K. Kelm

ABSTRACT A group of solids with the general composition A2In12Q19 (A = K, Tl, NH4; Q = Se, Te) is characterized by combined Xray single-crystal and high-resolution transmission-electron microscopy (HRTEM). Similar nanosize domains with variable sizes and complex internal structures are common to all three compounds. Although a partial ordering of domains for the bulk of K2In12Se19 is dominating, the observed ordering patterns in microdomains range from total random orientation to a pattern with a ninefold superstucture (rare precursor phase not stable under HRTEM conditions). In spite of testing various synthesis conditions it was not possible to avoid these unusual structural features for K2In12Se19, which are apparently intrinsic. The formation of significantly larger domains is observed for K2In12Se19–xTex and K2–yTlyIn12Se19 and results in a twofold superstructure that can be observed with X-ray diffraction also on a macroscopic scale. (NH4)- In12Se19 is a special case where an initial weak ordering is observed that is characterized by ring X-ray reflections forming hexagons around certain reciprocal lattice positions. This pattern has apparent similarities to K2In12Se19 but is not stable in the HRTEM. Instead, it disappears rapidly and is finally replaced by a twofold superstructure similar to K2In12Se19–xTex and K2–yTlyIn12Se19. The reason was identified as a combined process of domain broadening and NH3 evaporation. As observed for K2In12Se19 at T 473 K, the superstructure reflections disappear. Surprisingly, the pseudobinary phase In2Q3 (Q: chalcogen) shows strong structural similarities to K2In12Se19 with respect to the internal structure of the nanodomains. Their three-dimensional arrangement, however, and the resulting superstructure are closer related to K2In12Se19–xTex and K2–yTlyIn12Se19.

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Al-rich Ti-Al alloys attracted some attention during the past years due to the possibility of their application as light-weight, high-performance materials at elevated temperatures. The effect of the addition of Nb to Al-rich Ti-Al alloys has been studied for Ti36Al62Nb2 by a combined approach of transmission electron microscopy (TEM) techniques for unraveling the structure and composition at the nanoscale. Structural analyses on as-cast ternary alloys revealed the presence of h-TiAl2-, Ti3Al5- and γ-TiAl-type phases. After heat treatment, phase transformations like the replacement of the metastable h-TiAl2-type by the stable r-TiAl2-type were identified. Additionally, changes of the microstructural features like the formation of interfaces with different orientation relationships are apparent. The orientation and interfacial relationships involved are compared to those of binary Ti-Al alloys rich in Al.

High-temperature creep of a binary Al60Ti40 (at.%) alloy in the as-cast state and after annealing at 1223K for 200h which produced nearly lamellar γ-TiAl+r-Al2Ti microstructure was studied utilizing creep compression tests in a temperature range between 1173 and 1323K in air. The material was manufactured by centrifugal casting. Microstructural characterization was carried out employing light-optical scanning (SEM) and transmission electron microscopy (TEM) as well as X-ray diffraction (XRD) analyses. It is shown that the alloy exhibits reasonable creep resistance at 1173K, especially in relation to its low density of around 3.8g/cm3. Stress exponents calculated as n=Δlog (strain rate)/Δlog (stress)=4 were found to be relatively constant for the temperature and stress regime investigated. This indicates that dislocation climb may be the rate controlling creep mechanism. The assessment of creep tests conducted at identical stress levels and varying temperatures yielded activation energies for creep of around Q=457kJ/mol in the as-cast condition. This value is significantly higher than those found in literature for interdiffusion of Al or Ti in γ-TiAl. It is concluded that the difference is a due to the instability of the microstructure of the as-cast multi-phase alloy.