Materials Science Forum Vols. 830-831

Abstract: An understanding of the plastic deformation behavior of Ti6Al4V (Ti64) is of great interest because it is used in aerospace applications due to its high specific strength. In addition, Ti alloys have limited slip systems due to hexagonal crystal structure; hence twinning plays an important role in plastic deformation. The present work focuses upon the grain size effect on plastic deformation behaviour of Ti64. Various microstructures with different grain size were developed via annealing of Ti64 alloy in α-β phase regime (825°C and 850°C) for 4 hours followed by air cooling. The deformation behavior of these samples was investigated at various deformation temperature and strain rate conditions. Detailed microstructure studies showed that (i) smaller grains undergoes twinning only at low temperature and high strain rate, (ii) large grain samples undergo twinning at all temperatures & strain rates, though the extent of twinning varied.

Abstract: The feasibility of employing the indigenously developed Ferroboron alloy as an alternate neutron shield material in combination with 9Cr-based ferritic steel clad in future Indian Fast Breeder Reactors (FBR), has been investigated from a metallurgical perspective. In this regard, extensive studies have been undertaken to estimate quantitatively the nature of interaction between Ferroboron and P91-ferritic steel at high temperatures. It is found that in the temperature range 550 to 600°C, 9Cr-based ferritic steel is fully compatible with Ferroboron. However, at higher temperatures, Feroboron interacts with ferritic steel; but the maximum estimated loss of clad thickness is restricted to about 250 μm for 60 years of service.

Abstract: The feasibility has been demonstrated for achieving high-strain-rate superplasticity with elongations beyond 2000% in Al-Mg-Sc-Zr alloy with the partially recrystallized structure, produced by warm equal-channel angular pressing to strain of e~3 only. Subsequent alloy ambient temperature rolling up to e~1.6 enhanced the superplastic elongations and moved the optimum of superplasticity toward the higher strain rates.

Abstract: In this paper, microstructural evolution of Indian Fast Reactor Advanced Clad (IFAC-1) steel during thermo-mechanical processing has been investigated. Hot isothermal forging has been simulated in a Gleeble® thermo-mechanical simulator in the temperature range 1173-1473K and true strain rate range 0.01-100s^-1. Instability map has been developed using the stress-strain data obtained. Effect of major forging parameters on various microstructural features has been studied quantitatively. Results from this study have been used to construct various maps (‘μ-maps’) representing different aspects of microstructural evolution. An analogy between the μ-maps and instability maps provides essential insights into possible instability mechanisms operative in the material. The μ-map analysis shows potential as a tool for optimisation of industrial-scale forging parameters.

Abstract: High strength fasteners of Titanium alloy Ti-6Al-4V are widely used in both launch vehicle and satellite structures. Ti-6Al-4V fasteners having three different types of microstructures were analysed for mechanical properties to understand the role of two phase α+β or transformed β phase in the head region. Based on the microstructure-mechanical property correlations, it was concluded that the presence of transformed beta phase will not affect the strength properties, but will produce scatter in percentage elongation, percentage reduction in area and failure torque values. Therefore, it is recommended that the microstructure of aerospace titanium alloy Ti-6Al-4V fasteners should contain homogeneous two phase α+β throughout.

Abstract: In the present investigation, microstructural characteristics and dry sliding wear behaviour of T6 heat treated conventionally cast Al-25Mg₂Si-2Cu alloy have been discussed. The as cast alloy was subjected to solutionizing at 500°C for 5h and isothermal aging treatment at 190°C for different aging times. The micro structural characterization was studied using Scanning Electron Microscope with EDS analysis. The microstructure of as cast alloy consists of intermetallics of coarse block like sharp edged β-(Mg₂Si), θ-(Al₂Cu) and Q-(Al-Mg-Cu-Si) in the form of Chinese scripts and needles distributed randomly in the Al-matrix. The microstructure of heat treated alloy shows spheroidization of β phase and fine precipitation of θ-(Al₂Cu) and Q phases. The dry sliding wear test was carried out using pin-on-disc machine. Age hardened alloy exhibits high wear resistance and minimum coefficient of friction over the entire range of applied loads and sliding velocities. Furthermore, high wear resistance was observed in the under aged condition compared to over ageing conditions.

Abstract: Cu-Cr-Zr-Ti alloys are widely used for fabrication of thrust chamber in liquid rocket engines, because of their high thermal conductivity and adequate strength. The alloy should be used in peak aged condition to achieve the best combination of strength and conductivity. However, realization of final component involves different stages of forming, intermediate heat treatment and brazing operations, which limit the usage of the alloy in peak aged condition. The formability of material depends on grain size to a large extent. Larger grained metals have better formability, but the roughened surface that results from stretching metal with coarse grain structure results in surface defects like orange peel. Copper alloys on cold working sometimes exhibit a surface pebbling effect termed as ‘orange peel’ or alligator skin’.The plates of this copper alloy, finish hot rolled at 925 to 975°C and annealed at 980±10°C for 10 ±2 min followed by air cooling are subjected to different stages of forming to get the final profile of thrust chamber. During the course of forming of plate for divergent thrust chamber, orange peel defect is observed in areas of higher deformation. Detailed characterization of these formed shells in terms of microstructure, grain size and mechanical property has been carried out to find out the exact cause of this defect. Study revealed that the plate has coarse grain structure, which has resulted in orange peel defect during forming. To overcome this effect, the plates were realized with modifications in finish hot rolling and annealing temperatures, which has shown improvement in grain refinement and mechanical properties. It has suppressed the orange peel defect during forming.

Abstract: Ni-Cr-Fe based alloy Inconel 690 is widely used in power plant, marine, chemical and nuclear applications due to its excellent mechanical properties, resistance to thermal creep deformation, good thermal stability and resistance to corrosive and oxidizing environments. In order to study the microstructure of the alloy and the precipitates formation during thermal exposure, the alloy was subjected to in-situ high temperature X-ray diffraction technique (HT-XRD) in the temperature range 298-1273K. Results of high temperature XRD patterns show (111), (200), (220) and (311) reflections confirming the stability of fcc structure in the temperature of investigation. With increase in the temperature, a shift in peak positions towards lower 2θ values due to lattice expansion was noticed. The average thermal expansion coefficient (TEC) of the alloy increased from 1.33 x 10^-5 K^-1 to 1.53 x 10^-5 K^-1 in the temperature range 298-1273 K. Scanning electron microscopy indicates austenitic grains of sizes in the range 100-150μm and chromium carbide precipitate at grain boundaries after the HT-XRD heat treatment.

Abstract: Copper thin film was deposited on double zincated aluminium using electrodeposition route. To understand the growth behavior of copper nodules, deposition was done by varying coating current density. The topography and cross section of the coating was analyzed by using SEM. The structural details of copper nodules were studied by using TEM. Deposited copper thin film was made up of pomegranate like nodules with size varying from 5 μm to 8 μm, varying as a function of coating current density. At higher current density, the coating was made up of multiple layers of nodules having defects like twins, dislocations and stacking faults. The nodules had fine crystallites of size in the range of 50 nm.

Abstract: This article presents the effect of individual addition of Sb, Sn and Pb on the precipitation sequence at room as well as high temperature tensile properties of AZ91 alloy. The results show grain refinement, formation of Mg₃Sb₂ and Mg₂Sn phases when Sb and Sn are added to AZ91 alloy. Lamellar precipitate is significantly suppressed while Pb added. Improved room and high temperature tensile properties are observed in Sb and Sn addition. Maximum tensile properties are noticed with AZ91 alloy having 0.5 wt.% Sb addition.