L.L. He

Si-induced twinning of TiC and formation of Ti3SiC2 platelets

Abstract Effects of silicon in titanium carbide (TiC) have been investigated using high-resolution electron microscopy combined with high-spatial-resolution analytical electron microscopy. The results demonstrate that Si can reduce the twin boundary energy of TiC, leading to the formation of a lot of two-dimensional (2D) defects in TiC grains containing Si. These defects were identified as microtwins of four (111) spacings thick and structural-related Ti 3 SiC 2 platelets of only one unit cell thick. The twin-stabilizing effect of Si is discussed in terms of coordination environments of Si. Very thin Ti 3 SiC 2 platelets are formed, accompanied by the segregation of Si atoms and carbon vacancies to the twin boundaries. Microtwins and Ti 3 SiC 2 platelets were found to grow in 2D with Si totally confined in the defects.

Effects of Si and Al on twin boundary energy of TiC

Effects of aluminum and silicon on the twin boundary energy (TBE) of titanium carbide (TiC) have been investigated using transmission electron microscopy and first-principles calculations. The results demonstrate that Al is more effective relative to Si to reduce the twin boundary energy of TiC. By electronic structure analysis, the high TBE of pure TiC can be attributed to the energy penalty accompanying the formation of trigonal prismatic coordination of Ti atoms at twin boundaries. For the SUM doped TiC, the lower TBEs are partly due to the absence of the energy penalty accompanying the formation of trigonal prismatic coordination of Si/Al atoms, and are partly due to the stabilizing effect via the formation of pi bonding between carbon neighbors of Si/Al atoms. The pi bonding between carbon neighbors of Al is stronger than that of Si, providing an explanation for the larger twin-stabilizing effect of Al

Orientation relationship and interfacial structure between ζ-Ti5Si3 precipitates and γ-TiAl intermetallics

Abstract The orientation relationship (OR) and the interface structure between Ti 5 Si 3 precipitates and the γ -TiAl phase have been investigated systematically. The habit plane of Ti 5 Si 3 in TiAl was determined to be (0001) ζ ‖(111) γ . However, there is no low-index direction of the two phases parallel to each other in the plane. This “abnormal” OR has been predicted precisely from a recently developed geometrical method, in which the overlap of reciprocal lattice points of two adjoining crystals is utilized to obtain the optimum OR. In spite of the significant difference in crystal structure between TiAl and Ti 5 Si 3 , the interface was found to be semi-coherent with good matching and has the largest possible displacement shift complete lattice corresponding to the bicrystal. The energy of the interface is also discussed. The translational state between the two lattices and the chemistry of the terminating plane of Ti 5 Si 3 have been determined by using high-resolution transmission electron microscopy.

Polymorphism of Ti3SiC2

We investigated the crystal structure of Ti3SiC2 by means of high-resolution electron microscopy (HREM). Two polymorphs, alpha- and beta-Ti3SiC2, were identified. The amount of the a phase was larger than the P phase, indicating that the former has lower energy than the latter. We also found that the bright spots in HREM images of Ti3SiC2 do not necessarily correspond to the atomic columns; thus an intuitive interpretation of the image contrast in terms of the stacking sequences of the close-packed layers should be made cautiously.

Topology of charge density and elastic anisotropy of Ti3SiC2 polymorphs

Using an all-electron, full potential first-principles method, we have investigated the topology of charge density and elastic anisotropy of Ti3SiC2 polymorphs comparatively. By analyzing the charge density topology, it was found that the Ti-Si bonds are weaker in beta than in alpha, resulting in a destabilizing effect and lower Youngs modulus in directions between a and c axes for beta. On the other hand, the Si-C bonds (absent in alpha) are formed in beta in the c direction. The formation of the Si-C bonds not only mitigates the destabilizing effect of the weaker Ti-Si bonds, but also results in larger Youngs modulus in the c direction. In contrast to the high elastic anisotrophy, the elastic anisotropy of Ti3SiC2 is very low.

B2 precipitates and distribution of W in a Ti-47Al-2W-0.5Si alloy

The precipitation behavior of B2 particles and the distribution of W in a Ti-47Al-2W-0.5Si alloy were investigated using high-spatial resolution analytical electron microscopy (AEM) and high-resolution electron microscopy (HREM). The ledges at the alpha(2)/gamma interfaces were found to be enriched in W and therefore the preferable nucleation sites for the B2 precipitates. In the B2 phase, as high as one third of Al is substituted by W in comparison with the composition of the 7 phase. The segregation of W to the alpha(2)/gamma and B2/gamma interfaces was also detected. These results provide explanations for the previous observations that additions of tungsten are effective in promoting the creep resistance of TiAl alloys

Stacking faults and grain boundaries of Ti3SiC2

Stacking faults and grain boundaries of Ti 3 SiC 2 have been investigated by means of high-resolution electron microscopy. The stacking faults were found to result from insertion of additional TiC layers, instead of dissociation of dislocations conventionally observed in other materials. The stacking faults with many TiC layers inserted can also be viewed as TiC platelets. A glass phase is not formed at large-angle grain boundaries of Ti 3 SiC 2, manifesting the weak directionality of bonding in Ti 3 SiC 2 . The results also indicate that the softening of Ti 3 SiC 2 at high temperatures does not result from intergranular glass films but from an intrinsic property of Ti 3 SiC 2 . Moreover, it was found that one grain contacts the other with its base plane for most of the investigated grain boundaries.

Indentation induced amorphization in gallium arsenide

Vickers indentations were carried out on the surface of GaAs single crystal with the load of 0.049 N and were observed using high-resolution electron microscopy in the present experiment. The experimental results reveal that many defects such as dislocation, microtwin and stacking-fault occurred and amorphization took place beneath the indentation. High-pressure induced amorphization and shear deformation induced amorphization were proposed for the transformation from crystalline to amorphous structure.

On orientation relationship of the Ti5Si3 precipitates in a TiAl alloy

The orientation relationship (OR) and interface structure between zeta-Ti5Si3 precipitates and gamma-TiAl phase in a Ti-Al based alloy composed of gamma-TiAl and alpha(2)-Ti3Al lamellae have been investigated using transmission electron microscopy. Various orientation relationships defined by a pair of parallel directions and planes are discussed with the method of basic vector transformation matrix in the reciprocal space from gamma-TiAl to zeta-Ti5Si3 precipitate phase and two new kinds of orientation relationships between zeta-Ti5Si3 and gamma-TiAl phases have been found. Periodical interface fringes at gamma-TiAl/zeta-Ti5Si3 interface are analyzed according to the Moire fringes and interface misfit dislocations

Boron nitride nanotubes filled with zirconium oxide nanorods

Boron nitride nanotubes (BNNTs) filled with zirconium oxide (ZrO2) nanorods were synthesized by the improved solid-gas multiphase reaction method. The structure of ZrO2 nanorods was monoclinic single crystal or multi-twin crystal. The diameters of ZrO2 nanorods varied from 20 to 40 nm. The inner diameters of BNNTs were similar to those of corresponding ZrO2 nanorods. The BNNTs exhibited open end, closed end, or an end connected with a short tube grown from the tip of the ZrO2 nanorod. No preferred orientation was observed for the growth of ZrO2 nanorods.