METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 28A, JULY 1997—1429 Amorphization Reaction of Ni-Ta Powders during Mechanical Alloying PEE-YEW LEE, JU-LUNG YANG, CHUNG-KWEI LIN, and HONG-MING LIN This study examined the amorphization behavior of Ni x Ta 100-x alloy powders synthesized by me- chanically alloying (MA) mixtures of pure crystalline Ni and Ta powders with a SPEX high energy ball mill. According to the results, after 20 hours of milling, the mechanically alloyed powders were amorphous for the composition range between Ni 10 Ta 90 and Ni 80 Ta 20 . A supersaturated nickel solid solution formed for Ni 90 Ta 10 , as well. X-ray diffraction analysis reveals two different types of amor- phization reactions. Through an intermediate solid solution and by direct formation of amorphous phase. The thermal stability of the amorphous powders was also investigated by differential thermal analysis. As the results demonstrated, the crystallization temperature of amorphous Ni-Ta powders increased with increasing Ta content. In addition, the activation energy of amorphous Ni-Ta powders reached a maximum near the eutectic composition. I. INTRODUCTION PREPARING amorphous alloys based on IVa to VIa group refractory transition metals is highly attractive, since amorphous phases of these high melting temperature alloys possess high crystallization temperatures. [1] However, the high cooling rate necessary to bypass the nucleation and growth of crystalline phases in the undercooled alloy melts always severely restricts, for instance, amorphization of the alloy with a high melting temperature by the liquid quench- ing method. [2] As previous investigations demonstrated, amorphous materials can be obtained by destabilizing a solid phase’s crystallinity by a solid-state reaction. [3,4] Var- ious techniques to synthesize amorphous alloys via solid- state amorphization include hygrogenation, multilayer interdiffusion, and mechanical alloying (MA). [5,6] Mechanical alloying was developed by Benjamin in the early 1970s to produce superalloy powders strengthened by oxide dispersions. [7] It is a high energy ball milling process consisting of repeated mechanical mixing, cold welding, fracturing, and rewelding of powders during ball-powder collision events. [8] Owing to this unique novel milling be- havior, MA allows materials scientists to circumvent ma- terial limitations and manufacture alloys that are difficult or impossible to produce by conventional melting and cast- ing techniques. For instance, other investigators used MA to successfully synthesize many equilibrium and/or non- equilibrium phases in many alloy systems, including amor- phous materials, quasicrystals, nanocrystalline materials, rare earth magnets, and intermetallics. [9,10] Since the discovery of amorphous phase in mechanically alloyed Ni 60 Nb 40 powder, [11] this technique has been applied to numerous amorphous materials. [6,9,10] However, the Ni- Ta binary system has received only limited attention. PEE-YEW LEE, Professor and Director, and CHUNG-KWEI LIN, Visiting Associate Professor, are with the Institute of Materials Engineering, National Taiwan Ocean University, Keelung, Taiwan 202. JU-LUNG YANG, Engineer, is with Sino-American Silicon Products, Inc., Hsinchu, Taiwan 300. HONG-MING LIN, Professor and Chairman, is with the Department of Materials Engineering, Tatung Institute of Technology, Taipei, Taiwan 104. Manuscript submitted November 22, 1996. Therefore, this study investigates the feasibility of prepar- ing amorphous Ni-Ta powders by MA, starting from ele- mental Ni and Ta powders and using a shaker ball mill. II. EXPERIMENTAL DETAILS A. MA Processes Elemental powders of Ni (99.98 pct, 300 mesh) and Ta (99.98 pct, 325 mesh) were weighed to yield the de- sired compositions, Ni x Ta 100-x (x = 10, 20, 30, 40, 50, 60, 70, 80, and 90), and then canned into an SKH 9 high speed steel vial (40 mm in diameter and 50 mm in height) to- gether with Cr steel balls (7 mm in diameter with ball-to- powder ratio = 2:1) under an argon-filled glove box, where a SPEX* 8000 shaker ball mill (1200 rpm) was employed *SPEX is a trademark of SPEX Industries, Edison, NJ. for MA. The overall MA processes lasted 20 hours and were interrupted every 15 minutes for the first hour and every 30 minutes thereafter. Each interruption was followed by an equal length of time (30 minutes) to cool down the vials. An appropriate amount of the mechanically alloyed powders was extracted to examine the progress of amor- phization. B. Characterization of Mechanically Alloyed Powders Techniques used to examine the status of amorphiza- tion include X-ray diffraction, scanning electron micros- copy (SEM), Vickers microhardness, and differential thermal analysis (DTA). The X-ray analysis was per- formed using a SIEMENS* D-5000 diffractometer with a monochromatic Mo K  radiation (40 kV and 30 mA). The microstructure and the morphology of the mechani- cally alloyed powders were examined using a Hitachi S-4100 scanning electron microscope. Next, the Vickers microhardness (five tests for each specimen) was mea- sured with a Matsuzawa MXT50-UL machine using a load of 25 g. The thermal stability of the as-milled pow-