Heating Conditions in Electrostatic Levitation Experiments for Grain Refinement of Ti-6Al-4V with TiC

Abstract

To elucidate the grain refinement mechanism of a titanium alloy (Ti-6Al-4V) with titanium carbide (TiC) heterogeneous nucleation site particles, the space mission Hetero-3D is being carried out in the electrostatic levitation furnace in the International Space Station (ISS-ELF). In this study, as preliminary ground experiments for Hetero-3D, samples of Ti-6Al-4V with TiC were melted and solidified in a ground-based electrostatic levitation (ESL) furnace to establish the heating conditions for the prior-β grain refinement. The number of prior- β grains can be counted on the reconstructed prior-β grain maps in the cross sections of the samples using orientation imaging microscopy software. As a result, the effect of the prior-β grain refinement due to TiC can be confirmed through the degree of undercooling, ΔT, of less than 50 K. TiC particles act effectively with respect to the grain refinement of the sample with 5 mass% TiC in the ESL experiments under the following two heating conditions: (1) the maximum temperature, Tmax, is lower than 2050 K, and (2) the integral value of heating, I (T, t), calculated based on the square of temperature, T, and melting time, t, is smaller than 1.5 × 108 K2∙s. Furthermore, voids are generated on prior- β grain boundaries near the center of the refined sample due to shrinkage during solidification.