Volume 104, Issue 9

The interrelated effect of activation energy and grain boundary energy on grain growth and thermal stabilization in nanocrystalline materials was investigated. Departing from Borisov's equation and Kirchheim's grain boundary energy model, an activation energy model was derived incorporating solute segregation and grain growth. An integrated thermokinetic model for nanoscale grain growth was then developed by incorporating the coupled effects of activation energy and grain boundary energy. By application of the model to nanoscale grain growth of dense nanocrystalline gadolinia-doped ceria, a good agreement between the model predictions and the experimental results was obtained. It is shown that the interrelated effect of increased activation energy and decreased grain boundary energy induced thermal stability in the nanocrystalline materials.

Friction stir welding was tried on a modified 9Cr-1Mo (wt.%) ferritic steel in an as-rolled condition. The microstructure of the resultant weld was divided into four distinct regions according to each thermo-mechanical history experienced during welding; i.e., stir zone, thermo-mechanically affected zone, inner heat-affected zone, and outer heat-affected zone. The first three zones showed distinct martensite morphologies depending on the different recrystallization phenomena during the heating cycle in the single-phase austenite region. In the outer heat-affected zone, however, only tempering occurred without phase transformation owing to a relatively low heating temperature. Hardness distribution of the weld closely reflected such microstructural differences, indicating that a considerable softening occurred in the thermo-mechanically affected zone and outer heat-affected zone owing to the coarsening and tempering effects, respectively.

The present work describes the effect of milling duration on the properties of a powder metallurgy processed Cu–Al–Ni–Ti shape memory alloy employing mechanical alloying. Powder mixtures milled for different durations were sintered in order to investigate the formation of solid solution and evolution of martensitic structure. The idea was to optimize the duration of milling (mechanical alloying) to obtain chemical homogeneity as well as shape memory properties in the processed material without undergoing extensive post homogenization treatment. The martensitic structure was noted to evolve in the powder mix milled for at least 16 hrs, whereas complete transformation to martensite occurred after milling for 40 hrs. Interestingly, the dissolution of alloying elements (to form the β phase prior to the formation of martensite) was noted to complete partially only during mechanical alloying for 40 hrs and remaining during subsequent sintering for 1 hr. The hot pressed compacts of the powders milled for 40 hrs were chemically homogeneous and consisted of fully martensite phase, which is essential for the realization of shape memory properties. They also revealed almost 100% shape recovery at the applied pre-strain levels of 1 and 2%.

The phase equilibria in the Co–Cr–W ternary system were experimentally investigated by using optical microscopy, electron probe microanalysis and X-ray diffraction on equilibrated alloys. In this study, isothermal sections of the Co–Cr–W ternary system at 1000°C, 1100°C and 1200°C were determined. The intermetallic compound R phase with its composition region were determined in the isothermal section at 1200°C, whereas it was absent at 1000°C and 1100°C. The Co 7 W 6 phase was found to possess a large solubility of Cr. The trapezoid-shaped composition region of σ phase was determined from 1000°C to 1200°C in the present work. The newly determined phase equilibria of the Co–Cr–W in the present study will provide useful information for Co-based alloys and Co–W base cemented carbides.

The isothermal section at 370°C of the Zn–Al–Ti ternary system in the Zn-rich region was investigated experimentally using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and X-ray diffraction. The experimental results reveal that T + TiZn 16 + TiZn 8 , T + TiZn 16 + η-Zn, T + TiZn + TiZn 8 , TiZn + TiZn 3 + TiZn 8 , T + η-Zn + (α-Al), T + (α-Al) + TiAl 3 and T + TiAl 3 + TiAl 2 are the seven three-phase equilibria in the Zn–Al–Ti system. T phase is a true ternary phase and it is in equilibrium with TiAl 3 , α-Al, η-Zn, TiZn, TiZn 8 and TiZn 16 at 370°C, respectively. The content of Zn in T phase varies from 12.8 to 56.8 at.%, Al varies from 17.7 to 59.5 at.% and Ti varies from 24.2 to 27.8 at.%. Ti 2 Al 5 was not found in the Zn–Al–Ti system at 370°C.

The enthalpies of mixing in binary liquid alloys of iron with the lanthanides (Pr, Nd, Gd, Tb, Dy and Lu) were determined at 1550–1950 K by means of isoperibolic calorimetry. They show exothermic effects (in the Fe–Pr system partially endothermic) which increase across the lanthanides row from the Fe–Pr system (Δ H min = −0.35 ± 0.10 kJ · mol −1 at x Pr = 0.22; Δ H max = 0.75 ± 0.24 kJ · mol −1 at x Pr = 0.76; T = 1829 K) to the Fe–Lu system (Δ H min = −11.58 ± 0.50 kJ · mol −1 at x Lu = 0.35; T = 1950 K).

The structural, mechanical and thermal properties of CeB 2 , PrB 2, NdB 2 , and PmB 2 compounds in hexagonal AlB 2 -type (P6/mmm) and ReB 2 -type structures (P6 3 /mmc) are investigated using density functional theory. The results indicate that those compounds with hexagonal AlB 2 structure are the most stable among the considered structures. The mechanical properties are calculated according to the elastic constants by means of the Voigt–Reuss–Hill averaging scheme. The temperature dependence of various quantities such bulk modulus, Debye temperature, thermal expansion, heat capacity, and Grüneisen parameter have been analyzed using the quasi-harmonic Debye model.

A comprehensive understanding of the solidification process is of paramount importance for the control and prediction of actual casting characteristics. The present work presents the potential of cooling curve analysis to characterize the solidification path of a cast hypoeutectic series of Al–Si–Cu alloys. The aim of this work was to examine how variation in chemical composition of Si (from 5–9 wt.%) and Cu (from 1–4 wt.%) may affect characteristic solidification temperatures, their corresponding solid fraction, and thermal freezing ranges of the investigated alloys. All solidification parameters that have been calculated using cooling curve analysis show good correlation with the corresponding parameters calculated using commercial Pandat software. These parameters, either collected from the cooling curve analysis or calculated using Pandat software, can be easily incorporated into existing simulation software packages in order to improve their accuracy. In addition, cooling curve analysis can be used to estimate the effect of cooling rate on the above mentioned solidification parameters and used as additional input data for simulation.

The degree of undercooling of three Pb-free solders Sn-4.5%Ag-0.5%Cu, Sn-4.0%Ag-0.5%Cu and Sn-2.5%Ag-0.5%Cu (wt.%) are determined using differential scanning calorimetry in two configurations (i) solder balls and (ii) solder joint samples prepared by reflowing solder balls on copper substrates (Ball Grid Arrays package). A large degree of undercooling was observed with solder balls, while the corresponding undercooling was significantly reduced in the case of solder joints. Samples were characterized using optical microscopy and scanning electron microscopy. The microstructure of the solder alloy is greatly affected by the reaction at the solder/copper interface. The role of the interfacial reaction between liquid solder alloy and copper substrate on the undercooling degree of the liquid alloy as well as on the microstructure of the solder joint is discussed.

Sub-stoichiometric (nitrogen-deficient) and Nb-substituted (Ti 1 – y Nb y N, 0 ≤ y ≤ 1) titanium nitride thin films were deposited by means of radio frequency magnetron sputtering on SiO 2 and Si (311) substrates and compared. Thickness of TiN x films varied in the range 116–230 nm, while the value was constant, at 500 nm, for Ti 1 – y Nb y N films. The sub-stoichiometric TiN x films deposited at room temperature are amorphous, independent of nitrogen partial pressure (in the range of 1.6 to 3.33 Pa). Annealing of the films at 600°C resulted in amorphous–crystalline transition only in the film deposited at 3.33 Pa, which crystallized into tetragonal ∊-Ti 2 N phase. The other films remained amorphous. The hardness and Young's modulus were highest for the film deposited at a nitrogen pressure of 3.33 Pa, viz., 5.9 GPa and 105 GPa respectively. In contrast, all as-deposited Nb substituted TiN (Ti 1 – y Nb y N) films crystallized into rock salt structure with (111) orientation up to y = 0.77. The hardness and Young's modulus for Ti 1 – y Nb y N films increased with increase in Nb concentration. The highest values of hardness and Young's modulus obtained were 29 GPa and 320 GPa respectively. This study shows that nitrogen deficient titanium nitrides are more difficult to crystallize and exhibit lower hardness than metal substituted nitrides.

The results of investigation of the isothermal oxidation wear mechanism of Ti-46Al-7Nb-0.7Cr-0.1Si-0.2Ni intermetallic alloy with AlCrN coating are presented. Tests in 9%O 2 + 0.2%HCl + 0.08%SO 2 + N 2 atmosphere were performed at a temperature of 700°C. The structure of the specimen and chemical composition of the oxidation products were analysed using scanning electron microscopy and energy dispersive X-ray analysis. In addition, mass changes were investigated.

The membrane electrode assembly determines the performance of the polymer electrolyte membrane fuel cell. Degradation of the membrane electrode assembly has been the subject of recent research with impedance measurements, cyclic voltammetry measurements, scanning electron microscopy, tunneling electron microscopy as the techniques currently in use for characterization. This paper considers the membrane electrode assembly as a laminate and uses the study of mechanical properties as a tool for identification of delamination. An attempt has also been made to compare the changes in the mechanical properties obtained due to delamination with the results obtained from impedance measurement, cyclic voltammetry studies and performance in the fuel cell. In this study mechanical properties such as stress, strain and modulus have been measured and the decrease in modulus as a result of delamination has been characterized. It has been found that study of the mechanical properties of the membrane electrode assembly compliments the other techniques and can be used in the post-mortem analysis of causes for degradation.

Three kinds of epoxy resins, i.e. tetraglycidyl diaminodiphenyl methane (AG80), difunctional diglycidyl ether of bisphenol-A (E51) and novolac type epoxy resin (F46) were selected as matrices for carbon fiber/epoxy composites. The objective of this work is to study the mechanical properties of fiber/epoxy composites by using these three kinds of epoxy resins with different physical and chemical performance. The results show that the composites fabricated with AG80 present the best stiffness and the composites prepared with E51 have the best toughness. The stiffness and toughness of the composites prepared with F46 are middle values located between those for AG80/epoxy and E51/epoxy composites. Thus, the mixed epoxy resin is a promising approach for industrial production.

Ceramic hollow microsphere reinforced metal matrix syntactic foams were produced by means of pressure infiltration. The reinforcing ceramic hollow microsphere's (SL300) microstructure was investigated. The microspheres contained various oxide ceramics, mainly Al 2 O 3 and SiO 2 . The samples were characterized using X-ray diffraction measurements and energy dispersive spectroscopy maps. The results showed that the Al 2 O 3 and SiO 2 distribution in the wall of the microspheres is not equal; Al 2 O 3 needles are embedded in the surrounding mullite and SiO 2 phase. Line energy dispersive X-ray spectroscopy measurements showed that, due to the above mentioned uneven distribution of Al 2 O 3 rich particles, a chemical exchange reaction took place between the aluminium alloy matrix and the microspheres and because of that the wall of the hollow microspheres became damaged.

Copper/bamboo fabric composite prepared via a silver catalytic electroless plating process is reported. The microstructure of the composite was analyzed by means of scanning electron microscopy, which illustrated that the copper coating was composed of spherical particles and clusters. The composition and chemical state of the metal layer were measured using X-ray diffraction and energy-dispersive X-ray analysis spectra; copper and a small amount of nickel were detected. Mechanical properties were measured based on a standard (ISO 13934-1:1999) for the fabrics with and without copper coating. The breaking force for the composite was improved by about 16.8% compared to uncoated bamboo fabric. The electromagnetic interference shielding effectiveness of the composite was more than 40 dB at frequencies ranging from 0.2 to 1000 MHz. The copper coating on bamboo fabric passed the Scotch®-tape test.