Decrease of Ductility in Al-10 Pct Mg Alloys During Long-Term Natural Aging
- PDF / 1,219,479 Bytes
- 5 Pages / 594 x 774 pts Page_size
- 11 Downloads / 148 Views
ml-10pct Mg
alloys are among the best for sand casting in respect of mechanical strength and resistance to corrosion, but require special casting techniques. 1 With these casting alloys a good ductility can only be achieved by solution treatment (T4-treatment), and they exhibit due to subsequent natural aging over a period of several years such unfavorable changes as a decrease in ductility and an increase in the susceptibility to stress corrosion cracking, together with an increase in the mechanical strength. 2,3 There would appear to be some relation between thses changes during natural aging and the formation of GP zones,/3' or 13 phase. In a previous paper, 2 it was concluded that the changes in various properties during natural aging were caused mainly by the formation of G P zones within grains, and that the contribution from the precipitation of fl' or fl phase at grain boundaries was small. This conclusion was arrived at from the observation that with a reversion treatment following a period of natural aging of several years, all these properties returned to nearly the same values as those immediately after solution treatment. In that paper, 2 however, the presence of G P zones after natural aging could not be confirmed by direct evidence, such as electron microscope observations. Until comparatively recently, the formation of GP zones in A1-Mg alloys was investigated mainly by means of electrical resistivity measurements, 4-6 because studies using X-ray diffraction and the electron microscope were complicated by the neighboring atomic numbers of AI and Mg. However, recently some investigators have overcome this difficulty and have succeeded in identifying GP zones by X-ray scattering, 7,8 neutron scattering 9 and electron microscopy/~ In particular, Gault e t a l . H identified the limit of the miscibility gap for G P zone formaY. KOJIMA, formerly with Tokyo Institute of Technology, is now Associate Professor, The TechnologicalUniversity of Nagaoka, Nagaoka, T. TAKAHASHI is Professor, Tokyo Institute of Technology, Tokyo, M. KUBO, formerly with Graduate School of Tokyo Institute of Technology, is now with Honda Motor Company Ltd., Tokyo, and T. MORINAGA is with Japan Light Metal Association, Tokyo, Japan. Manuscript submitted Sept. 8, 1980. METALLURGICALTRANSACTIONSA
tion, in addition to demonstrating the existence o f G P zones, by ultrasonic measurements of elastic properties and electron microscopy. The purpose of this paper is to show how the ductility of AI-10 pct Mg alloy decreases with natural aging up to about 13 years, and to confirm by direct evidence from electron microscopy that the decrease in ductility during natural aging is caused by the formation of G P zones. II. E X P E R I M E N T A L P R O C E D U R E S AI-10 pct Mg alloys with the chemical composition 9.80 pct Mg and 0.04 pct Ti were prepared using Federal-type sand-molds and then heat treated. The specimens were quenched in boiling water and held there for 5 min after a two-step solution treatment at 430 ~ for 10 h and at 490 ~ for
Data Loading...
