Suzuki Segregation to Stacking Faults in a Cu-Si alloy
- PDF / 2,249,111 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 53 Downloads / 222 Views
P5.10.1
Suzuki Segregation to Stacking Faults in a Cu-Si alloy B.G. Mendis, I.P. Jones and R.E. Smallman Department of Metallurgy and Materials, University of Birmingham, Edgbaston, B15 2TT, UK. ABSTRACT Field Emission Gun Transmission Electron Microscopy (FEG TEM) is used to measure solute atom segregation to stacking faults in a Cu- 7.15 at% Si alloy annealed at temperatures of 275oC, 400oC and 550oC. A highly localised increase in the silicon concentration was detected at the stacking fault plane. Segregation of the higher valence silicon atoms increases the local electron to atom ratio in the crystal thereby lowering the stacking fault energy as expected for a Cu-Si alloy system. Measurements across low angle boundaries showed hardly any change in solute concentration, which suggests that the segregation observed at stacking faults is due to a genuine chemical interaction rather than an elastic interaction which can occur in for example high angle grain boundary segregation. The segregation was greatest for the 275oC annealed alloy, where the enrichment is more than 2 at% Si above matrix composition, and was found to decrease monotonically with increasing temperature. The binding energy, as determined from a McLean isotherm, was measured to be -0.021 eV/atom. This is significantly different to the theoretical estimate of -0.0022 eV/atom, as calculated by data on phase equilibria. This could be due to several reasons including experimental error, assigning thermodynamic data of a bulk phase to a stacking fault and inadequacies in the Suzuki segregation model itself. INTRODUCTION In fcc crystals unit dislocations are able to dissociate into smaller partial dislocations which are bounded by a metastable stacking fault of 1/6 displacement vector. This shear changes the local {111} atom plane packing from an …ABC… sequence to an …AB… sequence which is hcp like. This change in crystallography results in a different chemical potential for the stacking fault compared to the rest of the matrix. For an alloy containing atoms of different type, segregation or desegregation of the solute atoms to the stacking fault plane is expected to take place in order to balance the chemical potential throughout the crystal. The sense of solute redistribution is determined by the criterion that at equilibrium the stacking fault must have the lowest energy possible. This is the mechanism originally proposed by Suzuki [1]. However, experimental evidence directly confirming the occurrence of Suzuki segregation is limited due to its highly localised nature (of the order of the stacking fault thickness, which is two interplanar spacings thick). The development of the FEG TEM with the ability to form nanometre or subnanometre size electron probes has neveretheless largely overcome this difficulty. In this paper Suzuki segregation measurements are reported for a Cu- 7.15 at% Si alloy. The reasons for selecting the Cu-Si alloy system are twofold. First there is only a large difference in valence between Cu and Si, with other parameters such as
Data Loading...
