• PDF / 3,151,002 Bytes
• 9 Pages / 598.28 x 778.28 pts Page_size
• 14 Downloads / 251 Views

DOWNLOAD

REPORT

---

I.

INTRODUCTION

NANOCRYSTALLINE metal powder may be generally defined by particles whose lineal dimensions fall between 5 and 100 nm. Numerous techniques have been used to produce these novel powders, which may be sintered by various methods to create bulk material that is neither amorphous nor classically crystalline.l~-4l Among others, synthesis methods include evaporation/ condensation, I3,4,sl high-energy ball milling, 16,7,81 sol-gel processing and reduction annealing, 19j sputtering, t]~ laser ablation, I~2'131 and spray deposition, t~4J Nanocrystalline metals offer new physical and mechanical properties based microstructurally on significant fractions of atoms occupying surface states. A common approach to bulk production is cold isostatic pressing in the synthesis apparatus at pressures up to 5 GPa (e.g., Reference 4). Clearly, it is desirable from manufacturing, user, and flexibility perspectives to perform conventional powder processing steps at sites remote to the synthesis apparatus. However, this creates new obstacles in terms of powder handling, as many metal powders are reactive in air. The purpose of this investigation was to study the classical sintering behavior of nanocrystalline iron powder after removal from the synthesis apparatus and to follow the evolution of microstructure during the densification process. II,

EXPERIMENTAL

Iron powder was obtained commercially from the Ultram Company (Olten, Switzerland). The powder was manufactured using the evaporation-condensation method. Lots of nominally 50 g powder were collected into glass transport tubes and sealed for shipping. Upon receipt, tubes were placed in a glove box with oxygen content and dew point controlled and monitored to less D.L. BOURELL, Professor and Temple Foundation Fellow, is with the Center for Materials Science and Engineering, The University of Texas at Austin, Austin, TX 78712. W.A. KAYSSER, Director, is with the DLR Institute for Materials Research, Cologne, Germany. Manuscript submitted February 22, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A

than 8 ppm and 195 K, respectively. The powder was removed from the tubes as needed and stored. Throughout the investigation, powder continued to be reactive when exposed to air, spontaneously combusting if the transfer to air was uncontrolled. Oxygen content of the powder was assessed using a LECO* TC-436 oxygen analyzer. Small portions of *LECO is a trademark of LECO Corporation, St. Joseph, MI.

powder were loaded into preweighed nickel cans which were sealed by double crimping. The average of eight measurements was reported. For sintered samples, no container was used, and the average of at least four measurements was reported. As-received powder surface area was measured using the Brunauer, Emmett, and Teller (BET) method, llSj with nitrogen as the adsorbing-desorbing gas. Two measurements were taken at each of three partial pressures using a Quantachrome Quantasorb Sorption System. The powder sample was a small portion of iron powder carefully brought into air under cont