Sharif Digital Repository

A study has been made to determine the influence of the carbon content on the kinetics of dynamic and static recrystallization during and after hot deformation of carbon steels. For this purpose, single- and double-hit hot compression experiments at various strain rates and temperatures together with the Avrami-type kinetics equation and Bergstrom approach have been utilized to investigate recrystallization behavior. The results show that the apparent activation energy of hot deformation decreases with increasing carbon content and this phenomenon results in a faster dynamic recrystallization at high temperatures and/or low strain rates. Also, increasing carbon content leads to a higher rate... 

The α-FAPbI3 perovskite nanocrystals exhibit interesting optical properties, which make them attractive for versatile optoelectronic applications; however, the spontaneous phase transformation to the non-perovskite phase (δ-FAPbI3) in humid conditions requires a new strategy to improve the phase stability of the material. We employed 3-Aminopropyl triethoxysilane (APTES)-assisted reprecipitation and sol-gel methods at ambient temperature to prepare stable α-FAPbI3 nanocrystals embedded in a silica matrix. Transmission electron microscopy (TEM) and X-ray diffraction analysis determined that ultrafine perovskite nanocrystals (11.5 ± 3 nm) were uniformly distributed in a silica matrix.... 

The microstructure evaluation and phase transformation of multi-pass friction stir-processed (FSP) commercially pure titanium were investigated using optical microscopy, scanning electron microscopy equipped with a backscatter detector, and electron backscatter diffractometer. The microstructure characterization shows that the grain refinement mechanism changed with the increasing number of passes. The equiaxed α grains in the untreated sample changed to lath-shaped grains surrounded by serrated grain boundaries, and produced Widmanstätten morphology during the cooling cycle of FSP. The higher micro-hardness values and bio-corrosion resistances of the FSP-treated samples are due to their... 

316L stainless steel is widely used as cathode blank in copper electrorefining. These cathode blanks become distorted through the cathode stripping in the cathode stripping machine and must be straightened before using again. After several times repeated cold straightening, the cathode blank spring backs to its distorted position during electrorefining resulted in short circuits and current efficiency decreases. The objectives of this study were to: i) investigate the cathode blank spring back behavior and properties; ii) propose a solution adhere to the subject. To achieve these goals, more than 20 samples are selected and cut into small pieces to simulate the cold straightening process.... 

The strain-induced martensitic transformation greatly affects the plastic behavior of the metastable austenitic stainless steels. The martensitic transformation continuously changes the initially homogeneous material into a strongly heterogeneous bi-phase one. In addition to the hardening behavior, this phenomenon would influence the damage growth and load-carrying capacity of the material during the plastic deformation. In this study, plastic behavior of the material AISI 304 including the hardening and damage growth, has been examined at low temperature; where a high rate of martensitic transformation affects the microstructure strongly. Experimental analysis and microscopic observations... 

Turn-to-turn fault (TTF) is one of the most prominent reasons for transformer failure. This fault typically initiates by involving only a few turns and can progress to a much more severe fault in a very short time. Therefore, it is critical for the transformer protective relays to detect the TTF in the early stages of its occurrence. The conventional differential relays, in general, cannot detect a minor TTF, and thus, a supplementary protective algorithm is usually required to improve the sensitivity of differential relays against TTFs. This paper proposes a percentage-based fault-related incremental currents method for the precise detection of a low-level TTF. The proposed method employs... 

The phase field approach (PFA) for the interaction of fracture and martensitic phase transformation (PT) is developed, which includes the change in surface energy during PT and the effect of unexplored scale parameters proportional to the ratio of the widths of the crack surface and the phase interface, both at the nanometer scale. The variation of these two parameters causes unexpected qualitative and quantitative effects: shift of PT away from the crack tip, "wetting" of the crack surface by martensite, change in the structure and geometry of the transformed region, crack trajectory, and process of interfacial damage evolution, as well as transformation toughening. The results suggest... 

This paper addresses the phase transformations and mechanical performance of dissimilar resistance spot welds between 2304 duplex stainless steel (DSS) and 1.2-GPa martensitic (MS) advanced high-strength steel (AHSS). The solidification mode and transformation path of the fusion zone (FZ) were analyzed. The key metallurgical feature of the FZ was the formation of a duplex microstructure consisting of delta ferrite and martensite. The FZ size at the sheet/sheet interface was the critical factor controlling the fraction of pullout failure during the partial thickness-partial pullout mode, the load-bearing capacity, and the energy absorption capability of the DSS/MS dissimilar resistance spot... 

Abstract: Nickel–titanium (NiTi) nanoparticles are ultrafine smart materials manifesting shape memory effect at very small scales. We have produced NiTi nanoparticles surrounded by an amorphous carbon shell using an innovative spark discharge system. The resulting nanoparticles were studied using various characterization methods to systematically study their size, morphology, size distribution, composition, structure, and thermal behavior. Field-emission scanning electron microscopy and dynamic light-scattering results indicated that the average size of the produced nanoparticles was about 13 nm. High-resolution transmission electron microscopy, energy-dispersive spectroscopy (EDS), and... 

This paper proposes a straightforward control method for voltage control of a three-phase transformer-based inverter in uninterruptible power supplies or distributed generation systems. The approach offers a dual-loop design consisting inner current control loop and outer voltage loop. Sliding mode current controller provides desired bandwidth for voltage controller which consists of a state feedback term for stabilization and resonant term for harmonic damping. The proposed scheme provides fast dynamic response and low total harmonic distortion even for high power inverters with the limitations of switching frequency and LC filter components. Experimental studies for 2KVA linear and... 

Recently, an increasing concern has been raised about turn-to-turn faults (TTFs) in power transformers, because these faults can lead to severe transformer insulation failure and consequently, its outage. Generally, it is impossible to experimentally analyze the transformer behavior under such faults, since the implementation of those experiments may be substantially destructive. Therefore, computer-aided models should be developed to investigate the performance of transformer protective relays under turn-to-turn faults. So far, existing transformer models are mainly formulated to implement in the EMTP-based softwares. However, most of power system protection engineers and researchers... 

The paper addresses the microstructure and failure characteristics of dissimilar resistance spot welds between austenitic stainless steel and ferritic stainless steel. The fusion zone (FZ) of dissimilar welds exhibited complex microstructure consisting of ferrite, austenite and martensite. The development of this triplex structure in the FZ was explained by analysing the phase transformation path and austenite stability. Results showed that all dissimilar welds failed in partial thickness-partial pull-out failure mode. It was shown that the fraction of the nugget fracture of the weld is reduced by increasing the FZ size improving the mechanical performance of the weld. Peak load and energy... 

Porous shape memory alloys are a class of very interesting materials exhibiting features typical of porous metals and of shape memory alloys. In contrast to dense shape memory alloys, considerable plastic strain accumulates in porous shape memory alloys even during phase transformation. Moreover, due to the microstructure of porous materials, phase transformation and plasticity phenomena are significantly pressure-dependent. In this article, we propose a three-dimensional phenomenological constitutive model for the thermomechanical behavior of porous shape memory alloys able to predict shape memory effect, pseudo-elastic behavior and plastic behavior under proportional as well as... 

Implementation of new materials in automotive body-in-white requires through knowledge of their metallurgical response to welding process thermal cycle. This two-part paper aims at understanding the physical and mechanical metallurgy of stainless steels, as interesting candidates for automotive application, during resistance spot welding. The second part addresses the phase transformations in the heat affected zone of three types of stainless steels including austenitic, ferritic and duplex steels. Failure modes and mechanical properties of stainless steel resistance spot welds are discussed. The peak load and energy absorption of stainless steel resistance spot welds are compared with... 

Weldability is one of the key requirements for automotive materials. This two-part paper aims at understanding the metallurgical phenomena during resistance spot welding of stainless steels, as interesting candidates for automotive body in white. Part I addresses the phase transformations in the fusion zone of three types of stainless steels including austenitic, ferritic and duplex types. The solidification and solid state phenomena including columnar to equiaxed transition, ferrite– austenite post-solidification transformation, martensitic transformation and carbide precipitation are discussed. Particular attention is given to the effect of high cooling rate of resistance spot welding... 

Different concentrations of CaF2 were incorporated in mica glass-ceramics to evaluate their effects on the crystallization and microstructure. The kinetics of phase transformations, and the microstructures of the final crystalline phase were found to be dependent on the concentration of nucleation agents. The results showed that CaF2 in high concentration had a synergistic effect and enhanced the formation of interlocked mica crystals. Non-isothermal DTA experiments showed that the crystallization activation energies of base glasses were changed in the range of 235-405 kJ/mol, while the crystallization activation energies of samples with addition of CaF2 were changed in the range of 419-747... 

Thermal oxidation of commercial ilmenite concentrate from Kahnouj titanium mines, Iran, at 500-950 C was investigated for the first time. Fractional conversion was calculated from mass change of the samples during oxidation. Maximum FeO to Fe2O3 conversion of 98.63 % occurred at 900 C after 120 min. Curve fit trials together with SEM line scan results indicated constant-size shrinking core model as the closest kinetic mechanism of the oxidation process. Below 750 C, chemical reaction with activation energy of 80.65 kJ mol-1 and between 775 and 950 C, ash diffusion with activation energy of 53.50 kJ mol-1 were the prevailing mechanisms. X-ray diffraction patterns approved presence of... 

In this paper, the shear deformation behavior of A286 Iron-based superalloy was studied with an emphasis on the influence of η phase on shear strength. The η (Ni3Ti phase precipitates at high temperature heat treatment or during services at the expense of gamma prime phase. According to the microstructural features, no evidences of η phase were found at 650 and 720°C. η phase precipitated at 780 and 840°C and the amount of it increased with an increase the time and temperature. Because of using the alloy as fasteners, investigation of shear properties and the influence of η phase on it are indispensable. The shear strength of the alloy with different volume fractions of η was examined. It... 

In this study, a particular nickel-based superalloy has been studied by transmission electron microscopy and the results are discussed in terms of phase transformations that may have taken place. The alloy has been found to consist of three major phases; a FCC γ phase containing a random distribution of nearly spherical γ′ - Ni3Al(Ti) - precipitates and elongated thin Ni3Ti(Ta) platelets with a hexagonal structure. The orientation relationships between the different phases have been determined; loss of coherency and possible occurrence of misfit-relieving interfacial dislocations are discussed and compared with experimental observations, and possible mechanisms of phase transformations are... 

In this study, the effects of stress-assisted heat treatment on the microstructure and phase transformation of a Ti-rich (Ti-49.52 at.% Ni) shape memory alloy were investigated. For this purpose, the alloy was heat treated at temperature of 500 °C for 10 h under applied stresses of 100 and 200 MPa. XRD, TEM, and repeated thermal cycling were employed to study the microstructure and transformation behavior of the heat-treated materials. Room temperature XRD diffractogram of the stress-free heat-treated material showed a weak reflection of austenite (B2), while that for the stress-assisted heat-treated materials had a high intensity implying the presence of residual austenite in the...