Al-Zn-Mg-Cu合金における巨大クローム金属間化合物について

抄録

It is well known that very coarse crystals, up to 2 mm in length, found in large extrusions of 7075 alloy, have been identified as primary crystals of CrAl₇ which grew under semi-continuous casting, and have caused the decrease in fatigue strength.
Experiments were carried out to make a study of the prevention of the occurrence of “giant” chromium compounds without decreasing the resistance to stress corrosion cracking in Al-Zn-Mg-Cu alloys.
The results obtained are as follows:
(1) When the Cr content is less than 0.23% in Al-5.5%Zn-2.5%Mg-1.6%Cu alloy, the occurrence of “giant” chromium compounds is suppressed in commercial practice. However, the addition of >0.3%Mn, >0.3%Fe or >0.05%Ti promotes their occurrence.
(2) The addition of Zr 0.1% has no influence on the occurrence of “giant” chromium compounds, and the addition of 0.2 to 0.3%Zr prevents their occurrence. It might be due to the increase in solubility of Cr in the Al-matrix caused by the formation of primary crystals of ZrAl₃ before CrAl₇ at a higher temperature.
(3) It is concluded that Al-Zn-Mg-Cu alloy containing both 0.2%Cr and 0.1%Zr is a material having an excellent resistance to stress corrosion cracking, without fear of the occurrence of “giant” intermetallic compounds in industrial production.