EFFECT OF SOLIDIFYING RATE ON INTEGRALLY DI-RECTIONALLY SOLIDIFIED MICROSTRUCTURE AND SOLID/LIQUID INTERFACE MORPHOLOGY OF AN Nb--Ti--Si BASE ALLOY: EFFECT OF SOLIDIFYING RATE ON INTEGRALLY DI-RECTIONALLY SOLIDIFIED MICROSTRUCTURE AND SOLID/LIQUID INTERFACE MORPHOLOGY OF AN Nb--Ti--Si BASE ALLOY