PRESSURE-ASSISTED REACTIVE SYNTHESIS OF TITANIUM ALUMINIDES FROM DENSE 50AL-50TI ELEMENTAL POWDER BLENDS

In the present research, dense gamma-TiAl based intermetallic samples were fabricated by reactive synthesis of fully dense elemental 50 at. pet Al-50 at. pet Ti powder blends. Two different processing routes we re attempted: thermal explosion under pressure (combustion consolidati on) and reactive hot pressing. In both approaches, relatively low proc essing or preheating temperatures (less than or equal to 900 degrees C ) were used. The entire procedure of thermal explosion under pressure could be performed in open air without noticeable oxidation damage to the final product. The application of a moderate external pressure (le ss than or equal to 250 MPa) during synthesis was shown to be enough t o accommodate the negative volume change associated with TiAl formatio n from the elemental components and, thereby, to ensure full density o f the final product. Microstructure and phase composition of the mater ials obtained were characterized employing X-ray diffraction and scann ing electron microscopy with energy dispersive analysis. It was found that at elevated temperatures (e.g., 900 degrees C), the equiatomic 50 Al-50Ti alloy lies beyond the homogeneity range of the gamma-TiAl phas e in the Ti-Al binary and contains, in addition to gamma-TiAl, Al-rich Ti3Al. Mechanical properties of the materials synthesized were evalua ted in compression tests at different temperatures and by microhardnes s measurements. Due to its very fine microstructure, the Ti-Al materia l synthesized via reactive hot pressing exhibited superplastic behavio r at temperatures as low as 800 degrees C.