial layers was made in order to identify the microstructure of the ion nitrided layer using X-ray diffraction, and also the determination of the superficial hardness and hardness gradient. The nitrided samples were subjected to two thermal shock regimes, one in the range of 500-20°C and the other in the range 600-20°C. The crack and crack networks initiation process as a result of thermal fatigue was analyzed quantitatively and qualitatively using optical microscopy. The study of the cracking process highlighted the fact that thermal shock resistance is influenced by the characteristics of the superficial layer and also by the maximum temperature of the thermal stress cycle.

The Study of Fatigue Behaviour at the Contact of some Sintered Materials from Powder Based on Iron

The analysis of a large range of sintered parts shows the fact, they are strained at contact fatigue, in most of the cases. The Hertzian pressure, at which materials in contact are subjected, establishes the appearance of variable stress, which records a maximum at some depth under the contact surface. In the case of sintered materials this stress variation conducts to a faster degradation of strained surface than in the case of compact materials. This is because the stress variation establishes a critical state at the level of stress concentrators, generated by pores, standing in material structure, leading to formation of some cracks which may propagate at the level intergranular necks, producing removal for some particles of material. The accomplished research on sintered materials, having different chemical compositions, revealed the fact, at the beginning, that the materials undergo a cold hardening, indicated by a hardness increasing, followed by some cracks generating, accompanied by their extension and material removal. This process develops in the same mode for all investigated materials, but with different intensity.

Mechanical Alloying of Fe30Cu70

Fe-Cu system is a binary alloys system, nevertheless very difficult. This paper presented the milling duration influence on ball-milled Fe30Cu70 alloys. After 16 hour of milling it has been concluded that true alloying at atomic level occurs during milling. The average grain size depends by milling time. Varying the milling time changes the powder morphology, their size and structure. We found that the complete fcc Fe –Cu solid solution is formed when the grain size of Fe-bcc reach a value about 10 nm, because at this value of crystallite the free energy for interface become less than interfaces energy. The milling duration have a strongly influence on solid solubility and phases form in Fe-Cu system. The phase formation for Fe30Cu70 (mass %) has been investigated by X-ray diffraction (XRD). The mixing enthalpy (positive in this system) also depends on alloy composition.

On Massive Powder Coating in Ball Milling

During ball milling of ductile-brittle systems, often is reported a massive coating of milling devices (balls and walls of the vials). Therefore, to slow the powder deposit on the milling devices, it is useful to analyze the causes that initiate and favour the phenomenon. It seems that the massive powder coating of milling devices is very similar to build-up edging of the materials on the cutting tools in machining. The aim of the paper is to find what differences appear in the coating when are used different powder compozitions, or compozitions with fractions of powder previously milled compared to initialy unmilled compozitions.