| Summary: | 碩士 === 國立中山大學 === 材料與光電科學學系研究所 === 101 === In this work, the effect of heat treatment on mechanical properties of ultrafine grained (UFG) Al-15Zn alloy is studied. The UFG Al-15Zn alloy was processed by equal channel angular extrusion (ECAE) with route Bc. A pre-heat treatment after homogenization was applied to adjust the hardness of the Al-15Zn alloy so that it could be extruded at room temperature. The Al-15Zn alloy processed by 12 ECAE passes was subjected to post-heat treatment at 150-250 ⁰C. The microstructure of the UFG Al-15Zn alloy after various processing conditions was revealed by using transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD), and mechanical properties were studied by using microhardness test and tensile test.
Increasing ECAE passes, the aluminum grain size was reduced, and there were coarsening of large Zn particles and precipitation of fine Zn particles at the same time. The effect of increasing ECAE passes on the hardness of the UFG Al-15Zn alloy is hardening in the first two passes and softening in the following passes.
The effect of post-ECAE heat treatment can be summarized as follows:
(1) After post-heat treatment at temperatures below 200⁰C, the fully solution temperature for Al-15Zn, the size of Zn particles and the aluminum grain size increased. As compared with the as-ECAE condition, both yield stress and tensile strength decreased with little change in elongation.
(2) After post-heat treatment at temperatures near the fully solution temperature (200-225 ⁰C), the aluminum grain size increased and considerable amount of low angle boundary (LAB) appeared. The alloy exhibited lower yield stress, enhanced work-hardening rate and superior elongation comparing with the as-ECAE condition.
(3) After post-heat treatment at 250 ⁰C, above the fully solution temperature, the aluminum grain size increased considerably. The alloy exhibited lower ultimate tensile strength and elongation as compared with that being heat treated at 225 ⁰C.
|
|---|
