مجله مهندسی متالورژی
پیاپی 69 (بهار 1397)

In the present study, crystallization behavior of amorphous (Al90Ni8Zr2)98MM2 alloy have been investigated using X-ray diffraction, differential scanning calorimetry (DSC) and micro-hardness test. For this purpose, amorphous ribbons were manufactured using a melt-spinning apparatus. The average thickness of the produced ribbon was 20 micrometers. The results of the kinetic test showed that the crystallization in the amorphous alloy (Al90Ni8Zr2)98MM2 occurs at least in three steps. DSC graphs were analyzed using Kissinger's method and the activation energy for the first, second and a third stage of crystallization was 331, 241, and 330 kJ/mole, respectively. The results of micro-hardness test showed that the amorphous phase hardness at room temperature was 413 Vickers, which was reduced to 276Hv by isothermal heat treatment at temperatures close to first transformation temperatures. It has been observed that, after heat treatment at a temperature above α-Al phase transformation temperature, the hardness increases to 453Hv. Formation of intermetallic phases at 800K, cause the hardness drop to 269Hv.

The constitutive equations based on the dislocation mechanics are applicable at relatively low temperatures. Nevertheless, considering the diffusion processes is necessary for modeling the hot flow stress. In other words, the softening effects of dynamic recovery and recrystallization should be taken into account. In the present work, a modified Zerilli–Armstrong constitutive equation for predicting the hot flow stress of a microalloyed steel was proposed, in which the effects of hardening and softening phenomena were contemplated. It was shown that the original equation is not able to model the softening part of flow curves related to dynamic recrystallization and it was clarified that the constants of the model should be modified for appropriate consideration of the effects of dynamic recovery. On the other hand, it was found that the hardening and softening stages should be separated and the peak strain can be utilized into the flow stress formula. While retaining the general form of the original Zerilli–Armstrong model, the developed constitutive relation was able to appropriately predict the hot flow stress. Conclusively, this constitutive model can be considered as a simple and viable one for modeling the flow stress of steels.

In this present study, the mechanical alloying technique was used to the amorphization of Al80Fe20 system. The particle size, the thermal behavior, and the magnetic properties were investigated on the milled specimens in the different milling times. The performed tests on the milled specimens included the X-ray diffraction (XRD), determine the magnetic properties, and the Differential scanning calorimetry (DSC). The results were shown that the milling time for the amorphization was 70 h, in this system. The peaks of DSC were demonstrated that the mechanical alloying caused to formation of the amorphous phase. It is noteworthy that increasing the milling time after 70 h caused to formation of the crystalline Al and Al3Fe phases. In addition, the amorphization of used alloy at 70 h caused to decreasing the residual magnetism to 0.11 T and improving the soft magnetic behavior.

In this research, the effect of first step age temperature during two steps aging process on the strength, ductility, hardness and microstructure of Al-Cu-Mg alloy has been investigated to obtain an optimum combination of strength and ductility. After solution treatment and quenching the samples in the water, they have artificially been aged for 2 hours in 175, 190, 205°C. Then the samples were naturally aged for 10, 50, 100 hours. To investigate the mechanical properties, all the samples were subjected to tensile test and microstructure analysis after each cycle. The data of one and two steps aging treatment and also the effect of first step temperature were compared. The results show that second step of natural aging by affecting the stability of precipitates and microstructure leads to change in tensile properties. With increasing the first step aging temperature, the impact of second step aging process decreases. Also two steps aging process with temperature of 190°C in first step and time of 50 hours in naturally aging results in and optimum combination of strength and ductility.

In recent years, methods based on sever plastic deformation have been considered to improve the structure and mechanical properties of various metals, especially aluminum. Among the different methods, accumulative roll boding process is appropriate to improve the structure of metals sheet. Using this method, nanostructured aluminum alloys and aluminum nanocomposites reinforced by particles can be manufactured and simultaneously improve mechanical properties such as tensile strength and hardness. In most researches, after applying of 6 to 8 cycle of accumulative roll boding process the average of grain size reduced to less than 500 nm and the tensile strength increased to more than 2 times of the strength of the annealed aluminum. Accumulative roll boding process can be applied to different sheets in different routes, which will have a different effect on structural modification and mechanical properties. In this paper, the effect of various methods of accumulative roll boding process on microstructure and mechanical properties of aluminum alloys and aluminum based composites has been investigated.

Friction stir welding is a new and effective method for joining the alloys with welding problems and also for the dissimilar alloys. In the present study, friction stir welding is used to join st37 low carbon to 304 stainless steel plates at different tool rotation speeds. The stir zone in the 304 steel shows evidence of dynamic recrystallization with a moderate dislocation density. The stir zone in the st37 steel appears to experience dynamic recrystallization too, although the allotropic transformation during cooling cycle of the welds removes the features of dynamic recrystallization and produces a fine ferrite-pearlite microstructure with a low dislocation density. The relationship between hardness and microstructure is investigated through Hall-Petch equation for the stir zone of both steels. The results of weighted least-squares fit also show that the average hardness of austenite in the 304 steel and ferrite in the st37 steel inside the stir zones has a reverse relation with hardness according to the Hall-Petch equation. The hardness of the base metal and the stir zones of 304 steel does not stand on a same Hall-Petch line that can be attributed to the relatively higher dislocations due to the dynamic recrystallization of stir zones.

In this research, the effect of different filler metals on microstructure and mechanical properties of dissimilar joint between AISI316 and AISI430 stainless steels was studied. For this purpose, GTAW process with ER308L, ER309L and ERNiCrMo4 filler metals with diameter of 2.4 mm were used. Microstructure and fracture surfaces of the welded samples were analyzed by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and ferritoscopy. Also, the mechanical properties of the joint were evaluated by tension, impact and microhardness tests. The results show that the microstructure in the welded sample with ER308L filler metal was austenitic with lathy and skeletal ferrite and Widmanstatten austenite. In the welded sample with ER309L filler metal the microstructure was composed from austenite with skeletal ferrite and in the welded sample with ERNiCrMo4 filler metal was fully austenitic. In tension test, all samples fractured from AISI430 base metal in a ductile manner. ER309L filler metal indicated low impact energy of about 49 J and ER308L and ERNiCrMo4 filler metals indicated higher impact energy of about 120 and 73 J, respectively. The fracture of the weld metal in the welded samples with ER308L and ERNiCrMo4 filler metals was ductile and in the welded sample with ER309L filler metal was brittle. The results of microhardness test indicated that ER308L and ERNiCrMo4 filler metals had higher microhardness as compared with ER309L filler metal due to the presence of alloying elements, finer microstructure and higher grain boundaries.

Solidification microporosity forms within dendritic space of gas turbine blades at large section in investment casting. Microporosity content more than permissible limit significantly reduces mechanical properties of turbine blades at the service conditions. In these work effects of Hot Isostatic pressing (HIP) followed by standard heat treatment cycle process on microstructural characteristics such as grain boundaries serration, microporosity content and mechanical properties of Ni-base superalloy IN738LC have been investigated by optical, Scanning Electron Microscopy (SEM) and practical experiments. The results of studies have shown that hot isostatic pressing provides more serration at grain boundary and could mostly eliminate or reduce microporosity, was generated during solidification by means of sintering. HIP is able also to improve creep life and ductility but has no effect on yield stress at tensile experiment.