Some remarks concerning the WLF-relation

Summary The limitations of the WLF-relation are discussed with respect to their inherent demands concerning universality. The problem of the reference temperature is discussed in some detail, and their constants are related to the temperature T where the apparent activation energy of flow gets infinite, as well to the characteristic activation energy of viscous flow at infinitely high temperature, E.It is concluded that the use of experimentally derived temperature dependent apparent activation energy of flow, E(T), from the slopes of crossing isochrone and isotherm viscoelastic curves substitutes the WLF-relation favourably. Thus, the problematic choice of a reference temperature is avoided. The two kinetically specific constants, E and T, may be admitted to characterize the energetic and entropic contribution, respectively, to the viscous flow. Experimental data are presented for some polymers.Herrn Prof. Dr. B. Vollmert zum 65. Geburtstag herzlichst gewidmet     

Vorbildliche Holzpalatzorganisation bei einem amerikanischen Fußbodenwerk

Zusammenfassung Organisation und Bewirtschaftung von Schnittholz-lagerpl?tzen in den USA weichen sehr wesentlich von den Verh?ltnissen in Europa ab. An dem Beispiel einer gro?en Fu?bodenfabrik in der Appalachian-Gegend werden die besonderen Merkmale der Lagerplatzorganisation behandelt. Der Einschnitt erfolgt in der Gegend in kleinen und kleinsten S?gewerken. Die Fabriken sind deshalb darauf angewiesen, sehr gro?e Lagerpl?tze zu unterhalten. Vermessung, Sortierung und Klassifizierung werden auf den Lagerpl?tzen der Firmen durchgeführt. Vor der künstlichen Trocknung in den Trockenkammern mu? das Holz, das grün angeliefert wird, l?ngere Zeit gestapelt lagern. Die Stapelung erfolgt unter Benutzung besonderer Stapelmaschinen. Zum Transport innerhalb der Pl?tze dienen die sogenannten “Straddle Trucks”. Die Entstapelung zur Beschickung der Trockenkammern erfolgt auf zweierlei Weise. Entweder wird der Stapel bereits vorher so aufgebaut, da? er durch gabelstapler paketweise abgetragen werden kann, oder es wird eine gro?e Anzahl Trockenwagen beladen und auf einem umfangreichen Gleissystem bereitgestellt.     

Core/Rim Structure of Liquid-Phase-Sintered Silicon Carbide

Plasma etching in conjunction with scanning and analytical transmission electron microscopy was used to characterize the microstructure and microchemistry of silicon carbide sintered with yttrium-aluminum garnet (YAG). The SiC grains comprise a core/rim structure with small amounts of excess yttrium, aluminum, and oxygen being present in the rim while these elements are missing in the core. The core/rim interface was found to be coherent, and both the core and the rim are composed of the same polytype, predominantly the 6H structure. These results suggest that Ostwald ripening by solution and reprecipitation controls the sintering mechanism in this system.     

Untersuchungen zum SwRK-Verhalten des unlegierten Stahles Ck 45 in chloridionenhaltigen wäßrigen Medien

Corrosion fatigue behaviour of steel Ck 45 in aqueous solutions containing chloride ions The fatigue behaviour of carbon steel Ck 45 (comparable to AISI 1045x) is investigated for cyclic tension and rotating bending load with a frequency of 25 Hz. The fatigue limits under cyclic tension load in air are 410 N/mm² for smooth specimens and 290 N/mm² for notched specimens. For rotating bending load a value of 200 N/mm² is found for smooth specimen. The fatigue limits for N = 10⁷ in two different environments (0.3% and 3% NaCl-solution) reach only 37–57% of the according values in air. Cathodic protection of smooth specimens causes an improvement to 95% of the air fatigue limit. The evaluation of the free corrosion potential for all corrosion fatigue tests and the appearence of fracture show typical attributes of corrosion fatigue in the active state.     

Deformation behavior of isotactic polypropylene crystallized via a mesophase

The mechanical behavior of semicrystalline isotactic polypropylene (iPP) of different crystallinity, crystal morphology and superstructure was investigated by standard tensile stress–strain analysis, dynamic-mechanical analysis, and in situ observation of the deformation by atomic force microscopy (AFM). Emphasis is put on the comparison of the mechanical characteristics of specimens containing either non-isometric lamellae, being arranged in spherulites, or nodular isometric domains, which are not organized in a superstructure. The formation of lamellae/spherulites and of nodules was controlled by the conditions of crystallization. The replacement of cross-hatched monoclinic lamellae and a spherulitic superstructure by randomly arranged isometric nodules leads to a distinct increase of the ductility and toughness, even if the crystallinity is identical. The modulus of elasticity and the yield stress increase as expected with increasing crystallinity. Slightly lower values of Young’s modulus and yield strength are detected if samples contained non-lamellar crystals in a non-spherulitic superstructure, proving an effect of the crystal shape on the deformation behavior. For the first time, tensile deformation of semicrystalline iPP which contains nodular ordered domains instead of lamellae has been followed by in situ AFM.     

Sintering Mechanisms of Zirconium and Hafnium Carbides Doped with MoSi2

The microstructure of two pressureless-sintered ultra-high-temperature ceramics, namely ZrC+20 vol% MoSi₂ and HfC+20 vol% MoSi₂, was characterized by scanning and transmission electron microscopy. With regard to the ZrC–MoSi₂ system, Zr _x Si _y compounds and SiC were detected. In the HfC–MoSi₂ system, a mixed phase was detected at the triple points and identified as (Mo,Hf)₅Si₃. For both the systems investigated, the high wettability of the silicide-based phases on the matrix grains suggests that sintering is assisted by a liquid phase. This contribution reports for the first time on the sintering mechanisms of early transition metal carbides doped with MoSi₂ as a sinter additive, on the basis of the microstructural evolution observed upon sintering and in the light of phase diagrams and thermodynamical calculations.     

Field-Assisted Densification of Superhard B6O Materials with Y2O3/Al2O3 Addition

B₆O is a possible candidate of superhard materials with a hardness of 45 GPa measured on single crystals. Up to now, densification of these materials was only possible at high pressure. However, recently it was found that Al₂O₃ can be utilized as an effective sintering additive, similar to the addition of Y₂O₃/Al₂O₃ that was used in this work. The densification behavior of the material as a function of applied pressure, its microstructure evolution, and the resulting mechanical properties were investigated. A strong dependence of the densification with increasing pressure was found. The material revealed characteristic triple junctions filled with amorphous residue composed of B₂O₃, Al₂O₃, and Y₂O₃, while no amorphous grain-boundary films were observed along internal interfaces. Mechanical testing revealed on average a hardness of 33 GPa, a fracture toughness of 4 MPa·m^1/2, and a strength value of 520 MPa.