Mechanical Properties of Gamma Titanium Aluminides

Doktorsavhandling, 2000

In recent years intermetallic compounds have attracted a great deal of attention as structural materials. Among the intermetallics, gamma titanium aluminides (.gamma.-TiAl) are considered to be closest to application. Compared to Ni-based materials these alloys offer opportunities for substantial weight reductions, and compared to titanium alloys they offer better creep, oxidation and burn resistance, and increased strength at elevated temperatures. The mechanical properties of .gamma.-TiAl alloys are characterised by fairly constant yield and ultimate tensile strengths up to about 700-800°C, accompanied by low ductility. Moreover, the mechanical properties are strongly related to the microstructure. As an example, the fully lamellar microstructure consisting of lamellar colonies built up by .alfa.2- and .gamma.-platelets is characterised by high fracture toughness, high crack propagation resistance and superior creep strength when compared to the duplex microstructure consisting of lamellar colonies and single phase .gamma.-grains. Yet, higher tensile strength and ductility and longer total fatigue life are obtained with the duplex structure. The possibility to heat treat .gamma.-TiAl alloys achieving vastly different mechanical properties is unique among intermetallic compounds. The key is to find the right balance of properties through chemical composition, processing and heat treatment for the envisaged application. In addition, a robust and well controlled production process is a necessity to achieve the desired properties. Relatively small variations in chemical composition and/or processing parameters may cause considerable effects on the microstructure and, thereby, on the resulting properties. Tensile testing of Ti-48Al-2W-0.5Si at temperatures up to 800°C showed maintained strength levels up to the brittle to ductile transition temperature positioned above 700°C. The influence of the microstructure on the tensile properties is large. A duplex fine grained microstructure has superior properties in comparison with a material with lower contents of aluminium and tungsten where coarse grained, nearly lamellar structures develop. The nearly lamellar structure shows large scatter in tensile strength due to the anisotropic properties of the large lamellar colonies. Low cycle fatigue testing of five different alloys and microstructures showed that the fatigue performance can roughly be correlated to the yield strength in combination with the ductility of the material. A fine grained nearly lamellar microstructure results in the highest yield strength among the tested alloys leading to superior low cycle fatigue properties. Duplex structures resulted in similar yield strengths and low cycle fatigue properties. A coarse-grained nearly lamellar structure showed the lowest low cycle fatigue performance due to a combination of low yield strength, low ductility and anisotropic properties of the large lamellar colonies leading to early fracture when oriented with the lamellar laths perpendicular to the loading direction. At cycling, all the tested alloys showed a cyclic hardening behaviour. Microstructures containing a large volume fraction single phase .gamma.-grains show larger hardening. Tensile testing of diffusion bonded joints between Ti and .gamma.-TiAl showed a strengths similar to the .gamma.-TiAl base material at temperatures up to 600°C. At creep testing most of the creep elongation occurs in the Ti-alloy, but failure is initiated in the joint bond line. Creep causes degradation and pore formation in this line. Interlinkage of these pores creates a crack, which grows slowly until the fracture toughness of the .gamma.-TiAl is exceeded terminating the creep life.