Browsing by Author "D.S. Sarma"

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A microstructural characterization of solution-treated titanium alloy Ti6Al4V
(1987) G. Sridhar; R. Gopalan; D.S. Sarma
A transmission electron microscopic study has been carried out to characterize the microstructural features of a titanium-6 wt% aluminum-4 wt% vanadium alloy after treatments at three different solutionizing temperatures of 900°C, 955°C, and 1065°C, followed by subsequent cooling at various rates in air, furnace, or water. Solutionization at 900°C and 955°C produced duplex microstructures consisting of equiaxed α and partially transformed β phases, while solutionization at 1065°C resulted in coarse Widmanstätten structures on furnace or air cooling and a fully martensitic structure on water quenching. The present studies revealed the morphology, size and distribution of the α, β, and martensite phases and also the presence of an interface phase with an fcc structure at almost all α-β interfaces. This interface phase has a width of about 0.15 μm and occurred in all slowly cooled specimens. Selected area diffraction patterns revealed the Burgers relationship between α and β and Nishiyama-Wassermann relationship between β and γ. The martensite is typically of lath structure with alternate laths in different orientations when quenched from 1065°C but observed as a fine platelike structure when quenched from 900° and 955°C. © 1987.
Age hardening studies in a Cu-4.5Ti-0.5Co alloy
(Elsevier BV, 2001) S. Nagarjuna; K.K. Sharma; I. Sudhakar; D.S. Sarma
Age hardening in a Cu-4.5Ti-0.5Co alloy has been studied at different aging temperatures and times. It has been observed that this alloy exhibits considerable age hardening with hardness increasing from 225Hv to a peak value of 320Hv on aging. Yield strength increases from 360 to 710 MPa and tensile strength from 610 to 890 MPa on aging the solution treated alloy for peak strength. The electrical conductivity of the alloy is found to be 4 and 8% International Annealed Copper Standard (IACS) in solution treated and peak aged conditions, respectively. Addition of cobalt to Cu-4.5Ti alloy reduces the aging temperature and time for attaining peak hardness. Ordered, metastable and coherent Cu4Ti (β1) precipitate is found to be responsible for maximum strengthening of the alloy. Interestingly, absence of equilibrium precipitate Cu3Ti and presence of Cu4Ti phase have been noticed in the overaged condition. The absence of Cu3Ti is attributed to the addition of cobalt. In addition, intermetallic phases of Ti and Co like Ti2Co and TiCo have been observed in solution treated, peak aged and overaged conditions. Cold work prior to aging enhances the hardness, strength and electrical conductivity of the alloy. For example, 90% cold work followed by aging at 400°C for 1 h increases the hardness from 320 to 430Hv; yield and tensile strengths, from 710 to 1185 and 890 to 1350 MPa, respectively, and electrical conductivity, marginally by 1% IACS. While mechanical properties are comparable, electrical conductivity of Cu-4.5Ti-0.5Co is less than that of the binary Cu-4.5Ti alloy in the solution treated as well as peak aged conditions. © 2001 Elsevier Science B.V. All rights reserved.
Analysis of flow behaviour of an aluminium containing austenitic steel
(Elsevier BV, 2007) D.V.V. Satyanarayana; G. Malakondaiah; D.S. Sarma
Experimental true stress-true strain data, at both ambient and elevated temperatures, of an Fe-Ni-Cr-Al alloy in solution treated as well as aged conditions have been analysed using different flow relationships. Ludwigson relationship provides the best fit of the data for all the conditions investigated, though all the relationships, except Voce, describe the flow behaviour well at a high temperature of 673 K. The transition in macroscopic flow behaviour of the alloy with strain, in solution treated condition, can be correlated with the transition in dislocation mechanism from planar slip in low strain regime to slip plus deformation twinning in the high strain regime. Although ageing does not appear to alter the macroscopic flow behaviour, it causes considerable change in flow parameters of the Ludwigson relationship and substructural evolution. The flow data of the aged alloys fitted according to Ludwigson model not only yield a unique set of flow parameters for each ageing condition but also exhibit a systematic trend with ageing time. The transition in macroscopic flow behaviour of the alloy with strain, in aged conditions, can be correlated with a change in dislocation mechanism from dislocation-precipitate interaction at lower strains to dislocation-dislocation interaction at higher strains leading to formation of a dense dislocation tangled networks in the matrix regions surrounding the precipitates. © 2006.
Bainitic microstructures in a 0.2%C1%Mo steel
(1993) A. Joarder; D.S. Sarma
[No abstract available]
Cold cracking studies on low alloy steel weldments: Effect of filler metal composition
(2003) G. Madhusudhan Reddy; T. Mohandas; D.S. Sarma
The resistance to hydrogen cracking of Cr - Mo, Ni Cr - Mo and Si - Mn steels which are employed in armoured vehicle construction was evaluated under implant test conditions. Austenitic stainless steel filler (AWS E312), which is reported to be resistant to hydrogen assisted cracking, was used to study the cracking tendency of all the three steels. Four other fillers, namely a nickel based filler (ENiCrFe-3), a low carbon, low alloy steel, a mild steel (A WS E6013) and a matching filler for Cr - Mo, were employed to evaluate their relative cracking tendency. Cr - Mo and Ni - Cr - Mo steels exhibited high cracking tendency while Si - Mn steel was resistant to cracking with the E312 filler. Cr - Mo steel was resistant to cracking with the nickel based filler, the low carbon, low alloy steel and the matching filler. The observed cracking tendency of the steels is linked to a susceptible interface/fusion boundary microstructure. © 2003 IoM Communications Ltd.
Correlation of structure and flow behaviour of Cu-Ti-Cd alloys
(Elsevier BV, 2006) R. Markandeya; S. Nagarjuna; D.V.V. Satyanarayana; D.S. Sarma
The work hardening behaviour of Cu-3Ti-1Cd and Cu-4Ti-1Cd alloys has been studied by conducting tensile tests at room temperature in solution treated, peak aged and overaged conditions. The flow behaviour has been analyzed using various true stress-true strain relationships. The Ludwigson relationship provides the best fit for the true stress-strain data of the two alloys at 298 K in all the conditions studied. The Hollomon equation as well as Ludwik equation fits the experimental σ-ε data for both the alloys reasonably well in highly overaged condition. While both the alloys exhibit transient flow behaviour at lower strains in solution treated condition, Cu-3Ti-1Cd alloy only exhibits this behaviour in peak aged condition. No such transition was observed in overaged condition of both the alloys. Further, these alloys display higher flow stress and strain hardening rates in peak aged condition than that in solution treated condition. Cu-3Ti-1Cd alloy shows higher flow stress and lower strain hardening rates in overaged condition than in the solution treated condition. On the other hand, Cu-4Ti-1Cd alloy exhibits lower flow stress as well as lower strain hardening rates in overaged condition when compared to that in solution treated condition. The observed behaviour in Cu-4Ti-1Cd alloy is related to the modulated structure with fine precipitates of Cu4Ti present in solution treated condition as compared to coarse and diffuse lamellar structure in overaged condition. © 2006 Elsevier B.V. All rights reserved.
Discussion of "the interface phase formation in titanium alloys"
(1987) G. Sridhar; D.S. Sarma
[No abstract available]
Effect of alloying content on high cycle fatigue behaviour of Cu-Ti alloys
(Elsevier Ltd, 1997) S. Nagarjuna; M. Srinivas; K. Balasubramanian; D.S. Sarma
The high cycle fatigue (HCF) strength of Cu-Ti alloys of four compositions viz. Cu-1.5 wt% Ti, Cu-2.7 wt% Ti, Cu-4.5 wt% Ti and Cu-5.4 wt% Ti, has been investigated in solution treated as well as peak aged conditions. The fatigue strength was found to increase from 108 MPa for the solution treated Cu-1.5 Ti, to 295 MPa, for the peak aged Cu-5.4 Ti alloy. Though the fatigue ratio did not vary with Ti content in peak aged condition, it was slightly higher for low Ti alloys (1.5 and 2.7 Ti) than the high Ti containing ones in the solution treated state. The fatigue strength and resilience of Cu-Ti alloys are comparable with those of the commercial Cu-2Be-0.5 Co alloys as well as the annealed 0.75% carbon steels. Basquin equation is followed by these alloys. Copyright © 1997. Published by Elsevier Science Limited.
Effect of austenitizing temperature and cooling rate on the structure and properties of a ultrahigh strength low alloy steel
(2006) N. Kishore Babu; M.R. Suresh; P.P. Sinha; D.S. Sarma
A 0.3C-CrMoV(ESR) steel is being developed primarily for making pressure vessels used for aerospace applications. Since it is important to understand the range of microstructures and mechanical properties that will be obtained in the heat affected zone of welds, the steel has been subjected to different austenitizing treatments (temperatures ranging from 925°C to 1250°C) followed by cooling at various rates to room temperature. It has been shown that the austenite grain size increased from about 10 to 250 μm as the austenitizing temperature is increased from 925°C to 1250°C (1 hr) and that the hardness, YS, UTS,% elongation and% reduction in area as well as CVN energy for 450°C tempered condition decrease as the austenitizing temperature is increased for all cooling rates (furnace cooling, air cooling, oil quenching, quenching and tempering at 450°C). This is attributed mainly to the increase in austenitic grain size. The ranges of microstructures that can be obtained in the heat-affected zone are massive ferrite, fine pearlite, upper as well as lower bainite and martensite. The Charpy impact energy for the oil-quenched steel tempered at 200°C, however, did not vary significantly with austenitizing temperature. © 2006 Springer Science + Business Media, Inc.
Effect of carbon on structure and properties of FeAl based intermetallic alloy
(2001) A. Radhakrishna; R.G. Baligidad; D.S. Sarma
The benefits of carbon addition achieved in iron aluminides based on Fe3Al, could not be achieved in the case of iron aluminides based on FeAl because of the precipitation of soft graphite phase as against hard Fe3AlC0.5 at high concentration of carbon. This study also indicates that the strength of iron aluminides containing carbon appears to be determined by the volume fraction of Fe3AlC0.5 precipitates in these alloys. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.
Effect of cooling rate on the as-quenched microstructure and mechanical properties of HSLA-100 steel plates
(Minerals, Metals and Materials Society, 2003) S.K. Dhua; D. Mukerjee; D.S. Sarma
The effect of cooling rate on the as-quenched microstructure and mechanical properties of a 14-mm-thick HSLA-100 steel using various cooling media such as brine, water, oil, air, and furnace has been stud- ied. While quenching in brine, water, and oil resulted in lath martensite structures, the granular bainite and martensite-austenite (M-A) constituents were found in air- or furnace-cooled specimens. The aver- age lath spacing increased slightly on decreasing the cooling rate (300 nm in brine-quenched specimen to 400 nm in oil-quenched specimen). The precipitates of Cu and Nb(C, N) were observed in all the quenching conditions except in the brine-quenched specimen. The as-quenched strength and toughness of the brine-, water-, and oil-quenched specimens were higher (yield strength: 894 to 997 MPa, ulti- mate tensile strength: 1119 to 1153 MPa, and Charpy V-notch energies: 65 to 70 J at -85°C) than -those of air- and furnace-cooled specimens (yield strength: 640 to 670 MPa, ultimate tensile strength: 944 to 1001 MPa, and Charpy V-notch energies: 10 to 20 J at -85°C). For industrial production of HSLA-100 steel plates, oil or water quenching is recommended in lower thickness plates (<25 mm). For production of thicker plates, however, water quenching is more suitable.
Effect of modulations on yield stress and strain hardening exponent of solution treated Cu-Ti alloys
(Elsevier Ltd, 1998) S. Nagarjuna; M. Srinivas; K. Balasubramanian; D.S. Sarma
[No abstract available]
Effect of oxygen content of powder on microstructure and mechanical properties of hot isostatically pressed superalloy Inconel 718
(Elsevier BV, 2006) G. Appa Rao; M. Srinivas; D.S. Sarma
Inert gas atomized (IGA) superalloy Inconel 718 powders with varied oxygen levels of 275, 180 and 140 ppm were consolidated by hot isostatic pressing (HIPing) at 1200 °C/120 MPa/3 h. The microstructural characterization of as-HIPed alloys has shown that the densification phenomenon of the powder does not depend on its oxygen content. However, the formation of deleterious phases such as prior particle boundaries (PPBs) occur more predominantly in the HIPed material produced from the powder with high oxygen content. It was also observed that the recrystallization and formation of annealing twins in the HIPed alloy are greatly influenced by the oxygen content. Transmission electron microscopy (TEM) studies have revealed that the precipitation of γ″, γ′ and δ phases does not depend on oxygen level of HIPed alloy during heat treatment as per AMS 5662J standard schedule but the MC carbides enriched with Nb and Ti precipitated preferentially at the PPBs for the alloy with high oxygen content of 275 ppm. In contrast, the carbides were found to be precipitated more uniformly in the matrix of the alloy with low oxygen content of 140 ppm. Tensile properties of the as-HIPed and HIP + heat treated alloys have shown that the yield strength (YS), ultimate tensile strength (UTS) do not get influenced by the oxygen content, but the ductility was found to be deteriorated drastically at elevated temperatures with increasing the oxygen content of the alloy. Stress rupture properties of the heat treated alloys at 650 °C under a stress level of 690 MPa have maintained a direct relation with the oxygen content, as the alloy with 275 ppm of oxygen content has shown inferior rupture life of 27 h with 2.1% ductility as compared to 84.5 h of life with 4.5% ductility and 116 h of life with 6% ductility offered by the alloys with 180 and 140 ppm of oxygen content, respectively. A better combination of mechanical properties achieved by use of prealloyed powder with low oxygen content makes it possible to explore the near net shape advantage of HIP technology to its maximum potential for alloy 718 components. © 2006 Elsevier B.V. All rights reserved.
Effect of prior cold work on age hardening of Cu-4Ti-1Cr alloy
(Elsevier BV, 2005) R. Markandeya; S. Nagarjuna; D.S. Sarma
The influence of cold rolling to 50, 75 and 90% reduction on the age hardening behaviour of Cu-4Ti-1Cr alloy has been investigated by hardness and tensile tests and optical as well as transmission electron microscopy. Hardness of Cu-4Ti-1Cr alloy increased from 222 Hv in solution treated condition to 416 Hv with 90% cold work and peak aging. Cold deformation reduced the peak aging temperature from 450 °C for the undeformed alloy to 400 °C for cold deformed alloys. The yield strength (YS) and ultimate tensile strength (UTS) of the alloy with 90% cold work followed by peak aging at 400 °C were found to be 1165 and 1248 MPa, respectively. The microstructure of the deformed alloy exhibited elongated grains and deformation bands. The maximum strength on peak aging was obtained due to precipitation of ordered, metastable and coherent β1, Cu4Ti phase in addition to high dislocation density and deformation twins. Overageing of the alloy resulted in reduced hardness and strength due to the formation of incoherent and equilibrium β, Cu3Ti phase in the form of cellular structure. However, the morphology of the discontinuous precipitation was changed to globular shape with high deformation. © 2005 Elsevier B.V. All rights reserved.
Effect of prior cold work on mechanical properties and structure of an age-hardened Cu-1.5wt% Ti alloy
(Springer Netherlands, 1997) S. Nagarjuna; K. Balasubramanian; D.S. Sarma
The effects of prior cold work on hardness, tensile properties, electrical conductivity and microstructure of an aged Cu 1.5 wt % Ti alloy have been studied by employing hardness and resistivity measurements, tensile tests and scanning and transmission electron microscopy. The hardness increased from 80 VHN in the solution-treated condition, to 210 VHN on peak ageing and 280 VHN with prior cold work followed by ageing. While a similar trend has been observed in yield and tensile strengths, the ductility (percentage elongation) decreased from 45% to 9%. The electrical conductivity of the alloy also increased up to 26% International annealed copper standard upon ageing the cold-worked alloy. Maximum strengthening of the alloy was associated with the precipitation of metastable, coherent and ordered Cu4 Ti, β′ phase having body-centred tetragonal structure. The differences in the properties and microstructural evolution between low and high titanium alloys (for example, the absence of composition modulations and deformation twins in Cu 1.5 Ti alloy, while they are present in Cu 4.5 Ti alloy) have been discussed. Prior cold work did not change the fracture mode of microvoid coalescence.
Effect of prior cold work on mechanical properties, electrical conductivity and microstructure of aged Cu-Ti alloys
(Kluwer Academic Publishers, 1999) S. Nagarjuna; K. Balasubramanian; D.S. Sarma
The mechanical properties, electrical conductivity and microstructure of Cu-2.7wt%Ti and Cu-5.4wt%Ti alloys have been studied in different conditions employing hardness and resistivity measurements, tensile tests and optical, scanning and transmission electron microscopy. Ageing of undeformed as well as cold worked alloys raises their hardness, strength and electrical conductivity. The hardness increased from 120 VHN for solution treated Cu-2.7Ti to 455 VHN for ST + cold worked + peak aged Cu-5.4Ti alloy. While tensile strength increased from 430 to 1450 MPa, the ductility (elongation) decreased from 36 to 1.5%. A maximum conductivity of 25% International Annealed Copper Standard (IACS) for Cu-2.7Ti and 14.5% IACS for Cu-5.4Ti is obtained with the present treatments. Peak strength was obtained when the solution treated alloys are aged at 450°C for 16 hours due to precipitation of ordered, metastable and coherent β′, Cu4Ti phase having body centred tetragonal (bct) structure. While mechanical properties of Cu-Ti alloys are comparable, electrical conductivity is less than that of commercial Cu-Be-Co alloys.
Effect of tempering temperatures on the mechanical properties and microstructures of HSLA-100 type copper-bearing steels
(Elsevier BV, 2001) S.K. Dhua; Amitava Ray; D.S. Sarma
Two copper-bearing high-strength low-alloy (HSLA) steels with chemistry similar to HSLA-100, were made on a laboratory scale, one in an air induction (100 kg) furnace and the other in a vacuum induction (50 kg) furnace. The ingots cast were hot-rolled to 25 mm thick plates which were subsequently austenitized and tempered at different temperatures (400-700°C) for 1 h. Evaluation of mechanical properties and microstructure of as-quenched and tempered plates revealed that substantial improvement in strength (YS-1024 and 1025 MPa; UTS-1079 and 1111 MPa for steels 1 and 2) occurred at the expense of impact toughness on tempering at 500°C owing to profuse Cu precipitation in the matrix. With increase in tempering temperature however, the notch toughness improved considerably, reaching peak values of 53 and 123 Joules (J) at - 85°C for steels 1 and 2 at 650 and 700°C tempering temperatures, respectively. The partially recovered matrix and the coarsened Cu precipitates in this temperature range presumably enhanced dislocation movement and notch toughness. © 2001 Elsevier Science B.V. All rights reserved.
Effect of thermomechanical working on the microstructure and mechanical properties of hot isostatically pressed superalloy Inconel 718
(Elsevier BV, 2004) Goudu Appa Rao; M. Srinivas; D.S. Sarma
Due to the high oxygen content (275 ppm) of prealloyed powder, the hot isostatically pressed (HIPed) superalloy 718 consisted of prior particle boundary (PPB) networks affecting the tensile ductility and stress rupture properties adversely. To improve the mechanical properties, the HIPed alloy was hot rolled at two different temperatures, viz. 1150 °C and 950 °C with a reduction of 40% in each case. The material rolled at 1150 °C was heat treated as per the conventional schedule consisting of solution treatment at 995 °C/1 h, followed by water quenching (WQ) and ageing treatment at 720 °C/8 h/furnace cooling (FC) to 620 °C and holding at 620 °C/8 h/air cooling (AC) to room temperature, while the alloy rolled at 950 °C was directly aged without solution treatment. Optical and transmission electron microscopy (TEM) studies revealed that while these treatments eliminated the PPB networks, the fine-scale precipitation of γ″ and γ′ phases was similar to that of the HIPed and heat-treated alloy. Hot rolling at 1150 °C followed by conventional heat treatment offered a very good combination of yield strength (YS; 938 MPa), ultimate tensile strength (UTS; 1033 MPa), ductility (18% elongation (EL) and 36% reduction in area (RA)) and excellent stress rupture properties at 650 °C. On the other hand, rolling at 950 °C + direct ageing treatment showed tremendous improvement in YS (1185 MPa) and UTS (1277 MPa) with acceptable level of ductility and stress rupture properties required for critical components. © 2004 Elsevier B.V. All rights reserved.
Effect of Ti additions on the electrical resistivity of copper
(Elsevier BV, 1997) S. Nagarjuna; K. Balasubramanian; D.S. Sarma
Electrical resistivity of Cu-Ti alloys containing 1.5, 2.7, 4.5 and 5.4 wt.% Ti, has been determined from the resistance values measured using Kelvins Bridge apparatus at room temperature. The resistivity in solution-treated alloys increases with Ti content linearly up to about 4.0 wt.% Ti, beyond which it decreases with further additions of Ti. However, in peak-aged condition, the resistivity continues to increase linearly up to 5.4 wt.% Ti without showing any decrease. Nordheim's rule of resistivity is followed up to approximately 4.0 wt.% Ti in the solution-treated alloys. Further, Nordheim's rule modified with the incorporation of the law of mixtures for two-phase systems (ρt = ρmυfm + ρpυfp) is obeyed right up to 5.4 wt.% Ti in the peak-aged alloys. The difference in behaviour is attributed to the fine scale precipitation formed during quenching in solution-treated Cu-4.5Ti and Cu-5.4Ti alloys, as revealed by transmission electron microscopy (TEM). The contribution to the total resistivity by β′-Cu4Ti precipitate and prior cold deformation is considerable in deformed and peak-aged Cu-Ti alloys. © 1997 Elsevier Science S.A.
First order reaction kinetics for transient creep in NiAl hardened austenitic steel
(2009) D.V.V. Satyanarayana; G. Malakondaiah; C. Phaniraj; D.S. Sarma
Transient creep of an NiAl hardened austenitic steel was analysed in the temperature range of 823 to 923 K at stresses ranging from 150 to 450 MPa in the frame work of first order reaction kinetics. The present analysis is aimed: to correlate various transient creep parameters with steady state creep rate following first order reaction rate theory to obtain correlation constants; and to arrive at a unified equation to describe primary and steady state regimes of the creep curves in terms of correlation coefficients thus derived. Good correlation of transient creep parameters with steady state creep rate has been obtained over the test conditions studied indicating that the basic mechanism of deformation is the same for all the three stages of creep. Unified equation that fits the experimental creep strain time data for different test conditions over transient and steady state regimes has been obtained in terms of correlation coefficients. © 2009 Institute of Materials, Minerals and Mining.