Papers by Author: Daniel Caillard

Abstract: An unified structural model has been devised [1] that describes both icosahedral phasesi-AlPdMn and i-AlCuFe. The model suggests that possible icosahedral structures could be found inalloys with general composition Al61:8(PdjCu)21:35(MnjFe)8:29(AljFejMn)4:28(AljCu)4:28. Applied to the systems AlCuMnFe [2] and AlPdFe [3], thisformula leads to the compositions Al66:08Cu21:35Mn8:29Fe4:28 and Al70:36Pd21:35Fe8:29 that have bothbeen synthetized by rapid quenching and annealing.The as-quenched quaternary alloy Al66:08Cu21:35Mn8:29Fe4:28 exhibits a F-icosahedral phase that trans-forms into a decagonal phase after annealing. Its 10-fold axis aligns along one of the 5-fold axes ofthe icosahedral phase. This implies a group-subgroup transformation, with the kernel group ̅5m, anal-ogous to the one observed during CFC to HCP transformations, with kernel ̅3m.The as-quenched and annealed ternary alloy Al70:36Pd21:35Fe8:29 is similar to i-AlPdMn with an iso-morphic substitution of Mn with Fe. A study of the phase diagram of the system AlPdFe [3] showsthat the results are compatible with the structural model proposed here in strong relations with thosepresented in [1] for i-AlPdMn.

Abstract: The phase transformations due to a sequence of chemical treatments leading to the nucleation and biomimetic growth of hydroxyl carbonated apatite (HCA) at the surface of titanium implants were studied by scanning and transmission electron microscopy in cross-section. In the first step, an acid etching forms a rough titanium hydride layer which remains unchanged after subsequent treatments. In the second step, soaking in a NaOH solution induces the growth of nanobelt tangles of nanocrystallized, monoclinic sodium titanate. In the third step, soaking in a simulated body fluid transforms sodium titanate into calcium and phosphorus titanate, by ion exchange in the same monoclinic structure. Then HCA, of a hexagonal structure, grows and embodies the tangled structure showing a preferential direction growth along its “c”-axis, perpendicular to the substrate surface. The interfaces between the different layers seem to be strong enough to prevent interfacial decohesion. The role of the titanate phase in the nucleation of HCA is finally discussed.

Abstract: The motion of steps during the growth of hydride precipitates has been observed by in situ transmission electron microscopy. Precipitates in different orientation relationships (OR) are shown to obey to the rules of three-dimensional edge-to-edge matching. They form clusters in order to realize a more isotropic distribution of the volume expansion, and to decrease their total elastic energy.

Abstract: Scanning (SEM) and cross-sectional transmission (TEM) electron microscopy analyses have been performed to study the transformations induced on the surface of titanium implants by a sequence of chemical treatments having as goal to induce the nucleation and growth of hydroxycarbonated apatite (HCA). In the first step, an acid etching forms a rough titanium hydride layer, which remains unchanged after subsequent treatments. In a second step, soaking in a NaOH solution induces the growth of nanobelt tangles of nanocrystallized, monoclinic sodium titanate. In a third step, soaking in a simulated body fluid transforms sodium titanate into calcium and phosphorus titanate, by ion exchange in the monoclinic structure. Then, HCA grows and embodies the tangled structure. The interfaces between the different layers are shown to be strong enough to prevent from interfacial decohesion. The role of the titanate structure in the nucleation of HCA is finally discussed.

Abstract: Simulated body fluid (SBF) solutions are widely used for in vitro bioactivity tests and to coat bioinert materials with biomimetic calcium phosphates. In this study SBF solutions with varying HCO3 - content were used to precipitate hydroxy carbonated apatite (HCA) on a bioactive titanium surface. XRD as well as cross-sectional TEM analyses revealed that the biomimetically derived crystallites show a preferred growth orientation in direction of their c-axis and perpendicular to the surface of the substrate. FTIR and Raman analyses revealed that, as long as the HCO3 - concentration in the testing solutions is below 20 mmol/l, only B-type HCA precipitates. Using SBF with a HCO3 - concentration equal to human blood plasma (27 mmol/l) leads to a AB-type substitution where, in accordance with bone mineral, CO3 2- substitutes PO4 3- as well as OH-.

Abstract: Quasicrystals can deform Plastically at high temperatures by the movement of dislocations like in crystals. Little is known, however, on the mechanisms that control dislocation movements in these structures. In a first part, the crystallography of quasicrystals is introduced as the projection, in the physical space, of a periodic structure in a hyperspace with more than three dimensions (6 in the case of icosahedral structures). The main properties of dislocations are defined in the physical space and in the six-dimensional space. New results are then presented, showing that dislocation movements may have a large component of climb, in contrast with all existing models that are based on glide processes only. Dislocation movements in two-fold and five-fold planes are compared and discussed in connection with macroscopic mechanical properties.