Title

Real time microradiography of dendrite growth and coarsening in Sn-13%Bi alloy

Date of Completion

January 2004

Keywords

Engineering, Metallurgy

Degree

Ph.D.

Abstract

A novel microradiography technique has been applied to observe in real time dendrite growth and coarsening in Sn-13%Bi alloy. High intensity synchrotron radiation combined with high resolving power negative film has been used to image growing dendrites during solidification. The images obtained in 120–150 micrometer thick samples reveal the evolution of dendrite morphology and allow quantitative measurement of dendrite growth and coarsening kinetics. ^ Dendrite coarsening by remelting from the tips toward the roots during continuous cooling, and dendrite coalescence during isothermal holding were observed in real time. Sporadic dendrite fragmentation was also captured during directional solidification. Coarsening kinetics, such as change in secondary dendrite arm spacing with respect to solidification time, rate of remelting and rate of coalescence, were measured quantitatively based on the real time observations. ^ Temperature gradient zone melting (TGZM) during directional solidification was also observed, and the rate of TGZM was measured. A significant amount of remelting was seen on cold sides of dendrite arms, whereas precipitation on hot sides of dendrite arms during continuous cooling and isothermal holding. TGZM has a strong influence on dendrite morphology. ^