Crack Paths 2012

DISCUSSION

The trend of stress-strain curves, of peak stress and ductility allow to affirm that the

deformation of the alloy is controlled by dynamic recovery (DRV). The presence of a

plateau in the curves indicates the formation of subgrains inside elongated grains that is

in dynamic equilibrium [19-22]. In some cases, microstructural changes during test do

not allow the obtainment of a plateau and stress values increase or decrease with

straining as it is the case for the curves at 250°C (Fig.1.) [19-22 ]. The peak stress

reduces with increasing temperature and with decreasing strain rate; both of these

conditions result in an improved recovery. The variations of ductility with strain rate

and temperature confirm that the D R Vcontrols the deformation. In fact, the ductility of

aluminum alloys increases from 200°C to 400°C and is for each maximumtemperature

at low strain rate [23].

More interesting is to considere that stress variation with strain rate is maximumat

250°C and minimumat 400°C (Fig.2). The opposite occurs for ductility. This can be

explained by analyzing the response to thermal treatment carried out at 250°C and

400°C (Fig.5) on the as received (as cast) alloy.

The exposure at 250°C (Fig. 5) induces a rapid reduction in hardness with time and,

correspondingly, an increase in electrical conductivity. Both these factors indicate that

at 250°C the as cast alloy develops a microstructural evolution associated with

reduction in dissolved atoms and the presence of precipitation/coarsening

precipitates.

These microstructural changes are associated with a strong reduction in hardness and

explain both the absence of a plauteau in Fig.1 and the significant reduction in stress by

reducing the strain rate. In fact, the coalescence of the precipitates is usually associated

with a decreasing in stress and the deformation enhances precipitation kinetics

(heterogeneous nucleation (not hardening) of the precipitates).

After exposure at 400°C (Fig. 5) there is a decrease in hardness in the first 30 min of

treatment and a corresponding decrease in electrical conductivity, to indicate an increase

of atoms in solid solution. The kinetics of this transformation at 400 ° C is very fast and

then the microstructure (and / or the values of hardness and conductivity) stabilizes at

short times and remains unchanged up to 16h of treatment. In this case, therefore, the

effect of the microstructural changes is less relevant on tensile stress values.

The microstructural analysis (Fig. 3) shows that the cracks are located primarily near the

particles. Inclusions and constituent particles, infact, constitute points of stress

concentrations that favor the onset of cracks during the deformation. Regarding the role

of Mg-Znprecipitates seems to be ruled out their important contribution to cavitation.

In fact at 250°C cavitation remains almost constant compared to precipitation progress

(i.e as strain rate decreases) (Fig. 4 and 7). The role of nucleation point of the cavity

seemstoberelatedtotheconstituentparticlesandsegregation.Thesampledeformedat

250 and 400°C aftersolutioning at490°C for2hexhibits areduction ofcavitation

(Fig.6) probably due to the reduction of interdendritic and grain boundary segregation.

The solubilization treatment does not eliminate constituent particles (practically

insoluble) that remain therefore the only possible points for starting cracks.

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