For AA2024, AA6061, and A356 alloys, samples with different
liquid fractions were prepared by RAP processing for tensile
tests. Optical microstructural observations revealed that the structure
components present in the zone of material beneath the
fracture surface were globular grains and a quenched liquid
eutectic phase. It appeared that intergranular fractures mainly
occurred in the samples. Two typical decohesion characters were
classified in the fractures: (A) intercrystalline fracture of the quenched liquid eutectic phase located at the grain boundaries (liquid
film) and (B) direct decohesion at the interface between the
grains and liquid film.
As the liquid fraction increased, the strength and ductility of
the alloys noticeably decreased initially. This was very evident for
AA2024 alloy. However, a further increase in the liquid fraction
resulted in an increase in the strength of the semisolid AA6061 and
A356 alloys that was also accompanied by a simultaneous increase
in the ductility. For AA2024 alloy, the samples with higher liquid
fractions failed during the RAP processing, which made it impossible
to conduct a tensile test.
The fracture mechanism depended on the liquid phase fraction
within the thixotropic microstructure. The weakening effect of the
skeleton (interconnected grains) was presumed to be the reason for
the initial decrease in the tensile properties caused by the increasing
liquid fraction. On the other hand, the improvement in the
tensile properties of the alloys was attributed to the improvement
of the cohesion strength between the coexisting phases. Specifically,
it was supposed that the interface cohesion force between
the grains and liquid film was high enough that the tensile stress
was transferred to the grains, resulting in their deformation.
It was postulated that the interface cohesion between the grains
and the liquid film was the determining factor that controlled the
tensile properties of the semisolid RAP processed alloys.