Materials Science Forum Vols. 830-831

Abstract: In the present work the effects of Si (0.5, 1, 1.5 wt%) on the as cast microstructure of Mg-2Sb alloy have been investigated. Si addition forms new Mg-Sb-Si ternary compound in Mg-Sb alloys besides the morphology of Mg₃Sb₂ changes from needle to chinese scripted and polygonal shaped Mg-Sb-Si ternary compound. The improved room temperature tensile properties are obtained with Mg-2Sb-1Si alloy which is due to the presence of polygonal shaped Mg-Sb-Si ternary compound.

Abstract: Light weight aluminium alloys and low-density materials have drawn the attention of researchers as potential structural materials for transportation sector due to the requirement of effective reduction in fuel consumption, stringent emission norms and higher payload capacity. Functionally Graded Materials (FGM) provides variation in properties and better functional performance within a component. Sequential casting is fairly a new technique to produce functionally graded materials and components by controlled mould filling process. Bimetallics of aluminium alloys are prepared by sequential casting using A390-A319 alloy (cast-cast alloy) and A390-A6061 alloy (cast-wrought alloy) combination and solidified under gravity. The effect of temperature of the two melts and gap between pouring of the melts on microstructure and properties of the bimetals are investigated. The microstructures show good interface bonding between the two different alloy metals. The hardness testing shows higher hardness at hypereutectic alloy region. The process described in this study shows potential and effective approach to create good bonding between two different aluminium alloys to develop advanced functional and structural materials which can be used in various automobile components to reduce the overall weight of the vehicle, by which better fuel efficiency and performance can be achieved.

Abstract: The Al-Zn-Mg system is a familiar age-hardenable 7xxx series of aluminium alloy. Aluminium alloys are gaining wide popularity in aeronautical, automotive, and transportation industries. Scandium (Sc) has the ability to refine grain size of cast aluminium structure. It has been possible to achieve an ideal combination of strength, density, and thermal stability because of the unique age-hardening characteristics of Sc. Moreover, low solid solubility of Sc in aluminium is responsible for the improvement of the microstructure and mechanical properties when added in small amounts (≤0.6 wt.%). Further, inoculation is an effective means of grain refinement in liquid state of as-castaluminium alloys. So, density of GP zones formation and early stage of ageing effects assessment main priority in the present work. However, coherent precipitates like ScAl₃are finely dispersed to provide thermal stability by increasing recrystallization temperature. Hence, the improvement in the high temperature stability of aluminium alloys (7xxx series) may be attributed to the grain boundary pinning (e.g. Zenerdrag mechanism) by the fine precipitates.In this paper, the relationship between the mechanical behavior and microstructure characteristics of Al-Zn-Mg-Sc based alloys are investigated to understand the thermal stability mechanism of grain refinement and dispersive precipitation.

Abstract: Present study reports the effect of electromagnetic stirring (EMS) on reciprocating wear characteristics of A390 alloy. The microstructure features showed the refinement of eutectic silicon and reduction in size of primary silicon particles due to the effect of EMS. Hardness of the A390 alloy (100.28 BHN) prepared by EMS was higher than conventional A390 alloy (66.86 BHN). Reciprocating wear tests were carried out at different contact loads (15N to 90N) and speeds (0.2m/s to 1m/s) using pin-on-reciprocating plate tribometer for a constant sliding distance of 500m. As the reciprocating velocity increases from 0.2m/s to 1m/s, the wear loss of both EMS stirred and without stirred A390 alloys were found to decrease gradually up to the critical velocity 0.6m/s. It was noticed that above critical reciprocating velocity 0.6m/s, the wear loss was found to increase abruptly. Also, as the load increases from 15N to 90N, the wear loss of both the alloys were found to increase gradually up to 45N and thereafter abrupt increase in wear loss was noticed up to 90N. This clearly indicates a transition in the mode of wear from mild oxidative to severe metallic wear regime. Wear rate of without stirred A390 alloy in the region 45N to 90N showed predominant increase compared to EMS stirred A390 alloy. This trend suggests that EMS stirred A390 alloy is more coherent and stable under dry sliding wear conditions than without stirred A390 alloy.

Abstract: Gyroscope is an orientation determining sensor whose performance is mainly dependant on one of the critical element called the gimbal flexure. Thinner the flexure, better is the performance. To have sufficient strength, M300 maraging steel is used to fabricate the thin flexures machined by Electrical Discharge Machining (EDM) process down to the required thickness of 0.05 mm in various stages. After the EDM processing, the recast layer formed can cause degradation in the microstructure and mechanical properties. Micro-abrasive machining was carried out to remove the recast layer. An attempt was made to understand the changes that occurred after EDM and after micro-abrasive machining with respect to the microstructure using optical and Scanning Electron Microscopy (SEM) and with respect to mechanical properties using nanoindentation. Nanoindentation technique was adopted as the recast layer was just few microns thick. Indentations were carried out on the base material, as EDM cut flexure, and the micro-abrasive machined EDM cut flexure to obtain elastic modulus and hardness values for each condition and the results were analyzed.

Abstract: Effects of furnace and short electric pulses (SEP) annealing with current density of ~10⁴ A/mm² on the microstructure of the cold-rolled 01570c aluminum alloy, containing high densities of Al₃(Sc,Zr) dispersoids, were investigated. The high potential of the SEP annealing to process fine-grained sheets from aluminum alloys with high additions of transition metals has been shown.

Abstract: ZnO nanostructures such as nanowires and nanorods are beneficial in solar energy harvesting because they provide a structure with a large surface area. Also, they provide a direct pathway to electron transportation, eliminating the scope for grain boundary scattering. In this investigation, thin ZnO films were prepared by thermal evaporation of the Zn metal, which was followed by oxidation. Evaporation parameters and oxidation temperatures were fixed. The oxidation duration was the variable. The effect of oxidation time, on the morphology and structural properties were studied by using scanning electron microscopy and X-ray diffractometry. The study shows that with increase in oxidation time, the morphology changed to nanorods from initial flake morphology. As a function of oxidation time, the crystallinity and texture became more dominant. By using, I-V characteristic curves, it was found that the morphology changes alter the surface electrical conductivity of the prepared ZnO films.

Abstract: This paper demonstrates a unique application of DIC wherein the structural performance and structural integrity have been evaluated together almost in real-time. The results obtained from the ground level tests, simulating the bonding between the TPS to the base structure of a space crew module, are reported to find the adequacy of two different adhesives prior to actual bonding. The test objective was to characterize the adhesives based on the structural performance (deflection and strain behavior) of the TPS and conduct health monitoring in real-time (i.e. abort the test whenever the TPS fails). The dual objective could be met using DIC in a full-field and non-contact manner, which was essential due to the limitations of the contacting type measurements.

Abstract: Carbon fibre composites used in many safety-critical applications experience damage due to operating environment such as bird strike, hailstorm etc., In some cases, the damage is barely visible and its presence in the structure can cause accelerated damage leading to catastrophic failure. In this paper, the results of fatigue damage progression in woven CFRP laminate subjected to constant amplitude loading for un-impacted specimens as well as impacted specimens are presented. The stiffness of the specimens was periodically monitored during fatigue testing to arrive at the rate of damage progression. Special experiments were carried out under a programmed version of an equivalent fighter aircraft loading – FALSTAFF (Fighter Aircraft Loading Standard for Fatigue and Fracture) and it was observed that the damage progression is slower in case of spectrum loading compared to constant amplitude loading. The effect of load sequencing on damage is investigated in this paper.

Abstract: Solid rocket motor nozzles and liquid engine throats use ablative composite materials to protect the structures from the extremely severe operating conditions of high temperature, pressure and particle impingement. These ablative nozzle liners are processed from carbon phenolic/silica phenolic prepregs using a complex processing cycle. The process starts from impregnating the fibres with phenolic resin followed by either moulding or wrapping/winding prepreg tapes over metallic mandrels followed by polymerization under pressure. Traditionally, prepreg is cut into straight tapes and wound on mandrels to form the divergents. For throats and convergents, prepreg patterns are cut and stacked for curing. Plies of shape conforming to the development of base cone are cut from the prepreg and wound on the mandrel. After polymerization, the final required configuration is machined from this cylindrical blank after the liners are cured. The new method, described in this paper, replaces the straight-cut prepreg tapes by bias-cut prepreg tapes. These can be used for ablative liners in which the plies are at an angle to the direction of the flow. Bias tapes are cut at 45° to the warp and weft directions of the fabric to the required width and formed to the required frustum of cone. Since the cone is formed from straight cut bias tape, the fibre construction will be uniform around the circumference unlike the pattern cut tape. The efficiency of this process depends on the wrapping ratio which is in turn a function of the diameter, ply angle and tape width. To get the best wrapping ratio, shaped mandrels are used which brings down the material wastage and machining allowances. This paper describes the attempts to develop the Bias wrapping technology for angle wound liners. The details of process trials, challenges faced and how they were overcome, advantages of the process over conventional process etc are explained. Exhaustive material characterization of the material was done. Property comparison and validation of performance in subscale tests and a full scale static test are also discussed. The analysis indicates that this new technology has good potential in replacing the traditional tape-winding technology for rocket motor nozzle liners.