مجله مهندسی متالورژی
پیاپی 70 (تابستان 1397)

Ni-Cu (70.4-29.6 ; wt%) alloy was prepared by a Vacuum Arc Remelting (VAR) furnace and the effect of thermal treatment on structure and Curie temperature was investigated. The results of Inductively Coupled Plasma (ICP) and EDS analysis showed that specimens with 4 times remelting were homogenized. The Curie temperature of alloy that measured by vibrating sample magnetometry (VSM) under condition of slow cooling across from order-disorder transition of alloy was higher than specimens quenched from recrystallization temperature. Curie temperature (TC) of alloy that quenched after melting was 60 ͦ C ,TC of sample that annealed in 1000 ͦ C for 24 hours then quenched in water was 40 ͦ C and sample that after annealing twice heated up to 70 ͦ C was 45.5 °C .
The d-spacing of the specimen without heat treatment, calculated by X-Ray Diffraction, had a little increase due to clustering. In addition, the surface morphology by optical microscope confirmed this behavior of Cu–Ni alloys in the different heat treatment.

ZrN thin films were deposited on silicon (111) and 304 stainless steel substrates using direct current (DC) reactive magnetron sputtering. Effects of the substrate bias voltage on the films’ structure, morphology and hardness were investigated. The films were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM) and microhardness tester. XRD patterns showed grain size refinement from 19 to 13 nm with an increase of bias voltage from 0 V to 150 V. In addition, (111) and (200) diffraction peaks were only present and other orientation were omitted. FESEM cross section of ZrN thin films showed a well aligned columnar structure. Based on AFM images, the surface roughness was also increased at higher bial voltages. The increase of bias voltage also resulted in hardness increase. Maximum hardness of 1720 Vickers was obtained at 100 V bias. Negative bias voltage induces some residual stress in the films due to the increase in film's density by the elimination of porosity and voids.

Viscosity measurement could provide valuable information for melting and solidification base processes. In this paper a falling ball viscometer is used for measuring molten zinc viscosity. The molten metals are not transparent, some changes are done in the structure of conventional method for tracing ball movement. To solve this problem and in order to trace the movement of the ball (as a falling object) in the melt, a thin steel rod was connected to the ball. The displacement of the ball could then be evaluated from the rod end, which is outside the melt.Viscosity changes versus temperature is studied and as expected the viscosity of the liquid tends to decrease by increasing the temperature. In the beginning the rate of viscosity reduction was high (0.004 cP/°C) but then with increasing temperature the rate of viscosity reduction was decreased (0.0001 cP/°C). Molten zinc is a shear thinning fluid which means by increasing shear rate and mean speed of falling ball the viscosity of molten zinc would drop.

In this research effect of pouring temprature on the microstructure charactristics and hot tensile properties of an experimental Ni-Fe based superalloy N-155 has been investigated. the the melt was made in induction furnece by vacuum melting . All samples were poured in the preheated ceramic mould in different pouring temprature include of 1420, 1460, 1500, 1540 and 1580 °C and cooled down to room temprature in insolated conditions. The microstructure charactristics was analised by SEM and EDS tecniques. The hot mechanical properties was caried out in 816°C for each casting condition. The results show that the main microstructure consist of austenite as only dominate phase and the secand perciptation phase concist of MCN carbunitride as shown in continuse films precipteted in a long of grain boundries. A fine equiaxed / cellural structure can be shown in pouring temprature less than 1460°C due to initial dissolved carbids. The microstructure was modified to coarse dendritic structure inlongated in heat transpher direction at higher pouring temprature around of 1500 °C. The SE images showed that the carbides morphology are blocy and short script-like in matrix respectivly was preciptated in lower superheat while in higher temprature that change to the thin and non-continouse films which preciptated around of grains boundries. The hot UTS strength reached to 310 Mpa in 1500 °C due to modified structure and reduced to 245 Mpa because of enlarged grains and also elongation increases as pouring temprature rises.

Techniques of severe plastic deformation have been of continual interest in the production of novel metallic microstructures. Among these, accumulative roll bonding and simple shear extrusion have been extensively used in modern industry. In severe plastic deformations, applied strains have higher values than usual forming methods. Accumulative roll-bonding (ARB) and simple shear extrusion (SSE) processes are two severe plastic deformation processes capable of developing nanostructures. In this research, pure commercial copper was processes with ARB and SSE processes to obtain nanostructure and improve its structure, strength and hardness. Five passes of extrusion and seven passes of rolling were applied on the specimens with the same conditions. After preparing the specimens using both methods, mechanical and structural analyses were performed to evaluate the properties. Mechanical properties were measured using standard tensile and hardness tests. The results show that hardness, yield strength and ultimate tensile strength of the extruded and rolled specimens are increased by increasing the number of cycles. It was concluded that mechanical and structural properties of the specimens produced under ARB conditions appear to be better than SSE process. The layers structures were also studied by optical microscopy.

In this study, microbiologically influenced corrosion because of metabolite sulfate reducing bacteria (SRB) has been investigated in terms of its electrochemical behavior and surface phenomena. According to the results of electrochemical impedance spectroscopy, SRB through changing the bacteria culture medium during the first hour that sample was contacted to the bacteria solution, leads to an increase in corrosion resistance of HSLA X70 steels from 235 to 1651Ω.cm2. Microstructural examinations on the sample surface embedded in culture medium without bacteria, in comparison to the sample exposed to the culture medium with bacteria, indicate that corrosion products disperse widely which is consistent with the electrochemical tests results. On the contrary, sample exposed to the SRB included bacterial colonies, although shows a reduction in the uniform corrosion rate via the alkalization of the culture medium, provides sites under colonies which are prone to the pitting corrosion. The results of the cyclic polarization test confirm the susceptibility to pitting corrosion of the sample in the bacteria containing solution.

Damage caused by the two-phase gas-solid flow on the immersion tubes into the cyclone in cement industry that involved the environmental corrosive and abrasive factors is very important. In this work, the effect of particle flow and impact on the corrosion of the immersion tube was studied by the simulation of two-phase gas-solid into the cyclone. The results of gas-solid particle flow simulation and investigation on the particle abrasion on the immersion tube were shown that the impact of particle and shear stress is more in the upper half compared to the lower half. In addition, microstructural studies conducted by optical and scanning electron microscopies on the severely corroded sections of immersion tubes were shown that chromium carbide in the grain boundaries of austenite is formed and the cracks nucleated and growth along the grain boundaries of austenite. Formation of chromium carbide in the grain boundaries of austenite caused to decreasing the chromium from the areas adjacent to the grain boundaries, and as a result, intergranular corrosion occurred. Furthermore, the abrasion of the particle impact on the upper half of the immersion tube leading to severe corrosion based on the erosion corrosion mechanism.

Changes in strength and hardness of alloys are the main factor affecting the strain rate during creep. Thus measuring these properties and studying their correlations to creep resistance is an important issue. In the current study, the relationship between hardness and creep resistance for H13 steel was investigated during short-term creep. Creep fracture and 1% creep ductility tests were carried out at 500-600 C and stress of 872-926.5 MPa. Hardness test was carried out on all specimens and tensile test was performed on 1% creep ductility specimens at room temperature. The stress power of 4.5 indicated that dislocation creep was the dominant mechanism. The creep activation energy was lower than the activation energy of the self diffusion of alpha iron (109 kJ/mol), which indicated the role of stress in the apparent activation energy of this alloy. A linear relationship has been observed between hardness variation ratio and creep life ratio, although the slopes for the head and gage were slightly different. This linear relationship makes it possible to more accurately predict the life of H13 steel in creep condition.