| Summary: | 博士 === 國立成功大學 === 材料科學(工程)學系 === 85 === Extrusion of commercial purity aluminum and many Al-based
composites often leads to [111] fiber texture. This
preferred orientation may not only enhanc e the tensile
strength, but also affect other mechanical behaviors such as
uniform deformation and formability. Based on this
consideration, a widely used 99.8 wt.% pure Al and the Al-
Al3Ni eutectic alloy, a potential new structural
material, were chosen in this study to explore texture hardeni
ng and superplastic-like deformation, with special emphasis
on the roles of [111] fiber texture and second
phase particle distribution. The tensile tests were
performed at room temperature and 500(C, all wi th the
tensile direction taken along the [111] fiber axis (i.e. along
ex trusion direction). TEM results of the deformed specimens
show that primary slip is the main dislocation activity, but
it can only be observed in limit ed subgrainsand most of the
subgrains are virtually dislocations-free. This p lastic
inhomogeneity, which gives rise to the room temperature texture
h ardening and 500(C superplastic-like deformation, can be
rationalizedby comp aring the Taylor factors of primary
slip and multiple slip for the subgr ains of different
misorientations. The Taylor factor decrease with in creasing
misorientation of the [111] direction from the tensile axis,
where the rate of decrease is faster for primary slip. The
smaller the Tay lor factor, the easier the dislocation slip.
The pole figure data indicate that fewer
subgrains have larger misorientation. Owing to the fact that
their Taylor factors are smaller, these subgrains should be
the se enco untered primary slip of parallel
dislocations. The pure Al and
the Al-Al3Ni eutectic alloy with pure i ntergranular
distribution of the Al3Ni particles in the as-extruded
condition and only the latter has superplastic-like
behavior at 500 (C. The above two materials reveal a
plateau-like flow behavior at the initial stage of
yielding at room temperature since primary slip is the main
dislocation activity. In contrast to the usual Hall-
Petch str engthening, the yielding strength of the extruded
aluminum with [111] fib er texture dose not very with
subgrain size. At 500(C, the textures of t he Al-Al3Ni
eutectic alloy before and after superplastic-like
deformation show characteristics that only small portion
of the subgrains are misali gned larger from the fiber axis.
Therefore, the TEM microstructures show plast ic inhomogeneity
in which parallel dislocations can be observed in a f
ew subgrains even in the final fracture stage. Rather than
the degree of strain hardening, the
superplastic-like behavior is associated m ainly with high
strain softening resistance. Strain rate harden
ing dose not govern the superplastic-like behavior
although it p lays a role. Intragranular particles
can eliminate the plastic
inhomogeneity by causing large amount of dislocations to
tangle around the particles, which will not only su
ppress texture hardening at room temperature but also
easily introduc e dynamic recovery and thus strain
softening at 500(C. To restrict th e number of intragranular
particles is therefore required in order to have
superplastic-like behavior. Through the subgrain
refining inv olving particle bonding and ladder-like
subgrain formation by pinning of primary slip dislocations,
this restriction can be achieved by trapping the particles
in the newly formed subgrain boundaries.
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