Issue 22

A. Brotzu et alii, Frattura ed Integrità Strutturale, 22 (2012) 20-25; DOI: 10.3221/IGF-ESIS.22.03 21 E XPERIMENTAL n alloy having the nominal composition Ti-48Al-2.5Cr-0.5Nb-2Mo (at. %) was prepared by induction melting under an Ar atmosphere from pure Ti, Al, Cr and Nb. The molten metal was cast directly into the rotating mould. Microstructure and mechanical properties of castings are affected by melting temperature and rotation speed of the mould. In this experimental work the mould rotation speed was 1500 rpm for all the specimens. As far as the melting temperature is concerned, because of the geometry of the used induction furnace and because of the poor precision of the optical pyrometer it was not possible to fix a casting temperature but we proceeded to carry out the casting 30 seconds after the complete melting of the charge material. The melting process was observable through a quartz window facing the ceramic crucible. Metallographic specimens were prepared using conventional methods and were etched with the Keller's reagent to reveal the grain structure. Metallographic structure was inspected by both optical microscope and scanning electron microscope (SEM) and microanalyses were carried out by energy dispersion spectroscopy (EDS). Phases were examined by X-ray diffraction (XRD). The tensile and CT specimens, whose average composition is reported in Tab.1, were produced by means of centrifugal casting using a graphite mould: graphite cores were used to obtain the holes. Al (at.%) Ti (at.%) Cr (at.%) Nb (at.%) Mo (at.%) Specimen 1 48.83 45.88 2.81 0.58 1.90 Specimen 2 47.80 47.54 2.05 0.35 2.28 Specimen 3 47.32 47.31 2.66 0.54 2.17 Specimen 4 47.85 46.62 2.84 0.47 2.21 Table 1 : Average composition of the CT specimens. Fig. 1 shows the obtained ingot. The average dimension are: thickness 9.7 mm, width 33 mm, length 40 mm, distance between the centre of the holes 14.3mm, hole diameter 5.7 mm. The upper portion of the ingots, where the shrinkage cone is located, has been removed and a crack has been obtained by using a diamond blade. Fracture toughness testing was carried out in general accordance with ASTM E399 using an Instron-3367 instrument with a cross-head speed of 0.5 mm/min. Load and displacement data were recorded during the tests and the fracture strength P max was calculated from the obtained curves. Fracture surfaces were analysed by SEM in order to evaluate fracture mode and fracture sites. After toughness test 2 mm thick slices were cut from CT specimens. These slices were machined in order to obtain small non proportional dog bone tensile specimens with rectangular section. The calibrated zone length was about 10 mm with a rectangular section area S 0 of about 15 mm 2 . Tensile tests were carried out in general accordance with ASTM E8 M and UNI EN 10002-1 using an Instron 3367 instrument (cross-head speed of 0.5 mm/min). Hardness tests have been carried out by using a Wolpert hardness tester with a Vickers diamond pyramidal indenter. Figure 1 : Macrograph showing the CT specimen. A