Large-Scale Heterogeneity and Long-Term Relaxation in Aluminum–REM Melts.

The long-term relaxation of large-scale heterogeneities in aluminum–rare earth metal (REM) melts with a REM content of 5–10 at % was observed earlier on measuring viscosity and density but did not manifest itself on measuring electrical resistance and magnetic susceptibility. This behavior could be related to both the specifics of the measured properties and the sample sizes. In the case of viscosity and density measurements, they are much larger: 12–15 g for a cylindrical crucible diameter of ~15 mm for viscosity and density measurements and smaller than 1 g for a crucible diameter of 6 and 4 mm for electrical resistance and magnetic susceptibility measurements, respectively. To solve this problem, we measure the time dependences of the electrical resistance of the Al₉₁La₉ melt at 1060°C for samples of various sizes, namely, for standard (crucible diameter of 6 mm) and large (crucible diameter of 10.5 mm) samples, using the rotating magnetic field method. When the sample size increases, the random measurement error increases; therefore, we have to take additional measures to stabilize the electric current in the coils creating a magnetic field. When the sample mass increases to 2.15 g at a crucible diameter of 10.5 mm, a large-scale heterogeneity is found to occur during melting. It is interpreted as a compact "cloud" of intermetallic microparticles surrounded by a melt with a high REM content. The sample relaxes to an equilibrium homogeneous state in a few hours. To accelerate the process, an additional action is required: heating to a high temperature, about 1500°C, which reduces the time to less than one minute. This heterogeneity does not occur in small samples (0.7 g at a crucible diameter of 6 mm). This heterogeneity is likely to appear due to a REM atom flux to the surface and the reverse flux of aluminum atoms into the volume during crystallization, which is similar to segregation during the crystallization of cast iron and steel. Our measurements make it possible to estimate the scale of the appearing heterogeneities, which corresponds to the large sample size. [ABSTRACT FROM AUTHOR]