نشریه فرآیندهای نوین در مهندسی مواد
پیاپی 56 (بهار 1400)

In the present work, effect of WO3 dopant on the sintering behavior, microstructure evolution, and microwave dielectric loss of Ba(Co1/3Nb2/3)O3 ceramics were systematically investigated. (1-x) Ba(Co1/3Nb2/3)O3 – (x) WO3 compounds, where x=0, 0.002, 0.004, 0.008, and 0.02, were prepared by the conventional solid state synthesis route followed by sintering at 1300-1450ºC for 10h at air atmosphere. Solid solution limit of WO3 oxide in the Ba(Co1/3Nb2/3)O3 compound and formation of any secondary phase were determined by X-ray diffraction (XRD) technique. In addition, the obtained XRD patterns were simulated by Rietveld refinement and degree of 1:2 cation ordering was calculated based on the refinement results. Scanning electron microscopy (SEM) was employed to study microstructural development of the ceramic samples and to directly identify secondary phase formation and their morphology. XRD results demonstrated that WO3 could solve into Ba(Co1/3Nb2/3)O3 structure for x<0.02, while detailed investigation by SEM directly indicated that even for Ba(Co1/3Nb2/3)O3 – 0.002 WO3 (x=0.002) composition additional phases were precipitated during high-temperature sintering. According to the XRD results, it was found that BaWO4, and Ba9CoNb14O45 compounds were formed as secondary phases. On the other hand, Rietveld refinement simulation showed that addition of WO3 into Ba(Co1/3Nb2/3)O3 results in a significant decline in the 1:2 cation ordering degree, where it was deceased from 95% to 59% when x was increased from x=0 to x=0.02. Quality factor, Q, (inverse of dielectric loss, 1/tanδ) of the prepared ceramics were measured at the microwave frequency range and it was found that incorporation of WO3 noticeably lowered the quality factor of Ba(Co1/3Nb2/3)O3 materials, where Q×f (f denotes resonance frequency) was measured to be 61,000 GHz for x=0 composition, whereas, measurements did not show any resonant peaks for x=0.02 ceramics, which means the ceramics suffer from a huge microwave dielectric loss.

In this research microstructural evolution during high temperature brazing of dissimilar bonding of IN738 Ni-base superalloy to TiAl intermetallic compound using an amorphous Ni-Si-B ternary alloy was investigated. Phase transformations via solidification and solid state reactions are discussed. Observations indicated that the microstructure of IN738/MBF-30/TiAl joint consist of four different zones; isothermal solidification zone in both sides, athermally solidified zone in the bond center, diffusion affected zone in the IN738 side and reaction layer in the TiAl side. γ-Ni solid solution phase in ISZ of the IN738 side and binary isostructural solid solutions in ISZ of the TiAl side were formed during holding time at bonding temperature. Ni-Cr borides have been formed due to binary eutectics associated with γ-Ni solid solution in the ASZ during cooling. Cr-Mo borides and Ni-rich boride with different morphologies were precipitated in the DAZ. Ni element from MBF-30 molten interlayer reacted with γ-TiAl base, leading to the formation of the reaction layer containing single phase δ-Ti2Ni and triple phase τ2-Al2TiNi+τ4-AlNi2Ti+β1-NiAl layers adjacent to TiAl substrate. Microhardness evaluation of different zones indicated that some high hardness phases have formed in the bond region and presence of the γ-Ni solid solution in the ASZ cause to decrease the detrimental effects of them.

In this research paper, principal attention is given to the effect of interstitial nitrogen on the precipitation of secondary phases and tensile strength of gas tungsten arc welded (GTAW) Inconel 718. Welding was performed using Ar+(0-5%)N2 shielding gas mixtures. Secondary phases were characterized by optical microscope, field-emission gun scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). The results revealed that with increment of nitrogen content, the volume fraction of Nb-bearing phases like Laves and carbide increased due to increased microsegregation of Nb in interdenderitic region. Moreover, nitrogen was found to have negative role in the size of gamma double prime particles within γ core. According to the tensile test results, Ar+1%N2 weld samples showed the maximum ultimate tensile strength and yield strength compared to other samples. While more nitrogen leads to reduction of both strengths due to increased Laves phase quantity and decreased size of gamma double prime precipitates.

This paper reports an investigation carried out to determine the effect of aluminum oxide Nano-particles coated with manganese on the microstructure and mechanical properties of single-pass butt joint of low carbon steel plates of 6.0 mm thicknesses using gas metal arc welding process. After selecting appropriate welding parameters and adding 0.25 gr and 0.5 gr of Nano-particles into the joint line and carrying out the welding, the samples were prepared for micro hardness and tensile tests. Furthermore, the sample with optimum Nano-particles having the highest ultimate tensile strength and without having any visible and metallurgical defects in the microstructure was selected for further investigation.  Subsequently, microstructures of the weld without and with optimum Nano-particles were studied using optical microscopy and scanning electron microscopy (SEM) and the fractured surface of the weld obtained from tensile testing was investigated for the samples without Nano particles and the optimum sample with Nano materials were studied. The results show that Nano-particles added to the weld pool penetrated into the weld zone and helped in formation of acicular ferrite in the microstructure. Based on the results obtained, ultimate tensile strength and percentage of elongation of samples without Nano-particles and with optimum Nano-particles were increased from 387 MPa and 6.8% to 408 MPa and 13.6%, respectively. In addition, the average hardness of the weld metal without Nano-particles and with optimum Nano-particles were increased from 158 VHN to 172 VHN respectively

The surface properties of industrial parts and especially wear behavior of them plays an important role in life service of them. Chromium coat and plasma nitriding are two common ways of applying hard coat on diverse substrates. In this paper, both of these coats applied on titanium substrate as an advanced engineering materials and the wear behavior of them were compared. For this aim, Pin on disk test was carried out and the weight loss was measured. For determining the wear mechanism of the sample, field emission scanning electron microscopy study was carried out on wear traces. Results show that although the achieved surface micro hardness value of chromium coats is higher than nitride samples, 594 HV at current density of 50 A/dm2 and 60 min versus 528 HV at 600 oC for nitride sample, but they show the weaker wear resistance. Adhesive wear mechanism is determined mechanism at both samples. Also, the lowest coefficient of friction is obtained at sample nitride at 600 oC

Biodegradable porous scaffold polymers are good candidates for tissue engineering. In this research, a three-dimensional porous poly-lactic acid (PLA) scaffold was prepared using a Fused Deposition Modeling (FDM) including about 70% porosity. The study of the phases of primary filament and scaffold using the X-ray diffraction (XRD) test shows that no significant phase difference has been created due to the manufacturing process and the polymer retains its phases properties. The results of the mechanical properties evaluation by the compression test show that the mechanical properties of the scaffold are different in both the parallel and perpendicular directions of the Z axis during the printing, and the mechanical properties of the scaffold are of anisotropic property. Microstructural study by Scanning Electron Microscope (SEM) also shows that the morphology of scaffold porosities is different in two directions, and this is the main cause of anisotropic mechanical properties. Anisotropy of the mechanical properties of FDM produced scaffolds should be considered during load bearing applications in vivo.

Magnesium alloys, as the lightest structural alloys, due to their high strength-to-weight ratio offer significant potential for improving energy efficiency of various transportation systems. This paper addresses the influence of friction stir processing (FSP) treatment on the microstructure and mechanical properties of cast AZ91 Mg alloy. It is demonstrated that FSP treatment enables elimination of dendritic structure, significant grain refining, break-up and partial dissolution of coarse β and formation of ultra-fine sub-micron Mg17Al12 particles. These microstructural modifications resulted in enhancement of mechanical properties in terms of tensile strength and energy absorption by 48% and 283%, respectively. It is shown that FSP treatment altered the failure mechanism of the alloy from brittle cleavage-dominant mode to ductile dimple-dominant mode which can increase the potential of Mg alloys to use in safety-critical application. Therefore, it can be concluded that FSP, as a process of sever plastic deformation at high temperature, has a great potential to tailor the microstructure and enhancing the mechanical properties of cast Mg alloys.