The synthesis and a joint experimental and theoretical study of the crystal structure and physical properties of the new ternary intermetallic compound TiGePt are presented. Upon heating, TiGePt exhibits an unusual structural phase transition with a huge volume contraction of about 10 %. The transformation is characterized by a strong change in the physical properties, in particular, by an insulator–metal transition. At temperatures below 885 °C TiGePt crystallizes in the cubic MgAgAs (half‐Heusler) type (LT phase, space group F$\bar 4$3m, a=5.9349(2) Å). At elevated temperatures, the crystal structure of TiGePt transforms into the TiNiSi structure type (HT phase, space group Pnma, a=6.38134(9) Å, b=3.89081(5) Å, c=7.5034(1) Å). The reversible, temperature‐dependent structural transition was investigated by in‐situ neutron powder diffraction and dilatometry measurements. The insulator–metal transition, indicated by resistivity measurements, is in accord with band structure calculations yielding a gap of about 0.9 eV for the LT phase and a metallic HT phase. Detailed analysis of the chemical bonding in both modifications revealed an essential change of the Ti–Pt and Ti–Ge interactions as the origin of the dramatic changes in the physical properties.
The mechanical properties of a series of six fiber-reinforced ceramics and glasses have been evaluated with the objective of critically assessing present understanding. A mdor parallel theme has been the characterization of the interface and an assessment of the thermomechanical properties of the interfaces as they relate to composite behavior. The results establish that the available mechanical property models correlate well with experiments, provided that independent measurements are made of the residual stress, the interface sliding stress, and the in situ strength properties of the fibers. In addition, trends in the sliding stress are found to be qualitatively consistent with those expected for sliding along debonded surfaces. [
The effect of the adatom surface diffusivity on the evolution of the microstructure and the intrinsic stress of thin metal films was investigated for the case of growth of polycrystalline Ag films on amorphous SiO 2 (a-SiO 2) and amorphous Ge (a-Ge) substrates, with high and low Ag adatom surface diffusivity, respectively. The surface diffusivity of the deposited Ag adatoms on the aGe substrate is suppressed also after coalescence of Ag islands due to the continuous (re)segregation of Ge at the surface of the growing film as evidenced by in-situ XPS. An assessment could be made of the role of adatom surface diffusivity on the microstructural development and the intrinsic stress evolution during film growth. As demonstrated by ex-situ TEM and ex-situ XRD, the Ag films grown on the a-SiO 2 and aGe substrates possess strikingly different microstructures in terms of grain shape, grain size, and crystallographic texture. Nevertheless, the real-time in-situ stress measurements revealed a compressive ! tensile ! compressive stress evolution for the developing Ag films on both types of substrates, however on different time scales and with stress-component values of largely different magnitudes. It was concluded that (i) the microstructural development of metallic thin films is predominated by the surface diffusivity of the adatoms and (ii) the intrinsic stress evolution is largely controlled by the developing microstructure and the grain-boundary diffusivity. V
The current interest in tough, high-temperature materials has motivated fiber coating development for brittle-matrix composites with brittle reinforcements. Such coatings are needed for controlled interface debonding and frictional sliding. The system investigated in this study was sapphire fiber-reinforced alumina. This system is thermochemically stable for severe use conditions, exhibits little thermal expansion mismatch, and utilizes the excellent strength and creep resistance of sapphire reinforcements. Porous oxide and refractory metal coatings which satisfy requirements for toughness improvement in these composites were identified by employing a variety of newly developed mechanical testing techniques for determining the interfacial fracture energies and sliding resistances.
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