Papers by Author: Radu L. Orban

Abstract: The effect of Fe, respective Cu, additions as substitute for Ni in NiTi shape memory alloys (SMAs) on the delaying of its phase transition and narrowing hysteresis are well known, NiTi-Fe and NiTi-Cu SMAs having applications especially to the actuators that require such properties. These SMAs are currently produced by conventional melting methods, which are energo-intensive and impose very severe processing conditions to avoid contamination. The results of researches presented in this paper prove the possibility of these SMAs obtaining by powder metallurgy via reactive sintering – more advantageous from both technical and economic point of view. A beneficial effect on both sintering and homogeneity of the obtained SMAs proved to have a controlled mechanical alloying applied to powder mixture before compacting and sintering.

Abstract: Directional solidification techniques have been applied to produce Ni based intermetallic alloys with preferentially oriented columnar crystals extended along the complete length and parallel to the solidification direction. Enhanced ductility is expected from such alloys. In this paper we present the research results concerning the application of this technique to some complex Ni3Al- Fe-B alloys obtained from compacted mixtures of elemental powders. The corresponding master alloys have been obtained in a vacuum induction furnace by the known Exo-Melt process [1]. The directional solidification of these alloys was subsequently performed on cylindrical samples, at two solidification rates, 30 and 15 mm/h. The influence of the rate and composition used on the dimensional variations, densities, microstructure, constituent phases and lattice parameters, as well as on the surface chemical composition have been documented and are presented in the paper.

Abstract: There are investigated the possibilities to avoid or at least to reduce the Al2O3 scales formation on NiAl powder particles at its plasma spray deposition on steel substrates. The optimum processing parameters and the necessity to surround the plasma jet by an inert gas have been established. In appropriate processing conditions, the obtained coating layer is formed by flattened particles, welded together and to the substrate, proving their melting during spraying. It is dense and adherent, consisting of NiAl with only small Al2O3 inclusions, proving the NiAl stability preserving without decomposition or a notable oxidation, as premises of its desired functionality achievement.

Abstract: There is investigated the influence of Al2O3 content on the density and mechanical properties of Cu-Al2O3 (2.0 ÷ 10.0 [vol.%] Al2O3) cermets processed by classical routes of powder metallurgy. The obtained results show that the cermets relative density remains quite constant, at a value of over 95 [%], up to a content of ~4 [vol.%] Al2O3. Together with reinforcing effect of the hard Al2O3 particles, proved by the apparition of an elastic zone on the Stress-Strain curve of tensile test, determine a notable increasing of ultimate tensile strength (UTS) and hardness with a small variation of elongation. Over about 4÷5 [vol.%] Al2O3, a decrease in UTS and also in toughness, proved by the Fracture energy decreasing, occurs. Consequently, from the considered points of view, is not recommended to adopt higher reinforcing levels. For electrical applications, the relative density of the processed cermets can be further increased with 3 ÷ 4 [%] by die cold repressing.

Abstract: The influence of cooper powder as reinforcing phase on the main mechanical and physical properties of recycled polyethylene matrix composites has been investigated. The obtained elastic modulus increases up to the upper adopted reinforcing level (∼10 [vol. %]), while elongation and fracture energy decrease. The UTS values remain quite constant within the reinforcing content interval. Electrical resistivity proved to depend, to a great extent, on the reinforcing particle distribution inside the polymer matrix. It has a sudden drop when a continuous electrical conductive path is formed at a critical volume fraction of metal particles. At a lower content, instead, it slowly decreases as reinforcing powder content increases. Both the mechanical properties and values for electrical resistivity are similar to those published for polymers obtained directly from monomers.

Abstract: This paper investigates the effect of Fe, Cr and B additions, in small proportions, as alloying elements in Ni3Al with the purpose to reduce its intrinsic fragility and extrinsic embrittlement and to enhance, in the same time, its mechanical properties. It represents a development of some previous research works of the authors, proving that Ni3Al-Fe-Cr-B alloys obtained by reactive synthesis (SHS) starting from Mechanically Alloyed powder mixtures have superior both room temperature tensile strength and ductility, and compression ones at temperatures up to 800 °C, than pure Ni3Al. These create premises for their using as superalloys substitutes.

Abstract: Research results focussed on the combined influence of iron and boron, in proportions of 0.5 and up to 10 wt.% respectively, in complex alloyed Ni3Al synthesised by Self Propagating High Temperature Synthesis (SHS) in the thermo-explosion mode at two ignition temperatures, 950 and 1150 oC, are presented. By XRD and Mössbauer spectroscopy it was established that for 950 oC ignition temperature, the evolved heat is not high enough for the added Fe to be fully incorporated into the synthesised Ni3Al phase, a temperature of 1150 oC being required. For this temperature, the density of the synthesised alloys, their capacity to be cold deformed by re-pressing, hardness and bending strength variations as a function of B and Fe contents, proved their cumulative effects of the ductility and mechanical properties of complex alloyed Ni3Al enhancing.

Abstract: The behavior of stoichiometric and near-stoichiometric NiAl at plasma spray deposition, without and with a bond coat, for coating layers realization on a low alloyed steel substrate has been investigated and is presented. In all variants, NiAl particle melting and their welding at the impact with substrate were observed. Furthermore, a relatively compact and adherent coating layer was formed and the NiAl was found to maintain its stability. These are all important factors for assuring the coating layer oxidation and corrosion resistance. Good results from the coating structure and adherence to the substrate points of view were obtained for the 45:55 Ni:Al composition, without a bond coat but adopting an Ar protective surrounding of plasma jet. The high resistance to corrosion of 45:55 NiAl composition was further validated through corrosion tests.

Abstract: Property modification of Ni3Al with alloying addition of Fe and Cr was investigated through MA and subsequent SHS processes. Fe addition brought about high temperature strengthening effect and formed a Ni-Fe compound in the liquid grainboudary phase of Ni3Al structure and Cr addition showed a retarded homogenization. Homogenized Ni3Al composition with Fe and Cr addition by MA and SHS showed better mechanical resistace and deformability in the compression tests at temperatures up to 800°C than the same composition processed by classical PM routes.

Abstract: This paper investigates the possibilities of niobium using for the mechanical properties of the common low alloyed Cu-Ni-Mo-C structural sintered steels enhancing by a similar technique with that used in the classical HSLA steels obtaining. For this purpose, both thermodynamic and experimental investigations were carried out, their results showing that the added Nb to these steels form at sintering in a nitrogen containing atmosphere, rather then the other alloying elements, carbides, nitrides, carbonitrides. They are solved in austenite and precipitate at the sintered steels cooling / artificial aging, forming precipitates of an expected strengthening effect. The results for 0.2 ÷ 1.0 % fine Nb powder and 0.3 % graphite additions to Distaloy AB powder cold compacted and sintered in dissociated ammonia proved that such a strengthening effect, which leads to higher mechanical properties of the processed steels than of the Cu-Ni-Mo-C ones, can be achieved.