미세조직이 반응고 357-T5 합금의 기계적 특성과 부식거동에 미치는 영향
Effect of microstructure on mechnical and corrosion behaviors of thixoformed 357-T5 semi-solid al alloy
미세조직 반응고 357-T5합금 부식거동 금속재료공학;
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The effects of microstructural features on the fracture behaviors, including tensile, impact, high-cycle fatigue, fatigue crack propagation, corrosion and stress corrosion cracking, of thixoformed 357-T5 (Al-7%Si-0.6%Mg) alloy were examined. It was found that the tensile elongation of 357-T5 alloy increased exponentially from 3.1 to 8.6% with increasing the solid volume fraction from 55 to 68%. The yield and tensile strength, however, showed no notable dependancy on the volume fraction of primary α. The resistance to pitting corrosion with high volume fraction of primary α was substantially lower than that with low volume fraction primary α, which is believed to be due to the area effect as related to the galvanic corrosion between Si particle and eutectic Al phase. The resistance to impact and high-cycle fatigue of thixoformed 357-T5 tended to improve greatly with increasing volume fraction of primary α. An almost three-fold increase in impact energy value was, for example, observed with increasing volume fraction of primary α from 59 to 70%. The improvement in both impact and fatigue properties of thixoformed 357-T5 with increasing volume fraction of primary α in the present study appears to be related to the magnitude of stress concentration at the interface between primary and eutectic phase, by which the fracture process is largely influenced. The higher volume fraction of primary α was also beneficial for improving the resistance to stress corrosion cracking in 3.5% NaCl solution. The in-situ slow strain rate test results of thixoformed 357-T5 in air and 3.5% NaCl solution at various applied potential values demonstrated that the percent change in tensile elongation with exposure decreased linearly with increasing volume fraction of primary α within the range studied in the present study. Based on the fractographic and micrographic observations, the mechanism associated with the beneficial effect of high volume fraction of primary α in thixoformed 357-T5 alloy was discussed.