Effect of microstructure on tensile behavior of thixoformed 357-T5 semisolid Al alloy
- PDF / 5,110,442 Bytes
- 4 Pages / 606.28 x 786.28 pts Page_size
- 111 Downloads / 195 Views
CHUL PARK, Graduate Student, and SANGSHIK KIM, Associate Pro fessor, are with Division of Materials Science and Engineering, Engineer ing Reseaich Institute, Gyeongsang National University, Chinju 660 701, Korea. Contact e mail: [email protected] YONGNAM KWON, and YOUNGSEON LEE, Senior Resemchers, and JUNGHWAN LEE, Principal Resemcher, me with the Materials Engineering Department, Korea Institute of Machinery and Materials, Changwon 641 010, Korea. Manuscript submitted August 6, 2002. METALLURGICALAND MATERIALSTRANSACTIONSA
mens at a nominal strain rate of 1 × 10 3/s on a universal testing machine. In the present study, the microstructural features were documented directly from near the fractured area of each specimen after testing to ensure the better microstructural correlation with tensile behavior of the present alloy. The volume fraction of each phase was gaged by using UTHSCSA image analyzing software. The error range for the measurement was estimated to be less than 1 pct. Figure 2 shows the changes in (a) yield strength (YS) and ultimate tensile strength (UTS), and (b) tensile elongation for thixoformed 357-T5 as a function of the volume fraction of primary c~phase. This figure represents that the overall dependency of both YS and UTS values on the volume fraction of primary c~ phase was not significant within the volume fraction range studied, even though the YS appeared to decrease slightly with an increase in the volume fraction of primary c~. The tensile elongation, however, was strongly dependent on the volume fraction of primary c~. The tensile elongation, for example, increased exponentially from 3.1 to 8.6 pct with an increasing volume fraction of primary c~ from 55 to 68 pct. Similar plots were drawn in Figure 3 for both (a) YS and UTS, and (b) tensile elongation as a function of average size of primary c~. Again, both YS and UTS values did not show any notable dependency on the size of primary c~. Moreover, any reasonable correlation between the tensile elongation and the size of primary c~was not possible, as demonstrated in Figure 3(b). In order to understand the reason for the variation of tensile elongation with different volume fractions of primary c~, the SEM fractographs and the optical micrographs of cross-sectional area of tensile fractured specimens were examined. Figure 4 shows the representative optical micrographs showing the tensile fracture paths and the matching SEM fractographs of thixoformed 357-T5 specimens with different primary c~volume fractions of (a) 55 and (b) 68 pct. Regardless of the volume fraction of primary c~, the tensile fracture occurred by either passing through the clusters of eutectic Si phases or following the primary c~/eutectic phase boundaries. Some nonglobular shaped primary c~phases, which were favorably located with the long axes perpendicular to the tensile direction, were intermittently sheared during tensile fracture. However, they appeared to be trivial considering the number of occurrences. The present study also suggested that the entrapped eutectics d
Data Loading...
