Tensile Property Analysis
Mechanical properties were evaluated under uni-axial ten- sile loading condition. An INSTRON 8800® testing ma- chine was used and the rate of elongation for each sample was 1 mm/min. No pre-load was applied to the samples at the beginning of the each test. Five samples were evaluated for each condition.
Results and Discussion
The hypothesis for the various stages and kinetics of the pre- cipitation reaction during incubation followed by artificial aging at high temperature formulated from the results the transient micro-hardness of the primary Al phase are pre- sented in the following sub-sections. Further, the results and discussion for the uni-axial tensile properties obtained for various combinations of incubation process coupled with artificial aging at high temperature are also presented as a sub-section.
Sequence of Precipitation During Incubation at Room Temperature
Figure 4 is a schematic representation that illustrates the ob- served variation of micro-hardness with time during incuba- tion of A356.2 Al alloy. Also, shown are the critical stages in the precipitation sequence that highlight the formation of specific clusters or precipitates.
A hypothesis of the critical stages in the precipitation sequence is proposed and based on our critical review of the literature as well as the results from this work. Our hypothesis is followed by the results, justification, and discussion.
Figure 4. Schematic of a typical variation of the micro- hardness data with time during the incubation of A356.2 alloy sample at room temperature showing the various critical stages in the precipitation reaction sequence.
There are five critical stages in the sequence of precipitation during incubation at room temperature. Points A to E in Fig- ure 4 denote these stages. ‘Point A’ denotes the beginning of incubation immediately after quenching the sample from the solution treatment temperature. At point A, the SSSS of Mg and Si exist in the primary Al phase. Immediately after point A, the Si and Mg atoms begin to diffuse in the Al ma- trix to form Si and Mg individual-clusters, respectively. The mobility of these atoms in the Al phase largely depends on the vacancy concentration present in the phase, which in turn depends on the rate of quenching after solution treatment. The mobility of the Si atom is greater than Mg atom due to three reasons: the diffusion coefficient of Si in the primary Al matrix is greater than that for Mg atom;42
the calculated
Figure 5. Schematic of the sequence of precipitation reaction during incubation at room temperature for A356.2 alloy sample showing; SSSS Individual-Clusters of Si atoms Individual-Clusters of Mg atoms Dissolution of Mg clusters Co-Clusters (GP-I) β” (GP-II).
International Journal of Metalcasting/Fall 2011 23
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