Abstract
Two gamma-TiAl alloy groups were the subject of this investigation. From the first group, Ti-40Al-5Mn and Ti-40Al-10Mn alloy ingots, produced by cold-hearth plasma arc melting were characterised using light and electron microscopy and XRD. Their dendritic microstructures comprised of alpha2/gamma lamellar areas and single-phase gamma, beta and Laves (Mn2Ti) grains or precipitates. The lamellar alpha2/gamma Volume fraction was less in the Ti-40Al-10Mn alloy, due to the influence of its higher Mn content. The alloys were heat treated at 1200°C for 4 hours, resulting in nearly lamellar and duplex microstructures in the Ti-40Al-5Mn and Ti-40Al-10Mn alloys, respectively, hi both cases, the lamellar colonies contained poorly formed, discontinuous alpha2 lamellae. Raising the heat treatment temperature to 1300°C provided both alloys with fully lamellar microstructures with well formed, linear and continuous alpha2 lamellae. From the second alloy group, the Ti-47Al-4(Nb, Mn, Cr, Si, B) alloy known as gamma-TAB, was produced as hot isostatically pressed billets. Its microstructure comprised of some "feathery" lamellar colonies and single phase gamma-matrix. Unique phases based on Ti-B and Ti-Si were identified as being TiB and Ti5Si3, respectively. The boride stringers were profusely spread in the microstructure, whereas the Ti5Si3 was very elusive and found only once in TEM analysis. Heat treatment at 1400°C for 6 hours resulted in a fully lamellar Ti-47Al-4(Nb, Mn, Cr, Si, B) alloy with wide interlamellar spacing and greater gamma lamellae volume fraction; however the lamellar morphology was generally linear and continuous. In order to assess microstructural and lamellar morphological stability of the fully lamellar alloys, isothermal ageing at 800°C for durations of 500, 1000, 1500 and 2000 hours was conducted. Major microstructural changes took place in the Ti-Al-Mn alloys, whereby Ostwald ripening of alpha2 agglomerates and beta grains in the early stages, gave way to Laves phase lamellae and grains in the latter stages of ageing. This was more extensive in the Ti-40Al-10Mn alloy due to phase equilibrium requirements, whereby more of the alpha2/gamma lamellar content was eliminated. However, the specific resilience of alpha2 lamellae was inferior in the Ti-40Al-5Mn, as greater serration, fragmentation and termination migration within lamellar regions was witnessed. Termination migration within lamellar regions was considered as being independent to termination migration of the colony boundaries, which was distinguished as recession or contraction of the lamellar region, with an equivalent extension of the gamma-matrix. Boundary splitting was not precisely seen, but must have been the precursor to termination migration. In the Ti-47Al-4(Nb, Mn, Cr, Si, B) alloy, no Ostwald ripening of alpha2 agglomerates or formation of beta or Laves phases was observed. The gamma-matrix did progressively extend to eliminate alpha2/gamma lamellar grains to some degree, and gamma lamellae coarsened profoundly. Fragmentation and termination migration of alpha2 lamellae was more prominent after 1500 hours ageing. Mean interlamellar spacing (lambda) and coarsening ratio (r) increased the most in the Ti-40Al-5Mn alloy, due to the extent of alpha2 lamellar fragmentation and termination migration, followed by the Ti-47Al-4(Nb, Mn, Cr, Si, B) and Ti-40A1-10Mn alloys, respectively. Modes of morphological evolution that had not taken place during thermal ageing in all alloys studied, were direct cylinderisation, edge spheroidisation and discontinuous coarsening. Overall, the Ti-47Al-4(Nb, Mn, Cr, Si, B) alloy provided the best model for development of gamma-TiAl alloys with enhanced morphological stability, if issues such as the low alpha2 phase volume fraction and coarse gamma lamellae could be resolved. The role of silicon in gamma-TiAl alloys also needs to be examined, and whether it has any bearing on lamellar morphological stability.