Microstructural effects on the tensile and creep behavior of Ti-48Al based gamma titanium aluminides

The major goal of this thesis was to develop a mechanistic understanding of the effects of microstructure on the tensile and creep behavior of binary Ti-48Al and quarternary Ti-48Al-2Cr-2Nb gamma titanium aluminides. Innovative thermo-mechanical and heat treatment procedures were designed to produce a host of microstructures from near gamma, duplex to near and fully lamellar microstructure. These microstructures were tested for tensile behavior from RT to 800{dollar}spcirc{dollar}C and creep tests were performed in the temperature range 700-815{dollar}spcirc{dollar}C at stress levels ranging from 100-300 MPa.; The results of this study indicate that fine-grained, near/fully lamellar microstructures can be successfully produced in Ti-48Al-based alloys by extrusion near/at/above the {dollar}alpha{dollar}-transus with attractive room temperature ductilities ({dollar}ge{dollar}2%), strengths ({dollar}ge{dollar}400 MPa) and creep resistance.; Microstructure exert a strong influence on the tensile properties; with the grain size and lamellar volume fraction playing connected, but complex, roles. In the near {dollar}gamma{dollar} and duplex structures, slip by motion of 1/2 {dollar}<{dollar} 110) unit dislocations and twinning are the prevalent deformation modes at RT, whereas twinning is more common in the near/fully lamellar structures. The low mobility of dislocations are the likely cause of low ductility of these alloys. The increase in ductility of the Ti-48Al alloy at high temperatures is due to the increased climb and/or cross-slip of 1/2 {dollar}<{dollar} 110) dislocations and twin thickening.; Microstructure exerts a strong influence on the creep rates also. The fully lamellar microstructure has the highest creep resistance followed in decreasing order by the near-lamellar, duplex, and near-{dollar}gamma{dollar} microstructures. An increase in the volume fraction of lamellar grains in duplex structures leads to a reduction in the creep rates up to 768{dollar}spcirc{dollar}C.; The creep behavior of all the microstructures is described by power law creep with activation energies (300-380kJ/mole) near that for self diffusion of Ti in TiAl, suggesting climb controlled creep processes. In the near-{dollar}gamma{dollar} and duplex structures, stress exponents (n) near 5.0 are obtained, whereas in the near and fully lamellar microstructures, a change in n value from {dollar}sim{dollar}3 at low stresses to {dollar}sim{dollar}8 at high stresses occurs. In the latter case the lamellae width/spacing, rather than grain size, has a greater effect on the creep resistance, with finer spacing leading to lower creep rates.; The crept samples with the near-{dollar}gamma{dollar} and duplex are dominated by 1/2 {dollar}<{dollar} 110) unit dislocations, their configuration indicating that dislocation climb and glide are active, consistent with the observed Q and n values. The fully lamellar structure indicate that both 1/2 {dollar}<{dollar} 110) and 1/2 {dollar}<{dollar} 112) dislocations are active and deformation is highly anisotropic and that the lamellar interfaces obstruct slip and slip transfer (depending on their orientation). The combination of low creep rates with a high stress exponent for the fully lamellar structure is the result of anisotropic deformation and lamellar boundaries obstructing slip through geometrical constraints; the dislocations that are piled-up at the interfaces, overcome these obstacles by local climb, which is reflected in Q values near that of self-diffusion of Ti in TiAl.