Abstract:
The -TiAl based intermetallic alloys play an important role in structural applications, such as
aerospace and automotive industries. The current work studied the novel development and
microstructural evolution, along with their mechanical properties of the -TiAl based alloys viz.
the binary (Ti--48Al), ternary (Ti--48Al-2Nb), quaternary (Ti--48Al-2Nb-0.7Cr) and quinary (Ti-
-48Al-2Nb-0.7Cr-0.3Si). The alloys were fabricated employing both powder and ingot
metallurgy routes. Consolidation of the alloys was achieved by vacuum arc melting. As-cast and
thermally treated sections of sample ingots were characterized using optical microscopy (OM) and
scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photo-electron spectroscopy
(XPS), atomic force microscopy (AFM), Raman spectroscopy (RS), transmission electron
microscopy (TEM), electron backscattered diffraction (EBSD). Thermal analysis, such as
differential scanning calorimetry (DSC) and differential thermal analysis (DTA) were
employed in order to give insight into phase transformations. Moreover, hardness, room temperature
tensile testing with subsequent fractography, cyclic oxidation, nitriding treatment and
incorporating first principle calculations with orientation relationships were some of the property
behaviors studied in the dissertation.
The key findings of the research were: 1) the addition of Nb and Cr to the -TiAl based alloys
promoted grain refinement and induced phase transformation, 2) slight overall Al loss of the
-TiAl based alloys was observed due to the compaction method prior to melting. Uni-axial cold
pressing of the blended metal powders indicated that Al particles migrated to the surface in
contact with the die facets, 3) of all the alloys studied, the quinary (Ti-48Al-2Nb-0.7Cr-0.3Si)
alloy exhibited good mechanical properties, 4) phase transformation and microstructural
evolution of an α-solidifying quinary alloy illustrated a homogeneous microstructure with
Spheriodized/Widmanstätten laths, 5) it has been shown that the formation of /α2/Ti5Si3 in the
Ti-48Al-2Nb-0.7Cr-0.3Si intermetallic alloy followed the Blackburn orientation relationship of both
the as-cast (-solidifying) and heat-treated (α-solidifying) phases, 6) surface cladding in a N/Ar
atmosphere indicated that the oxidation properties of the ternary and quinary showed improved
significance compared to the binary and quaternary alloys and, 7) Sn and Mn, were some of the
doping elements added to the quinary alloy excluding the Cr due to its inducement of the brittle
β-phase which exhibits low mechanical properties. As a result, -TiAl based Ti- 48Al-2Nb-0.3Si,
Ti-48Al-2Nb-1Sn-0.3Si, and Ti-48Al-2Nb-1Sn-0.7Mn-0.3Si intermetallic
alloys were developed and investigated.
