Intergranular fracture of gamma titanium aluminides under hot working conditions

Abstract

A comparative study of the hot workability of a near gamma titanium aluminide alloy Ti-49.5Al-2.5Nb-1.1Mn in the cast and wrought conditions was performed. Tension tests conducted on coarse grain, cast material, and fine grain wrought material revealed a pronounced variation in both fracture/peak stress and ductility with temperature and strain rate. Brittle, intergranular fracture occurring at high strain rates was found to be controlled by wedge crack nucleation, whereas the ductile fracture observed at low strain rates was controlled by the growth of wedge cracks and cavities. Dynamic recrystallization was shown to be the main restorative mechanism to accommodate grain boundary sliding and thereby control the crack growth rates. The ductile-to-brittle (DB) transition was found to be determined by the critical values of a grain size-based stress intensity factor given by the product of the peak/fracture stress and the square root of grain size. A processing map for the near gamma titanium aluminides was constructed based on the comparative analysis of the hot tension and compression test results.

Hot deformation of high-Nb-containing γ-TiAl alloy in the temperature range of 1000–1200 °C: microstructural attributes to hot workability

“…11a). It is observed that such cracks are found to be associated with coarse γ-grain present at the colony boundaries in accordance with reported studies [31]. It is interesting to note the high strain gradient associated with such coarse γ-grain ( Fig.…”

“…7c-inset). It has been proposed that grain boundary sliding plays a major role in the nucleation of both wedge cracks and cavities [29][30][31]. Due to the lack of adequate matrix slip and grain boundary diffusion in γ-TiAl alloys, especially at low temperature and high strain rates, grain boundary sliding leads to stress concentration at the colony boundaries and triple junctions.…”

“…Due to the lack of adequate matrix slip and grain boundary diffusion in γ-TiAl alloys, especially at low temperature and high strain rates, grain boundary sliding leads to stress concentration at the colony boundaries and triple junctions. Wedge cracks can initiate when such stress concentrations are not effectively relieved [31].…”

“…The vast body of literature suggests that the critical fracture stress of multi-component γ-TiAl-based alloys during hot deformation is apparently controlled by the microstructural modifications effected through variations in alloying additions such as aluminum (determining α 2 -γ two-phase structure) and β-stabilizing elements [23,27]. In order to overcome such limitations, Semiatin and Seetharaman [31,33] have developed a "microstructureinformed criterion" for the initiation of wedge cracking for brittle failure during hot deformation of γ-TiAl incorporating both peak flow stress (σ p ) and grain size (d). The approach relies on the critical value of the parameter "σ p √d" as the criterion for wedge cracking.…”

Comparative assessment of microstructure and compressive behaviours of PM TiAl alloy prepared by HIP and pseudo‐HIP technology

“…Figure 12 shows the relationship between the peak stress and T. The activation energy of the hot deformation Q p is determined to be about 319 kJ mol 21 . A plot of ln[sinh(as)]-lnZ is show in Fig.…”