D. Shechtman

Indexing of icosahedral quasiperiodic crystals

Since the definition of quasiperiodicity is intimately connected to the indexing of a Fourier transform, for the case of an icosahedral solid, the step necessary to prove, using diffraction, that an object is quasiperiodic, is described. Various coordinate systems are discussed and reasons are given for choosing one aligned with a set of three orthogonal two-fold axes. Based on this coordinate system, the main crystallographic projections are presented and several analyzed single-crystal electron diffraction patterns are demonstrated. The extinction rules for three of the five icosahedral Bravais quasilattices are compared, and some simple relationships with the six-dimensional cut and projection crystallography are derived. This analysis leads to a simple application for indexing powder diffraction patterns.

The microstructure of rapidly solidified Al6Mn

The microstructure of rapidly solidified Al−Mn alloys containing 18 to 25.3 wt pct Mn was studied by transmission electron microscopy. One of the phases found in the microstructure exhibits icosahedral symmetry manifested in electron diffraction patterns having five-fold symmetry. A new structural concept is proposed to account for the observed electron diffraction patterns. The structure is assumed to be composed of many connected polyhedra. Although not forming a regular lattice, such structures are able to produce sharp diffraction peaks. The terminal stability and transformation of the icosahedral phase was also studied and reported.

The deformation and fracture of Ti3Al at elevated temperatures

The tensile properties of the intermetallic compound Ti3Al have been determined in air at several temperatures within the range of 25 to 900 °C. The dislocation structures produced by the various testing conditions were studied in the electron microscope and the fracture modes were studied in the scanning electron microscope. These microstructural observations were correlated with the mechanical properties. The results indicate that Ti3Al has only limited ductility even at 900 °C. The apparent ductile-brittle transition which occurs above 600 °C is due to increasing amounts of intergranular cracking. Some increase in ductility above 600 °C is due to the onset of dislocation cross slipping. The fracture mode up to 600 °C is entirely cleavage. Above 600 °C the fracture shows increasing evidence of plasticity; however, cleavage remains the main fracture mode up to 900 °C.

The Effect of Rapid Solidification Velocity on the Microstructure of Ag-Cu Alloys

Electron beam solidification passes have been performed on a series of Ag-Cu alloys between 1 wt pct Cu and the eutectic composition (28.1 wt pct Cu) at speeds between 1.5 and 400 cm per second. At low growth rates conventional dendritic or eutectic structures are obtained. The maximum growth rate of eutectic structure is 2.5 cm per second. At high growth rates microsegregation-free single phase structures are obtained for all compositions. The velocity required to produce this structure increases with composition for dilute alloys and agrees with the theory of absolute stability of a planar liquid-solid interface with equilibrium partitioning. For alloys between 15 and 28 wt pct Cu, the velocity required to produce the microsegregation-free extended solid solution decreases with composition and is related to nonequilibrium trapping of solute at the liquid solid interface. At intermediate growth rates for alloys with 9 wt pct Cu or greater, a structure consisting of alternating bands of cellular and cell-free material is obtained. The bands form approximately parallel to the local interface.

Initial stages of corrosion within Mg-Zn-Y-Zr alloy in 1 g/l NaCl solution

Corrosion behavior of as-cast and heat treated Mg-Zn-Zr-Y alloy was studied in 1 g/l NaCl solution. The as-cast alloy was the least resistant to pitting corrosion. Its Mg-Zn solid solution matrix was the structural constituent responsible for corrosion susceptibility due to the presence of Zn-lean regions. Homogenization of the Zn distribution within the matrix significantly increased the corrosion resistance of the alloy. Grain boundaries, which contain intermetallic phases, were resistant to corrosion attack, thus impeding corrosion propagation.

Icosahedral and decagonal phase formation in Al-Mn alloys

The solidification conditions leading to the formation of the icosahedral phase in Al-Mn alloys have been investigated, using samples prepared by melt spinning and electron beam surface melting. It is found that the icosahedral phase can grow with a range of compositions, but that it grows in competition with another metastable phase which is decagonal. Both of these phases can displace the equilibrium intermetallic phases by nucleating ahead of them in the melt when the solidification velocity is greater than a few centimeters per second. The relative abundance of the icosahedral and decagonal phases varies with composition and solidification rate. Icosahedral crystals in electron beam melt trails are often about 25 μm in diameter, and they grow dendritically along a preferred crystallographic direction.

Growth defects in diamond films

Abstract : Growth defects in diamond films grown by plasma-assisted chemical vapor deposition (CVD) were studied by high resolution electron microscopy. Several features of the microstructure were resolved and their importance to the growth of the diamond film was evaluated. The observations included various twin boundaries of the type sigma=3 as well as sigma=9, sigma=27 and sigma=81, which form by an interaction of lower order twins. These higher order boundaries, are loci of intersection points of growing planes on two adjacent twins and can serve as an indicator for the local crystal growth direction. The central nucleation site for the growing planes in many cases can be traced back to a quintuplet twin point. A twin quintuplet has five reentrant angles and thus serves as a preferred nucleation site for new planes as the crystal grows.

Biocompatibility of rapidly solidified magnesium alloy RS66 as a temporary biodegradable metal

Biodegradable magnesium-based alloys are very promising materials for temporary implants. However, the clinical use of magnesium-based alloys is often limited by rapid corrosion and by insufficient mechanical stability. Here we investigated RS66, a magnesium-based alloy with extraordinary physicochemical properties of high tensile strength combined with a high ductility and a homogeneous grain size of ~1 μm which was obtained by rapid solidification processing and reciprocal extrusion. Using a series of in vitro and in vivo experiments, we analyzed the biodegradation behavior and the biocompatibility of this alloy. In vitro, RS66 had no cytotoxic effects in physiological concentrations on the viability and the proliferation of primary human osteoblasts. In vivo, RS66 cylinders were implanted into femur condyles, under the skin and in the muscle of adult rabbits and were monitored for 1, 2, 3, 4 and 8 weeks. After explantation, the RS66 cylinders were first analyzed by microtomography to determine the remaining RS66 alloy and calculate the corrosion rates. Then, the implantation sites were examined histologically for healing processes and foreign body reactions. We found that RS66 was corroded fastest subcutaneously followed by intramuscular and bony implantation of the samples. No clinical harm with transient gas cavities during the first 6 weeks in subcutaneous and intramuscular implantation sites was observed. No gas cavities were formed around the implantation site in bone. The corrosion rates in the different anatomical locations correlated well with the local blood flow prior to implantation. A normal foreign body reaction occurred in all tissues. Interestingly, no enhanced bone formation could be observed around the corroding samples in the condyles. These data show that RS66 is biocompatible, and due to its interesting physicochemical properties, this magnesium alloy is a promising material for biodegradable implants.

The formation of F.C.C. titanium in titanium-aluminum multilayers

The Ti layers in Al/Ti multilayers are shown to transform from hexagonal close packed in the as-deposited multilayers to face centered cubic on ion milling in cross-section. By examining the change in free energy of the system on transformation it is demonstrated that interfacial, surface and strain free energies cannot overcome the theoretically predicted 0.08 eV/atom difference in bulk free energy between f.c.c. and h.c.p. Ti. Based on experimental observations of f.c.c. Ti in multilayers, it is argued that this value is too large.

Precipitation in rapidly solidified Al-Mn alloys

Precipitation at 450 °C was studied in melt-spun ribbons containing up to 15 wt pct Mn in solid solution in Al. The as-spun ribbons were microsegregation-free at compositions up to 5 wt pct Mn, but in more concentrated alloys a cellular microstructure was present. Upon annealing, four precipitate phases are observed, some of them being found preferentially on cell boundaries and others being found within the cells. Al6Mn, G, and the Gℍ phase can coexist for long times at 450 °C, but the G phase appears to be slightly more stable. A less stable T phase was detected in Al-5 wt pct Mn foils following short annealing periods. The supersaturation of the Al matrix can persist for many hours in alloys containing up to 3 wt pct Mn, but is essentially gone after 1 hour in alloys with 5 wt pct Mn or more.