The development and microstructural evolution of the bulk yTi-45Al-2Nb (Sn Cr Mn & Si)-doped intermetallic alloys and their mechanical properties

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.