Dendrite Growth Within Supercooled Liquid Tungsten And Tungsten-Tantalum Isomorphous Alloys

The dendrite growth in both supercooled liquid pure W and binary W-Ta isomorphous alloys has been observed and measured by an electrostatic levitation technique. The liquid W and W-x% Ta (x = 25, 50, 75) alloys were substantially supercooled by up to 733K (0.2 T-m) and 773K (0.23T(L)), respectively. The measured density and the ratio of specific heat to emissivity displayed a linearly increasing tendency versus supercooling. The thermal dendrites in supercooled liquid tungsten achieved a maximum growing velocity of 41.3 m.s(-1), and the concurrent recalescence process exhibited Johnson-Mehl-Avrami type kinetics. Liquid W-Ta alloys showed stronger supercoolability but a lower maximum dendrite growth velocity of only 35.2 m.s(-1). The dendritic growth kinetics was always characterized by a power function relation to liquid supercooling. The microstructure of equiaxed grains transforms to the well-developed dendrites with the increase of supercooling. The grain refinement effect resulting from dendrite fragmentation took place in a moderate supercooling regime in rapidly solidified W-Ta alloys.