Effect of Temper and Test Temperature on Fatigue Crack Growth Properties of IM, PM and Particulate Reinforced 6061 Al Metal Matrix Composites

• Main Authors: Jui-Cherng Huang, 黃志成
• Other Authors: 單秋成
• Format: Others
• Language: zh-TW
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/44411346449118422264

博士 === 國立臺灣大學 === 機械工程學研究所 === 92 === The tensile, fatigue crack growth properties and overload retardation phenomenon of 6061 Al alloy fabricated by ingot metallurgy (IM), powder metallurgy (PM) routes and SiC particulate reinforced 6061 composites have been evaluated in both T4 and T6 tempers at temperatures ranging from 25 to 300 0C. In order to characterize the fracture behavior of the materials, the microstructure was observed by transmission electron microscope (TEM) and fracture surfaces of the specimens were examined with a scanning electron microscope (SEM). At 25 0C, the PM alloy and composites possess a higher strength, higher strain hardening rate and a lower elongation than the IM alloy and PM alloy, respectively. Raising the testing temperature from 25 to 200 0C greatly reduced the advantage in strength of the PM alloy and composites over that of the IM alloy and PM alloy, respectively. At 300 0C, both alloys and composites possess similar strength. The fatigue crack growth resistance in the TL orientation is inferior to that in the LT orientation for both alloys and composites in T4 and T6 tempers. The difference in crack growth resistance between the two orientations decreases with increasing temperature and is basically non-existent at 300 0C. Furthermore, the fatigue crack growth resistance in the T6 temper is superior to that in the T4 temper. In both alloys, fatigue crack growth resistance decreases with increasing temperature. At all temperatures, the PM alloy always has an inferior crack growth resistance as compared to the IM alloy. At low �寐 levels, the proportion of SiC particulates on the fracture surfaces was much smaller than that at high �寐 levels. Damage of composites was characterized by cracked particulate at low and intermediate temperatures and interfacial debonding at high temperature. It is evident that the extent of overload affected zone and amount of retardation was larger in T4 specimens and increased with the overload ratio (OLR), especially for the T4-IM alloy. A crescent shaped overload stretch zone was formed in the central region of the specimens in the T6-PM and composites, especially for the composites, where the crack particulate dominated. By raising the testing temperature, the momentary acceleration was increased and the maximum retardation decreased. For the T6-IM alloy, the maximum retardation became 12% and 3% at 200 0C for OLR=1.5 and 1.75, respectively, while that became 36% and 11% at 250 0C. If the overload was held for 3 hrs at 250 0C, the maximum retardation became 46% at OLR=1.5, while that momentary acceleration also increased from 2.25 to 2.85 times.