A holographic reconstruction system for measuring concentration profiles in an alloy metal model under directional solidification

The study presented here makes use of holography and interferometry to measure the refractive index and concentration variations within a transparent liquid metal model analogue undergoing directional solidification. Holography is used to record experimental systems under circumstances that prevents the simultaneous application of different optical processing techniques. Interferometry is used to make quantitative measurements of phase changes within the solidification system recorded holographically. Experiments performed under 1 g^e; and microgravity conditions are examined.

Holograms were recorded in NASA Marshall Space Flight Center's Holographic Ground System in Huntsville, AL, and in orbit in the space shuttle in NASA's Fluids Experiment System as part of the first International Microgravity Laboratory. Holograms were reconstructed at the University of Tennessee Space Institute and combined with interferometry to produce fringe patterns related to the phase change distribution within the solidification system. Two different procedures for extracting quantitative data from fringe patterns (Fourier transform and intensity) were used and compared.

The results indicate that a well-defined exponential solute distribution forms when solidification is performed without the influence of gravity. With gravity present, the density inversion in the liquid, a result of the solidification process, creates buoyancy driven convection. For this reason, recorded holograms of experiments performed on the ground were taken early in the experiment prior to the onset of convection; the measured solute distributions are not as well-defined as those measured from experiments performed without gravity applied.