The Microstrain Generation in Cryomilled Nickel by Impurity Nitrogen Atoms
- PDF / 299,806 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 79 Downloads / 164 Views
Q3.3.1
The Microstrain Generation in Cryomilled Nickel by Impurity Nitrogen Atoms Kyung H. Chung and Enrique J. Lavernia Department of Chemical Engineering and Materials Science University of California, Davis, CA 95616-5294, USA ABSTRACT The residual microstrains in cryomilled Ni powders, processed under various cryomilling conditions, are measured by XRD and analyzed using a single line approximation (SLA) method. The results show that the average residual microstrains are in the range of 2×10-3 ~ 6×10-3, and the residual microstrain on the (200) plane is higher than those on the other planes by 33 %. The measured microstrain is proposed to evolve from the introduction of N as impurity atoms in Ni lattice may be attributed to the evolution of the residual microstrain. The N atoms tend to stay in the octahedral sites of Ni, and the diameter of N atom is larger than that of the octahedral site of Ni by 48 %. Accordingly, a lattice strain field is expected around interstitial N atoms that are located at octahedral sites. By comparing the crystal structure around the octahedral site of Ni with that of Ni3N structure, the lattice strains are estimated to be in the range of 5 ~ 15 %. The results show that the (200) plane has the lattice strains that are two times higher than those in other planes, and this is argued to be the reason for the measured anisotropy of residual microstrain in Ni after cryomilling. INTRODUCTION Mechanical alloying (MA) has been widely used to synthesize nanostructured materials [1]. It is believed that MA produces nanostructures by the generation and re-arrangement of dislocation structures that evolve due to high strain fields that are generated during milling. More recently, MA under liquid nitrogen, described in the literature as cryomilling, has been used to manufacture nanostructured materials from a variety of systems, including Al [2], Ni [3], Zn [4] and Fe [5]. A major concern in the processing of materials via MA is the nature and amount of impurities that are introduced during MA. The small particle size of the powders, availability of large surface area, and formation of new fresh surfaces during milling are factors that contribute to the contamination of powder [6]. The present study was undertaken to provide fundamental insight into the factors that govern the development of residual microstrain and its relation with impurity atoms introduced as contamination in cryomilled materials. Cryomilling was selected in view of recent interest in this technique to synthesize nanostructured materials [2-4]. Ni was selected as a model fcc system. EXPERIMENTAL PROCEDURES Commercially available Ni powders with a purity >99.5% (Sulzer Metco Inc., Westbury, NY) and a nominal particle size of 45±11 µm were used in the present study. The powders were cryomilled using an Szegvari attritor model B in stainless steel tank with stainless steel balls. The diameter of balls were 6.4 mm and the ball to powder ratio was 20:1 or 32:1. The milling of powders was conducted in liquid nitrogen at a cryogenic
Data Loading...
