Surface Energy Driven Crystallization of Amorphous Pd 81 si 19
- PDF / 889,805 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 20 Downloads / 187 Views
ABSTRACT In-situ TEM investigations during thermal treatment of amorphous Pd 81 Si 19 have been performed. It was found that crystalline nuclei are formed near the perforation edge of the hole produced by electrochemical polishing. After impinging with neighboring crystals, a crystallization front formed which was aligned parallel to the perforation edge. The crystallization front moved in the direction perpendicular to the perforation edge. Crystal growth was found to proceed faster in thinner parts of the specimen than in thicker parts. The results are described qualitatively within a thermodynamic model taking into account volume-, surface- and interfacial free energies and an appropriate specimen geometry. INTRODUCTION Since the discovery of amorphous metals, their thermal stability is object of the scientific and technological interest because many physical properties deteriorate during crystallization. Few is however still known about the thermal stability of amorphous metals produced as very thin films where the influences of the free surface and of the film/substrate interface on the thermal stability cannot be neglected. The technological importance of metastable ultrathin films however requires a basic understanding of the influences of the free surfaces and of the film/substrate interface on their thermal stability. In-situ investigations by means of transmission electron microscopy (TEM) provide useful information about the influence of the free surface on crystallization. For example, it has been shown by TEM in-situ investigation that crystallization of the amorphous alloys Ni80 P20 and Ni 69Cr 14 P17 proceeds completely different in thick and thin parts of the specimens [1,2]. A thermodynamic description of crystal growth in ultrathin foils has been presented which takes into account volume-, surface- and interfacial free energies [3,4]. The specimen geometry was approximated by a wedge. It was found that the crystal growth rate increases dramatically with decreasing specimen thickness [4]. The specimen geometry used in that description did however not allow the description of crystal nucleation. In a further contribution, we present results of a crystallization study performed in-situ in a transmission electron microscope on free standing foils of amorphous Pd81 Sil 9 . Nucleation and growth in very thin foils are described qualitatively in a further developed thermodynamic model. EXPERIMENTAL Discs of 3 mm diameter were punched from a 30 gm thick ribbon of rapidly quenched amorphous Pd 81Sil 9 . These discs were electrochemically jet-polished from both sides at 288 K using an electrolyte which consists of a mixture of perchloric acid (20%), acetic acid (80 %) plus Thiorea (7g). TEM examinations have been performed in-situ at 300 kV in a Philips CM30 TEM. The specimen thickness at different positions was compared by measurement of the X-ray 477 Mat. Res. Soc. Symp. Proc. Vol. 481 ©1998 Materials Research Society
intensity created by the electron beam. Compositions have been measured by means of en
Data Loading...
