Issue 42

A. Brotzu et alii, Frattura ed Integrità Strutturale, 42 (2017) 272-279; DOI: 10.3221/IGF-ESIS.42.29 274 etching. The Keller’s reagent (water 95 ml, nitric acid 2.5 ml, hydrochloric acid 1.5 ml, hydrofluoric acid 1 ml) has been used in order to evaluate grain size and shape. Intergranular corrosion tests have been carried out according to the ASTM G110 standard (standard practice for evaluating intergranular corrosion resistance of heat treatable aluminum alloys by immersion in sodium chloride+ hydrogen peroxide solution). The ground surfaces have been chemically polished by immersion at 95°C for 60 seconds in an acidic solution (water 94,5 ml, nitric acid 5 ml, hydrofluoric acid 0,5 ml) followed by immersion in concentrated nitric acid at room temperature for 60 seconds. The chemically polished surfaces have been rinsed with water and then observed by means of optical microscope before the corrosion test. This chemical treatment reveals the grain microstructure. Intergranular corrosion tests have been carried out by immersing for 6 hours the prepared specimens in a solution that contains sodium chloride and hydrogen peroxide (sodium chloride 5.7 g, hydrogen peroxide (30%vol) 1 ml, 100 ml of water). After the test the surfaces have been examined in order to identify the corrosion areas. Then from the attacked specimens one metallographic cross section has been prepared and observed both before and after chemical etching with the keller’s reagent. R ESULTS Microstructure characterization ig. 1 shows the optical micrographs of the four etched alloys. The 7050 T7451 alloy (Fig. 1a) is characterized by areas with coarse and elongated grains that coexist with recrystallized bands characterized by a homogeneous distribution of fine grains. 2195 T8 alloy (Fig. 1b) and 7075 T6 alloy (Fig. 1c) have coarse grains elongated in the rolling direction. The 7075 T7351 alloy is characterized by a microstructure quite similar to that of 7050 T7451 alloy: the main difference between the two alloys is that 7075 T7351 alloy elongated grains are smaller. Figure 1 : Optical micrograph of the four studied alloys: a) 7050 T7451 rolled and mechanically machined, b) rolled 2195 T8, c) rolled 7075 T6, d ) 7075 T7351 hot ring rolled. F