Showing papers by "Defence Metallurgical Research Laboratory published in 2000"

Texture and mechanical property anisotropy in an Al–Mg–Si–Cu alloy

Abstract: This work describes the correlation of microstructure–texture and texture–mechanical properties of a hot rolled Al–Mg–Si–Cu alloy in T-6 condition. Texture measurement was carried out on both the surfaces and mid thickness levels of the specimen using conventional pole figure as well as orientation distribution function (ODF) methods. Tensile properties and fracture toughness of the alloys were evaluated at different orientations with respect to the rolling directions. High cycle fatigue properties were evaluated in the rolling direction. From these observations, the in plane anisotropy factor (%IPA), K IQ values and fatigue life were calculated and correlated with textures and microstructures/fracture features of the alloy.

Effect of prealuminizing diffusion treatment on microstructural evolution of high-activity pt-aluminide coatings

Abstract: The effect of prealuminizing (or prior) diffusion treatment on the evolution of Pt-aluminide coatings on the Ni-based superalloy CM-247 has been studied by using a single-step, high-activity aluminizing process. Coatings generated without any prior diffusion treatment, as well as those formed by adopting two extreme prior-diffusion schedules (at 850 °C for 0.5 hours and at 1034 °C for 5 hours), were investigated by analyzing the coating structures at various stages of aluminizing. When the dilution of the Pt layer, caused by its interdiffusion with the substrate during the prior diffusion treatment, is only marginal (as in the case of no prior diffusion and diffusion at 850 °C for 0.5 hours), the equilibrium Pt-aluminide coating structure evolves through the formation of two transient layers during the initial stages of aluminizing. In contrast, for diffusion at 1034 °C for 5 hours, which results in extensive dilution of the Pt layer, the two-phase equilibrium structure (PtAl2 in a matrix of NiAl) in the outer layer of the coating is found to develop during very early stages of aluminizing and remains unchanged, even over extended periods of aluminizing. Further, in the case of prior diffusion at 1034 °C for 5 hours, Pt is found to remain distributed to a greater extent over the entire thickness of the coating than in the cases of limited prior diffusion treatment. The present findings underline the significance of the nature of the prior-diffusion schedule on the microstructural evolution of Pt-aluminide coatings. It has also been found that a prealuminizing diffusion treatment is particularly important for Pt-aluminide coatings from the point of view of coating adhesion to the substrate.

Hot Corrosion of Protective Coatings

Abstract: Improvement in efficiencies of gas turbine engines requires a significant increase of gas inlet temperatures. This results in an increased service temperature for blade materials and consequently in enhanced oxidation and hot corrosion attack of the blade coatings, which are usually of MCrAlY type where M is Ni, Co or NiCo. This type of coating can provide protection against oxidation and hot corrosion and act as a bond coat for thermal barrier coating systems. In both cases slow growth rates and optimum adherence of the alumina scales forming on the MCrAlY coatings during high temperature exposure are significant for component life. The above mentioned properties for the alumina scales strongly depend on the coating base composition as well as on the presence of minor alloying elements. In the present paper the performance of existing superalloys during hot corrosion is briefly described followed by the results obtained on hot corrosion of MCRAlY type coatings explaining the effect of t...

A simple model for viscosity of powder injection moulding mixes with binder content above powder critical binder volume concentration

Abstract: A simple model is proposed for the variation of viscosity of powder injection moulding mixes with binder content above Critical Binder Volume Concentration (CBVC), a characteristic of powder. Coarse silica and fine alumina powders are mixed with different types of low viscosity binders at different binder proportions. The viscosities of these mixes are measured by capillary rheometry with a special die at different shear rates. The CBVC values of the powders estimated by the model from the viscosity data of the mixes are matching with those values arrived from the experimental torque rheometry and temperature measurement methods. The proposed model has shown excellent correlation between the calculated and experimentally measured viscosity data at different shear rates, binder contents with different binders and powders. The model also gave precise CBVC values with the viscosity data of the previous authors validating the model to other powders and binders. The model is useful for the prediction of viscosity of PIM mix with known binder content, viscosity of binder and CBVC of the powder.

Effect of volume fraction of SiC p reinforcement on the processing maps for 2124 Al matrix composites

Abstract: The hot working characteristics of 2124 Al alloy matrix composites reinforced with 0, 5, 10, 15, and 20 vol pct of SiC particulate, produced by the powder metallurgy route, were studied using processing maps. The maps based on the dynamic materials model were generated from the flow stress data obtained from hot compression tests, carried out at strain rates ranging from 0.001 to 10 s−1 and temperatures ranging from 300°C to 525°C. All the compositions studied exhibited domains of dynamic recrystallization (DRX) and superplasticity. Flow instabilities were found at higher strain rates and lower temperatures. The composite with 10 vol pct SiC showed a tendency for abnormal grain growth at lower strains, which manifested itself as a shift in the DRX domain to lower strain rates and the disappearance of the superplasticity domain.

On the dynamic embrittlement of copper-chromium alloys by sulphur

Abstract: The high temperature ductility of copper-chromium alloys has been examined in the temperature range of 25–700°C. Copper-chromium alloys exhibit severe intermediate temperature loss in ductility. The loss in ductility is associated with the ingress of sulphur along the grain boundaries under the influence of tensile stress, followed by decohesion of grain boundaries. Integranular facets of the fracture surface exhibit striations, indicative of quasi-static, step-wise crack growth process.

Study of plasma- and detonation gun-sprayed alumina coatings using Taguchi experimental design

Abstract: Atmospheric plasma spraying (APS) is a most versatile thermal spray method for depositing alumina (Al2O3) coatings, and detonation gun (D-gun) spraying is an alternative thermal spray technology for depositing such coatings with extremely good wear characteristics The present study is aimed at comparing the characteristics of Al2O3 coatings deposited using the above techniques by using Taguchi experimental design Alumina coating experiments were conducted using a Taguchi fractional-factorial (L8) design parametric study to optimize the spray process parameters for both APS and D-gun The Taguchi design evaluated the effect of four APS and D-gun spray variables on the measured coating attributes The coating qualities evaluated were surface roughness, porosity, microhardness, abrasion, and sliding wear The results show that the coating quality is directly related to the corresponding coating microstructure, which is significantly influenced by the spray parameters employed Though it is evident that the D-gun-sprayed coatings consistently exhibit dense and uniform microstructure, higher hardness, and superior tribological performance, the attainment of suitable plasma-sprayed coatings can be improved by employing the Taguchi analysis

Microstructure and mechanical properties of friction welds of an α + β titanium alloy

Abstract: The mechanical properties of friction welds of an α+β titanium alloy (Ti–6.5Al–1.9Zr–3.3Mo–0.25Si) were evaluated in the stress relieved and in the post weld heat treated (PWHT) conditions to understand the effect of post weld heat treatments on the microstructure and mechanical properties. Stress relieved welds exhibited mechanical properties comparable to the base metal except impact toughness. The impact toughness was ∼65% of the base metal. This was mainly due to a mixed microstructure of martensite and thin alpha+beta. Post weld heat treatment at 700°C that led to beta precipitation together with silicides exhibited poor impact toughness and trangranular fracture with shallow and under developed fine dimples. A PWHT at 960°C for 1 h followed by air cooling (AC) that led to the decomposition of α1 to equilibrium alpha+beta and coarsening of the transgranular alpha improved the toughness. This treatment improved all other properties. 960°C/1 h/water quenching (WQ) PWHT reduced the impact toughness and exhibited quasi cleavage fracture possibly due to α1 microstructure. Prolonged soaking at 960°C marginally reduced the toughness. This is thought to be due to a lean distribution of alpha consequent to its coarsening and possible compostional differences between the α and β phases that led to smooth and flat fracture in the transgranular locations. The smooth and flat fracture features were due to poor resistance of the microstructure to crack propagation.

Comparative Wear Behavior of Ceramic and Carbide Tools During High Speed Machining of Steel

Abstract: Two types of oxide-based ceramic cutting tools have been developed for high speed machining of hardened steel. These tools were made of alumina (A12O3) and zirconia toughened alumina (ZTA). Commercially available tungsten carbide (WC)-based tools were also used during machining for comparison. In general, ceramic tools exhibited superior performance as compared to the WC tools, especially at higher machining speeds, both in terms of tool life and surface finish of the work-piece. The worn-out tools were observed under a stereo-microscope for studying the role of different wear mechanisms on the tool life. While severe crater wear was observed in the WC tools, only a small amount of edge chipping and nose wear occurred in the ceramic tools during high speed machining. The correlation between the mechanical properties of the tool material, the tool lives and their wear behavior was also studied.

Processing Nickel Free High Nitrogen Austenitic Stainless Steels through Conventional Electroslag Remelting Process

Abstract: Nickel free high nitrogen austenitic stainless steels, made through air-induction melting were processed using conventional electroslag remelting (ESR) process without application of nitrogen gas pressure over the melt. It was found possible to retain the high nitrogen contents of the original steel. The loss in nitrogen content during ESR was found to increase with increasing melt rate. Electroslag remelting was carried out on eleven steels with a base composition at around 18wt%Cr–18wt%Mn–0.1 to 0.6wt%C–0.53 to 0.9wt%N. While the air-induction melted steel had extensive porosity, the ESR ingots were all sound and free from porosity. Thus, steels made in any other process route can be successfully remelted using conventional ESR. The cast structure analysis in a typical medium carbon high nitrogen steel showed that Cr and Mn has a tendency for microsegregation. The presence of microsegregation and residual carbides affect the ductility of the cast steel.

Influence of Thermal and Mechanical Processing on Room Temperature Mechanical Properties of Nickel Free High Nitrogen Austenitic Stainless Steels

Abstract: Nickel free high nitrogen austenitic stainless steels with a base composition around 18%Cr–18%Mn and varying in C and N contents and processing conditions were evaluated for their mechanical behaviour. A range of mechanical properties from high strengths to ultra high strength levels could be obtained. The steels develop ultra high strength levels in the as-hot worked condition, hot work1cold work condition and solution treated1cold worked conditions. They show high strength levels with very good ductility and impact toughness levels in the solution annealed condition. The Hall–Petch parameters and DBTT obtained in the solution annealed condition were found to be influenced by the carbon content of the steel. The work hardening behaviour of the steels have been examined at various processing conditions. Nitrogen steels with high carbon content show good mechanical properties in the solution treated and subsequently cold rolled conditions. In the as-formed condition, the high carbon steels show inferior ductility due to grain boundary lamellar nitride precipitation.

Evolution of texture in the β(B2) phase of a two phase titanium aluminide intermetallic alloy Ti-24Al-11Nb

Abstract: Evolution of texture in the high temperature β(B2) phase of the intermetallic alloy Ti-24Al-11Nb during thermomechanical processing has been studied. The temperature of rolling, amount of rolling deformation imparted, and the volume fraction of α 2 phase are found to influence the texture of the β phase. Textures observed generally do not correspond exactly to the commonly observed texture of bcc metals, alloys, or similar aluminide intermetallics.

Effect of carbon and processing on structure and mechanical properities of Fe-11 wt.% Al alloy

Abstract: Ingots of Fe–Al alloys containing 11 wt.% Al and 0.5 and 1.1 wt.% C were prepared by a combination of air induction melting with flux cover (AIM) and electroslag remelting (ESR). The ESR ingots were hot forged at 1373 K to 53% reduction and subsequently hot rolled at 1373 K to 53% rolling reduction. The cast ESR samples were placed in a hearth furnace at 873, 1073 and 1273 K for 24 h then furnace cooled to room temperature. The Fe-11 wt.% Al alloy containing 1.1 wt.% C exhibited a significant higher yield strength at test temperature up to 773 K than alloy containing 0.5 wt.% C. This may be attributed to the presence of large amount of hard Fe 3 AlC 0.5 precipitates in the alloy. The thermo-mechanical processing of cast ESR ingots has resulted in significant improvement in room temperature tensile ductility. This may be due to breaking of cast dendritic structure and more uniform distribution of Fe 3 AlC 0.5 precipitates. These high carbon Fe-11 wt.% Al alloys exhibited excellent resistance to decarburization up to 1273 K because of the formation of continuous protective Al 2 O 3 film on the surface. It would appear from the present work that thermo-mechanically processed high (1.1 wt.%) carbon ESR alloy exhibit an attractive combination of properties.

Effect of alloying additions on structure and mechanical properties of high carbon Fe-16 wt.% Al alloy

Abstract: Effect of quaternary alloying elements Mn, Cr, Ni and Ti on structure and properties of Fe 3 Al-based alloy containing about l wt.% carbon have been investigated. Four different alloys were prepared. The composition of the quaternary alloying element was proposed to be ≈4 wt.% and was substituted for iron. Processing of Fe-16Al-4.1Mn-1.0C, Fe-16.5Al-3.5Cr-0.94C, Fe-16Al-4.0Ni-0.9C and Fe-15.6Al-2.8Ti-1.0C alloys through a combination of air induction melting with flux cover (AIM) and electroslag remelting (ESR) yields a sound ingot free from macro and microporosity with very low sulphur, oxygen and nitrogen. This process route also exhibited excellent recovery of alloying elements. As-cast alloys were examined using optical microscopy, X-ray diffraction, electron probe microanalyses (EPMA) and scanning electron microscopy (SEM) in conjunction with energy dispersive X-ray analysis to understand the microstructure of these alloys. The as-cast ESR ingots of alloys containing Mn, Cr and Ni exhibited a two-phase structure of Fe 3 AlC 0.5 precipitate in the Fe 3 Al-based matrix. Both phases exhibited considerable amount of solid solubility for Mn, Cr and Ni, whereas the alloy containing Ti exhibited a three-phase microstructure of TiC particles and Fe 3 AlC precipitates in the Fe 3 Al-based matrix. This alloy has also exhibited very low solubility of Ti in the Fe 3 Al-based matrix and no solubility in the Fe 3 AlC precipitates. Several microcracks were observed in the as-cast ESR ingots of the high carbon Fe 3 Al alloy containing Ni and tensile tests could not be carried out for this composition. Tensile and creep tests were performed on the high carbon Fe 3 Al alloys containing Mn, Cr and Ti in the as-cast condition. No improvement in room temperature tensile strength and inferior high temperature strength and creep properties was observed by the addition of quaternary alloying elements.

Effect of post-weld heat treatment on fracture toughness and fatigue crack growth behaviour of electron beam welds of a titanium (α+β) alloy

Abstract: The effects of a post-weld heat treatment on the fracture toughness and fatigue crack growth behaviour of electron beam welds of an α + β titanium alloy, Ti–6.5Al–1.9Zr–0.25Si have been studied. Welds in the stress-relieved condition exhibited poor fracture toughness due to poor energy absorbing capacity of the thin α and α′ phases. Post-weld heat treatment which resulted in the decomposition of α′ to α + β and the coarsening of intragranular and intergranular α resulted in improved toughness. This improvement in the toughness is related to improved ductility leading to crack blunting, crack path deviation at the thick intragranular and intergranular α phase. Fatigue crack growth resistance of welds was superior to the base metal in the α + β heat-treated condition. The superior crack growth resistance of the welds is due to the acicular α microstructure which results in a tortuous crack path and possible crack closure arising from crack path tortuosity.

Effect of carbon content on elevated temperature stability and tensile properties of Fe–8.5 Al Alloys

Abstract: Fe–8.4Al–0.04C, Fe–8.26Al–0.46C and Fe–8.35Al–1.1C alloys were prepared by a combination of air induction melting and electroslag remelting. The low (0.04 wt.%) carbon alloy exhibited microcracks therefore it was not studied further. The ESR ingots of high (0.46 and 1.1 wt.%) carbon alloys exhibited a significant amount of Fe 3 AlC 0.5 precipitation. The cast ESR samples of Fe–8.26Al–0.46C and Fe–8.35Al–1.1 C were placed in a hearth furnace at 873, 1073 and 1273 K for 24 h and then furnace cooled. The high carbon alloys do not undergo decarburization and exhibit stable microstructure up to 873 K. Decarburization appears to be a problem only after exposure at temperatures of 1073 K and above. The ESR Fe–8.26Al–0.46C alloy exhibited greater elongation and significantly better elevated temperature strength up to 873 K than those reported for cast VIM low carbon multicomponent alloy with similar Al content. This may be due to the presence of a large volume fraction of stable Fe 3 AlC 0.5 precipitates. All the high carbon alloys exhibit a sharp drop in strength at 873 K regardless of carbon content. These alloys are therefore targeted for potential structural application at or below 873 K.

Effect of MoSi2 and Nb reinforcements on mechanical properties of Al2O3 matrix composites

Abstract: The room temperature mechanical properties of Al2O3 composites reinforced with 25 vol% of either MoSi2 or Nb particulates were investigated. It was found that addition of Nb particles resulted in a reduction in the elastic modulus, but caused a significant increase in both flexural strength and fracture toughness. On the other hand, the addition of MoSi2 particles resulted in only a marginal decrease in elastic modulus and marginal increase in both flexural strength and fracture toughness. The elastic modulus results were explained on the basis of Tsai - Halpin model. For both the composites, the increase in flexural strength was attributed to the grain refinement of the Al2O3 matrix as well as the load transfer to the reinforcement particles. The marginal increase in fracture toughness in Al2O3 / MoSi2 composites was attributed to crack deflection, whereas the threefold increase in fracture toughness in Al2O3 / Nb composites was attributed to crack blunting and bridging.

Influence of Hardness on Perforation Velocity in Steel Armour Plates

Abstract: In an earlier investigation3, the influence ofh'ardness on tempered steel armour plates of 20 mm thickness, impacted by. 20 mm diameter steel ogive-shaped projectile at normal , was studied. Additional data is investigated with relation to the perforation velocity of the plates. It is observed that the plate perforation velocity and the plate plugging velocity decrease with increasing plate hardness.

The effect of high-temperature oxidation on the creep behavior of a superalloy (NIMONIC-105)

Abstract: Enhanced creep damage occurring in a NIMONIC 105 superalloy at 750 °C due to prior heat treatment at 1150 °C was investigated. The medium of heat treatment and creep testing was air. Scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) examination indicated that the heat treatment resulted in a surface region consisting of oxide casing, γ′ depleted zone, and grain boundary oxides. Rule of mixture-based mathematical analyses and experiments revealed that this region (referred to here after as the oxygen-affected zone (OAZ)) extended much deeper than that perceived by the SEM and EPMA observations. The formation of OAZ reduced the creep ductility drastically while its influence on the minimum creep rate was low. The probable mechanisms affecting the secondary and tertiary regimes are discussed. Prolonged heat treatments did not damage the creep properties any more significantly than the shorter heat treatments, indicating that all the important structural variations occurred during the initial stages of exposure.

Influence of mechanical processing and heat treatment on microstructure evolution in nickel free high nitrogen austenitic stainless steels

Abstract: A series of nickel free high nitrogen austenitic stainless steels were processed by various mechanical processing and heat treatment conditions to study the evolution of microstructure at every stage of processing. About eleven steels varying in composition, especially in terms of carbon contents, were processed through forging, rolling, solution annealing, cold rolling and warm rolling etc. The microstructure at every stage of processing was studied. The hot rolled condition, hot rolled and cold rolled condition gave deformation induced fragmented grain structure. The high carbon steels in the hot rolled conditions showed banding and grain boundary carbo-nitride precipitation. There were also intragranular carbide prcipitation in some of the high carbon steels.The solution treated microstructure in all steels showed extensive annealing twins and the lattice parameter increased with carbon and nitrogen contents. Cold rolling followed by solution treatment lead to development of slip lines in the fully austenitic matrix due to strain hardening of the matrix. The 600°C warm rolled condition showed grain boundary carbide precipitation.

Nature of Precipitates in Peakaged and in Subsequently Crept Al-Ge-Si Alloy

Abstract: Transmission electron microscopy (TEM) and X-ray diffraction (XRD) have been used to study the morphology and the lattice parameters, respectively of the diamond cubic SiGe precipitates that are present in peak aged, and in subsequently crept samples of a ternary Al-Ge-Si alloy. TEM reveals that there exist variations in the morphology of the SiGe precipitates within the microstructures under all conditions, and that nucleation of SiGe precipitates occurs in the alloy during creep. X-ray diffraction studies reveal that the lattice parameter of the SiGe precipitates present in the peak aged alloy does not change due to either over ageing or subsequent creep deformations, whilst, formation of SiGe precipitates having a different lattice parameter is detected under certain creep conditions.

Sequence of Precipitation of T2 and δ Phases during Ageing of Al-Li Alloy 8090C

Abstract: The precipitation of two dominant equilibrium phases viz. T 2 (Al 6 CuLi 3 ) and δ (AlLi) in an Al-2.2Li-1.0Cu-0.7Mg-0.04Zr (wt.%) base 8090 C alloy has been studied. The sequence of precipitation of T 2 and 8 during ageing at 190°C was established. Further, the preferred nucleation sites for the equilibrium phases were identified for different ageing times. Faceted precipitates of β (Al 3 Zr) were shown to nucleate T 2 which, in turn, nucleated δ.