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.

Impact of Carbohydrate-Nitrogen Ratio and Photosynthetic Activities on Plant Growth, Fruit Yield and Quality of ‘Kinnow’ Mandarin Citrus Nobilis Lour. X Citrus Deliciosa Tenora

The current study was conducted to determine the effects of balance use of fertilization particularly nitrogen with or without biostimulant on Carbohydrate-Nitrogenratio (C:N) and photosynthesis and then effect of both on production and quality of fruits. Three different experiments were planned and executed. In the first experiment, different nitrogen doses (1200, 900 and 600 g) along with humic acid (HA) (120 ml of 8% solution) per plant were applied in three installments and these plants were compared with control plants (receiving 1200 g N in two installments) for vegetative and reproductive growth. Application of 900 g N per tree with 120 ml HA was found the best regarding the increase in photosynthesis (4.47) and low C:N ratio (4.78) of plant at fruit setting stage which improved the yield (956 fruit/plant) and other quality related parameter of ‘Kinnow’. Maximum TSS (12.20 oBrix), ascorbic acid (44.96 mg/100g), total sugars (13.69%), reducing sugars (5.44%), non-reducing sugars (7.57%), phenolic compounds (398.55 mg GAE/100 g) and antioxidants (90.13 Ic μg/L) were also recorded in the fruit of this treatment. This treatment had also effect on plant growth with an increase of 20.64 cm in plant height and 37.25 cm3 in canopy volume. In second experiment three concentrations of salicylic acid (SA) and ascorbic acid (AA) were used for the optimization of best dose. The results elucidated stimulating effects of SA (6 mM) and AA (120 ppm) on photosynthesis and nitrogen use and decreased the C:N ratio of the plants. These effects indicate maximum utilization of carbohydrates in growth and development of the plant. Plants treated with SA (6 mM) showed higher values of biochemical parameters of ‘Kinnow’ fruits such as TSS (12.00 oBrix), ascorbic acid (46.51 mg/100g), fructose (2.95%), glucose (2.85%), sucrose (7.90%), phenolic compounds (360.42 mg GAE/100 g) and antioxidants (89.55 (Ic μg/L). Application of AA (120 ppm) improved ascorbic acid (46.51 mg/100g), reducing sugars (4.23%), non-reducing sugars (8.50%), antioxidants (87.63 μg/L) and phenolic compounds (365.24 mg GAE/100 g) in fruit. Plant growth was significantly affected by the above mentioned treatment of SA (6 mM) with an increase of 22.46 cm in plant height and 23.45 cm3 increase in canopy volume. In third experiment combined effect of N, SA and AA was in confirmation with the previous results of photosynthesis, C:N ratio and productivity of the plant. Plants treated with combined dose of SA, AA and N showed higher biochemical parameters of ‘Kinnow’ fruits such as TSS (12.20 oBrix), ascorbic acid (54.64 mg/100g), total sugars (14.30%), reducing sugars (1.81%), non-reducing 2 sugars (12.49%). Maximum total phenolic compounds (394.58 mg GAE/100 g) were also noted in the fruit of this treatment. Effect of this treatment was depicted with an increase of 24.85 cm in plant height, 2.34 cm in stem girth and 35.65 cm3 in canopy volume. Correlation between fruit yield and biochemical/physiological parameters showed significant effect on fruit yield. Correlation was found significant between C:N ratio and physiological parameters, such as photosynthesis, stomatal conductance and transpiration in plants. All these physiological process have direct or indirect relation with fruit production. The yield potential of a plant can be measured directly from total number of fruit per plant, however in this study involvement of photosynthetic activities in relation to C:N ratio at different growth stages of plant could be proved a good predictor of fruit yield. Conclusively, application of 900 g N per tree and 120ml HA in equal three splits (before flowering, fruit setting and fruit maturation) along with recommended doses phosphorus (P) and potassium (K) (600 and 600g) per tree can be used effectively to build synergistic photosynthetic and C:N ratio to improve the leaf nutrient status, fruit yield and quality of ‘Kinnow’ mandarin.