SIMPLE CUBIC TO FACE-CENTERED-CUBIC, AS WELL AS TRIGONAL PHASE TRANSFORMATIONS, DUE TO INCLUSION OF THE BORON IN THE LATTICES OF Ni_3Al PHASE FOUND AT THE GRAIN BOUNDARY PRECIPITATE OF BORON-DOPED Ni_3Al ALLOY

摘要

Structure of the grain boundary precipitates in boron doped Ni_3Al alloys has been investigated by TEM techniques. Grain boundaries in the alloy showed a remarkable variation in their precipitation behavior. A large number of boundaries were found either precipitate free or containing isolated precipitates. Crystal data obtained from such an isolated precipitate identified a new stable phase, which is suggested to form when B atoms are entrapped into the unit cell of Ni_3Al. The incorporation of B into the body centered position of the ordered simple cubic Ni_3Al creates the stable face centered cubic lattice phase with the cell parameter, a=10.89(5)A, and space group Fm3m. Crystal data of some other location of the boundary precipitate also provided evidences of another type of meta stable structure that has a trigonal symmetry with cell parameters, a=9.55(5)A, c = 6.99(5)A, and Z= 9.