Issue 22

A. Brotzu et alii, Frattura ed Integrità Strutturale, 22 (2012) 20-25; DOI: 10.3221/IGF-ESIS.22.03 24 The second acceptance criterion requires that both the thickness B of the CT specimens and the crack length shall be higher than 2.5(K Q /  YS ) 2 where  YS is the 0.2% offset yield strength in tension. Considering that the load-displacement curves obtained do not highlight any plastic deformation, in this formula the ultimate tensile strength was used. For all the tests these calculated values were lower than the thickness B of the CT specimens and the crack length a 0 . The calculated K Q values can then be considered as K IC . The K IC values are close enough and comparable, showing a good reliability of the used methodology. These values are also comparable with those reported in literature [9] for similar alloys and obtained by using samples tested in 3-point bending. These tests were carried out on alloys thermomechanically treated by hot isostatic pressing at 1250 °C for 4 hours at 175 MPa and aged at 950 °C for 48 hours in order to eliminate porosity and residual stresses. The fact that the K IC values we found are close to those could suggest that the described post-casting treatment is not that effective. Fig. 6 shows the fracture surface macrographs (a) and the morphology of the fracture surfaces (b). Fig. 6b highlights that the fracture surface is characterised by the typical cleavage fracture morphology where lamellae are well visible. ( a) ( b) Figure 6 : Macrograph showing the fracture surfaces ( a) and SEM micrograph showing the fracture surface morphology ( b) . C ONCLUSIONS n this work a Ti-48Al-2.5Cr-0.5Nb-2Mo (at. %) alloy was produced via centrifugal casting in order to evaluate its as- cast structure and mechanical properties. The main objective of this work was to set up a reliable methodology to measure the toughness of these alloys by using CT specimens. To this end four CT specimens were cast and tested. The K IC values range from 15.5 to 23.8. This variability is attributable to small changes in melting time and temperature that affect lamellar colony size. The higher ultimate tensile strength was 435.9 MPa. These results highlight that the K IC values obtained for the as-cast structures are acceptable and comparable with data available in literature for similar alloys subjected to thermomechanical treatments. This suggests that the used methodology is reliable and that mechanical properties can be further improved by a careful selection of both alloying elements and thermal treatments. R EFERENCES [1] E. A. Loria, Intermetallics, 8 (2000) 1339. [2] X. H. Wu, Intermetallics, 14 (2006) 1114. [3] M. Yamaguchi, H. Inui, K. Ito, Acta Mater, 48 (2000) 307. [4] D. Pilone, F. Felli, Intermetallics, 26 (2012) 36. [5] D. Pilone, F. Felli, In: TMS Annual Meeting, 3 (2008) 413. [6] M. H. Loretto, A. B. Godfrey, D. Hu, P. A. Blenkinsop, I. P. Jones, T. T. Cheng, Intermetallics, 6 (1998) 663. [7] S. Shih Donald, Y. W. Kim, In: Ninomi M, Akiyama S, Ikeda M, editors. Ti-2007 science and engineering [C]. Kyoto, Japan: The Japan Institute of Metals; (2007) 1021. [8] A. Bartels, H. Kestler, H. Clemens, Mater Sci Eng A, 329-331 (2002) 153. I