Volume 33, Issue 1

This is a study of microalloyed steels for power plants and reactors. Components operate at coal dust fire temperature or thermal states of reactors, prone to creep during its service. This is to assess remaining life after passage of valuable life by variation in microstructure, e.g. cavity formation. Precipitation at the sub-grain boundaries and grain interior has increased high temperature strength. Coarsening of these appears at the end of life. Variation of heat treatment like spheroidising in place of solutionizing has been responsive to deteriorate performance. Dislocation interplay with precipitate has been acceptable while interaction among dislocations to forest dislocation has been unacceptable. Dislocation assisted nucleation of precipitates of fine size has been found to strengthen steel by thermo-mechanical control process with in greater heating temperature and lower finish rolling temperature. High temperature performance of materials has been assessed by creep, accelerated creep, creep-fatigue and fatigue performances. Increasing temperature for increasing efficiency has correlated the phase transformation of steel. Fatigue performances have been included in creep properties of materials when intermittent shut down–shut up schedules are operated, e.g. peaking power plants.

In this study, hot workability aspect for modeling the flow stress at specified deformation level is investigated. The proposed model contains the basic parametric relationship of Arrhenius power law. The developed mathematical model is validated on Al 2024 alloy to check its feasibility over a wide range of strain rate, strain, and temperature. When compared with Johnson-Cook (JC) model, it is found to give a more perceptible experimental output by limited evaluation of arbitrary constants. The proposed model uses the input from hot compression test without the necessacity to go to torsion experiment. This proves its supremacy over JC model which require both the inputs. It also limits the number of constants, so produces a forwarding mark to N.S. Babu Model which needs 48 numbers of constants to define the constitutive aspect of deformation.

Nanosilver paste with a low sintering temperature is a high-demand interconnection material for electronic devices and components due to its superior electrical, thermal, and thermomechanical properties, as well as the fact that it is lead free. This study presents a nanoscale silver paste that can be sintered at 300 °C using 60-nm silver particles and an ethyl cellulose solution. The rheological and thixotropic behaviors of the typical paste were characterized, and the paste's screen printing ability was analyzed in detail. The paste demonstrates typical shear thinning rheological behavior and thixotropy of pseudoplastic fluids. Thermal characteristics of the 60 nm silver particles were measured by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) to obtain a proper sintering temperature schedule. In this report, we also discussed the solid content, thermodynamic driving force for densification, and sintering process of the nanosilver particles. A 83 wt% of silver solid content at a heating rate of 20 °C/min was favorable for achieving a proper sintering densification.

Recent thermal spray processes such as plasma spraying are usually employed to deposit Nickel–Chromium coatings onto the AISI 316 steel substrates to improve the wear resistance. Ni-base alloys, which are widely used to obtain wear resistant coatings, are usually heat treated after thermal spraying to improve their tribological properties. The dry sling wear experiments were carried out in pin on disc tribometer at room temperature 35 °C (308 K), 150 °C (423 K), 250 °C (523 K) and 350 °C (623 K) against medium carbon steel (EN8). The worn surfaces were examined by X-ray diffraction and SEM (Scanning Electron Microscopy) with EDS (Energy dispersive spectroscopy). The applied load was 40 N and sliding speed was 2 m/s. In 350 °C, the wear rate was marginally lesser than the rest of the temperatures due to the initiation of oxide layer formation, which was also substantiated by SEM and EDS. Fe peaks and O peaks were found scarcely. Few Fe peaks were also predominantly noticed by EDS analysis. The presence of Fe 3 O 4 in the tribolayers formed during sliding wear acted as a protective film to retard the wear rate in terms of mass loss.

In this study, conventional WC-12Co cermet coatings were deposited on AZ91 magnesium alloy using High Velocity Oxy-Fuel spray method. The effects of different coating thicknesses (60 and 120 µm) and wear loads (20 and 30 N) on tribological properties of AZ91 magnesium alloy coated with WC-12Co were investigated. Structural, mechanical and tribological properties of WC-12Co coatings were analyzed by means of X-ray diffractometer, scanning electron microscope, micro-hardness tester and pin-on-disc tribotester. It was observed that WC-12Co coating is an effective method for improving the wear behavior of AZ91 magnesium alloy.

Ti aluminides based on γ + α 2 has industrial importance due to its high temperature properties coupled with low density. Processing difficulty involved in this alloy has limited its application. Alternate processing route like reaction synthesis in combination with isothermal hot working has lot of potential. Further, heat treatment of hot worked material governs the properties of the alloy. In the present paper, heat treatment of reaction synthesized plus hot worked material has been studied to obtain desirable microstructure and phases. Five alloys of Ti48Al2Cr2Nb with minor addition of boron and Ni-P coated boron is made with two different particle size of Ti powder. Homogenized and worked alloy samples are heat treated to obtain two-phase microstructure useful for high temperature application. Heat treated samples are characterized for individual phases through optical microscopy, hardness measurement and electron microscopy. An attempt has been made to study the role of boron on development of various phases and microstructure.

Polycrystalline skutterudite Ba 0.4 Co 4 Sb 11.7 Te 0.3 with a bcc crystal structure was prepared by the High-Pressure and High-Temperature (HPHT) method. The study explored a chemical method for introducing Ba atoms into the voids of CoSb 3 to optimize the thermoelectric figure of merit ZT in the system of Ba 0.4 Co 4 Sb 11.7 Te 0.3 . The samples were characterized by X-ray diffraction, electron microprobe analysis, and thermoelectric properties measurement. The electrical resistivity, Seebeck coefficients and thermal conductivities of the samples were measured in the temperature range of 300–743 K. The power factor and the figure of merit, ZT , of the samples all increased with the increasing temperature. A dimensionless thermoelectric figure of merit of 0.87 at 743 K was achieved for n-type Ba 0.4 Co 4 Sb 11.7 Te 0.3 at last. The results indicated Ba-filled CoSb 3 prepared by HPHT method is an effective method to greatly enhance the thermoelectric properties of skutterudite compounds.

The effect of Y and Mg on the solidification structure of Fe-36Ni invar alloy melted in a carbon tube furnace was investigated using digital camera, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results showed that Y or Mg addition could remarkablly refine the solidification structure and increase the fraction of equiaxed grains. Great amount of high-melting point compounds (Y 2 O 2 S, MgS and MgS-MgO) formed in the alloy with the addition of Y and Mg respectively. Based on the theory of lattice misfit, the effectiveness of high-melting point compounds as heterogeneous nuclei of γ -matrix was calculated. The calculation of the lattice misfit between Y 2 O 2 S, MgS and γ -matrix, which were far less than 12%, showed that Y 2 O 2 S and MgS acted as heterogeneous nuclei were highly effective. However, MgO could not serve as the heterogeneous nuclei because the lattice misfit between it and γ -matrix exceeded 12%.

By means of direct aged treatment, creep property measurement and high resolution TEM microstructure observation, the deformation mechanisms of Tandem Hot Rolled GH4169 superalloy during creep are investigated. Results show that, after direct aging treatment, fine γ ″ particles with different sizes and shapes dispersedly precipitate in the alloy, which is one of important factors for the alloy possessing good creep resistance. And the deformation mechanisms of the alloy are that the deformed twinnings with different orientations are activated on {111} plane by pole mechanism, thereinto, the twinning dislocation may continuously slip around a pole axis dislocation on the twinning planes when the applied stress exceeds the critical value, and the twinnings may multiply by the dislocation reactions and mutual indemnification. As creep goes on, the denser dislocations with single or double orientations slip in the different twinnings, which play an important role of coordinating the grain deformation to enhance the creep resistance of the alloy.

The melting temperature and viscosity of CaO-Al 2 O 3 -10 mass% SiO 2 based slag system with various concentrations of Ce 2 O 3 have been studied using the melting point detector and the rotating crucible viscometer. And X-ray diffraction analysis has been used for phase identification. The results show that cerium is stable in Ce 3+ state existing mainly as CeAlO 3 and Ce 4.67 (SiO 4 ) 3 O phase in slags and CeAlO 3 phase appears in green color. The melting temperature gently decreases with Ce 2 O 3 additions in 1.57 of CaO/Al 2 O 3 . Moreover, the melting temperature increases first and then decreases with the increasing of CaO/Al 2 O 3 from 1.17 to 1.52 at 4.47 mass% Ce 2 O 3 . In addition, at 1.57 of CaO/Al 2 O 3 , the viscosity increases at the beginning and then decreases with the increasing Ce 2 O 3 content from 4.39 to 11.48 mass%. Furthermore, at 4.47 mass% Ce 2 O 3 , the viscosity decreases at the first and then increases with the increasing CaO/Al 2 O 3 from 1.17 to 1.52. Meanwhile, from the slopes of the Arrhenius relationship for viscosity, the activation energy range of viscous flow is from 179.07 to 433.70 kJ/mol. On the basis of these results, slag composition of 45.64 mass% CaO-39.02 mass% Al 2 O 3 -10.73 mass% SiO 2 -3.83 mass% Ce 2 O 3 is melting temperature of 1361 °C and viscosity of 0.398 Pa·s (1500 °C), which has superiority and is more suitable for the actual refining process.

The microstructure evolution of aluminum alloy 2618 and its homogenization kinetics were investigated by optical microscope, scanning electron microscope, energy dispersive spectroscopy and differential scanning calorimeter. The results show that the main constituent phases in as-cast alloy 2618 are α–Al, Al 9 FeNi phase and non-equilibrium binary θ (Al 2 Cu) phase instead of typical S (Al 2 CuMg) phase. The θ phase was dissolved into α–Al gradually and the continuous dendritic-network structure was broken with the increase of homogenization temperature and time. DSC analysis shows that the overburnt and liquidus temperatures of as-cast alloy 2618 are 506.4 °C and 638.0 °C, respectively. After the alloy was homogenized at optimized temperature 500 °C for 16 h, the θ phase was completely dissolved in matrix. The size and morphology of Al 9 FeNi phase had little change, while the liquidus temperature shifted to 641 °C. The calculated homogenization time for alloy 2618 at 500 °C is 15 h, which is in accordance with that obtained in homogenization experiments.

In this study, a series of polymer/clay nanocomposites (PCNs) materials consisting of organic poly(amide-imide) (PAI) and organo-modified clay content varying from 0 to 10 mass%, were successfully prepared by solution dispersion technique. At first, the reactive organoclay was prepared by using protonated L-phenylalanine amino acid as a swelling agent for silicate layers of montmorillonite/Na + (MMT/Na + ) via ion-exchange reaction. Then, poly(amide-imide) used as a matrix of nanocomposite was prepared through the polymerization reaction of N,N′-(4,4′-diphtaloyl)-bis-L-α-aminophenyl acetic diacid and 1,2-bis(4-aminophenoxy)ethane in a medium of N-methyl-2-pyrrolidone (NMP) / triphenyl phosphite (TPP) / calcium chloride (CaCl 2 ) / pyridine (py). This polymer was end-capped with amine end groups near the completion of the reaction to interact chemically with organoclay. Finally, PAI/organoclay nanocomposite films containing 1%, 3%, 5% and 10% of organoclay were prepared via solution intercalation method through blending of organoclay with the PAI solution. The resulting nanocomposite films were characterized by FT-IR spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA). XRD and TEM results revealed the formation of exfoliated and intercalated organoclay platelets in the PAI matrix. TGA results indicated that the addition of organoclay into the PAI matrix increases in the thermal decomposition temperatures of the resulted nanocomposites.