High resolution electron microscopy of interfaces in ultrafine microstructures of Zr and Ti based alloys

Abstract

Nanosized grains have been produced in Zr based bulk metallic glasses and rapidly solidified metallic glasses by crystallization. The various types of interfaces generated in these microstructures have been examined by high resolution transmission electron microscopy (HREM). The nanocrystals produced by the crystallization of Zr54.5Cu20Al10Ni8Ti7.5 and Zr52Ti6Al10Cu18Ni14 bulk glasses have been found to lie in the size range of 15 to 50 nm and comprised phases isostructural with tetragonal Zr2Cu and Zr2Ni. These observations are similar to those made by Xing et al [1]. HREM examination of nanograin boundaries showed the extension of the lattice fringes right up to the grain boundary. The lattice could be resolved simultaneously in some of the grains in view indicating that grain boundaries are parallel to low index planes in these grains [2]. Crystallographic defects like stacking fault and antiphase domain boundary could be noticed in many of the nanograins. Figure 1 shows the high resolution image of a nanograin of Zr2Ni phase where besides the fundamental lattice fringes a domain like structure could also be revealed. A number of planar crystallographic faults could be observed in this nanograin. A detailed analysis of these crystallographic faults has been carried out in this study. The same alloys have been produced in the glassy state by rapid solidification. Crystallization of the rapidly solidified metallic glasses also led to the formation of nanocrystals where HREM examination revealed the presence of twins and twin-twin interaction. It has been possible to generate a very fine lamellar microstructure by combustion synthesis and by direct laser fabrication technique of TiAl with widths of the α2 and γ lamellae in the range of 5 to 20 nm. The interface between the α2 and γ phases has been investigated in detail in this microstructure (Figure 2). The interfaces between contiguous lamellae of the γ phase have also been examined. The interface across which one phase is transforming to the other could be deciphered by HREM.