Crack Paths 2012

Hot working of many engineering alloys is often accompanied by the formation of

internal cavities [11-15]. The cavitation process depends strongly on alloy composition

and microstructure as well as on the imposed processing condition [16 17 13 14 18].

Particularly large particles and inclusions, notably on GB, introduce new sources of

segregation and low melting

fissure nucleation lowering ductility; solidification

constituents, especially if they spread along the GB, create severe problems [18]. Such

cavitation may lead to premature failure (i.e., failure at strains lower than those expected

based on material properties such as the strain rate sensitivity index and the strain

hardening exponent) or result in a finished part with degraded mechanical properties.

The aim of the paper is explain the role of microstructure on nucleation of cavity and

mechanism of growth of cavities wich depends on deformation parameters.

E X P E R I M E N TPARLO C E D U R E S

The compositions of the alloy studied in this investigation are reported in Table1.

The materials were supplied in the form of D Ccast billet of 20 cm in diameter and 40

cm in length. Cylindrical samples with gage length of 13 m mand 5 m mdiameter were

cut parallel to the longitudinal axis of the billet for tensile tests.

The microstructure in the deformed state has been investigated by optical microscope

(Nikon Epiphot 200) and by electron microscopy (JEOL JSM-6480 LV, S E MFIB

ZEISS 1540, and IXRF System SphinX 130). For polarized light observation the

samples were ground according to standard methods, electropolished (80ml perchloric

acid, 120ml distilled water, 800ml ethanol, 20V) and anodized (Barker’s reagent).

Hot tensile tests have been performed on as-cast alloys in the range 250°C-400°C and

10-5to 10-3s-1 .The temperature was measured by two independent thermocouples placed

close to the sample. The true stress-true strain curves were calculated from recorded

load-displacement data according to the usual formula.

Cavity area has been evaluates using NIS software for imaging analysis and reported as

Cs (%) (cavity surface present in a area divided by the considered area). The area of

cavities on each fractured sample has been analyzed on its longitudinal polished mid

plane by considering steps of 0.5 m muntil a distance of 4 m mfar away from fracture.

Cavity size < 5 0 m 2have not been considered in the analysis.

Table.1: composition of the alloy (wt%)

Zn M g Fe

Si

Ti

7000 5.5 1.2 0.07 0.03 0.01

A N A L Y S IOSFR E S U L T S

The stress-strain curves are shown in Fig. 1. The analysis of flow curves at 250°C

(Fig. 1a) shows a peak stress followed by a rapid reduction of stress with strain with

1122