1,721,126 research outputs found
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
Development of a wrought Mg‒7Sn‒3Zn alloy with improved strength and ductility
No full textThe current focus of research in Mg alloys is to develop wrought alloys suitable for the automotive industry. The challenges to be overcome are low strength, poor ductility, and high cost associated with wrought Mg‒based alloys. With this objective, wrought Mg‒7Sn‒3Zn (TZ73) alloy was developed in the present work, which does not contain expensive rare‒earth (RE) elements and has much higher strength than the most commonly used commercial AZ31 alloy, with a reasonable ductility. The alloy was prepared by squeeze casting, followed by homogenization at 300°C for 24 h to dissolve the low‒temperature eutectic and achieve uniform composition in the as‒cast microstructure. The processing map was generated by conducting hot compression tests in the temperature range 200 ‒ 450°C and the strain rate range 10‒3 – 101 s‒1. Based on the results from the processing map, the homogenized TZ73 alloy was rolled at 350°C. It was also subsequently annealed at 215°C. A detailed investigation has been carried out at each stage of the thermo‒mechanical processes. The hot‒rolled sheet exhibited a high strength (0.2% PS: 315 MPa, UTS: 362 MPa) with reasonable elongation‒to‒failure (9%). Strengthening is contributed to the alloy by the combined effect of grain refinement, solid solution strengthening by Zn and Sn atoms, fine Mg2Sn particles, and crystallographic texture. After annealing, the strength was reduced but % elongation‒to‒failure was increased, the alloy exhibiting tensile properties ‒ 0.2% PS: 218 MPa, UTS: 311, and elongation‒to‒failure: 18%.
The properties of the Mg‒based alloys are highly dependent on the crystallographic texture. A strong basal texture develops on rolling, which is detrimental to formability. The crystallographic texture depends on the strain path during rolling. Therefore, cross rolling (CR)
and reverse rolling (RR) were also carried out at 350°C. The RR process with refined microstructure and weaker texture exhibited better tensile properties (0.2% PS: 325 MPa, UTS: 386, and elongation‒to‒failure (9%) with minimum anisotropy) than CR. The basal texture is also weakened by introducing high shear stresses by having different circumferential velocities of the upper and lower rollers, known as asymmetric rolling (ASR). ASR was also employed for this alloy and the ASR sample exhibited the same strength levels as in symmetric rolling but higher elongation‒to‒failure. The present work shows that the wrought TZ73 alloy can be a promising candidate for the automotive industry
Corrosion And Wear Behaviour of Plasma Electrolytic Oxidation And Laser Surface Alloy Coatings Produced on Mg Alloys
Full text linkIn the present investigation, surface coatings employing laser surface alloying (LSA) and plasma electrolytic oxidation (PEO) processes have been prepared on Mg alloys. The coatings have been investigated for corrosion and wear behaviour. Two important Mg alloys based on Mg–Al system were selected namely, MRI 230D and AM50 as substrates. LSA coatings have been prepared employing Al and Al2O3 as precursors using different laser scan speeds. PEO coatings were prepared in standard silicate and phosphate based electrolytes employing unipolar, pulsed DC. Hybrid coatings using a combination of the two processes were also produced and investigated for corrosion and wear behaviour. Hybrid coatings of LSA followed by PEO (LSA+PEO) were investigated for effectiveness of sealing the cracks in the LSA coatings by subsequent PEO process and consequent improvement in the corrosion resistance. Hybrid coatings of PEO followed by LSA (PEO+LSA) were prepared with an objective of sealing the pores in the PEO coating LSA treatment. In an attempt to produce more compact PEO coatings, electrolyte containing montmorillonite clay additives was employed for the PEO process of AM50 Mg alloy. The coatings were produced employing different current densities and the effect of current density on the microstructure and corrosion behaviour of coating was investigated.
Electrochemical corrosion tests of uncoated and coated alloys were carried out in 3.5 wt.% (0.6M)NaCl, neutral pH, solution with an exposed area of 0.5 cm2 for a time duration of 18.5 h. For the PEO coatings with clay additives, corrosion tests were conducted additionally in 0.5 wt.% (0.08 M) NaCl, neutral pH, solution for a time duration of 226.1 h. Wear behaviour of LSA coatings was analyzed by employing a pin on disc tribo–tester conforming to ASTM G–99 standard at ambient conditions with ground EN32 steel disc of hardness Rc 58 as the counterface. Tests were conducted under dry sliding conditions for a sliding distance of 1.0 km at a sliding velocity of 0.837 m/s employing normal loads of 10, 20, 30 and 40 N. Friction and wear behavior of PEO and PEO+LSA coatings were analyzed at ambient conditions by employing a ball−on−flat linearly oscillating tribometer conforming to ASTM G–133 standard. AISI 52100 steel ball of diameter 6 mm was employed as the friction partner. Wear tests were conducted under dry sliding conditions for a total sliding distance of 100 m at normal loads of 2 N and 5 N with oscillating amplitude of 10 mm and mean sliding speed of 5 mm/s.
LSA coatings could not improve the corrosion resistance of MRI 230D Mg alloy. This was attributed to the presence of cracks in the LSA coating, which resulted in the accelerated galvanic corrosion of the substrate. LSA coatings improved the wear resistance at all loads. The improved wear resistance was attributed to β (Mg17Al12) phase and Al2O3 particles in the coating which increased the hardness of the LSA layer. No trend in corrosion and wear resistance with laser scan speed was observed for LSA coatings.
PEO coatings improved the corrosion resistance of the MRI 230D Mg alloy significantly. The improved corrosion resistance was attributed to the enhanced barrier protection provided by dense barrier layer formed at the substrate/coating interface and to the insoluble phase constituents in the coatings. PEO coating was effective in improving the wear resistance at low loads/contact pressures. At higher loads, the coating underwent micro–fracture as a result of the porosity in the coatings.
Hybrid coatings of LSA followed by PEO (LSA+PEO) in silicate based electrolyte improved the corrosion resistance of LSA coatings. However, the corrosion resistance was not improved to the extent of PEO coatings on as–cast alloy as a result of cracks in the primary coatings, which were not fully sealed by the plasma conversion products. No trend in corrosion resistance with laser scan speed was observed for LSA+PEOcoatings.
In hybrid coatings of PEO followed by LSA (PEO+LSA), primary PEO coating was completely melted and mixed with applied precursor to form a single composite LSA layer. The corrosion resistance of the hybrid coatings was observed to be lower than that of the as–cast alloy. The presence of solidification cracks reduced the barrier properties and resulted in the accelerated galvanic corrosion of the substrate similar to LSA coatings. Hybrid (PEO+LSA) coatings exhibited improved wear resistance as compared to as–cast alloy at lower loads as a result of increase in the hardness due to β (Mg17Al12) phase and oxide/ceramic particles in the hybrid layer. At higher loads, hybrid coatings exhibited higher wear rate as compared to as–cast alloy and PEO coatings. This was attributed to three–body abrasive wear as a result of dislodged hard oxide/ceramic particles in the wear tracks. No trend in corrosion and wear resistance with laser scan speed was observed for PEO+LSA coatings.
PEO coatings on AM50 Mg alloy by employing clay additives in the electrolyte resulted in the reactive uptake of clay particles producing a predominantly amorphous coating at low current density. Clay additives were effective in improving the compactness of the coating at lower current density. At higher current densities, the porosity of the coatings increased. The clay particles got re–constituted producing increasing amount of crystalline phases with increase in current density. Long term impedance measurements showed that clay addition as well as increased current density employed for the PEO process was not effective in improving the corrosion resistance of the coatings. At low current density, even though the coating with clay additives was more compact, it was deficient in MgO and consisted predominantly of an amorphous phase, which underwent fast dissolution in electrolyte thereby resulting in an early loss of barrier properties. At higher current densities, even though the coatings consisted of increased amount of MgO and crystalline phases, which resist dissolution in the electrolyte, the increased porosity and defective barrier layer resulted in easy permeation of the electrolyte into the substrate/coating interface, which resulted in much earlier loss of barrier properties and inferior corrosion resistance
Creep, Wear And Corrosion Behaviour Of Novel Magnesium Alloys And Composites
No full textIn the present investigation, MMCs have been fabricated using the creep-resistant AE42 magnesium alloy as matrix and reinforcing it with saffil short fibres (essentially δ-Al2O2) and SiC particles in various combinations. These MMCs have been investigated for their creep, wear and corrosion behaviour. The above properties of the matrix AE42 alloy have also been investigated for comparison. Further, laser surface melting has been carried out on a creep-resistant MRI 230D Mg alloy and the corrosion and wear behaviour of this alloy before and after laser surface melting has been investigated.
The creep tests on the AE42 alloy were carried out in the temperature range of 1500 to 2400C at the stress levels ranging from 40 to 120 MPa and the composites were tested in the temperature range of 1750C to 3000 at the stress levels ranging from 60 to 140 MPa both in the longitudinal direction (LD) and in the transverse direction (TD). Wear tests were conducted on a pin-on-disc set-up under dry sliding condition at a constant sliding velocity of 0.837 m/s for a constant sliding distance of 2.5 km in the load range of 10 to 40 N for the AE42 alloy and the composites, which were tested both in LD and TD, and for a constant sliding distance of 1km in the load range of 5 to 20 N for the MRI 230D alloy before and after laser melting. All the materials were subjected to electrochemical corrosion tests in a 5 wt.% NaCl solution having ph value 11 for 22 hours.
All the composites in both LD and TD exhibit lower creep rate as compared to the AE42 alloy and it is higher in TD than in LD. The creep resistance of the hybrid composites, in which saffil short fibres are partially replaced by SiC particles, is observed to be comparable , i.e., of the same order of magnitude , to that of the composite reinforced with Saffil short fibres alone at all the temperatures and stresses employed in both LD and TD. Wear rate of all the composites in both LD and Td is found to be lower than the alloy at all the loads employed and it is higher in TD than LD, Wear rate progressively decreases with the partial replacement of Saffil short fibres by Sic Particles, and is lowest for the composites reinforced with 10 vol.% Saffil short fibres and 15 vol.% Sic particles in both LD and TD. It is 34% and 35% lower than the 20% Saffil composite at 40 N load in LD and TD, respectively. The Ae42 alloy exhibits the best corrosion resistance and the addition of the Saffil short fibres and/or Sic particles in the AE42 alloy deteriorates its corrosion resitance. The composite reinforced with Saffil short fibres alone exhibits slightly better corrosion resitance than the hybrid composites. However, there is no systematic trend of corrosion resistance with SiC particles content. The laser surface melting is found to improve the corrosion, hardness and wear resistance of the MRI 230D alloy.
High temperature climb of dislocation is found to be the dominant creep mechanism in the AE42 alloy in the stress and temperature range employed. Various glide and climb of dislocation are found to be the dominant creep mechanisms for all the composites in both LD and TD in the stress and temperature range employed. The presence of SiC particles in the hybrid composites improves the wear resistance in both LD and TD since these particles remain intact and retain their load bearing capacity even at the highest load employed in the present investigation. They promote the formation of iron-rich transfer layer and they also delay the fracture of Saffil short fibres to higher loads in case of the composites in LD. Under the experimental conditions used in the present investigation, the dominant wear mechanism is found to be abrasion for the AE42 alloy and its composites in both LD and TD. It is accompanied by severe plastic deformation of surface layers in case of the alloy, the fracture of Saffil short fibres as well as the formation of iron-rich transfer layer in case of the composites in Ld, and the fracture and pull-out of the Saffil short fibres in case of the composites in TD. The lower corrosion resistance of all the composites is not caused by the galvanic coupling between reinforcements and matrix, and is related to the microstructural changes, such as, distribution of precipitates and the nature of the film formed at the surface. The improved corrosion resistance following laser surface melting is due to the absence of the Al2Ca phase at the grain boundary, microstructural refinement and increased solid solubility, particularly of Al, owing to rapid solidification; the improved hardness and wear resistance is due to grain refinement and solid solution strengthening.
To conclude, the creep resistance of the hybrid composites is comparable, wear resistance is better and corrosion resistance is slightly inferior to the composite reinforced with Saffil short fibres alone. Therefore, from the commercial point of view, the use of the hybrid composites, replacing a part of the expensive Saffil short fibres by cheap SiC particles, is beneficial. The laser surface melting is beneficial for the corrosion and wear resistance of the MRI 230D alloy
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Evolution of plasma electrolytic oxidation coatings formed on AM50 Mg alloy utilizing various alkaline electrolytes with and without glycerol additive and its corrosion behavior
No full textThe widespread application for Mg and its alloys are severely restricted mainly due to its relatively low strength and ductility and poor corrosion behaviour, particularly in the aqueous environment. The plasma electrolytic oxidation (PEO) process for improving the corrosion resistance of the Mg-based alloys, has been widely studied/utilized in the research and commercial arena. Until now only a few studies have been carried on the evolution of the PEO coatings over Mg and its alloys and the effect of glycerol as an electrolytic additive on PEO coating evolution has not been investigated till date.
Therefore, this study aims to systematically investigate the evolution and electrochemical behavior of the PEO coatings synthesized under similar operating conditions from the silicate-based, phosphate-based, and mixed silicate-phosphate-based alkaline electrolytes (abbreviated as bSi-PEO, bP-PEO, and bm-PEO respectively) as a function of PEO processing time. Additionally, the influence of glycerol as an additive on the evolution of PEO coatings synthesized from the aforementioned electrolytes (abbreviated as gSi-PEO, gP-PEO, and gm-PEO respectively) is also investigated.
PEO processing voltage, maximum pore size, and coated layer thickness increase while pore density decreases and corrosion performance improves with the PEO processing time irrespective of electrolytes type (silicate-based, phosphate-based, and mixed silicate-phosphate-based) with and without glycerol additive. It was observed that the dielectric breakdown voltage, VBD, was in the order bSi-PEO coating > bP-PEO coating > bm-PEO coating propounding their higher dielectric properties. On glycerol addition, the VBD for all the three base electrolytes was higher indicating higher dielectric properties. The maximum pore size, in general, for all the PEO processing times, was in the order silicate-based electrolyte < phosphate-based electrolyte < mixed silicate-phosphate-based electrolyte with and without glycerol additive. This indicates that the micro-discharge intensity during the PEO processing increased in the same order for a given PEO processing time. The maximum pore size decreased with glycerol additive for all the three electrolytes at all the PEO processing times suggesting a reduction in the intensities of micro-discharges. It was also observed that the PEO coating thickness increases in the order silicate-based electrolyte < phosphate-based electrolyte < mixed silicate-phosphate-based electrolyte with and without glycerol additive. This can plausibly be attributed to the enhanced reactivity of PO43- anions than SiO32- anions with Mg2+ cations during the PEO processing. In the case of mixed silicate-phosphate-based electrolyte, it seems that the PO43- and SiO32- anions catalyzed the reactivity of each other with Mg2+ cations and hence resulted in relatively more volume of deposits and thereby resulted in thicker PEO coatings. The glycerol addition to all the three base electrolytes lowered the PEO coating growth rates possibly by increasing electrolytes viscosity, which impedes the migration of ions from the electrolyte towards the substrate. The elemental compositional analysis revealed homogeneous PEO coatings for all the three base electrolytes with and without glycerol additive at all the PEO processing times. The glycerol addition to all the three base electrolytes promoted the formation of the MgO phase in the resulting PEO coatings.
Electrochemical studies utilizing 0.5 wt.% NaCl solution revealed that the corrosion performance was in the order bSi-PEO coating > bm-PEO coating > bP-PEO coating during the shorter immersion time. While the corrosion performance during longer immersion time was in the order bP-PEO coating > bSi-PEO coating ≃ bm-PEO coating indicating the bP-PEO degraded relatively less with the immersion time. In general, on glycerol addition to all the three base electrolytes, the corrosion performance improved. This could be due to the thicker inner barrier layer and higher MgO phase content in the PEO coatings despite lower coating thickness on glycerol addition. On glycerol addition, it was observed that for the shorter immersion duration the corrosion performance was in the order gSi-PEO coating > gm-PEO coating > gP-PEO coating indicating the gSi-PEO coating, despite having the least thick PEO coating, had better corrosion performance plausibly due to thicker and compact inner barrier layer. For longer immersion duration the corrosion performance was in the order gP-PEO coating > gSi-PEO coating > gm-PEO coating, indicating that the gP-PEO coating offered better corrosion behaviour as its degradation was retarded plausibly due to the availability of more quantity of amorphous Mg3(PO4)2 phase to recrystallize in aqueous solution, which resulted in the formation of its insoluble crystalline phase
Fatigue Damage Characterization Of Carbon/Epoxy Laminates Under Spectrum Loading
Full text linkFibre Reinforced Polymer Composites are extensively used in aircraft structures because of its high specific stiffness, high specific strength and tailorability. Though Fibre Reinforced Polymers offer many advantages, they are not free from problems. The damage of different nature, e.g., service mechanical damages, fatigue damage or environmental damage can be observed during operating conditions. Among all the damages, manufacturing or service induced, delamination related damage is the most important failure mechanisms of aircraft-composite structures and can be detrimental for safety. Delamination growth under fatigue loading may take place due to local buckling, growth from free edges and notches such as holes, growth from ply-drops and impact damaged composites containing considerable delamination. Delamination growth can also occur due to interlaminar stresses, which can arise in complex structures due to unanticipated loading.
The complex nature of composite failure, involving different failure modes and their interactions, makes it necessary to characterize/identify the relevant parameters for fatigue damage resistance, accumulation and life prediction. An effort has been made in this thesis to understand the fatigue behavior of carbon fibre reinforced epoxy laminates under aircraft wing service loading conditions. The study was made on laminates with different lay-up sequences (quasi-isotropic and fibre dominated) and different geometries (plain specimen, specimen with a hole and ply-drop specimen).
The fatigue behaviour of the composite was analyzed by following methods:
. Ultrasonic C-Scan was used to characterize the delamination growth.
. Dynamic Mechanical Analysis (DMA) was done to study the interfacial degradation due to fatigue loading. In this analysis, the interfacial strength indicator and interfacial damping were calculated. The DMA also provides the storage modulus degradation under fatigue loading.
. Scanning electron microscope examination was carried out to understand the fatigue damage mechanisms.
. A semi-empirical phenomenological model was also used to estimate the residual fatigue life.
This research work reveals that the Carbon Fibre Reinforced Polymer laminates are in the safe limit under service loading conditions, except the specimen with a hole. The specimen with a hole showed delaminations around the hole due to stress concentration and higher interlaminar stresses at the hole edges and this delamination is found to be associated with fibre breakage and fibre pullout. The quasi-isotropic laminate is found to show poorer fatigue behaviour when compared to fibre dominated laminate and ply-drop also shows poor performance due to high stress concentration in the ply-drop region
Improvement of High Temperature Strength of Al and Co Alloy by L12 Type Coherent Precipitates
No full textThe present work aims at developing a new class of high temperature alloys based on ordered
intermetallic compound that forms coherently with the matrix during solid state transformation.
The chosen intermetallics have L12 ordered structure, which is a derivative of fcc unit cell.
Most popular example of this fcc derivative is Ni3Al that is critical in developing high strength
at high temperatures (~900°C) in commercially successful Ni based superalloys. Similar
ordered structures form either in stable or metastable form can act as a main strengthening
constituent in Al and Co matrices.
For example Al3Sc, Al3Zr, Al3Hf can be dispersed in fcc Al matrix that are stable at
temperatures ~ 400°C due to very low diffusivity of transition metals (Sc, Zr, Hf etc.) in the
matrix. However, due to low solid solubility of these transition metals, the obtained volume
fraction of these precipitates in the matrix is not sufficient to provide adequate room
temperature strength.
In fcc Co matrix, stable Co3Ti phase with L12 ordered structure forms with cuboidal
morphology. However, besides having lower melting point, the precipitates have large misfit
that lowers thermal stability at high temperatures. Recently, addition of Al and W with a proper
ratio in Co is reported to lead the formation of metastable Co3(Al,W) L12 ordered phase in fcc
α-Co matrix. This provides significant strength at high temperatures (~ 900°C). The main
drawback for these alloys is their high densities (9.6 to 10.5 gm.cm-3) due to the requirement of
compulsory addition of W (~ 15 to 25 wt%) for stabilising the ordered phase.
In the present work, these problems are overcome leading to the development of new class of
Al and Co alloys. The thesis is organized in three parts. In the first part, the principles of
strengthening that can be optimized to develop newer high temperature high strength alloys are
reviewed. The ordered L12 structure, which is the mainstay of the current effort of new alloy
development, is elaborated. In the second part we present the results of our effort to the
development a new class of high strength high temperature Al alloys. A new approach has been
adopted to get a microstructure that contains both high temperature stable and room
temperature strengthening precipitates. This has been illustrated by two Al rich compositions,
Al-2Cu-0.1Nb-0.15Zr and Al-2Cu-0.1Hf-0.15Zr (at% unless stated otherwise). Addition of
Nb/Zr or Hf/Zr in Al alloys leads to the formation of high temperature stable L12 ordered
spherical coherent precipitates in the fcc Al matrix. Cu addition gives room temperature
strengthening θ’ and θ” precipitates. The arc melted alloys were chill cast (suction cast) in the
form of 3 mm rods followed by a novel three stage heat treatment process, as shown below.
In the case of Al-2Cu-0.1Nb-0.15Zr alloy, the chill cast structure consists of Cu rich phase at
the boundaries along the α-Al dendrites while Zr and Nb partition inside the α-Al dendrites.
Aging at 400°C leads to an increase in the hardness of the cast alloy due to the precipitation of
coherent L12 ordered Al3(Zr,Nb) spherical precipitates (~5nm) in the α-Al dendrites. Zr
strongly partitions to the L12 ordered precipitate relative to the matrix. Nb exhibits weak
partitioning in the precipitate. Further solutionising was optimized at 535°C for 30 minutes
such that the segregation of Cu in the chill cast samples can be eliminated. The WDS mapping
shows that Cu dissolved uniformly in the α-matrix while the Zr/Nb enriched α-Al dendrites are
still present. The L12 ordered precipitates are mostly found in these Zr/Nb enriched dendrites
formed during solidification. The precipitates sizes are finer (~5 nm) in dendrites and larger in
the interdendritic region. The Nb partitioning increases in the ordered L12 precipitates relative
to the matrix after solutionising. On aging at 190°C, fine θ” precipitates nucleate on prior
Al3(Zr,Nb) precipitates present in α-Al dendrites while the interdendritic regions contain
coarser θ’ nucleated on larger size L12 precipitates. The θ”/θ’ are much finer and higher in
number density for the quaternary alloy compared to binary Al-2Cu alloy subjected to
conventional heat treatment. The quaternary alloy show higher peak hardness of 1500 ± 8 MPa
after 5 hours of aging at 190°C compared to binary Al-2Cu alloy with peak hardness of 1260 ±
11 MPa
- …
