Volume 107, Issue 12

This study combines geometrically necessary dislocation analysis and statistically-stored dislocation identification to investigate plastic deformation mechanisms in TA15 titanium alloy. Tensile tests were conducted up to different strains at room temperature. The slip lines were observed and identified using high resolution scanning electron microscopy. Geometrically necessary dislocations were calculated based on strain gradient theories. The statistically-stored dislocations were studied using transmission electron microscopy under two beam conditions. The results indicate that the ratio of <a>-type geometrically necessary dislocation density to <c+a>-type decreases from 5 – 8 to 1 – 3 with increasing strain from 0.8 % to 2.0 %. The <a>-type slip is the dominant deformation mechanism at the early stage of plastic deformation, whereas <c+a>-slips become dominant with further increase in deformation strain.

The effects of diffusion alloying on the microstructure and properties of TiC-reinforced Fe-based powder metallurgy materials were investigated via X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and mechanical testing. The results show that the distribution of Ti in diffusion-alloyed particles becomes more uniform with increasing diffusion time. The TiC particles formed in-situ during sintering, and the microstructure of Fe-based powder metallurgy materials prepared from diffusion-alloyed powders became rounder, more uniform and refined with increasing diffusion alloying time. The apparent hardness and the bending strength increased with increasing diffusion time. The fracture mode of the samples prepared from diffusion-alloyed powders had brittle and ductile characteristics.

Low volume fraction silica nanoparticulate-containing magnesium composites targeting structural and biomedical applications were synthesized using the blend–press–sinter powder metallurgy technique followed by hot extrusion, and subsequently characterized for their microstructural, mechanical and damping properties. The results of microstructural characterization revealed a maximum ∼32% reduction in grain size with 2 vol.% addition of SiO 2 nanoparticulates. The compressive properties of pure magnesium increased with the addition of SiO 2 nanoparticulates with Mg-2 vol.% SiO 2 nanocomposite exhibiting the maximum 0.2% compressive yield strength and compressive fracture strain. The addition of SiO 2 nanoparticulates enhanced the damping characteristics of pure magnesium with Mg-2 vol.% SiO 2 nanocomposite exhibiting the maximum damping capacity and damping loss rate with a minimum change in elastic modulus which is favorable when targeting magnesium for biomedical applications. An attempt has also been made in this study to compare the biomechanical properties of synthesized Mg–SiO 2 nanocomposites with those of natural bone.

Hybrid ceramic particulate reinforced A356 aluminium composites were prepared via the semi-solid stir casting process followed by hot rolling. Fine micron-sized SiC powders and coarse micron-sized Al 2 O 3 particles were used. The particles were coated in order to increase the ceramic incorporation and bonding with the metal. The corrosion behaviour of the composites was investigated using potentiodynamic polarization measurements. The results indicated that nickel coating and agglomeration of the fine SiC particles led to an increase in the corrosion current density in Tafel polarization curves. It was concluded that the composite manufacturing route employed reduced the corrosion resistance of rolled A356 alloy.

The phase equilibria in the Nb–Si–Ta ternary system at 1 373 K, 1 473 K and 1 573 K were investigated by means of back-scattered electron imaging, electron probe microanalysis and X-ray diffraction. The isothermal sections at 1 373 K, 1 473 K and 1 573 K consist of two three-phase regions and seven two-phase regions, without any ternary compounds. The compounds of NbSi 2 and TaSi 2 , αNb 5 Si 3 and αTa 5 Si 3 form continuous solid solutions, respectively. The solubilities of Nb in Ta 3 Si and Ta 2 Si phases are extremely large, whereas the solubility of Si in the β(Nb, Ta) phase is relatively small.

In the ternary Cu–Fe–Hf system, the mixing enthalpies of liquid alloys were investigated at 1873 K using a high-temperature isoperibolic calorimeter. The experiments were performed along the sections x Cu / x Fe = 3/1, 1/1 at x Hf = 0–0.47 and along the section x Cu / x Fe = 1/3 at x Hf = 0–0.13. The limiting partial enthalpies of mixing of undercooled liquid hafnium in liquid Cu–Fe alloys, ΔmixH¯Hf∞${\Delta _{{\rm{mix}}}}\bar H_{{\rm{Hf}}}^\infty $, are (−122 ± 9) kJ mol −1 (section x Cu / x Fe = 3/1), (−106 ± 9) kJ mol −1 (section x Cu / x Fe = 1/1), and (−105 ± 2) kJ mol −1 (section x Cu / x Fe = 1/3). In the investigated composition range, the integral mixing enthalpies are sign-changing. For the integral mixing enthalpy, an analytical expression was obtained by the least squares fit of the experimental results using the Redlich–Kister–Muggianu polynomial.

In this study, sintering was investigated for making SiAlON by utilizing low cost Si 3 N 4 , Al 2 O 3 and AlN powder precursors. These materials were mixed by mechanical ball milling, then processed by pressureless sintering or spark plasma sintering methods and the properties compared. XRD spectroscopy showed nano-scale and amorphous phases, which were produced after longer milling time. Investigation of the bulks after sintering revealed that increasing the pressureless sintering temperature resulted in an increase in density and mechanical properties of the specimens, as did changing the sintering technique from pressureless to spark plasma sintering. Furthermore, the wear mechanisms of adhesive or micro-abrasion of the rubbed surfaces were confirmed.

A sodium–lead–germanate glass system, with a composition of x Na 2 O-((100– y )– x )[80GeO 2 :20PbO]- y Er 2 O 3 ( x = 0–25 mol.%, y = 0, 1 mol.%), was prepared by melt quenching and used to investigate the effect of Na 2 O and Er 2 O 3 on the germanate anomaly. The structural and optical properties of the glass samples were investigated using X-ray diffraction, Fourier transform infrared, and UV–Vis spectroscopy analyses. Elastic properties of Er 2 O 3 -doped glasses ( y = 1) were studied by measuring longitudinal and shear velocities through the pulse-echo method at 5 MHz. Based on Fourier transform infrared spectroscopy analysis of Er 2 O 3 -doped glasses, the conversion of GeO 4 into GeO 6 indicates that the glass system possesses the germanate anomaly characteristic but has no density anomaly. Longitudinal, shear, bulk, and Young's moduli ( C L , μ , K , and E , respectively) increased to their maximum values at x = 10 mol.% but decreased with increasing amount of Na 2 O added. This finding reveals the elastic nature of the germanate anomaly. Increase in elastic moduli indicates enhanced network rigidity of the glass system in the germanate anomaly region, where the coordination number increased with the transformation of GeO 4 to GeO 6 . Subsequent decrease in elastic moduli ( x > 10 mol.%) denotes weakened network rigidity of the glass system because of enhanced formation of non-bridging oxygen. Furthermore, analysis using bulk compression and ring deformation models reveals the nonlinear trends of K bc /K e ratio and average ring size diameter as a result of the germanate anomaly. The anomaly also influenced optical properties of both Er 2 O 3 -doped ( y = 1) and Er 2 O 3 -free ( y = 0) glasses, where the optical energy gap ( E opt ) decreased with the addition of Na 2 O up to 10 mol.% and slightly increased with more than 10 mol.% Na 2 O. By contrast, Urbach energy ( E U ) and refractive index ( n ) showed opposite trends to that of E opt . The behavior of E U indicates changes in defect concentration, which affects E opt and n.

The effects of martensite cold work on the reverse austenite formation were analyzed using vibrating sample magnetometry. The samples were previously austenitized at 820°C for 30 min and then subjected to 85% height reduction. In sequence, the samples were aged at 600°C for different periods of time. For comparison purposes a set of non-deformed samples – labelled as 0% deformation – was submitted to the same aging conditions. The mechanical deformation initially retarded the reverse austenite formation in the matrix.

Silicon carbide nanowire forests were successfully synthesized via a vapor–liquid–solid process. The synthesized SiC nanowires had planar stacking faults perpendicular to the nanowire axis, diameters of 70 – 180 nm and lengths of tens to hundreds of micrometers, and grew along the [111] orientation. The SiC nanowire forest was characterized in a field emitter application and exhibited excellent field emission properties. Furthermore, we carried out in-depth investigations into the growth mechanism of the SiC nanowires through adjusting the growth conditions.