Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Materials Science and Engineering


Directional solidification experiments have been carried out in the Pb-Bi peritectic alloys to study the (alpha) (--->) (beta) phase transition, primary spacing and band structure formation. The (alpha) to (beta) phase transition has been shown to occur at low growth rates as well as at high growth rates. The critical growth velocity at which the (alpha) phase and (beta) phase will grow at the same growing front has been determined experimentally and it is found to follow a relationship of the form: V(,cr) (alpha) G('m) C('n), at a fixed solid fraction. The experimental results on the relationship between G/V(,cr) vs. C were found to agree with the theoretical model which predicted the planar single (beta) phase stability;The characteristics of primary spacing for Pb-Bi peritectic alloys were found to be the same as those in eutectic systems. An empirical relationship (lamda) (alpha) G('-a) V('-b) has been found at higher growth velocities to hold true. As V is decreased, the spacing tends to become constant and even to begin decreasing. For constant G and V, the effect of composition on primary spacing in peritectic region was found to be quite small. The effect of convection was found to affect the primary spacing relationship at low velocities;In Pb-Bi peritectic alloys, the band structures have been observed within the entire peritectic composition regime when the growth conditions were such that the (alpha) phase first formed as a planar growth front. Band structures were also observed when the conditions were slightly greater than the critical conditions for the planar (alpha) phase formation. These band structures were shown to be transition structures when the (alpha)-phase tends to transform to the (beta) phase at low growth rate conditions. The distance over which the band structures form has been found to decrease as the peritectic alloy composition increased.



Digital Repository @ Iowa State University,

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Bor-Ching Fuh



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137 pages