Experimental study of ordering kinetics in aluminum alloys during solidification

摘要

The microscopic structure and crystallization behavior of liquid Al and Al-0.3Ti-0.02B (wt. percent) are studied by time-resolved neutron diffraction measurements during the liquid-solid phase transformation for continuous cooling. A specially developed furnace insert was used to obtain a temperature stability of 40 mK in the vicinity of the solidification temperature of T_0 = 933 K. The evolution of the static structure factor S(Q) has been monitored during the liquid to solid phase transformation as a function of the cooling rate. The evolution of the liquid fraction f_L. during the transformation is determined from the value of the first peak in the liquid structure factor. The evolution of the solid volume fraction f_L = 1-f_L is analyzed in terms of the Johnson-Mehl-Avrami model. The Avrami exponent n is found to change for pure Al as well as for Al-0.3Ti-0.02B alloy with cooling rate and the rate constant k decreases by an order of magnitude for Al-0.3Ti-0.02B alloy compared to pure Al. Anomalous temporal oscillations were observed in the Bragg peak intensity of the solid grains during the solidification of the Al-0.3Ti-0.02B alloy.