Transactions of The Indian Institute of Metals            Effect of Zn concentration on diffusion induced grain boundary migration in Cu –Zn system A.K. Pradhan, S.P. Gupta and K. Mondal Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur-208 016, UP, India. E-mail: kallol@iitk.ac.in Received 19 January 2009 Revised 24 April 2009 Accepted 29 May 2009 Online at www.springerlink.com © 2009 TIIM, India Abstract Cu-Zn alloy system with three different compositions has been chosen to study the time, temperature and composition dependence of the Diffusion Induced Grain boundary Migration (DIGM) in the temperature range of 277-427 o C. The grain boundary migration follows parabolic rate law as a function of time. The diffusivity, D b α, was calculated from concentration-distance profile using growth rate, v. The activation energy for diffusion is found to be 101kJ/mol which is nearly half of the activation energy required for volume diffusion indicating that preferential grain boundary diffusion is more favorable than volume diffusion leading to grain boundary migration in Cu-Zn system. 1. Introduction Diffusion Induced Grain boundary Migration (DIGM) is a part of the more general Chemically Induced Interface Migration (CIIM), which includes Liquid Film Migration (LFM) and simple Interface Migration (IM) between two different phases 1 . CIIM plays an important role during many different practical processes like welding, preferential oxidation, sintering, and isothermal heat treatment of systems involving two phases which are not in equilibrium with each other [1]. The preferential diffusion of solute atom into the grain boundary than that into the grain is the basis for DIGM. When a poly-crystalline material is exposed to any solute vapor at low temperature where volume diffusion is negligible, the solute diffusion will be more through the grain boundary because of its open structure compared to that of the packed structure of the grain, which leads to a concentration gradient between the grain boundary and adjacent grain interior and this causes the grain boundary to migrate leaving behind the solute atoms in order to maintain homogeneity [1]. Rhines and Montgomery [2] first discovered DIGM in Cu-Zn system. However, composition change was not observed. After that many researchers have shown that at low temperature the composition change due to DIGM is much faster than due to the volume diffusion process [3-5]. Diffusion coefficient of solute in DIGM has been observed to be 8-10 orders of magnitude higher than that in volume diffusion process. Though many theories have been proposed for the occurrence of DIGM such as coherency strain energy theory [6], chemical free energy theory [7], grain boundary energy theory [8] etc, it is still not clear what guides the grain boundary movement. Recently, Sivaiah et al. [4] have shown the DIGM in Cu-Zn system using Cu- 38wt% Zn alloy as source and they have concluded coherency strain energy to be the driving force for DIGM. In Keywords: diffusion induced grain boundary migration; SEM EDAX; activation energy; diffusivity; optical metallography the present case, Cu-Zn system with three different compositions has been chosen to study the composition dependence of DIGM in the temperature range of 277-427 o C. It has been shown that the preferential grain boundary diffusion of solute atom is the main reason for DIGM. 2. Experimental High purity Cu in rod form and Zn in small pieces of 99.9% purity was used. The Cu rod of 9mm diameter was first swaged into 5mm diameter. The swaged Cu rod was then rolled into 0.9mm thickness plate and then cut into pieces with approximately 7.62 cm (3 inch) length each. These pieces were polished by using 4/0 emery paper and then ultrasonically cleaned first using methanol for 10 minutes and then using water for 10 minutes. The water cleaning was done to remove methanol completely and this would avoid the formation of dark spots on the samples during heat treatment. Cu pieces were vacuum-encapsulated in quartz tube at a pressure less than 10 -5 mbar and annealed at 900 o C for 24 hours to relieve any internal stress present in sample and to obtain large grains. This helps to study DIGM under optical microscope easily. Internal stress may alter the result since coherency strain produced by diffused solute atoms is supposed to be the driving force for DIGM. After annealing the Cu sheets were cut into pieces with 6mm width using a slow speed diamond cutter and thus avoid any stress generation during cutting. The final dimension of the sample remains approximately 9mm x 6mm x 0.8mm (L x W x T). Alloy with desired compositions (33 wt% Zn, 20 wt% Zn and 10 wt% Zn) were prepared by encapsulating the Cu sheet and Zn pieces in required amount inside quartz tube under vacuum. The alloy compositions are so chosen as per binary phase diagram of Cu-Zn system 9 in order to avoid the