M. Cornet

Nanocrystal evolution in bulk amorphous Zr57Cu20Al10Ni8Ti5 alloy and its mechanical properties

Abstract Bulk alloys of composite microstructures of nanocrystals and amorphous phases were produced by annealing the bulk Zr57Cu20Al10Ni8Ti5 amorphous alloy prepared by copper mold casting. The mechanism for the evolution of the nanocrystalline microstructures was discussed with respect to the particular crystallization behaviors of the amorphous alloy. The changes of the microstructures due to the annealing conditions were studied. The mechanical properties of the partially nanocrystallized alloys were studied by compression test at room temperature. The fracture stress and the Youngs modulus increase with the crystalline fraction to ≈45% (i.e. corresponding to a heat release of 45% of the total heat of crystallization) and then decrease. The yield stress also increases with the crystalline fraction to ≈45%, but then the deformation shows only an elastic part and no yield point appears with a further increase of the crystalline fraction. The hardness increases linearly with the crystalline fraction. Fracture morphology examination indicates that the fracture mechanism transforms from ductile to brittle nature at a crystalline fraction of about 40–45%.

Influence of nitrogen on the microstructure and mechanical properties of Ti–48Al alloy

Abstract The influence of nitrogen on the microstructure and on the mechanical properties of a two-phase lamellar Ti–48Al alloy was studied. The prominent part played by nitrogen in influencing grain growth has been underlined. The effect of nitrogen, in solid solution in the α 2 phase or in precipitates in the γ phase, on the microstructural parameters has been addressed. The studied mechanical properties (microhardness, compression yield stress and creep) are improved when the nitrogen content increases and when this element is in solid solution. This improvement is due to combined hardening factors such as the interlamellar spacing, the presence of nitrogen in solid solution or its precipitation. These factors are more or less effective, so the effects of each of them on the three kinds of studied mechanical properties have been specified.

Deformation behavior of partially crystallized Zr57Ti5Al10Cu20Ni8 bulk amorphous alloy

Abstract Annealing of the Zr 57 Cu 20 Al 10 Ni 8 Ti 5 amorphous alloy near the glass transition temperature produces microstructures of nanocrystals embedded in an amorphous matrix. The crystallization becomes retarded when the crystal amount reaches about 40 vol%. The deformation behavior of the partially crystallized alloys were measured by compression test at room temperature and at elevated temperature. The partially crystallized alloy shows higher yield stress and fracture stress (with the highest value of about 1800 and 1950 MPa, respectively) than the as-cast amorphous alloy (1560 and 1650 MPa, respectively). The fracture stress and the elastic deformation begin to decrease when the crystalline fraction is above 45 vol%. Compressive tests at elevated temperature shows that the stress for viscous deformation of the partially crystallized alloy (with about 40% crystals) does not change significantly in comparison with the amorphous alloy. These results indicate that such bulk amorphous alloy can be hot-worked (shaped) at elevated temperature.

Mechanical properties of Zr57Ti5Al10Cu20Ni8 amorphous and partially nanocrystallized alloys

Abstract Zr 57 Cu 20 Al 10 Ni 8 Ti 5 amorphous cylinders of 8 mm diameters were cast with copper moulds. The mechanical properties of the amorphous alloys obtained were measured by compression tests. The as-cast amorphous alloys show a high yield stress of about 1560 MPa and fracture stress of 1650 MPa. Structural relaxation, through annealing below the glass transition temperature, for example, at 573 K for 40 min, increases these values to about 1700 and 1760 MPa, respectively. Annealing of the amorphous alloy in the supercooled liquid region causes initial crystallization, but the residual amorphous phase becomes relatively stable. The partially crystallized amorphous alloy, for example with about 40% (vol) crystalline phase, still shows excellent strength, with a yield stress of 1700 MPa and a fracture stress of 1850 MPa. Both the as-cast and annealed amorphous alloys show ductile deformation with serrated flow before final fracture. These results indicate that amorphous alloys with such crystallization behaviour can be quite safely hot-worked near the glass transition temperatures.

Transmission electron microscopy analysis of the early stages of damage in a γ/γ′ nickel-based alloy under low cycle fatigue

Abstract Ni-based γ/γ′ single crystals containing a medium volume fraction of cuboidal γ′ precipitates, were deformed under low cycle fatigue at room temperature in order to characterize the early damage due to fatigue deformation. The selected alloy exhibits mechanical properties similar to those shown by commercial superalloys. During the early hysteresis loops, the dislocation microstructure contains essentially single ( 1 2 )〈110〉 dislocations localized in their slip plane and forming dipoles in places. Reproducible evidence for planar slip is found which can be related to short-range order in the matrix together with the presence of ordered precipitates. In samples subjected to further plastic deformation, additional sessile entanglements are formed which tend to exhaust slip in deformation bands.

Ductile phase precipitation in the L12 ternary intermetallic alloy Ni3(Al, Si)

Abstract Mechanical properties of the Ni3Al intermetallic alloy could be improved by fine ductile precipitates. Four hypostoichiometric Ni3 (Al, Si) intermetallic alloys were prepared by levitation melting. The chemical compositions were chosen as close as possible to the assumed phase boundary between the γ′ and binary (γ + γ′) domains. Conditions for the precipitation of fine ductile γ particles in the γ′ intermetallic phase were established in preparation for further studies of mechanical and dislocation behaviours in this kind of microstructure.