I. Soletta

Mechanical alloying of the Al–Ti system

Abstract We have investigated the possibility of an amorphization reaction by mechanical alloying for two compositions of the Al–Ti binary system. While the Al-rich composition Al75Ti25 appears to give, after milling for 21 h, an Al(Ti) highly cubic phase, the Ti-rich composition Al25Ti75 does amorphize using various milling conditions. The progress of the amorphization as a function of time of milling was monitored by X-ray diffraction. At first, Al atoms diffuse into the host lattice of hexagonal Ti; subsequently, the milling accumulates a critical density of disorder that causes the Ti(Al) crystalline phase to collapse into an amorphous phase. The formation of amorphous alloys is discussed on the basis of thermodynamic models. The Miedema model is compared with a calculation of phase diagrams approach which has been modified to account for dependence of thermodynamic properties of the liquid upon temperature. T 0 curves are presented, showing a glass-forming range in agreement with experiments.

Structural changes induced by the mechanical alloying of crystalline metal powders

The authors report structural results of mechanical alloying binary mixtures of pure elemental powders as a function of milling time. X-ray line broadening techniques are used to follow the lattice destabilization caused by interdiffusion of elements, which is favoured by a negative heat of mixing. Experimental evidence is outlined on the formation of highly distorted solid solutions and intermetallic compounds, which, due to the milling action, collapse to an amorphous condition when a critical density of defects is reached.

X-Ray absorption spectroscopy and diffraction study of miscible and immiscible binary metallic systems prepared by ball milling

Abstract A series of copper-based alloys (Cuue5f8Ti, Cuue5f8V, Cuue5f8Mn, Cuue5f8Co of equiatomic composition) were obtained by ball milling under a protective atmosphere with the aim of exploring the effect of milling on systems showing very different miscibilities in the solid state. The alloys were studied by means of X-ray spectroscopy (EXAFS), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The mechanical treatment produces an amorphous phase in Cuue5f8Ti. The other systems, displaying a positive heat of mixing, cannot be amorphized, although enhanced reactivity leads to the formation of supersaturated solid solutions. The effect of milling time has been studied on Cu 50 V 50 following the increase in lattice parameters of both fcc and bcc solutions. The fragmentation of the crystallites to a size as low as 300 A represents a necessary condition for the solid state reaction in Cu 50 V 50 .

Nanostructured Fe3Al intermetallic obtained by mechanical alloying and thermal ageing

Mechanical spectroscopy, XRD, TEM and EDS have been employed to follow the structural transformations of bulk samples obtained by mechanical alloying of Feue5f8Al powders. The results show that it is possible by properly combining milling, cold consolidation and thermal ageing to synthesize a nanocrystalline Fe3Al intermetallic phase. Related driving mechanisms and underlying structural transformations are examined and discussed.

Thermal behaviour of CuTi and CuTiH amorphous powders prepared by ball milling

Abstract Solid state amorphization reactions in Cuue5f8Ti have been studied by means of DSC and structural techniques. The influence of hydrogen from the parent titanium powder on the amorphization and crystallization processes has been investigated. For Cuue5f8Ti a diffusion-controlled process can be inferred for solid state amorphization from the parabolic trend of the heat of crystallization, as a function of the milling time. The presence of hydrogen in the alloys is found to modify the crystallization behaviour of the amorphous phase. A DSC method for the determination of the amount of hydrogen present in the alloys is given.

The influence of hydrogen contamination on the amorphization reaction of CuTi alloys

Abstract Neutron diffraction experiments have been made on amorphous Cu x Ti 1− x alloys prepared by mechanical alloying (MA) over a wide composition range, 0.75 > x > 0.30. An unexpected result has emerged because the neutron diffraction patterns are exceptionally sensitive to hydrogen contamination in the samples. A comparison between a series of CuTi amorphous alloys contaminated with hydrogen and a similar series that were hydrogen-free shows that in titanium-rich alloys the hydrogen appears to act as a catalyst for the amorphisation reaction and in its absence the reaction in inhibited.

STRUCTURAL AND THERMODYNAMIC ASPECTS OF GLASS-FORMATION IN CU-TI-H - ROLE OF HYDROGEN IN MECHANICAL ALLOYING

Abstract The results of experiments of milling elemental powders of Cu and Ti in the presence of hydrogenated Ti are reported. A mixture of an amorphous phase and TiH2 was obtained by grinding γ-CuTi under H2. Ball milling was performed on ternary mixtures of Cu, Ti and TiH2 under Ar. Complete amorphization occurred at low concentrations of TiH2; otherwise, an amorphous phase coexisting with unreacted hydride was obtained. The structure of the powders was checked at steps during milling by correlating X-ray diffraction and thermal analysis data. The free energy of formation of the alloys, estimated at 300 K along a section of the Cuue5f8Tiue5f8H phase diagram, shows a driving force for amorphization at low hydrogen content. All ternary Cuue5f8Tiue5f8H hydrides, both crystalline and amorphous, are metastable with respect to a mixture of TiH2 and Cuue5f8Ti intermetallics and the possibility of a phase separation in the amorphous hydrogenated alloy is outlined.

Thermodynamic Aspects of Amorphization and Deformation in Cu-Ti-TiH2 Mixtures

Mechanichal alloying of Cu, Ti and TiH2 and grinding of -CuTi in H2 atmosphere lead to a mixture or a hydrogenated amorphous phase and TiH2. From the heat of transformation of metastable phases, their enthalpy of formation is determined along a section of the Cu-Ti-H phase diagram. The free energy of formation, estimated at 3 00 K, shows a driving force for amorphization at low hydrogen concentration. A phase separation is predicted for higher hydrogen contents. The effect of deformation of pure elements on amorphization is discussed.