Comparative Study of Mechanical Alloying Induced Nanocrystallization and Amorphization in Ni-Nb and Ni-Zr Systems
- PDF / 954,091 Bytes
- 15 Pages / 593.972 x 792 pts Page_size
- 111 Downloads / 240 Views
UCTION
MECHANICAL alloying (MA) is a process of continued ball milling of a mixture of different powders. The severe plastic deformation, repeated flattening, cold welding, and fracturing of powder particles during mechanical alloying led to a fine composite of constituents that is often accompanied with transformation to metastable phases such nanocrystalline and amorphous structures.[1,2] MA has the advantage that it does not require miscibility of the constituents in the liquid state. Thereby, MA offers a greater flexibility in the choice of constituent materials. Grain refinement of powders to the nanometer size in MA is governed by the plastic deformation induced during milling. Eckert et al.[3] observed that the final grain size in a series of face-centered cubic (fcc) metals, including Al, Cu, Ni, Pd, Rh, and Ir, scales inversely with the melting point of the respective metals. Benghalem and Morris[4] studied ball milling nanocrystallization of a variety of elements including the fcc metals Al, M.H. ENAYATI, Associate Professor, and E. DASTANPOOR, MSc Student, are with the Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran. Contact e-mail: [email protected] Manuscript submitted August 24, 2012. Article published online April 11, 2013 3984—VOLUME 44A, AUGUST 2013
Ni, and Cu, and the body-centered cubic (bcc) metals Fe and W and found that metals with higher melting temperatures attained a smaller final grain size regardless of their crystal structures. However, Fecht et al.[5,6] did not observe a clear inverse correlation between the ultimate grain size and melting temperature for bcc metals Cr, Fe, Nb, and W, as well as for hexagonal close-packed (hcp) metals Hf, Zr, Co, and Ru. In fact, the ultimate grain size of these series of bcc and hcp metals suggested a constant size (9 nm and 13 nm, respectively) independent of melting temperature. Mohamed et al.[7–9] analyzed the minimum grain size that was obtained by ball milling by plotting the normalized minimum grain size, Dmin/b (where b is the Burger vector) as a function of melting temperature Tm, the activation energy for diffusion Q*, and normalized hardness H/G (where H is the hardness and G is the shear modulus). They observed that all curves are similar in trend and that the data for all metals and alloys, regardless of their crystal structure, define a single curve that is divisible into two regions. In the first region, Dmin/b decreases rapidly with increasing Tm, Q*, or H/G, while in the second region, Dmin/b decreases very slowly with each of these three parameters. The dependence of final grain size vs melting temperature has been discussed with respect to the competing rates of creating dislocations due to work hardening and METALLURGICAL AND MATERIALS TRANSACTIONS A
recovery phenomena, which scale with the melting point.[3,4,10] Thereby, the ultimate grain size achieved by ball milling is determined by the balance between creation and annihilation of dislocations during processing. Koch[10] has argued that a d
Data Loading...
