Issue 27

A. Brotzu et alii, Frattura ed Integrità Strutturale, 27 (2014) 66-73; DOI: 10.3221/IGF-ESIS.27.08 68 Alloy Al Ti Cr Nb Mo B Ni A 50.3 39.6 - 7.1 2.5 0.5 - B 47.1 46.5 2.5 3.1 - 0.8 - C 46.2 45.7 3.1 4.6 - 0.7 - D 49.0 41.1 2.5 4.9 2.5 - - E 46.7 43.2 2.4 4.7 2.3 0.7 - F 43.5 47.7 2.7 3.6 1.8 0.7 - G 47.7 42.9 2.7 0.3 1.0 - 5.4 H 53.0 29.6 1.6 0.5 1.3 - 14.0 I 46.0 43.5 3.5 3.5 3.5 - - L 43.0 48.0 2.5 5.2 1.3 - - M 42.5 45.6 2.8 5.3 3.8 - - N 46.7 40.3 4.2 3.5 5.3 - - O 38.3 47.6 4.5 3.5 6.1 - - P 39.3 47.5 4.8 3.3 5.1 - - Table 1 : Chemical composition (at.%) of fourteen TiAl based alloys. Specimens from “A” to “I” were cast under argon atmosphere, while specimens from “L” to “P” were cast under vacuum. (a) (b) (c) Figure 1 : Macrograph showing three fractured specimens (34x40x10 mm 3 ). Specimen shown in macrograph (a) was cast under vacuum while specimens in macrographs (b) and (c) were cast under argon atmosphere. This observation can be an important clue to identify the causes of sudden and, in some cases, explosive fractures. In particular centrifugal casting, used for manufacturing our specimens under a gas atmosphere, may generate a turbulent flow that produces gas entrapment and then air pockets or porosity formation after solidification. Other casting techniques allow a slower filling of the mould: if the liquid metal enters the mould with quiescent flow gas defects are greatly reduced. The analysis of cracks highlights that fracture paths seem to follow the trend of the turbulent flow and fracture surfaces are characterised by the presence of gas defects. Fig. 2d shows a gas defect found on the Alloy “C” fracture surface, while Fig. 3 shows gas porosity found on the external layer of Alloy “C” specimen. Those defects may result from entrapment of air during pouring or may be precipitated during solidification as a result of change in solubility with temperature. Defects take the form of internal blowholes, surface or subcutaneous pinholes or intergranular cavities. Gaseous elements may be absorbed by dissociation of compound gases in contact with the molten alloy. In our study the gas precipitated from the metal on cooling could be hydrogen whose source is moisture and organic compounds contaminating the charge materials. Since the most important safeguard against gas defects is a low gas content in the metal when poured, in this research the charge materials have been degreased and afterwards they have been preheated together with the crucible in a muffle to evaporate surface moisture.