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.

Nutritional and Biochemical Evaluation of Vitamin K Enriched Dietary Sources

The present investigation was an effort to explore the health claims of vitamin K enriched dietary sources i.e. cooked spinach and fermented soybean/natto against the menace of vitamin K deficiency. Initially, spinach and soybean were characterized with special reference to vitamin K contents followed by product development and finally bioefficacy study for the management of blood coagulation and vitamin K dependent proteins. The nutritional analysis indicated that spinach has ample amount of moisture followed by protein and fiber whereas soybean contains higher amount of protein and fat contents. The high performance liquid chromatography (HPLC) quantification of vitamin K revealed that phylloquinone was higher in spinach as compared to soybean. Amongst antioxidant extracts, methanolic extracts of spinach and soybean showed higher total phenolic, 2,2-diphenyl-1 picrylhydrazyl (DPPH) scavenging and antioxidant activities. During product development phase, four dietary products i.e. cooked spinach (T 1 ), reconstituted spinach (T 2 ), natto A (T 3 ) and natto B (T 4 ) were formulated. Characterization of soybean based prepared products showed improvement in nutritional status due to fermentation as compared to raw material. In the formulated products, cooked spinach (T 1 ) and natto A (T 3 ) attained higher scores for sensory profile from each category. On the basis of nutritional characterization, vitamin K contents and antioxidant potential, two best products i.e. T 1 and T 3 one from each raw material were selected for efficacy study. Bioevaluation study was carried out involving New Zealand rabbits through two sequential trials for validity of the results. Accordingly, two types of studies were conducted on the basis of different groups of rabbits i.e. study I (normal rabbits) and study II (vitamin K deficient rabbits). Moreover, three different types of dietary sources namely cooked spinach a source of phylloquinone (D 2 ), natto for menaquinone-7 (D 3 ) and synthetic menadione (D 4 ) along with control (D 1 ) were provided to the respective groups. Feed & water intakes and body weights of rabbits varied significantly (p<0.05) with vitamin K dietary sources in study II while these traits behaved non-significantly in study I. The blood coagulation parameters including bleeding, clotting and prothrombin & partial thrombinplastin times and international normalized ratio (INR) were significantly (p<0.05) reduced due to vitamin K enriched dietary sources in vitamin K deficient rabbits (study II) except fibrinogen level that was improved. The serum phylloquinone levels were increased momentously (p<0.05) during study I & II (trial 1) by 23.78, 16.71 & 10.64 and 47.58, 36.42 & 27.71% in D 2 , D 3 & D 4 groups, respectively as compared to control. Similarly, serum menaquinone-7 level was improved in D 2 , D 3 and D 4 groups by 2.59, 17.77 & 1.11 in study I and 12.74, 24.50 & 10.78% in study II (trial 1). The vitamin K dependent proteins i.e. osteocalcin was increased whilst, undercarboxylated osteocalcin (ucOC) and protein induced by vitamin K absence or antagonist-II (PIVKA-II) were decreased due to vitamin K enriched dietary sources. Likewise trend for these traits was noticed in trial 2 of both studies. The liver & kidney functioning tests and hematological values were within normal range. From the present exploration, it is concluded that vitamin K enriched dietary sources containing menaquinone-7 and phylloquinone are effective to improve the serum vitamin K status and ameliorate the coagulation and vitamin K dependent proteins related abnormalities.