Effects of Boron, Carbon and Gadolinium Additions on the Microstructure and Grain Size of Ti-48Al-2Cr-2Nb Alloy

摘要

To the two-phase Ti-48Al-2Cr-2Nb alloy, melted in a vacuum induction furnace, boron (0.5-1.0 at. %), carbon (0.2-0.4 at. %) and gadolinium (0.15-0.3 at. %) were added to refine the grain size for improving its ductility. The influence of those modifiers on the macro- and microstructure of the alloy was studied both after casting and after heat treatment. The research of the macrostructure has shown the occurrence of three zones on the ingot section: the chill zone of very fine, almost invisible, equiaxed grains at the surface, the columnar zone with elongated grains growing to the direction of carrying away heat in the casting mould, and the middle zone of coarse equiaxed grains. The influence of the type and amount of a modifier on the shape and size of particular zones, and the grain size reduction effect have been determined. An attempt has been made at explaining the mechanisms of changes of the macro- and microstructure of the alloy being modified. Also, the influence of the modifiers on selected mechanical properties of the alloy has been determined. For instance, it has been found that hardness of the investigated alloy, after adding carbon, boron and gadolinium, increases by 50, 40 and 25% respectively.