PHASE-TRANSFORMATIONS IN THE (TI,AL)3NB SECTION OF THE TI-AL-NB SYSTEM .1. MICROSTRUCTURAL PREDICTIONS BASED ON A SUBGROUP RELATION BETWEEN PHASES

Possible paths for the constant composition coherent transformation of b.c.c. or B2 high temperature phases to low temperature h.c.p. or orthorhombic phases in the Ti-Al-Nb system are analyzed using a sequence of crystallographic structural relationships developed from subgroup symmetry relations. Symmetry elements lost in each step of the sequence determine the possibilities for variants of the low symmetry phase and domains that can be present in the microstructure. The orientation of interdomain interfaces is determined by requiring the existence of a strain-free interface between the domains. Polydomain structures are also determined that minimize elastic energy. Microstructural predictions are made for comparison to experimental results given in Part II.