Improvement of Toughness at Cryogenic Temperature by Structure Controlling in Al-Zn-Mg Alloys

Proof stress, tensile strength, uniform elongation, work done for tensile fracture increase with decreasing temperature to about 20K and decrease gradually with temperature for the three alloys. Data of ductility and toughness at cryogenic temperatures of Al-Zn-Mg alloys are novel and very important. Temperature dependence of fracture toughness K_Q and R_<Sb> correspond to the uniform elongation and work done for tensile fracture. High ductility and toughness are obtained in order of alloy No.3, No.1 and No.2. Alloy No.3 with the typical lamellar structure, aspect ratio above 160, showed the highest ductility and toughness and the lowest strength. Alloy No.1 with aspect ratio of about 4 showed higher ductility and toughness than alloy No.2 with aspect ratio of about 9. Thus, correspondence with ductility or toughness and the degree of lamellar structure is not so strong.