| Summary: | 碩士 === 國立成功大學 === 材料科學(工程)學系 === 84 === In this investigation, the effect of nickel
content on the tensile strength, elongation,aging
phenomenon and self-hardening phenomenon in the
HAZ of the Al-5Zn-2Mg-Ni alloys was studied. Three
different nickel contents, including 1wt%,3wt% and 5wt%,
were addied into the 7N01 (Al-5Zn-2Mg) aluminum alloy.
In the as-cast microstructures of 1Ni and 3Ni alloys, second
phase particles are distributed around the dendritic boundaries
of matrix. Bundles of particles are observed in the solidified
eutectic structure of 5Ni alloy. As confirmed by the X-ray
diffraction, the second phase particles are Al3Ni.
The experimental results show that the tensile strength is
increased, the elongation is decreased drastically and the rate
of aging is increased with increasing nickel content. With
increasing volume fraction of Al3Ni particles, the aging time
required for obtaining peak aging strength is also shortened. As
to the remelting test, the alloy shows the rate of increasing
hardness in the HAZ is increased with increasing nickel content
after remelting. Since the thermal expansion coefficient of Al3
Ni particles is different from that of the matrix,
residual stress and dislocations are induced
locally around the Al3Ni particles during
quenching. Hence, the precipitation is enhanced by the
Al3Ni particles.
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