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Phase diagrams and glass formation in metallic systems
No full textThe basic thermodynamic aspects of glass‐formation in metallic systems are reviewed. In particular, the specific features of a phase diagram with respect to glass‐formation are evidenced. On the basis of the regular solution model, the effect of various thermodynamic quantities on the free energy difference between undercooled liquid and crystal phases are outlined. In order to describe the amorphous phase, a specific heat difference between liquid and solid phases in the undercooling regime is introduced in the CALPHAD assessment of various binary systems. The glass‐transition is described as a second order transition. Examples are given for different systems, including Fe‐B and Cu‐Mg. From the description of the free energy of various phases as a function of composition and temperature, the driving forces for nucleation of crystal phases and the T0 curves are estimated
Thermodynamic analysis of the undercooled liquid and glass transition in the Cu-Mg-Y system
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Thermodynamics of glass-formation in Cu-based alloys
No full textA thermodynamic analysis of glass-formation in binary Cu-Mg and Cu-Zr alloys has been carried out. To this purpose, the thermodynamic assessment of binary systems has been reviewed and suitable thermodynamic models for undercooled liquid alloys and for glass transition have been selected. Various binary Cu-Mg compositions have been prepared and amorphous samples have been obtained by rapid solidification. Experimental results both on equilibrium and non-equilibrium phases have been used in the assessment procedure. New parameters have been introduced in order to describe the liquid/amorphous phase. The agreement between calculated thermodynamic quantities and corresponding experimental data has been significantly improved. From calculated thermodynamic functions of Cu-Mg and Cu-Zr systems, driving forces for nucleation of various intermetallic phases have been estimated as a function of temperature and composition
STRUCTURAL AND THERMODYNAMIC ASPECTS OF GLASS-FORMATION IN CU-TI-H - ROLE OF HYDROGEN IN MECHANICAL ALLOYING
No full textThe 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 gamma-CuTi under H-2. 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 Cu-Ti-H phase diagram, shows a driving force for amorphization at low hydrogen content. All ternary Cu-Ti-H hydrides, both crystalline and amorphous, are metastable with respect to a mixture of TiH2 and Cu-Ti intermetallics and the possibility of a phase separation in the amorphous hydrogenated alloy is outlined
Rapid solidification of silver-rich Ag-Cu-Zr-Al alloys
Full text linkIn this work amorphous/crystalline Ag–Cu–Zr–Al composites with 80 wt.% Ag were obtained by rapid
solidification exploiting the presence of a miscibility gap in the liquid. Undercooling favored the separation
of a silver-rich liquid, that solidifies as f.c.c.-Ag solid solution, and a remaining liquid that falls in the
glass forming compositional range of the Cu–Zr–Ag–Al system. Alloys with nominal composition (at.%)
Ag73.0Cu12.7Zr12.1Al2.2 (alloy A), Ag73.1Cu17.2Zr9.7 (alloy B0), Ag72.5Cu16.7Zr9.4Al1.4 (alloy B1), Ag71.8Cu16.2
Zr9.1Al2.8 (alloy B2) were studied. All the alloys have a silver content (80 wt.%) corresponding to the
one required for commercial precious alloys. The effect of the cooling rate and the composition on phase
selection and microstructures was evaluated by comparing slowly cooled master ingots and rapidly
quenched ribbons. In alloys B0, B1 and B2, the amorphous fraction progressively reduces when increasing
the Al content. For alloy A, amorphization of the silver poor liquid can be obtained only at high quenching
rate. A ductile fracture behavior of the as-spun ribbons is combined with high hardness values (260–
290 HV)
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