Showing papers by "Defence Metallurgical Research Laboratory published in 2001"

Studies on the formation of black particles in rice husk silica ash

Abstract: The formation of black (carbon fixed) particles in rice husk silica ash has been studied in detail. The formation of black particles in the silica obtained by calcination of raw untreated rice husks, is higher than that in acid treated rice husks. The tendency to form black particles increases with increase in the heating rate and the temperature of calcination of the untreated rice husks. Potassium has been shown to cause the formation of black particles in rice husk silica. By treating with 3.0 N HCl acid the formation of black particles can be avoided. There is no effect of heating rate on the formation of black particles in silica from acid treated rice husks. Treatment with HCl acid was found to decrease the oxidation (burning) of carbon at lower temperature (400°C ).

Identification of hot corrosion resistant MCrAlY based bond coatings for gas turbine engine applications

Abstract: In the current paper, an attempt has been made to identify hot corrosion resistant coatings which can produce a thermodynamically stable and inert oxide scale during exposure to a hot corrosion environment and thereby enhance the coating life significantly. The results obtained from hot corrosion tests of a variety of MCrAlY-type coatings with different alloying elements and traces of silicon and hafnium in the presence of sodium chloride and vanadium containing environments revealed that a NiCoCrAlY coating exhibits maximum life time among the coatings studied. The results also revealed that presence of trace elements in the coating reduces coating life significantly. Based on the results, a novel electrochemical mechanism has been proposed. It has been shown that the hot corrosion of protective coatings is an electrochemical phenomena and hence the electrochemical techniques appear to be quite useful in evaluating coatings for hot corrosion resistance.

Synthesis of nanocrystalline copper–tungsten alloys by mechanical alloying

Abstract: Copper–tungsten exhibits total absence of solubility in both solid and liquid state. Mechanical alloying (MA) as a solid state, non-equilibrium process can be beneficial to the processing of such an immiscible system with the added features of refinement of structure. A study was undertaken to synthesise various Cu–W alloys and develop an ultrafine microcomposite structure of tungsten in copper matrix by mechanical alloying. Elemental powders of copper and tungsten were milled in high energy ball mills. The milling behaviour was found to depend on the composition, milling time and milling atmosphere. The milled powders were characterised for their particle size, microstructure and lattice parameters. Metastable mutual solid solubility in the system was confirmed. Crystallite sizes were found to be in the nanocrystalline regime. The conversion of milling energy effectively to generate deformed surfaces, which in turn led to metastable solid solubility and nanocrystalline structure, was aided by the presence of oxygen in the milling atmosphere.

High performance coatings for titanium alloys to protect against oxidation

Abstract: With a view to use titanium alloys more effectively at elevated temperatures, the oxidation and alpha case formation should be prevented. This can be achieved by the application of high performance oxidation resistant coatings on the titanium alloys. A high performance oxidation resistant coating, based on the combination of surface modification techniques, electro-deposition and pack aluminizing, has been successfully developed to protect the titanium alloy, IMI 834 from oxidation and alpha case formation. Negligible weight gain and elimination of the alpha case formation during the entire exposure period demonstrate the superior performance of platinum aluminide coating. The excellent behavior is due to the formation of a protective, adherent and continuous alumina scale on the surface of the coating. Based on the results obtained for both coated and uncoated alloys, the developed high performance coating can enhance the life of components fabricated from the titanium alloy, IMI 834 considerably by preventing oxidation and alpha case formation and thereby improve the efficiency of gas turbine compressor sections significantly

Some Theoretical Aspects on Designing Nickel Free High Nitrogen Austenitic Stainless Steels

Abstract: Nickel free high nitrogen austenitic stainless steel design calls for proper choice of alloying constituents that enhances nitrogen solubility and retention of the same after alloying. The nitrogen alloyed should be adequate enough to give a single phase austenitic matrix along with other alloying elemental constituents. The various studies that evaluated these aspects show certain inconsistencies. Certain empirical formulations have been evolved in this study by analyzing all various published data. The formulae obtained, enables choice of alloying elemental composition to get desired microstructure and strength in the solution annealed condition. The various inadequacies that exists in the data used for assessing nitrogen solubility calls for caution when they are applied to actual nitrogen steel production process conditions.

Age hardening studies in a Cu–4.5Ti–0.5Co alloy

Abstract: 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 225 H V to a peak value of 320 H V 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 Cu 4 Ti (β l ) precipitate is found to be responsible for maximum strengthening of the alloy. Interestingly, absence of equilibrium precipitate Cu 3 Ti and presence of Cu 4 Ti phase have been noticed in the overaged condition. The absence of Cu 3 Ti is attributed to the addition of cobalt. In addition, intermetallic phases of Ti and Co like Ti 2 Co 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 430 H V ; 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.

Effect of process variables on the structure, residual stress, and hardness of sputtered nanocrystalline nickel films

Abstract: Nanocrystalline nickel films of about 0.1 μm thickness grown by sputtering with and without substrate bias possessed average grain sizes of 9–25 nm. Variation in substrate bias at room and liquid nitrogen temperature of deposition strongly affected grain structure and size distribution. Qualitative studies of film surfaces showed variation in roughness and porosity level with substrate bias and film thickness (maximum of 8 μm). The films had tensile residual stress, which varied with deposition conditions. The hardness values were much higher than those of coarse-grained nickel but decreased with an increase in the film thickness because of grain growth.

Development of oxidation resistant coatings for titanium alloys

Abstract: This work addresses the development of a coating to protect the titanium alloy designated IMI834 from oxidation and alpha case formation. This is to enable the safe use of the alloy at relatively high operating temperatures and thereby significantly improve the efficiency of aeroengines. Two different types of surface modification techniques have been employed to coat the titanium alloy: electrodeposition and pack aluminising. Cyclic isothermal oxidation studies revealed that platinum aluminide coating obtained by electrodeposition followed by pack aluminising exhibits good oxidation resistance compared with that exhibited by a plain aluminide coating. Based on results obtained by different techniques, it is suggested that platinum aluminide is a prospective coating material for preventing alpha case formation and protecting against oxidation in components fabricated from titanium alloy IMI834.

Erosion-oxidation interaction in Ni and Ni-20Cr alloy

Abstract: The solid-particle erosion of metals and alloys at elevated temperatures is characterized by different mechanisms of material removal, depending on the temperature of exposure, impact velocity, impact angle, and flux rate of the eroding particles. The objective of this article is to delineate the prevalent erosion mechanisms in nickel and a nickel-20 chromium alloy over a large range of temperatures, impact velocities, impact angles, and particle feed rates. For this purpose, a specialized elevated-temperature erosion rig has been utilized. Nickel and a Ni-20Cr alloy have been chosen as the test materials, in view of their substantially different oxidation behaviors. Results from the present study indicate that while the erosion rate generally increases with increasing temperature, an increased particle feed rate causes a reduction in the erosion rate, especially at higher temperatures beyond 650 K. On the basis of a detailed examination of the morphology of the eroded surfaces and the subsurface features beneath the eroded surfaces, four different material removal mechanisms have been identified in the nickel, while only three material removal mechanisms were operative in the Ni-20Cr alloy. Utilizing the aforementioned information, erosion-oxidation (E-O) interaction mechanism maps (herein, termed E-O maps) delineating the regions of dominance of the various erosion mechanisms in an impact velocity-test temperature space have been constructed for Ni and a Ni-20Cr alloy. Finally, the differences in erosion behavior between the Ni and Ni-20Cr alloy have been identified and rationalized.

Studies on structural transformation and magnetic properties in Sm2Co17 type alloys

Abstract: Structural transformations and microstructural characterisation of Sm2Co17 alloys containing Fe, Cu and Zr at different stages of thermal processing have been investigated by X-ray diffraction, optical, scanning electron and transmission electron microscopes. Solution treated samples consist of a mixture of hexagonal TbCu7 (1:7 H) and rhombohedral Th2Zn17 (2:17 R) structure types of 2:17 phase. After isothermal aging, TbCu7 + Th2Zn17 structures transform into Th2Zn17 type structure with precipitation of Cu-rich hexagonal SmCo5 (1:5 H) and Zr-rich platelet phases. In addition to the main phases, a soft magnetic phase of composition Zr6(FeCo)23 is formed in alloys containing higher Zr composition. Isothermal aging studies reveal that magnetic properties show a peak value when aged at 1108–1123 K for 10 h. TEM studies show cellular precipitate structure with cell interiors having 2:17 R structure, while the fully coherent cell boundaries have the 1:5 H structure. Zr-rich platelets which run across many cells and cell boundaries were found to have 1:7 H structure.

Effect of extrusion parameters on structure and properties of 2124 aluminum alloy matrix composites

Abstract: Discontinuously reinforced aluminum matrix composites (DRA) have been attracting attention because of their amenability to undergo deformation processing by conventional metalworking techniques. Extrusion is used in processing of DRA composites for consolidation, redistribution of reinforcements, and shape forming. The important parameters that control the extrusion process are temperature and strain rate, which is a function of several equipment/extrusion parameters. Vacuum hot-pressed (VHP) 2124 Al/30 SiCp composite billets were extruded at different ram speeds (1, 10, 100 mm sec−1) and using different extrusion ratios (4:1, 10:1, and 20:1). The extruded samples were studied for their integrity, microstructure, and mechanical properties. The integrity of the extruded composite rod was very good at minimum extrusion speed of 1 mm sec−1, whereas 100 mm sec−1 extrusion speed resulted in extensive fir tree cracking. Extrusion of VHP billets, with necklace structure, resulted in elongated alternate stringers...

Effect of aluminizing on the oxidation behavior of the titanium alloy, IMI 834

Abstract: This article presents the development of a simple and cost-effective coating to protect the titanium alloy, IMI 834, from oxidation and alpha-case formation so that the alloy can be used safely, even at relatively high operating temperatures. A surface-modification technique, pack aluminizing, was employed to coat the alloy. Cyclic-isothermal oxidation studies at 800°C revealed that the plain aluminide coating obtained by pack aluminizing exhibits good oxidation resistance compared to the uncoated alloy. Based on the results obtained by different techniques, it is suggested that the plain aluminide is an economical, simple, and efficient coating for prevention of alpha-case formation and protection against oxidation of components fabricated from IMI 834.

Effect of molybdenum addition on structure and properties of high carbon Fe3Al based intermetallic alloy

Abstract: The effect of molybdenum addition on structure and properties of high (0.3 wt.%) carbon Fe 3 Al based intermetallic alloy containing about 16 wt.% aluminium has been studied. Three different alloys with the following compositions Fe–16.3 wt.% Al–0.27 wt.% C, Fe–16 wt.% Al–2.85 wt.%, Mo–0.3 wt.% C and Fe–15.8 wt.% Al–7.5 wt.% Mo–0.3 wt.% C were prepared by a combination of air induction melting with flux cover (AIM) and electroslag remelting (ESR). The alloys were characterized with respect to microstructure. Micro-analysis was carried out by electron probe microanalyser. A considerable amount of solid solubility for molybdenum was observed in the carbon containing Fe 3 Al alloy. At low (2.85 wt.%) concentration of Mo, Fe 3 AlC 0.5 precipitates with dissolved Mo and a small fraction of fine Mo 2 C precipitates was observed. At a high (7.5 wt.%) concentration of Mo, Fe 3 AlC 0.5 precipitates were totally absent and only Mo 2 C precipitates were observed. Significant improvements in strength on the addition of Mo has been achieved. This may be attributed to both solid solution strengthening and precipitation hardening.

Effect of hot rolling and heat treatment on structure and properties of high carbon Fe–Al alloys

Abstract: Iron–aluminium alloys with the following compositions, Fe–8.26 wt.% Al–0.46 wt.% C, Fe–7.2 wt.% Al–1.1 wt.% C, Fe–11.20 wt.% Al–0.5 wt.% C and Fe–10.83 wt.% Al–1.1 wt.% C were melted under a flux cover by air induction melting (AIM). The AIM ingots were subsequently processed through electroslag remelting (ESR). Both AIM and ESR ingots were hot forged and hot rolled at 1375 K to 92% reduction. ESR ingots exhibited better hot workability as compared with AIM ingots. This may be due to the axially oriented columnar grain structure relatively free from internal defects such as microporosity and non metallic inclusions observed in ESR ingots. About 14 ml thick ESR processed alloys in the hot-rolled condition also exhibited superior room temperature tensile elongation as compared with hot-rolled AIM alloys. This may be attributed to the comparatively homogeneous, clean ingot with a refined microstructure and a fine uniform distribution of Fe3AlC0.5 phase as observed after ESR. Further rolling of the ESR alloy to a 4-mm thickness has resulted in significant improvements in strength. The tensile elongation of hot rolled ESR alloys was reduced after heat treatment except in the case of Fe–7.2 wt.% Al–1.1 wt.% C alloy which exhibited a 17% improvement.

Effect of reinforcement size on low strain yielding behaviour in Al–Cu–Mg/SiCP composites

Abstract: The low strain yielding behaviour of SiC p reinforced Al–Cu–Mg alloy matrix composites with varying reinforcement sizes was investigated. Three different sizes (1.4, 15.8 and 62.8 μm) of SiC p were used to produce metal matrix composites (MMCs), namely, MMC1, MMC16 and MMC63, respectively, by employing a proprietary powder metallurgy technique. Matrix alloy was also produced in a similar manner for comparison and designated as Control. All materials were heat treated to peak aged condition and subjected to a controlled strain deformation of 1.5% in both tension and compression. The yield stress at 2.5×10 −4 offset strain, termed as ‘actual yield’, of all the MMCs was significantly lower than that of Control during both tensile and compressive deformation. The actual yielding in all the MMCs and Control occurred at lower stress levels in compression than in tension. The observed results were attributed to the distribution of residual stresses in the matrix of MMCs generated due to the large difference in coefficient of thermal expansion between the matrix and the reinforcement.

Synthesis of TiC Whiskers through Carbothermal Reduction of TiO2

Abstract: TiC whiskers were produced through carbothermal reduction of TiO2 in the presence of potassium (K2CO3) and nickel (NiCl2). The effect of potassium, nickel, and heating rate on the formation of whiskers was studied. Potassium was found to be an essential constituent for whisker formation. Nickel acts as a catalyst for TiC whisker formation only in the presence of potassium. The yield of whiskers was maximum at 1000–1200°C. At higher temperatures, formation of particulates of TiC was the dominant process. An increase in K2CO3 concentration during fast heating and decrease in K2CO3 concentration during slow heating was found to be beneficial in increasing the formation of TiC whiskers. A vapor–liquid–solid growth mechanism of whisker formation was explained.

Effect of strain rate on fracture toughness of mild steel

Abstract: The effect of strain rate on the yield strength and fracture toughness of mild steel was studied in the strain rate range 10-5–102 s-1. The lower yield strength was found to be independent of strain rate in the range 10-5–10-3 s-1. Beyond this strain rate, the lower yield strength was found to vary linearly with logarithm of strain rate up to a value of 102 s1. Fracture toughness was also found to increase with strain rate up to a value of 10-3 s-1 and subsequently decrease gradually with increasing strain rate. However, a drastic reduction in fracture toughness was observed at a strain rate of 102 s-1. This reduction was attributed to the change in fracture mode from ductile to cleavage.

SiC fibre by chemical vapour deposition on tungsten filament

Abstract: A CVD system for the production of continuous SiC fibre was set up. The process of SiC coating on 19 µ m diameter tungsten substrate was studied. Methyl trichloro silane (CH3SiCl3) and hydrogen reactants were used. Effect of substrate temperature (1300–1500°C) and concentration of reactants on the formation of SiC coating were studied. SiC coatings of negligible thickness were formed at very low flow rates of hydrogen (5 × 10−5 m3/min) and CH3SiCl3 (1.0 × 10−4 m3/min of Ar). Uneven coatings and brittle fibres were formed atvery high concentrations of CH3SiCl3 (6 × 10−4 m3/min of Ar). The flow rates of CH3SiCl3 and hydrogen were adjusted to get SiC fibre with smooth surface. The structure and morphology of SiC fibres were evaluated.

Influence of Cu additions on the morphology of GeSi precipitates in an Al-Ge-Si alloy

Abstract: Combined additions of Ge and Si to Al are known to produce higher precipitation hardening than that which occurs in the constituent binaries, when the total amounts of alloying atoms are the same for all the alloys investigated. In the resultant Al-Ge-Si alloys, the diamond cubic precipitates contain both Ge and Si and are designated as GeSi. During artificial aging at 160 °C, the GeSi precipitates are commonly present in three forms, i.e., equiaxed, 〈100〉Al lath, and triangular plate. The equiaxed form is the dominant one of the three. This article examines the influence of varying amounts (i.e., 2 to 4 wt pct) of Cu additions on the morphology of GeSi precipitates formed in an Al-2.6 wt pct Ge-1.04 wt pct Si alloy during artificial aging at 160 °C. It is shown that Cu additions have the remarkable effect of maximizing the nucleation frequency of the 〈100〉Al lath form and simultaneously suppressing the nucleation of the equiaxed and the plate forms of the GeSi precipitates. Increasing Cu additions also increase the homogeneity and cause refinement of the 〈100〉Al laths. These results are discussed in light of (1) the critical requirement of vacancies for the nucleation and growth of GeSi precipitates having an atomic volume larger than Al and (2) the crystallographic nature of the negative dilation strains that develop locally in the Cu-rich regions of the Al matrix. It is further shown that, in the alloys containing increased levels (i.e., exceeding about 2.5 wt pct) of Cu, the precipitation of ϑ′ (metastable ϑ-Al2Cu) phase occurs, and that the nucleation of Cu-rich ϑ′ precipitates occurs upon the 〈100〉Al laths of GeSi. The latter effect is discussed in terms of the attainment of both the nucleation site and the necessary solute supersaturation at the 〈100〉Al GeSi/α-Al interfaces.

Modelling of Interaction between Creep and Oxditaion Behaviour for Eegineering Materials

Abstract: Mechanical test data for design of high temperature components are mainly collected from tests conducted in air even though these are used in different environment. Therefore, in order to predict accurately the performance of a component in actual service it is necessary to develop appropriate constitutive equations taking into consideration the environmental effect. The paper briefly describes how our current knowledge on Continuum Damage Mechanics (CDM) can be utilized to formulate the same. In absence of sufficient experimental data, most of the existing approaches provide only qualitative support for their models. Creep test on specimens having different section sizes, under identical conditions, can provide valuable information on creep/environment interaction. A simple analytical method has been developed to extract the material parameters determining the kinetics of environmental attack from a creep strain-time plot. It has been shown that the material constants thus obtained provide good explanation of the observed section size and geometry effects in steels and nickel base superalloys.

Effect of annealing on microstructure, residual stress, and hardness of Al-Ti multilayered films

Abstract: Al–Ti multilayered films (12 at.% Ti) with bilayer period of 16 nm were deposited by magnetron sputtering. The films were annealed in vacuum at 350 or 400 °C between 2 and 24 h. During annealing, a diffusion-controlled chemical reaction between Al and Ti layers led to Al3Ti precipitation. Differential thermal analysis studies showed an exothermic reaction associated with Al3Ti formation, taking place between 320 and 390 °C, depending on the heating rate. The evolution of microstructure with annealing was examined with transmission electron microscopy and x-ray diffraction. The hardness and residual stress of the films in the as-deposited and annealed conditions were studied in relation to the microstructural changes on annealing.

Structure–property relationships during high temperature deformation of Cu–4·5Ti alloy

Abstract: A Cu–4·5Ti (wt-%) alloy was subjected to hot compression tests at temperatures ranging from 750 to 900°C and strain rates from 100 to 10-3 s-1 A hardness of 180 HV10 was observed after deformation at a low temperature of 750°C, at small as well as large strain rates, whereas the alloy deformed at high temperature (900°C) exhibited high hardness values, ie 300 HV10 at a strain rate of 100 s-1 and 280 HV10 at 103 s-1 Furthermore, a reduction in grain size from 110 µm at a strain rate of 103 s-1 to ∼55 µm at 100 s-1 was observed when the alloy was deformed at 900°C Optical and SEM observations revealed a lamellar structure at 750°C, while equiaxed grains with some lamellar growth were seen in the alloy deformed at 900°C, at both strain rates The difference in behaviour is attributed to the presence of equilibrium precipitate β-Cu3Ti at 750°C, whereas it is almost absent at 900°C The high hardness observed at 900°C is due to the formation of fine scale precipitation during air cooling of the d