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.

Integration of Seeding Rates and Weed Management Techniques for Weed Suppression and Some Agronomic Traits of Chickpea Cicer Arietinum L.

Chickpea (Cicer arietinum L.) crop has gained attention of the arid region farmers across Pakistan because of the low water requirement. Meanwhile, the chickpea is poor competitor of weeds in its early growth stages due to its slow growth rate and leaf area development. Taking under consideration the negative impact of weeds on chickpea crop, field, pot and laboratory studies were conducted during 2012-13 and 2013-14. During field studies, the effects of different chickpea seeding rates and different weed control techniques were tested against chickpea weeds under rain-fed conditions. The results of the field experiments showed that during both the years the lowest weed density (115.18 and 64.40 m-2) was recorded in Stomp 330 EC followed by Dual Gold 960 EC. The maximum seed yield (1226 and 1411 kg ha-1) was recorded in Stomp 330 EC for both the years, respectively followed by the application of Dual Gold 960 EC and manual weeding. Among the chickpea seeding rates, the maximum number of branches plant-1, 100 seed weight, biological yield and seed yield kg ha-1 were observed for 70 kg ha-1 seed rate during both the years. The economic analysis exposed that the maximum net profit to the farmers in case of added cost was obtained from the application of pre-emergence herbicides i.e., Stomp 330 EC and Dual Gold 960 EC as compared to the other weed control techniques. In pot studies the sensitivity of chickpea was tested against two pre-emergence herbicides (Stomp 330 EC and Dual Gold 960 EC) and three post-emergence herbicides (Topik 15 WP, Puma Super 75 EW and Isoproturon 500 EW). In pot studies, chickpea was more sensitive to pre-emergence than post-emergence herbicides in terms of their toxicity. All the studied parameters of chickpea showed a significant response to the herbicides applied except nodules fresh and dry biomass. Moreover, the least chickpea plant height (cm), number of nodules plant-1 and protein content (%) in seed were recorded under the pre-emergence application of Stomp 330 EC. In the third experiment, the allelopathic potential of five common weeds including Parthenium hysterophorus L., Cyperus rotundus L., Datura stramonium L., Phragmites australis (Cav.) Trin. and Convolvulus arvensis L. was examined against the different chickpea varieties (Karak-I, Karak-III and Sheenghar) under laboratory conditions. The results of the lab. studies demonstrated that among the chickpea varieties the “Karak-III” showed more susceptibility against the phytotoxic effects of weed extracts. However, among the tested extracts, C. arvensis showed more allelopathic effects in terms of germination failure in chickpea and just gave 43.33% germination as compared to control treatment (97.50%). However, the extract of P. australis showed a little stimulatory effects on all the tested chickpea varieties. Hence, it is concluded that an integrated weed management approach involving cultural practices and herbicides is needed to develop an effective and economical weed control technique and to achieve maximum chickpea yield.