Direct microscopic observation of the isothermal bainite evolution in terms of nucleation events, the location of the nuclei, as well as their growth is very valuable for the refinement of models predicting the kinetics of bainite transformation. To this aim, the microstructural evolution in a Fe-0.2C-1.5Mn-2.0Cr alloy during isothermal bainite formation at temperatures between 723 K and 923 K is monitored in situ using high temperature laser scanning confocal microscopy (LSCM). Both the nucleation and the growth kinetics of the bainitic plates are analyzed quantitatively. Bainitic plates are observed to nucleate on three different types of locations in the grain: at austenitic grain boundaries, on newly-formed bainite plates and at unspecific sites within the austenite grains. Grain boundary nucleation is observed to be the dominant nucleation mode at all transformation temperatures. The rate of nucleation is found to vary markedly between different austenite grains. The temperature dependence of the average bainite nucleation rate is in qualitative agreement with the classical nucleation theory. Analysis of plate growth reveals that also the lengthening rates of bainite plates differ strongly between different grains. However, the lengthening rates do not seem to be related to the type of nucleation site. Analysis of the temperature dependence of the growth rate shows that the lengthening rates at high temperatures are in line with a diffusional model when a growth barrier of 400 J mol −1 is considered.
A comparison between slow heating to aging temperature and direct charging at aging temperature on the microstructure and mechanical properties obtained after the aging was established for the metastable beta (β) titanium alloy Ti-15V-3Cr-3Al-3Sn. The alloy was subjected to two single aging (SA) and two duplex aging (DA) conditions, with two heating rates to aging temperature: (i) low heating rate of 5 oC/min (ii) direct charging into a furnace heated to aging temperature. The microstructure analysis was carried out using Field Emission Scanning Electron Microscopy. Mechanical Testing was carried to evaluate Ultimate Tensile Strength (UTS), 0.2% Yield Strength (YS), % Elongation (%El.), % Reduction in area (%RA) and hardness. In the case of SA samples aged at 500 °C for 8 h and 500 °C for 10 h, heating rate of 5 °C/min to aging temperature resulted in a finer microstructure but did not help in achieving better strength-ductility combination compared to direct charging. Lower rate of heating allows enough dwell time in the temperature range 250-300 oC for pre-precipitation reaction to occur which aids in fine scale precipitation of alpha phase during aging. In the case of DA samples aged at 250 oC for 24 h followed by 500 oC for 8 h and 300 oC for 10 h followed by 500 oC for 10 h, no tangible difference between lower rate of heating and direct charging was observed in mechanical properties or microstructure. This is believed to be due to the pre-aging steps 250 oC/24 h or 300 oC/10h in the two DA treatments, which create finely distributed precursors thereby leaving no scope for the heating rate to play a role.
Corrosion inhibiting conversion coating formation is triggered by the activity of micro-galvanic couples in the microstructure and subsequent local increase in pH at cathodic sites, which in the case of aluminium alloys are usually intermetallics. Ceria coatings are formed spontaneously upon immersion of aluminium alloys in a cerium conversion coating solution, the high pH gradient in the vicinity of intermetallics drives the local precipitation of ceria conversion compounds. Cu-rich intermetallics demonstrate a highly cathodic nature and have shown the local precipitation reaction to occur readily. Fe-rich intermetallics are, however, weaker cathodes and have shown varied extents of localized deposits and are in focus in the current work. Model cast Al-7wt.%Si alloys have been designed with 1 wt.% Fe, solidified at different cooling rates to achieve two different microstructures, with big and small intermetallics, respectively. Upon subjecting the two microstructures to the same conversion coating treatment (immersion in a 0.1 M CeCl3 solution) for a short period of 2 h, preferential heavy deposition on the boundaries of the big intermetallics and light deposition on the small intermetallics was observed. Based on these observations, a mechanism of localized coating initiation at these Fe-rich intermetallic particles (IM) is proposed.
This report dissects the analysis of annual performance of a cluster of wind turbine generators operated by Suzlon Energy Limited to troubleshoot shortfalls in the predicted generation of individual machines for the fiscal year of 2011-12. The first phase involved an estimation of the annual energy production using wind resource assessment on the commercial software Wind Atlas, Analysis and Application Program (WAsP). Comparison of the Annual Energy Production (AEP) with direct extrapolation of monthly generation to centum machine and grid availability highlighted negative inconsistency between the two in the case of five machines. Detailed study of their data histories indicated the reason to be coarse extrapolation and generation curtailment, although the latter is a minor contributor. The second phase consists of quantification of these losses for the concerned machines using statistical extrapolation and a variety of data approaches. A qualitative comparison of the methods is presented based on accuracy and utility. It is concluded that correlation of average hub-height wind velocities with the concerned machine's generation yields the most reliable and professionally useful results. Also, the advantages and shortcomings of the other methods have been discussed.
Micro-galvanic activity between the cathodic intermetallics and the anodic matrix in an aluminium alloy drives the localized deposition of cerium conversion coatings. It is at the cathodic sites that conversion coating deposition is initiated due to the local pH gradient created because of cathodic reduction reaction over them. The intermetallics’ chemical, electrochemical and geometrical properties critically influence the deposition, but very little is empirically known about the latter. With the objective of understanding the effect of intermetallic geometry, this work looks closely into the deposition on nearly 550 eutectic θ-Al2Cu type intermetallics with different size regimes to decipher the controlling geometrical variables. While this work investigates influence of intermetallic geometry on localized deposition, parallel conclusions may also be drawn about the corrosion activity around intermetallics as a function of their geometry due to these coatings being triggered by localized corrosion.
Cast aluminium alloy 354 has extensive applications in the automobile industry. Due to its attractive combination of mechanical properties and excellent castability, it is being used in production of automobile components like the compressor wheel for turbochargers. Performance of this component under fatigue loading conditions is a critical issue. The present study explores the possibility of improving the fatigue life of the component by bringing in process changes (i) adopting a two-step aging treatment in place of the normally used single step aging treatment (ii) adopting a lower artificial aging temperature (171°C) instead of the temperature normally used for artificial aging (188°C) while performing T61 treatment. In all cases Weibull analysis of fatigue test results was carried out. Weibull analysis of Ultimate Tensile Strength (UTS) values obtained after artificial aging at 171°C and 188°C was also carried out. Among the four variants of two-step aging treatment carried out, the one consisting of 100°C for 5 hours followed by 170°C for 5 hours was found to have the best characteristic fatigue life for the components. The modified T61 treatment where aging was carried out at 171°C instead of the normally used 188°C yielded better characteristic fatigue life as well as better Ultimate Tensile Strength (UTS).
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