Papers by Author: Mariana Lucaci

Abstract: The shape memory alloys are materials that can change their shape by applying thermomechanical treatments. In order to design SMA materials, the stress-strain-temperature relationship is necessary. Modeling the constitute behavior of these materials supposes a few phenomenological models that can provide a quick and reasonable approach to assess the behavior of SMA. The proposed constitutive equation expresses well the properties of the shape memory effect, pseudoelasticity and recovery stress.

Abstract: FeTi has one of the highest hydrogen volume storage capacity and its hydride - very favourable absorption/desorption conditions and a high reversibility. However, for applications its low hydrogen mass storage capacity (HMSC) has to be improved. The authors of this paper supposed this would be possible by Al and Ni additions and by a nanocrystalline state. However, FeTi synthesis and alloying with Al and Ni is difficult in a Fe-Ti-Al-Ni system due to the undesired secondary compounds formation. The presented researches proved the possibility of this impediment overcoming and of NiTi-Al-Ni alloy obtaining in a nanocrystalline state, from a mixture of elemental powders of components, by mechano-synthesis carried out in a planetary ball mill, for 24 h, in Ar atmosphere. The obtained alloy proved to have a HMSC of ~3.5 % at 50 0C and 1.5 atm, higher than of FeTi, its dehydrogenation occurring at the same temperature and 1.0 atm.

Abstract: Nb additions to NiTi smart alloys are known to lead to the PTT-hysteresis broadening and transformation temperatures raising – required in numerous applications. As Nb has a high affinity for oxygen, NiTi-Nb alloys processing by powder metallurgy, via SHS, from elemental Ni-Ti-Nb powder mixtures, seems to be more advantageous and cost-effective than by classical one. However, its application encounters difficulties determined by the NiTi higher Gibbs Free Energy of Formation than of Ni3Ti and NiTi2, possible Ni-Nb compounds formation, Nb acting as diluent in SHS. This research proved the possibility to overcome these difficulties and of NiTi-Nb alloys obtaining by SHS using energetically activated powder mixture by controlled Mechanical Alloying. Also, it was proved the possibility to reduce Nb content from the common one of 9 at.% to 5 at.% without a significant effect of transformation temperatures and hysteresis decreasing.

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: 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: The paper presents some considerations concerning the obtaining and characterisation of some new electrical contact pieces of Ag-SnO2-MeO type, where the MeO is an additive chose from the series of metallic oxides WO3, Bi2O3 and CuO. These have a SnO2 content of 9.5 wt. %, a MeO content of 0.5 wt. %, the balance being Ag. The electrical contact materials were manufactured by powder metallurgy techniques using a new type of powder mixtures obtained from SnO2 and MeO powders which were wet mixed, simultaneously with “in situ” Ag synthesis. These present a very uniform and fine dispersion, which in turn leads to improved functional characteristics of the final electrical contact pieces in comparison to the dry mechanical powders mixtures. The new products were designed for low voltage power engineering switching devices working in air at In = 40, 125 and 200 A.

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.