Sharif Digital Repository

The effects of particle size and alloying elements on the size and morphology of secondary/intermetallic phases in gas atomized Al-20Si-5Fe-2X (X = Cu, Ni, Cr) powders were studied. The microstructure composed of particle-like primary Si with average diameter of 0.5-2.7 μm, δ-Al4FeSi2 intermetallic compound containing small amount of dissolved alloying elements with the size of 1.4-7.7 μm, and fine Al-Si eutectic matrix with 0.25-0.4 μm spacing. As the powder particle size decreases, the primary silicon becomes finer while its morphology is not affected. For instance, 58-75% reduction in the size of primary silicon was observed when decreasing the average particle size from 89 to 18 μm. A... 

The effect of bonding temperature on microstructure of a transient liquid phase (TLP)-bonded GTD-111 nickel base superalloy, using a Ni-Si-B interlayer, was investigated. At low bonding temperatures microstructure of the joint centerline is controlled by B diffusion. However, at high bonding temperature effect of base metal alloying elements on the joint microstructure development was more pronounced. Effects of bonding temperature on the formation of secondary precipitates in diffusion-affected zone and isothermal solidification time were also analyzed. It was found that above a critical temperature, formation of borides in diffusion-affected zone is suppressed and the time required for... 

Al-20Si-5Fe melt was rapidly solidified into particles and ribbons and then consolidated to near full density by hot pressing at 400°C/250 MPa/1 h. According to the eutectic-growth and dendritic-growth velocity models, the solidification front velocity and the amount of undercooling were estimated for the particles with different sizes. Values of 0.43-1.2 cm/s and 15-28 K were obtained. The secondary dendrite arm spacing revealed a cooling rate of 6 × 105 K/s for the particles with an average size of 20 μm. Solidification models for the ribbons yielded a cooling rate of 5 × 107 K/s. As a result of the higher cooling rate, the melt-spun ribbons exhibited considerable microstructural... 

Al-20Si-5Fe-2X (X = Cu, Ni and Cr) ribbons were produced by melt-spinning and consolidated by hot pressing at 400 °C for 60 min. The microstructure of the ribbons and the consolidated alloys was investigated using optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffractometry (XRD) method, and transmission electron microscopy (TEM). The hardness and compressive strength of the specimens at ambient and elevated temperatures were examined. The microstructure of the ribbons exhibited featureless and dendritic zones. Results of XRD and TEM showed formation of spherically shaped Si particles with an average diameter of 20 nm. Ultrafine Si (110-150 nm) and iron-containing... 

In this study, the effects of milling time, chemical composition and heat treatment on microstructure and magnetic properties of nanocrystalline Fe-Si (6.5-25 at.% Si) alloys prepared by mechanical alloying have been investigated. The results show that increasing the milling time or the Si content, decreases the lattice parameter and increases the internal microstrain. The prepared powders mainly consist of micron-sized particles with an average grain size of less than 20 nm. The specific saturation magnetization values are slightly less than those of single crystal or conventional Fe-Si alloys and decrease as Si content increases. The coercive force values of the nanocrystalline as-milled... 

The aim of this work is to study the effect of cooling rate and subsequent hot consolidation on the microstructural features and mechanical strength of Al-20Si-5Fe-2X (X = Cu, Ni and Cr) alloys. Powder and ribbons were produced by gas atomization and melt spinning processes at two different cooling rates of 1 × 105 K/s and 5 × 107 K/s. The microstructure of the products was examined using optical microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The particles were consolidated by hot pressing at 400 °C/250 MPa/1 h under a high purity argon atmosphere and the microstructure, hardness and compressive strength of the compacts were evaluated.... 

The effect of Al content and Si addition on the microstructural and creep properties of Mg-Al-RE alloys was investigated in this study. The steady state creep rates were specified and it was found that the creep behavior of the alloy, which is dependent on the stability of the near grain boundary microstructure, was improved by the RE and Si addition. For the AZ91 alloy, the results indicate a mixed mode of creep behavior, with some grain boundary effects contributing to the overall behavior. However for the RE and Si added samples, sliding of grain boundaries was greatly suppressed and the dislocation climb controlled creep was the dominant deformation mechanism. Analysis of creep rates...