Effect of Stress Level on the High Temperature Deformation and Fracture Mechanisms of Ti-45Al-2Nb-2Mn-0.8 vol. pct TiB2: An In Situ Experimental Study

Abstract

The effect of the applied stress on the deformation and crack nucleation and propagation mechanisms of a c-TiAl intermetallic alloy (Ti-45Al-2Nb-2Mn (at. pct)-0.8 vol. pct TiB 2 ) was examined by means of in situ tensile (constant strain rate) and tensile-creep (constant load) experiments performed at 973 K (700°C) using a scanning electron microscope. Colony boundary cracking developed during the secondary stage in creep tests at 300 and 400 MPa and during the tertiary stage of the creep tests performed at higher stresses. Colony boundary cracking was also observed in the constant strain rate tensile test. Interlamellar ledges were only found during the tensile-creep tests at high stresses (r > 400 MPa) and during the constant strain rate tensile test. Quantitative measurements of the nature of the crack propagation path along secondary cracks and along the primary crack indicated that colony boundaries were preferential sites for crack propagation under all the conditions investigated. The frequency of interlamellar cracking increased with stress, but this fracture mechanism was always of secondary importance. Translamellar cracking was only observed along the primary crack.

Analysis of crystallographic slip and grain boundary sliding in a Ti–45Al–2Nb–2Mn (at%)–0.8vol%TiB2 alloy by high temperature in situ mechanical testing

“…These observations suggest the operation of boundary sliding during deformation. This is consistent with previous reports >14, 15]. Due to the relatively small size of the J grains, a large surface area of boundaries was present.…”

“…The J-TiAl intermetallic alloy studied in this work had a nominal composition of Ti-45Al-2Nb-2Mn(at.%)-0.8vol.%TiB 2 (Ti4522XD) and it was processed by centrifugal casting (CC) and powder metallurgy (PM Several tensile specimens were electrodischarge-machined from the CC and PM materials with a geometry provided in previous works >14, 15]. In order to evaluate the influence of the microstructure on the dominant deformation mechanisms, different microstructures were generated by heat treating selected CC and PM tensile specimens for two hours within the Dphase field (1300 ºC) followed by furnace cooling.…”

Recent progress in the high-cycle fatigue behaviour of γ-TiAl alloys

“…Indeed, though there is a clear consensus that room temperature ductility should be raised, that is, increasing the elongation to failure, it is not clear how this influences the HCF life. This is not in the least because of conflicting interpretations of results, whereby almost identical electron images of interlamellar movement have been interpreted in different ways: microcracking [64,65,122], interlamellar sliding to form ledges [123] or longitudinal plasticity near the lamellar interface (see Figure 13). Evidently, these mechanisms also have a varying degree of impact on fatigue crack formation and growth, as has been explored in 'Fatigue Crack Nucleation Mechanisms in γ -TiAl Alloys' section.…”

“…In contrast, due Figure 13. SEM images of similar deformation features near lamellar interfaces, interpreted as (a) ledges at lamellar interfaces [123], i.e. interfacial sliding, (b) interlamellar cracking [122], i.e.…”

Microstructure characterization, mechanical properties and toughening mechanism of TiB 2 -containing conventional cast TiAl-based alloy

“…This appears to be contrary to the results in the mentioned researches. Moreno [29,43,44] reported that in the Ti-45Al-2Nb-2Mnþ 0.8 vol pct alloy, colony boundaries were observed to be the preferential sites for crack nucleation during deformation, and colony boundary cracking was proved to be the main fracture mechanism. However, the mentioned colony boundary crack was not observed in the present study ( Fig.…”