Filipe Neves

Ni4Ti3 Precipitation during Ageing of MARES NiTi Shape Memory Alloys Studied by FEG-SEM

Martensitic transformation determines almost all important properties of NiTi shape memory alloys (SMAs), including shape memory effect. Moreover, any compositional inhomogeneities in the matrix of NiTi SMAs affect the martensite start temperature, Ms, because it depends on the concentration of Ni in the matrix: for Ni content exceeding 51.5 at.% Ms is lower than -200 oC [1]. There is a very good way to adjust the transformation temperature of NiTi alloys for Ni-rich alloys even after the alloys have been made. It is possible to use ageing treatment to rectify the transformation temperature due to incorrect initial composition of Ni-rich NiTi SMAs. The principle behind this method is the (metastable)-equilibrium between TiNi and Ni4Ti3 precipitates since the formation of those Ni-rich precipitates affects the composition of the retained matrix [1-2]. Although Ni4Ti3 is considered as a metastable phase compared with the equilibrium Ni3Ti precipitate, it is quite stable at temperatures below 600 oC and under normal ageing condition only Ni4Ti3 is observed. The ageing temperature and time dependence comes from the evolution of the density and size of Ni4Ti3.

X-Ray Diffraction Study of NiTi Produced by Mechanically Activated Reactive Extrusion Synthesis (MARES)

This study reports the use of X-ray diffraction quantitative phase analyses in NiTi alloys produced by MARES (Mechanically Activated Reactive Extrusion Synthesis). These analyses were performed with the PowderCell 2.4 software. The mechanically activated powders heated in a DTA furnace at 500 °C had as main phases Ni (27 wt %) and Ti (30 wt %) and the major intermetallic phase was Ni3Ti (20 wt %). Above 500 °C the intermetallic phases were predominant. At 600 °C the major phase was Ni3Ti (29 wt %) and at 700 °C was NiTi2 (32 wt %). In this temperature range the NiTi was a minor intermetallic phase (14-20 wt %). No changes in the constitution or in the amount of the phases were detected between the degassed powder samples and the extruded materials. The intermetallic phases were always predominant and the major was Ni3Ti (27-32 wt %). The NiTi phase content was in a range of 15-22 wt %. The weighted residual error, Rwp, of the fittings ranged between 17 and 27. Using the Williamson and Hall plot, crystallite sizes within the range of 26-53 nm and of 12-25 nm were evaluated for the metallic and intermetallic phases, respectively. Vickers micro-hardness measurements were virtually unchanged with the extrusion parameters but increased relatively to the mechanically activated powders.

Role of oxygen and nitrogen in mechanical alloying mechanism of Ni–Ti powder mixtures

Abstract In this study, the critical milling behaviour of Ni–Ti powder mixtures was evaluated in relation to the effect of atmospheric gases, more specifically to oxygen and nitrogen. Within the experimental conditions used, it is shown that both gases play an important role in the alloying process and that not only oxygen gas reacts with the mechanically alloyed powders but also nitrogen. The most effective mixing occurred for the mixtures with the highest contaminant contents.

Effect of Mechanical Activation on Ti-50Ni Powder Blends Reactivity

In the present study, equiatomic powder blends of Ni and Ti were mechanically activated for a short period of time in a planetary ball mill using different levels of energy input. The characterization of the mechanically activated materials was achieved by scanning electron microscopy, X-ray diffraction, differential thermal analysis and chemical analysis (oxygen and nitrogen measurements). During mechanical activation no phase transformation was induced and the high temperature reaction between Ni and Ti elemental powders was shifted to lower temperatures. Moreover, the temperature and the intensity of the exothermic reaction, i.e. the reactivity observed in the powder blends, decreased with the increase in the level of milling energy input. A maximum oxygen content of 0.39 wt% was measured after mechanical activation.

Measurement of Dynamic Young´s Modulus by Impulse Excitation of Vibration: Application to Al MMCs and Intermetallics

Dynamic Young ́s Modulus (E) was measured by impulse excitation of vibration in rectangular bars of near -TiAl alloys and in rods of AlNi/50v% SiC composites. Stainless steel 316 was used to assess the E results obtained with this method for a rod specimen cut from the uniform cross section length of a threaded end specimen, against the ones obtained previously or afterwards from that specimen in a conventional tensile test. The impulse excitation method lead to Young ́s modulus of around 160-170 GPa for the TiAl alloys and 133-257 GPa for the AlNi/50v% SiC composites, consistent respectively with some variation in the amount of the 2 phase present in the first case and varying porosities in the second.

Characterization of Cu2ZnSn(SSe)4 monograin powders by FE-SEM

The design and synthesis of high-efficiency materials to convert solar to electrical energy is an increasingly important research field. Within the photovoltaic technologies, crystalline Si have an 80% share while the remaining 20% are mostly thin film solar cells based on Cu(In,Ga)(S,Se)2 (CIGSSe) and CdTe [1,2]. However, the cost, the abundance and the environmental impact of the elemental components cannot be neglected. For these reasons, Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe) and their solid solutions CZTSSe has attracted much attention recently since they can provide the development of cost competitive solar cells. The CZTS-based solar cells consist of earth abundant and relatively inexpensive elements and represent an environmentally friendly alternative compared to the above mentioned systems [3]. The energy conversion efficiency of the CZTS-based solar cells has increased from 0.66% in 1996 to 11.1% recently [4].

Powder Metallurgical Processes for NiTi Shape Memory Alloys

Two promising powder metallurgy (PM) processes were used for the fabrication of NiTi shape memory alloys (SMA): Mechanically Activated Reactive FOrging Synthesis (MARFOS) and Mechanically Activated Reactive Extrusion Synthesis (MARES). In these two processes, equimolar powder mixtures of elemental Ni and Ti are first mechanically activated and then forged/extruded at relatively low temperature. Afterwards, heat treatments are used to promote homogenization and to adjust the composition of the NiTi matrix. When MARFOS and MARES processes are compared some differences have been observed but only in relation to the extent of phase transformation and to the degree of densification. The crystallite size was less than 100 nm for all the phases which indicates nanostructured materials and multi-step martensitic transformations could be observed in heat treated materials.

Effect of Milling Energy Modulation on the High Temperature Synthesis of FeTi

Depending on the energy level used during mechanical alloying, the constitution of the resulting products can vary extensively. With high energy input, full transformation to the equilibrium phase, FeTi, is achieved. In contrast, for low levels of energy input, the process is akin to mixing without any phase transformation even for extended milling periods. In the present work, nanostructured FeTi powders were produced by mechanical alloying, avoiding the unfavourable agglomeration problem, by using a relatively low level of energy (e.g. 300 rpm) to mill the pure metallic constituents, Fe and Ti, followed by subsequent heat treatment at 800°C. A major achievement of this research was to show that, by modulating the milling intensity and total milling time, the high temperature synthesis reaction of FeTi (1100°C) can be partially or totally suppressed, reverting instead to a metastable reaction path at low temperature (650°C). The mechanical “activation” modifies the reactivity of the system, producing a very thin Ti /Fe layers. That in conjunction with a high level of defects induced mechanically may be responsible for the metastability. Partial substitution of Fe with Ni (10%) resulted essentially in the same phase constitution, indicating solid solution of Ni in FeTi replacing partially Fe lattice positions.

Consolidation of Copper-Copper Nitride Nanocomposite Powders via Warm Extrusion

Copper nitride films prepared by sputtering have applications such as optical data storage material, insulation barriers in micro electronic devices and coatings for mechanical applications. The present study examines nanocomposites prepared by mechanical alloying of copper with copper nitride under nitrogen atmosphere, at room temperature, in order to establish a comparison with properties of Cu-N sputtered films. The powders were consolidated into bulk samples via warm extrusion at temperatures ranging from 300 to 500°C (0.42-0.64 Tf) after encapsulation without degassing. The as-milled powders and the extruded materials were studied using X-ray diffraction, optical microscopy, scanning and transmission electron microscopy and microhardness measurements. Also, the TEM observation of the extruded sample indicates a mean grain size of about 50 nm. This evidences a higher thermal stability of the as-milled powders and the advantage of using a fast consolidation process, at a relatively low temperature. Therefore, the consolidated material did not show the dramatic softening associated with recrystallization. The consolidation of nanostructured copper-copper nitride composite powders via warm extrusion, without major grain coarsening, was demonstrated.

Process Development of γ-TiAl Based Alloys and their Consolidation

The present paper describes part of the work that is being carried out to investigate the formation of Ti-Al-Ag nanostructured intermetallic compounds using Mechanical Alloying (MA). Mixtures of elemental powders with nominal compositions 52TiH2-(48-x)AlxAg (x=0, 2 and 4 at. %) were milled for 25h at 500rpm in a planetary ball mill. The MA led to the formation of a TiH2 (Al) solid solution coexisting with L12-TiAl3 phase. In all cases, Hot Isostatic Pressing (HIP) at 900oC/150MPa/2h and the subsequent heat-treatment at 1200oC/4h resulted mainly in �-TiAl phase formation. When compared with similar alloys produced in previous work by MA at 200rpm for 50h and consolidated in the same conditions more homogeneous microstructures were obtained. Addition of Ag led to a formation of Ag-rich phases preferential located at the grain boundaries.