P. Ochin

Experimental investigation and thermodynamic calculation of the Al–Mg–Zn system

Abstract On the basis of a critical assessment, experimental investigations by EPMA on ternary Al–Mg–Zn alloys were specifically performed to provide missing data of the ternary solubilities of the Al–Mg and Mg–Zn phases as well as to improve the knowledge of the extensions of the homogeneity ranges of the ternary τ- and Φ-phases. A thermodynamic description for the Al–Mg–Zn system was obtained taking into account those experimental data together with constitutional, thermodynamic and crystallographic literature information. The binary intermetallic phases are modelled to have ternary solubilities. The ternary τ-phase is modelled according to its crystal structure with cubic symmetry as (Mg) 26 ( Mg , Al) 6 ( Al , Zn , Mg) 48 (Al) 1 in the compound-energy-formalism. The Φ-phase is described by the sublattice formula Mg 6 ( Al , Zn ) 5 .

Structural study of the hydrides NiTiHx (X = 1.0 and 1.4)

Abstract The hydrides NiTiH x ( x = 1.0 and 1.4) have been prepared by a two-step method comprising an electrochemical treatment followed by a hydrogen gas pressure charging. The hydrides so obtained have a crystal structure of tetragonal symmetry and the hydrogen location has been determined by powder neutron diffraction. Hydrogen atoms occupy two octahedral sites. The hydrides do not present any structural phase transition as a function of temperature, in contrast to the parent alloy NiTi. The crystal structure is compared with other filled alloys of CsCl-type structure.

Cast bulk ZrTiAlCuNi amorphous alloys

Abstract Cylindrical bulk amorphous samples with diameters up to 10 mm have been prepared by casting Zr Ti Al Cu Ni alloys in a copper mould. In order to rank glass-forming ability as a function of alloy composition, alloys were also cast into wedge-shaped moulds; to a first approximation, the thickness of the amorphous region obtained can be taken as an indication of glass-forming ability. The compositions which lead to the production of bulk glasses all have reduced glass transition temperatures in excess of 0.65 and the extremely high glass-forming ability of these compositions is discussed. We suggest that both the Al and Ti contents are determining factors for the production of bulk amorphous samples and these are believed to reduce the driving force for, and hence the rate of, crystallisation. These amorphous alloys have been found to display high thermal stability and can be annealed for several minutes in the supercooled liquid region. They are ductile at room temperature and have a high value of yield stress.

Transformation and ageing behaviour of melt-spun Ni–Mn–Ga shape memory alloys

Abstract The effects of rapid quenching on the transformation behaviour of three melt-spun ribbons of Ni–Mn–Ga alloys has been studied by X-ray and electron diffraction, calorimetry, thermomechanical and magnetisation tests. One step martensitic transformation (MT) has been detected for each ribbon accompanied by the formation of modulated martensitic crystal structures with 7- or 10-layer periods, which differ from those observed for single crystals of the same compositions. The MT temperatures can be raised or lowered by ageing in the parent phase, depending on composition. The ageing also causes a significant increase of both Curie temperature and magnetization in all alloys. These effects indicate the occurrence of some atomic rearrangement processes leading to a more uniform element distribution and, probably, change of atomic order. The activation energies for these processes have been estimated to be between 0.2 and 0.7 eV.

Influence of the martensitic transformation on the hydrogenation properties of Ti50−xZrxNi50 alloys

Abstract Ti50−xZrxNi50 alloys with 0≤x≤24 develop either austenitic or martensitic crystal structures when prepared by melt-spinning or induction melting, respectively. This outcome is a consequence of the particular alloy microstructure resulting from each preparation method, which induces a difference of 100°C on the martensitic transformation temperatures for alloys with the same composition. Austenitic alloys absorb hydrogen up to 1.5 hydrogen atoms per AB unit (H/AB) at 130°C and 20 bar, without displaying any plateau pressure for hydrogen pressures between 0.1 and 10 bar. In contrast, martensitic alloys exhibit a plateau pressure with hydrogen concentrations between 1 and 2.1 H/AB, and reach a maximum hydrogen concentration of 2.6 H/AB under the same thermodynamic conditions. Consequently, martensitic alloys form a dihydride compound that, for the representative case of Ti32Zr18Ni50 alloy, has a formation enthalpy of −12.3±0.2 kcal mol H2−1.

Enthalpy of formation of Al–Fe–Si alloys (τ5, τ10, τ1, τ9)

Abstract In order to optimise the phase equilibrium of the ternary system Al–Fe–Si it is necessary to use thermodynamic data. In this paper we present the enthalpy of formation of ternary alloys at 298 K. These values were measured with a Tian-Calvet calorimeter (Setaram) by dissolution in an aluminium bath at 800°C (1073 K). After levitation melting, the alloys were annealed until they were monophasic, they were then checked by X-ray diffraction and the precise composition was determined by atomic absorption spectroscopy.

Ni49.8Ti42.2Hf8 shape memory alloy strips production by the twin roll casting technique

Abstract The twin roll casting technique has been used for the first time to produce semi-finished Ni49.8Ti42.2Hf8 shape memory alloy thin strips. Some cracks appear on these new products but can be removed by optimization of the operational parameters. The aspect, dimensions and thermodynamical behavior of the strips are studied and interpreted according to the operational parameters and the resulting microstructure, taking into account the complex heat and mass flows during the process.

Alloys of high glass-forming ability

Abstract A series of Zr-Ti-Al-Cu-Ni alloys, e.g., Zr 57 Ti 5 Al 10 Cu 20 Ni 8 , were found to have high glass-forming ability (GFA). Cylindrical bulk amorphous samples with diameters of 10 mm were produced by casting the melts in a copper mould. The Zr 65 Cu 17.5 Ni 10 Al 7.5 alloy, which was reported to have the highest GFA heretofore, was found to be amorphous only in the surface with radius dimensions of less than 1 mm under the same cooling conditions. The alloys were also cast in a chevron-shaped copper mould to compare the GFA of the alloys. Ti and Al were found to be the critical components favorable to the GFA of the Zr-Ti-Al-Cu-Ni alloys, which could greatly reduce the crystal growth rate of the alloys. Cu and Ni appeared to be the less critical components. However, it was necessary to modify the contents of all the components according to their effects in order to design an alloy of high GFA. The amorphous alloys displayed very high strength, thermal stability and superplasticity at elevated temperature.

Effects of Al on the glass-forming ability of Zr-Cu based alloys

Abstract Effects of Al on the glass-forming ability (GFA) of Zr-Cu alloys were investigated by studying the solidification characteristics of the Zr65Cu27.5Al7.5 alloy in comparison with those of the Zr65Cu35 alloy. It is presumed that the much higher GFA of the Zr65Cu27.5Al7.5 alloy than that of the Zr65Cu35 alloy is due to the much lower diffusion coefficient of Cu and Al in the Zr63Cu27.5Al7.5 melt, the low nucleation rate of the Zr2(Cu, Al) nucleating phase due to its small nucleation driven force and long nucleation reduction time resulting from the addition of Al. The higher GFA of the Zr65Cu27.5Al7.5 alloy does not result from the crystal growth difficulty caused by the long-range redistribution of Al at the solid-liquid interface. In fact, Al does not have long-range redistribution at the solid-liquid interface but is trapped by the growing solid to form Zr2(Cu,Al) and Zr(Cu,Al) phases under rapid quenching conditions.

Structure and properties of Ti-Ni-Zr and Ti-Ni-Hf melt-spun ribbons

Abstract Rapidly solidified ribbons of Ti–Ni–Zr and Ti–Ni–Hf shape memory alloys for high-temperature applications have been produced using a planar flow casting technique. Calorimetric and mechanical measurements, as well as transmission electron microscopy (TEM) observations have been performed. The characteristics of martensitic transformation in the ribbons are studied and compared with those of arc-melted bulk samples. The microstructure features and their influence on the martensitic transformation are analyzed.