Showing papers by "Grenoble Institute of Technology published in 1992"

Effect of microstructure on

Abstract: Good candidates to replace the conventional titanium alloys and nickel-based superalloys currently employed in gas turbine engines are the titanium aluminides. These ordered structures are attractive because of their good modulus retention at high temperatures, low densities, and improved creep resistance. The purpose of this research was to investigate how the micro- structure of the titanium aluminide, Ti-24Al-11Nb (at. pct), influences the creep rate. Study of the stress dependence of the steady-state creep rate has shown that microstructures with a coarse α2 lath size, in general, display improved creep resistance over fine-lathed structures. Specimens with an equiaxed α2-phase microstructure exhibit much greater creep rates at all stresses and temperatures, implying that dislocation slip length, proportional to α2 phase dimension, does not always control creep in these materials and that texture may adversely affect creep resis- tance. When microstructures and dislocation substructures were investigated by transmission electron microscopy (TEM), observations of crept specimens indicated that the majority of dis- locations have a-type Burgers vectors and are located in the α2 phase.

Metastable phases of the Zr-Al system obtained by mechanical attrition and rapid soldification

Abstract: The equilibrium phase diagram of the Zr-Al system, shows the existence at room temperature of the L1{sub 2} line compound Zr{sub 3}Al The peritectoid reaction which leads to its formation, starting from the mixture {alpha}Zr + Zr{sub 2}Al, was thoroughly investigated Apart from the interesting technological aspects several researchers have been concerned with the disordering, ie, destruction of the long-range crystalline order, and amorphization of this intermetallic compound Probably because of the links with nuclear reactors, the first experiments in this field involved electron and ion irradiation More recent experiments using Kr{sup +} ions extended the early findings In this paper, amorphization was observed and instability deduced from measurements of the shear modulus For this reason, a parallel was drawn between this solid-state reaction and melting Another factor considered necessary to a successful amorphization is the increase in the free energy of the system caused by the reduction of long-range order A critical value of the long-range order parameter, S, measured from the intensity of the electron diffraction spots, of 02 has been reported as necessary to the appearance of the first amorphous regions

Structural characteristics and performances of rf-sputtered Mo/Si and Co/Si multilayers for soft x-ray optics

Abstract: Precise structural analysis of Mo/Si multilayers deposited by a diode rf-sputtering system has been made using in-situ kinetic ellipsometry, grazing x-ray reflection, x-ray diffraction, Auger profile analysis, Rutherford backscattering, and high-resolution electron microscopy. The main structural imperfections (interface roughness and interdiffusion) have been related to the Mo crystallization and to the molybdenum silicide formation at the interfaces. The comparison to Co/Si multilayers deposited in the same conditions was useful to deduce the influence of the intrinsic properties of these systems on their structural behavior. Silicide layers are formed in real-time during the growth of the samples. They are completely amorphous and their composition is not far from defined compounds (MoSi2 and CoSi2). In Mo/Si multilayers the Mo on Si interface is always thicker than the other interface (approximately equals 15 angstrom compared to 8 angstrom). It is not due to the deposition conditions but to the crystallization of the molybdenum layers which reduces the silicon diffusion at the Si on Mo interface. The higher reactivity of cobalt with silicon produces thicker quasi-symmetric silicide layers (approximately equals 25 angstrom). The thermal behavior of the two systems is also controlled by the interdiffusion and the crystallization of the silicide layers. Absolute soft x-ray reflection was measured on different Mo/Si x-ray mirrors by synchrotron radiation at various wavelengths above the Si L-(alpha) line and related to the structural characteristics. In spite of the occurrence of thin silicide layers at each interface, reflectivities as high as 55% in normal incidence have been obtained at 130 angstrom.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.