一方向凝固したAl-Cu合金のミクロ偏析に関する定量的研究

  The influence of solidification conditions on microsegregation of dendritic structures was investigated with unidirectionally solidified Al-Cu alloys. In order to study the result of microsegregation, solute distribution at each different distance from the chill surface was determined as a function of the fraction of solid which was given by two dimensional solidification model. Also the mechanism of solute redistribution during the growth of dendrite was discussed. <br>  The segregation index (S) increased by decreasing the solidification rate (R), and the S between primary dendrite arms was more remarkable than that between secondary dendrite arms. The relationship between S and R was generally obtained as follows; <br>          S=A logR+C <br>where, A and C are the constants determined by concentration. Cu-isoconcentration curves around the dendrite element demonstrated Cu concentration to be minimum at the center of primary dendrite stalk. With decreasing distance from the chill surface (increasing R) the effective partition coefficient K_e (also minimum Cu concentration) increased, but K_e was little affected by concentration. The relationship between K_e and R was obtained as follows;<br>          K_e=0.21 logR+0.2 <br>in which, R is cm/min. <br>  The influence of diffusion layer on the solute movement ahead of the solid-liquid interface near the dendrite tip which was not obtained by means of the prior linear analysis in Al-Cu alloys was clearly demonstrated by determining the fraction of solid from two dimensional solidification model, and it was found that the diffusion layer has a remarkable effect upon the solute redistribution. The degree of microsegregation was dependent upon the change of the effective partition coefficient with the change in the rate of solidification, and could be interpreted qualitatively by the partially mixed liquid model.