Characterization of Microstructure in Semisolid Slurries

Abstract

It has been understood for some time that the flow behavior of semisolid metal slurries, and the properties of parts shaped from the slurries, depend both on the fraction solid and on the size, distribution, and morphology of the solid particles within the liquid matrix. The fraction solid f _s is essentially determined by the temperature for a given alloy; the other features, however, are a function of the history of preparation and it is known that slurry flow is enhanced by achieving fine spherical particles. Fraction solid (f _s) can be determined from phase diagrams for simple alloy systems if equilibrium may be assumed, or from direct measurement on rapidly quenched specimens, using metallographic techniques on sectioned surfaces, such as line intercept: f _s = L _a, where L _a is the total intercept length in the a particle phase per unit length of test line. Likewise, the average particle size (assuming spheres of constant diameter d) may be obtained by the measurement: d = L _a/N _a, where N _a is the number of grains per unit area of sectioned surface. It should be noted that the fraction solid obtained from phase diagrams is strictly a weight fraction and only coincides with the volume fraction obtained from metallographic measurements above when the densities of liquid and solid are considered equal and shrinkage is therefore ignored. Other techniques have been used to obtain these quantities, but are indirect, requiring calibration, and are probably less reliable. However, see in situ X-ray microtomography in Sect. 3.4, where the measurement is carried out in the semisolid condition to avoid quenching artifacts.