Stress corrosion cracking in Al-Zn-Mg-Cu aluminum alloys in saline environments

Copyright 2013 ASM International. This paper was published in Metallurgical and Materials Transactions A, 44A(3), 1230 - 1253, and is made available as an electronic reprint with the permission of ASM International. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplications of any material in this paper for a fee or for commercial purposes, or modification of the content of this paper are prohibited.Stress corrosion cracking of Al-Zn-Mg-Cu (AA7xxx) aluminum alloys exposed to saline environments at temperatures ranging from 293 K to 353 K (20 °C to 80 °C) has been reviewed with particular attention to the influences of alloy composition and temper, and bulk and local environmental conditions. Stress corrosion crack (SCC) growth rates at room temperature for peak- and over-aged tempers in saline environments are minimized for Al-Zn-Mg-Cu alloys containing less than ~8 wt pct Zn when Zn/Mg ratios are ranging from 2 to 3, excess magnesium levels are less than 1 wt pct, and copper content is either less than ~0.2 wt pct or ranging from 1.3 to 2 wt pct. A minimum chloride ion concentration of ~0.01 M is required for crack growth rates to exceed those in distilled water, which insures that the local solution pH in crack-tip regions can be maintained at less than 4. Crack growth rates in saline solution without other additions gradually increase with bulk chloride ion concentrations up to around 0.6 M NaCl, whereas in solutions with sufficiently low dichromate (or chromate), inhibitor additions are insensitive to the bulk chloride concentration and are typically at least double those observed without the additions. DCB specimens, fatigue pre-cracked in air before immersion in a saline environment, show an initial period with no detectible crack growth, followed by crack growth at the distilled water rate, and then transition to a higher crack growth rate typical of region 2 crack growth in the saline environment. Time spent in each stage depends on the type of pre-crack (“pop-in” vs fatigue), applied stress intensity factor, alloy chemistry, bulk environment, and, if applied, the external polarization. Apparent activation energies (E a) for SCC growth in Al-Zn-Mg-Cu alloys exposed to 0.6 M NaCl over the temperatures ranging from 293 K to 353 K (20 °C to 80 °C) for under-, peak-, and over-aged low-copper-containing alloys (~0.8 wt pct), they are typically ranging from 20 to 40 kJ/mol for under- and peak-aged alloys, and based on limited data, around 85 kJ/mol for over-aged tempers. This means that crack propagation in saline environments is most likely to occur by a hydrogen-related process for low-copper-containing Al-Zn-Mg-Cu alloys in under-, peak- and over-aged tempers, and for high-copper alloys in under- and peak-aged tempers. For over-aged high-copper-containing alloys, cracking is most probably under anodic dissolution control. Future stress corrosion studies should focus on understanding the factors that control crack initiation, and insuring that the next generation of higher performance Al-Zn-Mg-Cu alloys has similar longer crack initiation times and crack propagation rates to those of the incumbent alloys in an over-aged condition where crack rates are less than 1 mm/month at a high stress intensity factor

PERBEDAAN TINGKAT PENGETAHUAN KESEHATAN REPRODUKSI PADA REMAJA MAHASISWA FAKULTAS KEDOKTERAN DAN FAKULTAS ILMU SOSIAL DAN ILMU POLITIK UNIVERSITAS DIPONEGORO

Background: The awareness of reproductive health are increasing, namely HIV/AIDS, unwanted pregnancy and unsafe abortion. This indicates that knowledge of reproductive health is important. This research aimed to identify reproductive health knowledge in adolescent difference, including aspects of anatomy and physiology of the reproductive system, gender and sexuality, pregnancy and risk of teenage pregnancy, contraception, and sexually transmitted diseases among students of the Faculty of Medicine and the Faculty of Social and Political Sciences Diponegoro University. Methods: This research design was observational analytic with cross-sectional method, conducted from March until June 2011. The research subjects were students who meet the inclusion and exclusion criteria, collected by purposive random sampling method. Data was obtained from the questionnaire and analyzed with univariate and bivariate analysis. Univariate analysis was used to determine frequency distributions analysis and bivariate analysis used chi square test (x2) to identify significance. Results: Faculty of Medicine students of whom reproductive health knowledge level was categorized as good (55,1%), average (39,8%), and less (5,1%). Faculty of Social and Political Sciences students of whom reproductive health knowledge level was categorized as good (5%), average (67,3%), and less (27,6%). There are significant differences in reproductive health knowledge levels between students of those two faculty (p=0.000). Conclusion: The reproductive health knowledge level of Faculty of Medicine students is higher than Faculty of Social and Political Sciences students and significantly different. Keywords: adolescent, reproductive health knowledg

THE EFFECT OF PLASTIC DEFORMATION ON T1 PRECIPITATION

The enhancement of T1 precipitation in Al-Li-Cu alloys by stretching prior to aging (i.e., cold work) and the subsequent increase in alloy strength has been documented [1]. The role of matrix dislocations in the nucleation and growth of T1 plates, however, has not been studied in detail. In this paper the effect of different levels of plastic strain on the T1 particle distributions as a function of aging time at 190 C will be quantified. Additionally, the precipitation mechanism will be explored in order to better understand the role of matrix dislocations as nucleation sites and their effect on T1 plate growth

DEPENDENCE OF ELASTIC MODULUS ON MICROSTRUCTURE IN 2090-TYPE ALLOYS

The Young's modulus, shear modulus and Poisson's ratio were determined using an ultrasonic puise echo technique. Three commercially fabricated aluminum-copper-lithium alloys and an aluminum-lithium binary alloy were examined. The elastic properties were measured as a function of aging time, aging temperature, amount of stretching and testing direction. An increase in Young's modulus due to delta prime and T1 precipitation has been measured and treated quantitatively including precipitation kinetics. A significant decrease of about 5% in the modulus of elasticity was found in the peak age condition. This decrease can be attributed to precipitation of the T2 phase. The shear modulus behaves similar to Young's modulus while the Poisson's ratio remains unchanged. There is no significant orientation dependence of the elastic properties on testing direction despite the fact that a typical. rolling texture was present