Investigation of Microstructure and Stress Corrosion Cracking in Al-6061-T6 Alloy at Different Loads

Stress corrosion cracking (SCC) refers to the damage of mechanical components which are under the combined action of static load and corrosive environment. This phenomenon occurs in various applications including naval and aerospace industry where aluminum and steel alloys experience mechanical loadings in the presence of corrosive environments. In this research work, microstructural and environmental influence on corrosion behavior of Al-6061-T6 at different static loads was investigated. A new test fixture was developed for stress corrosion cracking. Dog-bone shaped tensile specimens of Al-6061-T6 were manufactured using CNC milling machine. Tests were conducted at constant loads of 200 N, 500 N and 800 N, in three different environments: dry ambient conditions, distilled water and 3.5% NaCl solution. Testing continued for different intervals of time i.e. 96 hours, 68 hours and 4.5 hours respectively. After each set of experiments, specimens were observed for cracks using metallurgical microscope. Detailed fractographic investigation of all the tested specimens was carried out using Scanning Electron Microscope (SEM). Excessive corrosion and material degradation was observed in specimens tested in distilled water and 3.5% NaCl environments. Microstructural analysis depicted pitting corrosion and crack deformation.  Some regions clearly showed that grain boundaries were attacked due to oxidation and chemical attack causing weakening of grain boundaries and resulted into intergranular corrosion. Precipitates and grain boundaries in Al-6061-T6 served as a reason of crack initiation due to hydrogen diffusion. Fractographic investigation provided the evidence of trans granular fracture as well as intergranular fracture which was observed as dimples and extensive ductile tearing.

Physiological and Molecular Characterization of Two Genetically Diverse Spring Wheat Triticum Aestivum L. Cultivars for Salt Tolerance

Hydroponic experiments were conducted to appraise variation in the salt tolerance potential of two wheat cultivars (salt tolerant, S-24 and moderately salt sensitive MH-97) at different growth stages. Salinity stress caused a marked reduction in plant biomass and grain yield of both wheat cultivars. However, cv. S-24 was superior to cv. MH-97 in maintaining higher plant biomass and grain yield under saline stress. Furthermore, salinity caused a significant variation in different physiological attributes measured at different growth stages. For example, salt stress caused a marked reduction in net photosynthetic and transpiration rate in both wheat cultivars but to a varying extent at different growth stages. Higher photosynthetic and transpiration rates were recorded at the boot stage than at other growth stages in both wheat cultivars. The response of other gas exchange attributes was also variable at different growth stages. Salt sensitive wheat cultivar MH-97 was more prone to salt-induced adverse effects on gas exchange attributes as compared to cv. S-24. Salt stress caused considerable reduction in different water relation attributes of wheat plants. A significant reduction in leaf water, osmotic and turgor potentials was recorded in both wheat cultivars at different growth stages. Maximal reduction in leaf water potential was recorded at the reproductive stage in both wheat cultivars. In contrast, maximal turgor potential was observed at the boot stage. Salt-induced adverse effects of salinity on different water relation attributes were more prominent in cv. MH-97 as compared to those in cv. S-24. The integrity of PS II was greatly perturbed in both wheat cultivars at different growth stages and this salt-induced damage to PS II was more in cv. MH-97. A significant alteration in different biochemical attributes was also observed in both wheat cultivars at different growth stages. For example, salt stress caused a substantial decrease in chlorophyll pigments, ascorbic acid, phenolics and tocopherols. In contrast, it increased the endogenous levels of ROS (H2O2), MDA, total soluble proteins, proline, glycine betaine and activities of enzymatic antioxidants (SOD, POD, CAT, APX). These biochemical attributes exhibited significant salt-induced variation at different growth stages in both wheat cultivars. For example, maximum accumulation of glycine betaine and proline was recorded at the early growth stages (vegetative and boot). However, cv. S-24 showed higher accumulation of these two organic osmolytes and this could be the reason for maintenance of higher turgor than that of cv. MH-97 under stress conditions. The activities of various enzymatic antioxidants increased markedly in both wheat cultivars, particularly at the vegetative stage. However, cv. S-24 exhibited consistent increase in the activities of various enzymatic antioxidants, whereas, this phenomena occurred erratically in cv. MH-97 at different growth stages. Salt stress significantly increased the endogenous levels of toxic ions (Na+and Cl-) and decreased essential cations (K+ and Ca2+) in both wheat cultivars at different growth stages. Furthermore, K+/Na+ and Ca2+/Na+ ratios decreased markedly due to salt stress in both wheat cultivars at different growth stages and this salt-induced reduction was more prominent in cv. MH-97. Moreover, higher K+/Na+ and Ca2+/Na+ ratios were recorded at early growth stages in both wheat cultivars. It can be inferred from the results that wheat plants are more prone to adverse effects of salinity stress at early growth stages than that at the reproductive stage.