1,721,076 research outputs found
Dynamic Strain Aging and Creep in near-α Ti Alloy, IMI834
No full textIMI 834 is a near- Ti alloy used in high temperature applications such as compressor discs and blades of aero engines. Titanium alloys including IMI 834 are known to exhibit dynamic strain aging (DSA) over the temperature range of 623-773K. However, the interplay between DSA and creep has not been previously studied in detail in titanium alloys. The objective of the present study is to probe creep mechanisms and phenomenology at intermediate temperatures in IMI 834 where DSA is expected to play a prominent role.
Coupons of IMI834, cut from a hot rolled rod, were heat treated to obtain a microstructure of 80% equated , 15% of lamellar and 5% . The heat treatment was optimized to ensure Si retention in the solution as it is known to affect DSA in titanium alloys. Constant strain rate tests carried out in tension over a range of temperatures and strain rates identified the DSA regime in the temperature range of 623-823K as determined by serrated behavior in stress-strain curves and negative strain rate sensitivity. The dislocation structure in this domain is dominated by jogged screw dislocations in slip bands. Strain accumulation is shown to depend on conservative jog glide along the length of screw dislocations due to line tension forces.
We have tried to predict this domain of DSA using Friedel’s model for breakaway stress, i.e. the stress required to break free the dislocations from solute atmosphere, using estimates of solute concentration accumulating at the edge jogs on arrest during thermally activated glide of the jogs at static solute obstacles. There is a good agreement between model and experimental data showing a DSA peak in the temperature range of 673-723K. The solute species responsible for DSA is estimated to be Si and C, but dominated by C in this temperature and strain rate regime.
Creep behavior of the alloy was explored over similar temperature regime in tension. Over this temperature range, especially at lower temperatures, very low strain rates below the resolution limits of the strain gauge were reached before the onset of steady state and therefore current work is primarily focused on primary creep behavior. Different types of primary creep behavior were observed with temperature and stress. Anomalous primary creep behavior has been observed in the form of an abrupt decrease in strain rate beyond certain critical strains, as well as in a stress insensitivity of instantaneous strain rates on loading at 673K. At higher temperatures of 773K, conventional behavior showed strain rates approaching a steady state value. Dislocation structure in primary creep was again dominated by jogged screw dislocations. At the strain rates associated with creep, our model shows that transitions occur in jog glide from solute breakaway controlled glide to solute drag controlled glide and then again to thermally activated glide over static solutes with increasing temperature. However, in contrast to high strain rate behavior, Si solutes dominates the mechanisms of jog glide in creep.
The work of this thesis has established that a continuum of dislocation mechanisms based on jogged screw dislocation glide determine mechanical behaviour over a wide range of strain rates ranging from 10-2 to 10-9 s-1 and temperatures from 623-773K in engineering titanium alloys
An Evaluation of the Mechanical Behavior of some new High Temperature Materials
No full textTwo new high-temperature alloy systems have been discovered recently at the Indian
Institute of Science, which form the basis of this thesis. The first alloy system is an
Intermetallic-Intermetallic eutectic composite alloy in the nickel-rich end of the Ni-Al-
Zr ternary system, first reported by Tiwary et al. These eutectic composites have very
impressive room temperature yield strengths of about 2 GPa which are retained till
about 700 . Coupled with 2 to 5% tensile plasticity at room temperature, excellent
high-temperature oxidation resistance, good long-term microstructural stability and very
low densities of about 7.35–7.95 gm/cm3, these materials are exciting candidates for
high-temperature applications.
The second alloy system comprises of Tungsten free Cobalt based superalloys having
the classical
−
0 microstructure similar to that of nickel-based superalloys as described
by Makineni et al. The presence of the L12 phase in cobalt-based systems was reported
by Lee et al. and Sato et al. where the addition of 25 wt.% tungsten seemed to stabilize
the metastable Co3Al. This however pushed the density of the alloy to about 9–10
gm/cm3, making it too heavy for most high-temperature applications. A large body of
work followed trying to reduce or eliminate the presence of tungsten in these alloys. The
new alloys by Makineni et al. do not contain tungsten which reduces the density to about
8.0–8.4 gm/cm3.
The high-temperature mechanical behavior of some of the alloys from these two sysi
tems has been evaluated in the current thesis. This thesis is divided into six chapters
Effect of Thermomechanical Processing on Microstructure And Microtexture Evolution in Titanium Alloys
Full text linkThe properties of titanium alloys are based on alloy compositions and microstructures that consist of mixtures of the two allotropic modifications of titanium, the low temperature α (hcp) and the high temperature β (bcc) phases. This thesis deals with the hot working behaviour of three commercial titanium alloy compositions designated IMI834, Ti17 and Ti5553 with a focus and detailed analysis of the Ti5553 alloy. These alloys represent the differing uses of titanium alloys in the aerospace industry. IMI834 is a near α alloy used in high temperature creep resistant applications as compressor discs and blades in aeroengines. Ti17 is a high strength alloy α+β used at intermediate temperatures in fan and compressor discs of aeroengines, while Ti5553 is a high strength-high toughness metastable β alloy used in the undercarriages of aircraft. The three alloys have widely differing β transus temperatures (related to α phase stability) and compositions. Titanium alloys are vacuum arc melted and thermomechanically processed. This process involves ingot breakdown in β (bcc) phase, and subsequent thermomechanical processing in two-phase α+β (hcp+bcc) region at temperatures that typically involve volume fractions of α in lath or plate form ranging from 15% to about 30%. The thermomechanical processing breaks down lath α to spheroidal particles, a process known as globularisation. Chapter I of this thesis reviews the current understanding of the hot working of titanium alloys and microstructure evolution during the hot working process. Chapter II summarises the main experimental techniques used: the hot compression test, and subsequent microstructure and microtexture analysis by scanning electron microscopy and related electron back scattered diffraction techniques (EBSD), transmission electron microscopy and related precession electron diffraction techniques (PED) for orientation imaging.
The starting structure in the α+β domain of hot work is generally not a random distribution of the 12 variant Burgers Orientation Relationship (BOR) between the α and β phases, (11̅0)β || (0001)α and β || α . A variety of morphologies and distributions ranging from the typical colony structures of near α and α+β alloys to the fine distributions of variants arranged in a triangular fashion are observed with specific growth directions and habit planes. Chapter III describes a quantitative evaluation of α distribution that are typical of some of the starting structures for the hot working conditions used in this thesis, specifically in the Ti5553 alloy. For this purpose, a Matlab based script has been developed to measure the spatially correlated misorientation distribution. It was found that experimental spatially correlated misorientation distribution varies significantly from a random frequency for both pair and triplet wise distribution of α laths. The analysis of these structures by established techniques of analysis of self-accommodated structures based on strain energy minimisation shows that the observed variant distribution arise from the residual strain energy accommodation of the semi-coherent α plates.
The hot working process has been examined through hot compression tests of the 3 alloys at strain rates ranging from 10-3 s-1 to 10 s-1 over a temperature range designed to maintain constant volume fractions of the α and β phases during deformation ranging from about 30% α to a fully β structure. Since extensive prior work has been carried on the processing of titanium alloys, Chapter IV focuses on a comparative study of hot deformation behaviour of the three alloys with an emphasis on isolating microstructural and other effects. The macroscopic flow behaviour has been analysed in terms of conventional rate equations relating stress, strain, strain rate and temperature. The three alloys show very similar features in their stress-strain behaviour. β phase deformation exhibits yield points whose magnitude varies with strain rate and temperature. The flow stress curves are typical of materials undergoing dynamic recovery and recrystallization processes. The stress-strain behaviour in the α+β domain of hot work exhibits significant flow softening in the early stages of deformation with a subsequent approach to steady-state behaviour at true strain of about 0.5. Activation energy analysis of the steady state condition suggests that the rate controlling mechanism is related to recovery in the β phase in both α+β and β processing. Zener-Hollomon plots of the flow stress in the three alloys indicate that their flow stress can be normalized to a temperature-compensated strain rate and they differ only in the slopes of the plots that are related to the stress exponent. Empirical constitutive models were developed for a predictive understanding of the flow stress as a function of strain, strain rate and temperature using conventional rate equations for the flow stress
Chapter V and VI examine the evolution of microstructure and microtexture in detail during hot deformation and subsequent heat treatment in Ti5553. A combination of EBSD (micron and submicron scale) and PED (nano meter scale) is used in orientation imaging to examine the globularisation process of the α phase and the recovery and recrystallization in the β phase in both supertransus and subtransus hot compression. The understanding of these processes is enhanced by tracking the same starting β grain through the deformation process. The effect of strain, strain rate and temperature on the evolution of subgrains in α and its fragmentation into spheroidal α is quantified. In the absence of shear bands, the globularisation process is seen to evolve from a strain driven Raleigh instability of the plate α, by subgrain formation in α and β phases. The related microtexture evolution is analysed. The analysis of recovery and microtexture evolution in the β phase described here has not been attempted earlier in the literature. The overall evolution of structure and texture is seen to result from the complex interplay between recovery and recrystallisation in the α and β phases in substranus deformation.
While the Burgers orientation relationship between α and β is lost in the early stages of deformation, it appears to be restored at large strains as a consequence of ‘epitaxial’ recrystallisation processes that seem to result from the discontinuous nucleation of recrystallization of either phase at interphase interfaces in the Burgers orientation. The effect of substranus deformation on β texture following supertransus post deformation heat treatment is also examined and compared with β textures resulting from alternative strain paths such as friction stir processing.
Finally Chapter VII summarises these results and the new insights into the evolution of structure and microtexture during hot deformation of titanium alloys and suggests directions for future work
Microstructure and Shape Memory Behavior of quaternary Ni(50-x)-(Pt/Pd)x-Ti(50-y)-(Hf/Zr)y alloys
No full textShape memory alloys (SMA) are those class of alloys that can cause shape memory, pseudoelastcity and two-way memory effects due to reversible austenite to martensite phase transformations. The NiTi shape memory alloys exhibit large recovery strains, however, the transformation temperatures of these alloys are less than 100 degree Celsius and these alloys exhibit low detwinning stresses/transformation stresses. Detwinning stress is the stress at which detwinning of martensite variants is initiated and transformation stress is the stress at which austenite to martensite transformation starts to occur at any temperature. Ternary additions such as Hf, Zr, Pt, Pd, Au etc. increase the transformation temperatures and detwinning stresses/transformation stresses of NiTi alloys. Though Hf and Zr are cheaper than the rest, NiTiHf alloys exhibit brittleness, large thermal hysteresis, poor shape memory and superelastic behaviour. The poor shape memory behaviour of Ti rich NiTiHf alloys has been attributed to the presence of (001)B19’ compound twins in its substructure, low matrix strength and occurrence of plastic deformation along with detwinning of martensite variants/stress induced martensite transformation. It has been reported that among the ternary elements Hf, Zr, Pt, Pd and Au, Ni substitution by Pt results in highest transformation temperatures in NiTi alloys. The effect of Pt additions on the microstructure and shape memory behaviour of ternary NiTiHf alloys has therefore been studied in this thesis, along with some compositions with Zr/Pd additions. The compositions chosen are based on Pt substitution for Ni in the Ni(Ti40Hf10) base composition and Ni(Ti30Hf20) base composition as well the (Ni45Pd5)(Ti30Hf20) and (Ni40Pt10)(Ti35Zr15) compositions. Microstructure and shape memory behaviour of the extensively evaluated binary Ni50Ti50 and ternary Ni50(Ti40Hf10) alloys were also included for comparison since shape memory behaviour is sensitive to alloy preparation techniques.
Chapter 1 provides an overview of the shape memory effect, characterization techniques used to determine shape recovery ratio, the crystallography of martensitic transformation in NiTi, uniaxial deformation and indentation studies performed on NiTi, and shape memory studies performed on NiTiHf, NiTiZr, and NiPdTiHf alloys.
Chapter 2 describes the characterization techniques used to study the microstructure, structure, substructure, transformation temperatures, shape memory effect and pseudoelastic effect of the compositions studied in this thesis. Microstructure was examined by optical and scanning electron microscopy. Microsegregation in cast microstructures was determined by electron probe microanalysis. X ray diffraction both at room temperature and high temperature was used to determine the lattice parameters of the major constituent phases, that is the monoclinic B19’ martensite and parent B2 phase. Differential scanning calorimetry was employed to determine transformation temperatures. Substructure of martensite was examined by transmission electron microscopy. Along with constant strain rate compression tests, Vickers and spherical microindentation studies were also performed to determine mechanical properties. Compression tests were performed to determine the detwinning stress, transformation stress above Af temperature, shape recovery ratio of alloys containing the martensite phase at room temperature, and pseudoelastic strains for alloys with the parent B2 phase at room temperature. The shape recovery ratio and depth recovery ratio of B19’ containing alloys were determined using the values of sample dimensions and indentation depths, respectively, determined before and after heating above Af temperatures. An AFM operated in contact mode was used to measure the indentation depths. Depth recovery ratio and energy recovery ratio in pseudoelasticity of B2 containing alloys at room temperature were determined from spherical and Vickers microindentations from the load-displacement profiles obtained during indentation and unloading.
Since the alloys evaluated in this thesis have not been studied earlier in the literature, a detailed study of the microstructure of cast and homogenized structures of all the alloys are described in Chapter 3. The extent of micro-segregation in cast microstructures and the composition of secondary phases was determined. Partial substitution of Ni by Pt/Pd in NiTi(Hf/Zr) resulted in substantial reduction in transformation temperatures. The maximum theoretical shear strain associated with austenite to martensite transformation, calculated using the values of lattice parameters of B19’ and B2 phases. was found to increase with increase in Hf and Pt additions. (001)B19’ compound twins were present in the substructure of martensite of ternary Ni50(Ti30Hf20) and quaternary Pt containing (NiPt)(Ti30Hf20) alloys. The chapter concludes with a discussion on factors affecting transformation temperatures.
Chapter 4 describes the shape memory behaviour of those alloys that contain the B19’ martensite at room temperature, that is Ni50Ti50, ternary Ni50(Ti40Hf10) and Ni50(Ti30Hf20) alloys, quaternary (Ni(50-x)-Ptx)(Ti30Hf20) (x = 5 and 10 at. %) alloys, and the quaternary (Ni45Pd5)(Ti30Hf20) alloy. The detwinning stress at room temperature was found to increase with Pt/Pd and Hf addition, and significant hardening was observed in the room temperature stress strain behaviour of ternary NiTiHf alloys and quaternary Ni(Pt/Pd)TiHf when compared to Ni50Ti50. The shape recovery ratio at a constant plastic strain of 2% did not change with the addition of 10 at. % Hf to Ni50Ti50 but decreased with further addition of Hf. While the addition of Pt did not alter the shape recovery ratio of the ternary Ni50Ti30Hf20 alloy, Pd addition resulted in improvement of shape recovery ratio. The Vickers indentations were made to a constant load of 300 mN and spherical indentations were made to a depth of 0.5, 1, 1.5 and 2 μm. The recovery determined by these different techniques are assessed and the factors affecting stress plateau and recovery ratios are discussed.
The pseudoelasticity behavior of alloys with parent B2 phase at room temperature, namely (Ni(50-x)-Ptx)(Ti40Hf10) (x = 5 and 10 at. %), (Ni40Pt10)(Ti35Zr15) and (Ni35Pt15)(Ti30Hf20) alloys is described in Chapter 5. The maximum stress at which pseudoelasticity is exhibited increases with increase in Hf, Pt or Zr addition. The temperature range over which these alloys show pseudoelastic behaviour has been determined through compression tests. The transformation stresses for the alloys were determined as a function of temperature up to Md and these stresses increase with temperature in a non-linear fashion. The pseudoelastic strain of the alloys was determined at two deformation stresses at room temperature in compression. Ni35Pt15Ti30Hf20 exhibited the lowest values and fractured at a stress of 2.3 GPa. Microindentation studies show that Ni35Pt15Ti30Hf20 alloy exhibited the lowest pseudoelastic recovery ratio while Ni45Pt5Ti40Hf10 exhibited the highest recovery ratio at room temperature. It was found that the trend in variation of pseudoelastic recovery with alloying additions has an inverse relationship to trend in variation of slope of stress plateau and dissipation energy (hysteresis) with alloying additions. The factors that affect transformation stresses, stress plateau, and pseudoelastic strains are discussed.
A summary of the thesis and suggestions for future work are provided in Chapter 6
Microstructural Evolution of Directionally Solidified Ni-Al-Re/Ru Alloys
No full textNickel based superalloys are high temperature materials which find application as blade and disc components of jet engines due to their ability to retain their mechanical properties at temperatures close to 0.8 of their melting point. The modern day nickel based superalloys are multicomponent alloys, containing up to 12 alloying elements which are added in balanced proportions to achieve high temperature strength and microstructural stability, Re and Ru have been the recent additions made to superalloy compositions in the last two decades. But the roles of Re and Ru in terms of partitioning among the phases γ and γ’ and its effects on mechanical properties are still not clear. Hence an effort was made to study their effects under constrains of constant volume fractions of γ’ and misfit between γ and γ’, to isolate effects that result purely due to composition. Three model alloy systems (two ternary and one quaternary) of the type Ni-Al-xRe, Ni-Al-yRu and Ni-Al-xRe-yRu, x=2.5/4 and y=2.5/6 with minor additions of 1.4 Hf and 0.07 C for reasons of castability were synthesized by investment casting. This thesis deals with the solidification related aspects of these alloys.
Chapter 1 deals with a brief overview of Ni based superalloys in terms of their physical metallurgy, processing, microstructure and the effects of Re and Ru studied in literature. These are summarized and the objective of thesis laid out. The experimental details of the techniques employed for studying the cast structure such as optical microscopy, scanning electron microscopy, electron probe microanalysis and differential scanning calorimetry are discussed in Chapter 2
In Chapter 3, the experimental results concerning microstructure, chemistries (local and global), differential scanning calorimetry and supporting Thermo-Calc based simulations are presented. The composition of the directionally solidified alloys were estimated using quantitative EPMA spot analysis after homogenizing them to the best possible extent. The cooling rates experienced by the bottom, middle and tip sections (25mm, 150mm and 295mm respectively from chill plate) of the model alloys during casting was estimated using ProCASTTM simulations. The simulations show the cooling rates to vary from 12.5 K/min to 6 K/min, corresponding to the range of thermal gradients 3000 K/m to 1500 K/m from bottom to tip sections. These estimations were performed assuming a constant withdrawal velocity of 24 cm/hr.
The morphological evolution of cast structure from bottom to tip section of the alloys was studied at a coarse scale with the aid of optical microscopy. A tendency towards increased branching of dendrites to form well developed tertiary arms is observed on moving from bottom to tip sections. Quantitative measurement of primary and secondary arm spacings (PDAS and SDAS) indicate that PDAS values increase from bottom to middle section and then saturates, whereas the SDAS values show an increase from bottom to tip sections. Also, the PDAS and SDAS values of MA-3 were observed to be larger than that of MA-2 and 1 for a given section. Another observable feature at this coarse scale is the evolution of inter-dendritic area fractions, which was observed to decrease from bottom to tip sections. The inter-dendritic areas were also observed to be less interconnected on moving from bottom to tip sections. Also the area fraction of inter-dendritic areas were found to be lesser in MA-3 compared to MA-1 and 2.
Scanning electron microscopy (SEM) operated in the BSE mode reveals finer scale details observed within the dendrites and in the inter-dendritic areas. The γ’ nodules formed within the dendrites through solid state reaction were found to increase in size on moving away from the dendrite core towards the tip of the dendrites. The inter-dendritic areas were observed to form a fan like structure consisting of γ/γ’ lamellae showing a variation of γ’ size from fine to coarse depending on its location within the inter-dendritic region. The inter-dendritic regions also consisted of bright phases such as carbides, which were blocky in case of MA-1and in case of MA-2 and 3 it was found to be script like. The other bright phase observed was one consisting of alternate bright and dark contrast when viewed at higher magnifications. This phase was absent in the bottom section of the model alloys.
An estimate of composition of features observed in the cast structure was made using electron microprobe analyses, operated in spot analysis mode and X-Ray mapping mode to generate quantitative and qualitative data. Re, Ni and Ru partition to dendrite core, whereas Al and Hf partition to the liquid or inter-dendritic regions. The bright eutectic phase to be rich in Ni and Hf and also in Ru in case of MA-3. Compositional data from single and multiple dendrites collected in the form of a grid of spot analyses points were used to build iso-composition contour maps of elements. These maps provide complementary visual evidence of elemental partitioning, and they also reveal the anomalous distribution pattern of Re, Ni, Al and Hf. It is observed that Re and Ni are found to be enriched at the secondary dendrite arm to the same extent as at the core of dendrites, similarly Al and Hf are found to be depleted in these locations. Thermo-Calc based simulations were used to estimate the local melting temperatures from the EPMA composition contours, the contours of melting points complement the compositional data. The data were analyzed to obtain the variation of composition with fraction solidified.
The thermo-physical properties of the model alloys were determined through differential scanning calorimetric (DSC) studies. The estimated thermo-physical properties such as liquidus, incipient melting temperature, dendrite melting temperature and temperature for complete solidification of last liquid show an increase in value with Re addition the carbide formation temperature was observed to be the same for MA-1, 2 and higher than that of MA-3. The DSC method was also made use of to generate specific heat (Cp) vs temperature curves, through which the fraction solidified as a function of temperature during cooling was determined. These tests were performed at a heating/cooling rate of 12.5 K/min to simulate the solidification conditions of bottom sections. The Cp vs T curves indicate a decrease in amount of last liquid to solidify from MA-1 to 3.
In Chapter 4 the observations made in Chapter 3 are correlated with the help of Thermo-Calc based simulations. The efforts to predict critical transformation temperatures such as liquidus, incipient melting and final solidification temperature using Thermo-Calc resulted in an overestimation of these values by the software. However, the trends were predicted correctly. A comparison of the critical transformation temperatures with values reported in the literature concerning systematic studies on multicomponent Re/Ru alloys indicate the role of base composition on these effects. The estimated thermo-physical properties indicate the difficulty in homogenization of MA-1 due to its narrow homogenization window compared to MA-2 and 3. The EPMA results indicate that Re segregates strongly in comparison to the other elements.
Analysis of microstructural parameters such as PDAS and SDAS indicate the role of off-axial nature of heat flows to contribute to the development of secondary and tertiary arms far away from the bottom sections, where these effects are found to be prominent. These effect are thought to cause the saturation of PDAS from middle to tip sections. However, the SDAS is observed to depend only on local time for solidification which is found to increase linearly with decrease in cooling rate from bottom to tip sections. Also, among the model alloys the decrease in PDAS from MA-3 to 1 correlated well with the decrease in solidification range.
The thesis throws brings out some observed anomalies such as the saturation of PDAS from middle to tip section, the relation between well developed secondary and tertiary arms with reduction in eutectic volume fraction and the localized increase in Re, Ni and depletion of Al and Hf at the secondary arms. These observations provide scope for further investigation
TEM Characterization of Microstructural Evolution of GaN Grown by MOCVD on c-Sapphire
No full textIII-Nitrides (AlN, GaN, InN) find use in the high-power high frequency applications. Bulk GaN substrates are difficult to grow, therefore the GaN based structures must be grown on foreign substrates.
In this thesis, microstructural evolution of Gallium Nitride grown by Metal Organic Chemical Vapour Deposition (MOCVD) process on c-plane sapphire substrate has been systematically studied using transmission electron microscopy (TEM) and related spectroscopic techniques. The role of nitridation temperature, its effect on GaN film for the subsequent growth processes has been analyzed.
The first step in the growth process is the nitridation of the sapphire layer. It has been observed, that under low temperature (at 530oC) nitridation, an AlON complex with a cubic structure is formed and at higher nitridation temperatures (> 800oC), AlN with a cubic structure is formed. The crystallography and orientation of the nitrides are defined.
The low temperature GaN nucleation layer grown on these nitride layer templates have also been studied and it has been observed that zinc blende-GaN is the majority phase for low temperature nitridation and wurtzite-GaN forms for the higher temperature nitridation. The continuity, mosaicity, and crystallography of these low temperature GaN films have been characterized in the as grown and thermally annealed conditions.
Subsequent high temperature GaN films grown on these templates are found to contain Ga-polar GaN and N-polar GaN as the majority phases for the low and high temperature nitridation cases, respectively. Dislocations, stacking faults and inversion domains present in these high temperature GaN films affect the properties of the device as a whole, and have been studied. The differences in the defect structures in Ga-polar GaN and N-polar GaN epilayers are described
Microstructural, Mechanical and Oxidation Behavior of Ni-Al-Zr Ternary Alloys
Full text linkThe thesis introduces a novel alloy system based on submicron distributions of intermetallic phases realised through eutectic solidification in the ternary system Ni-Al-Zr. Various compositions in this system comprising of intermetallic phases distributed in different eutectic structures show ultra-high strength at temperatures upto 700°C combined with reasonable tensile plasticity, exceptional oxidation resistance and high temperature structural stability. Intermetallics have long been used in high temperature alloys systems such as in the classical Ni-base superalloys that derive their strength from nanoscale dispersions of the aluminide, Ni3Al(γ’) in a matrix of disordered fcc Ni (γ), alloyed with expensive, high density refractory elements such as Re and Ru. The high temperature applications of intermetallics derive from their strength retention to high temperatures, creep resistance enabled by low diffusion rates, and attractive oxidation resistance based on high concentration of elements such as Al that forms stable oxides. Several decades of effort on the development of new generation of intermetallic alloys through the 80’s and 90’s have gone unrewarded, with the exception of TiAl based alloys that are now used in recent generation aircraft engines. The promise of intermetallics as high temperature candidate materials is limited by their poor ductility or toughness arising from several intrinsic properties such as low grain boundary cohesive strength (in the case of Ni3Al) or an insufficient number of slip systems (as in NiAl) or extrinsic effects such as embrittlement by hydrogen (Fe3Al) that derive fundamentally from the existence of directionality in bonding.
However, low ductility or toughness can often be alleviated by limiting the length scale for slip. We have therefore examined the possibility of combining intermetallics in the form of eutectic structures, potentially limiting slip lengths within each intermetallic constituent. Eutectic structures in binary systems limit the choice of intermetallic combinations so that finding such combinations with engineering potential is difficult. On the other hand combinations of three elements or more would enable a significantly larger set of permutations of eutectic intermetallics, provided the constituent binary phase diagrams contain either eutectic or peritectic reactions involving intermetallic phases, as well as intermediate intermetallic phases.
The ternary Ni-Al-Zr system met our criterion in several ways. The Ni-Al binary phase diagram shows a peritectic reaction from liquid and NiAl (Pm 3m, B2 with a lattice parameter of 0.288nm) to form Ni3Al (Pm 3m, L12 with a lattice parameter of 0.356 nm), intermetallics that have been extensively investigated in earlier literature. The Ni-Zr system shows a peritectic reaction between liquid and the Ni7Zr2 (C12/m1 with a lattice parameters a=0.469nm, b=0.823nm, c=1.219nm) phase to form the intermetallic Ni5Zr
(F 43m with a lattice parameter of 0.670nm). Further the NiAl and Ni7Zr2 are both intermediate phases and should therefore form a mono-variant eutectic on the composition line joining these two phases in the ternary system. We note that Zr participates in many glass forming systems. In the Ni-Zr system, for example, glass forming ability has been associated with the structure of the liquid phase and associated low diffusivity. As a consequence, a fine scale eutectic structure may be expected. Zr has also been reported to strengthen and ductilise Ni3Al and NiAl. Finally, both Al and Zr form stable oxides and might promote oxidation resistance.
After introducing the thesis in Chapter 1, the experimental details are outlined in the Chapter 2. The experimental results and subsequent discussions are presented in three subsequent chapters. Chapter 3 reports the microstructural aspects of as cast alloys in this ternary system Previous literature and our analysis of phase equilibria in the Ni-Al-Zr system based on Thermo-Calc, suggested that solidification from the liquid to form the Ni3Al + Ni5Zr, Ni3Al + Ni7Zr2 and NiAl+ Ni7Zr2 eutectics is possible. We obtained eutectic structures involving combinations of these intermetallic phases along a constant zirconium section at 11 at. %. The alloy A (Ni-77 at.%, Zr-11at.% and rest Al) contains eutectic structures containing the Ni3Al and Ni5Zr phases in two morphologies, a planar, lamellar structure and a more irregular form. The alloys B (Ni-74 at.%, Zr-11at.% and rest Al) and C (Ni-71 at.%, Zr-11at.% and rest Al) contain two different eutectic structures that combine the Ni3Al and Ni7Zr2 phases, and the NiAl and Ni7Zr2 phases. These phases were identified by a combination of X-ray diffraction, transmission electron microscopy coupled with energy dispersive spectroscopy and electron probe microanalysis. The volume fraction of each eutectic constituent is different in the two compositions in that alloy B(Ni-74 at.%, Zr-11at.% and rest Al) contains significantly higher volume fractions of the eutectic containing the Ni3Al and Ni7Zr2 phases than the alloy C (Ni-71 at.%, Zr¬11at.% and rest Al). In order to understand effect of individual phases we have melted several other alloys (alloy D to I) bounding these eutectic alloys (7-25 at.% Al, 5-15 at.% Zr and rest Ni) that form primary solidification phases of the intermetallic structures that constitute the eutectics.
Chapter 4 discusses the mechanical behaviour of the fully eutectic alloys alloys as well as alloys with a combination of primary phases along with a eutectic. Mechanical behaviour was assessed in vacuum arc melted and suction cast material. The compressive strength of eutectic and off-eutectic compositions has been evaluated as a function of temperature. Very high strength levels of around 2 GPa could be achieved accompanied by reasonable room temperature tensile plasticity in the range 3-4%. The introduction of the respective primary phases of NiAl, Ni3Al, Ni5Zr and Ni7Zr2 results in decrease of strength. We have explored the origins of strength and tensile plasticity in alloys through micro and pico indentation (hardness) measurements and an examination of slip lines and crack initiation on pre-polished surface of the tensile tested samples as well as by transmission electron microscopy. Chapter 5 explores the oxidation resistance of these alloys in isothermal tests. The oxidation resistance of alloys compares well with recently developed cast single crystal alloys. Clearly, the oxide scale is extremely adherent and no spalling occurs. Electron microprobe analysis shows the presence of a fine scale, layered oxide structures and reaction zones within the substrate. The oxidation behaviour has been characterized using TGA, XRD and EPMA. We have attempted to understand the mechanism of oxidation through analysis of rate constants and activation energy coupled with microstructural observations. Chapter 6 presents a summary of the current work and present the scope for further work
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
India-NATO Dialogue : Addressing International Security and Regional Challenges
No full textThe institute of Peace and Conflict Studies (IPCS) has been engaged since 2004 in an annual informal strategic dialogue with the North Atlantic Treaty Organization (NATO). The strategic dialogue in 2009 was conducted in two parts. The first was a full day open seminar among the strategic community in collaboration with the United Services Institute (USI) of India. Presentations were made by leading experts which were discussed and debated. The second was a select meeting among smaller group of senior strategic experts including members of NSAB. The conference was chaired by the Chairman, NSAB, Ambassador Shankar Bajpai. This book is summary of these deliberations along with the keynote address by the then German Ambassador HE Bernd Mutzelburg, who soon after assumed the position of that country’s special envoy to Afghanistan. This book provides the needed backdrop and a valued addition to the ongoing strategic discourse on the future of the South Asian Security
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