Papers by Author: George Arghir

Abstract: The SmCo₅ has three sintering stages using spheres (0.6 – 1.0 mm diameter) at temperatures between 1030 and 1200 °C. During the first stage the neck radius increases as X² ~ t for less than 1200 °C temperature, the exponent is 3 at 1200 °C. Interdiffusion is sintering mechanism for exponent of 2. Sm diffuses from inside to the surface, where it is oxidized and oxides fills the neck between the spheres. Co diffuses through the oxides. At 1200 °C the sintering mechanism is evaporation-condensation of Sm. The activation enthalpy of the first stage is 582 kJmol^-1 for temperatures above 1130 °C and 210 kJmol^-1 bellow 1130 °C, respectively. The second stage is characterized by a plateau where the neck growth is arrested. The small pores in the neck and in the sphere surface layer (formed during the first stage) shrink. When these pores disappear a continuous α-Co layer forms and the third stage starts. It is essential growth of the neck formed by dense Co layer. The law of sintering is X⁴ ~ t. The activation enthalpy (276 kJmol^-1 at temperatures above 1130 °C) closes the activation enthalpy of Co self-diffusion. This (together with the exponent of 4) suggests that the Co layer plays a role similar to that of a liquid film. Making slight changes in the chemical composition of the alloys and substituting an argon atmosphere to vacuum have no influence or stages and sintering mechanisms.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.