Defect structures and nonbasal slip of C36 laves phase MgNi 2 in a two-phase alloy
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I.
INTRODUCTION
LAVES phases are considered as strengthening components for future high-temperature structural alloys and have received increasing attention in recent years. [lj Among the three structures of Laves phases, cubic C15, hexagonal C14, and dihexagonal C36, the deformation behavior and defect structures of the former two have been studied by many investigators, but C36 has received little attention. Study of single-phase C36 alloys of MgNi2 and MgCu0.4Znl.6 {21 showed that after compression at high temperatures, the dominant microstructural features were extended faults and dislocations, both on the basal plane. Studies of C36 Fe2Zr in a two-phase alloy containing a high density of faults showed that compression at room temperature could cause a stressinduced phase transformation between C36 and C15 structures, {31and that microhardness indentation can cause nonbasal slip in the Laves phase.t4] It has been found that the AB'A'C stacking t~] in the complex dihexagonal structure makes the deformation more difficult and very different from the other two Laves phase structures. The dislocation structure, slip system, twinning, or other defects arising from deformation of the C36 structure are far from clear. The present article studies a Ni-MgNi2 alloy with the soft nickel-rich phase particles dispersed in the brittle Laves phase. In this alloy, a small amount of plastic deformation (about 4 pct strain) may be induced by compression at room temperature. II.
EXPERIMENTAL
The Ni-15 at. pct Mg alloy was prepared by arc melting and cut with a diamond saw into 5 x 5 • 5-mm samples for compression testing. The compression sampies were annealed in a encapsulation tube of a vacuum of 10 -6 t o e at 900 ~ for 75 hours. Compression experiments were performed at room temperature with a crosshead speed of 2.5 x 10 -3 cm/min. YAPING LIU, Postdoctoral Associate, JAMES D. LIVINGSTON, Senior Lecturer, and SAMUEL M. ALLEN, Professor, are with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139-4307. Manuscript submitted August 12, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
Transmission electron microscope (TEM) specimens approximately 0.4-mm thick were cut from the undeformed and compressed samples using a diamond saw, ground to a thickness of 0.1 mm, and electropolished in a solution of 10 pct perchloric acid-90 pct methanol. Some samples were prepared using ion milling. A JEOL* 200CX *JEOL is a trademark of Japan Electron Optics, Ltd., Tokyo.
and an Akashi EM-002B high-resolution transmission electron microscope operating at an accelerating voltage of 200 kV were used in the microstructure analysis. A Link Analytical environmental scanning electron microscope (ESEM) operating at an accelerating voltage of 30 kV was employed for the surface observation of uncoated samples. III.
O B S E R V A T I O N S AND DISCUSSION
A. Phase Distribution Because of the eutectic point at 23 at. pct Mg, an annealed alloy of proeutectic composition contains large dendri
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