experimental measurements of correlation factor for solute diffusion

Basic data for this project

Description

Substitutional atoms diffuse by jumping into neighboring, vacant lattice sites. But an atom that has just jumped may jump backwards into the vacancy so that, compared with a hypothetical random walk process, the net "forward movement" of the atom is decreased by a so-called correlation factor. For host atoms, the correlation factor is simply a numerical constant reflecting the diffusion mechanism and geometry of the diffusion sublattice. Solute atoms, on the other hand, interact with vacancies and modify the local concentration of vacancies and microscopic jump rates. Such solute-vacancy interactions make the correlation factor for solute diffusion highly sensitive to details of the atomic interactions and temperature. Its measurement should help identify diffusion mechanisms, interpret the interactions, and facilitate reliable predictions of diffusion rates via atomistic simulations. As yet there has been no direct determination of a correlation factor for solute diffusion. The present proposal aims to do that for the first time by combining results of complementary measurements of the diffusion coefficient and jump-rate of a solute in the same matrix. These will be measured using the radiotracer technique and perturbed angular correlation spectroscopy, respectively. To within a constant, the correlation factor is simply the ratio of the diffusion coefficient and the jump-rate. A3B phases having the common L12 structure have been chosen for study as a benchmark system. Diffusion in these ordered phases has attracted enormous interest because there exists a number of possible diffusion mechanisms and likely occurrence of inter- and intra-sublattice jumps for solutes with different activation energies. The influence of composition and temperature on the correlation factor will be determined. Specifically, correlation factors for diffusion of cadmium solutes will be measured for In-rich and In-poor compositions of the In3Gd phase at a variety of temperatures. Different diffusion mechanisms may operate at the two compositions, which will be obtained using samples having the opposing phase boundary compositions. State-of-the-art facilities at Münster University (radiotracer laboratory) and at Washington State University, USA (perturbed angular correlation laboratory) will enable measurements to be made having the required accuracy. In the absence of any previous measurements, results are likely to surprise.