METALLURGICAL ENGINEERING
Year:2019 | Volume:22 | Issue:3 (75)|Papers Count:7

NiTi alloys (SMAs) are unique alloys, which have two attractive properties, shape memory effect and superelastic behaviour. Each property strongly depends on the composition, temperature and structure. The microstructure was investigated by optical microscopy, and chemical composition was analyzed using line scan analysis with energy dispersive X-ray spectroscopy. X-ray Diffraction (XRD) analysis of the bulk samples was conducted. The transformation temperatures were measured by Dilatometry. The shape memory and superelastic behavior of material were investigated using loading-unloading test and in-situ thermal cycling by applying electrical current at the end of sample and the shape changes of the bi-layers were recorded using a digital camera. The solute addition of Hf to NiTi alloy, led to a Two-way shape memory effect after tensile deformation. TheHf solute additions on NiTi alloy provides bell-curve shape memory behavior during stress-free thermal cycling after 12 percent tensile deformation which demonstrates the acute influence of Hf additions on the material’ s properties.

Fiber-metal laminate, is known as FML in the composite structures. It is widely used in the aerospace industry due to good properties, weight optimization and other industrial properties such as fire resistance, high strength limit, impact resistance and corrosion. In addition, these materials have special characteristics such as high strength and low weight. The discussion of buckling behavior is one of the most important discussions of mechanical science. The present study examines the effects of nanoparticles (nanocleys) on the mechanical characteristic (buckling) of metal/ composite materials of epoxy basalt. Multilayer fiber-epoxy base metal is made of two-layer aluminum alloy 2024 which the between of layers is located the nanocomposite core containing epoxy with nanoclay. The nanocomposite core consists of basalt fibers with the weight to area ratio (W/A) equal to 300 gr/m2 EPR1080 resin, EA1080 hardener and bentonite clay nanoparticles with 0, 1, 3 and 5 percentages. It should be noted that, all specimens were made using hand lay up. Also, buckling tests have done using a tensile test device at a speed of 0. 5 mm / min. The results of the buckling experiments indicate that the buckling load of FML beam with 5% of nanoclay rather than other percentages of it has better buckling load. Also, absorbed energy enhanced by increasing weight content of nano particle.

Fe5C2 is a recently attentioned iron carbide with high saturation magnetization and suitable chemical inertness that is known to have applications in different fields of engineering and medicine. One of the most important challenges in usage, is thermal stability of iron carbide nanoparticles. In this research, Fe5C2 nanoparticles were synthesized through a wet chemical route. XRD and TEM techniques were used to characterize chemical and morphological features of the sample at room temperature, respectively. TEM micrograph demonstrated the spherical morphology and the average size of the nanoparticles to be 30 nm. Thermogravimetric analysis (TGA) was performed to study the thermal behavior of this magnetic carbide. The activation energy of the phase decomposition was evaluated to be 139. 51 kJ utilizing Ozawa method. After heating, XRD test was repeated to investigate phase changes at high temperature. Finally, X-ray diffraction pattern proved the conversion of Hagg iron carbide to iron and magnetite after heat treatment.

The effects of niobium addition (about 0. 15 wt. %) and austenitizing temperature on the mechanical properties and microstructure of a low carbon steel (containing less than 0. 2 wt. % carbon) after martensite process were investigated in this research. After casting and hot rolling at 1050 ° C, the samples were austenitized at 900, 1000, 1100 and 1200 ° C for one hour followed by quenching in water. The as-quenched microstructures consisted of martensite along with a small amount of widmanstatten ferrite. The maximum hardness and strength were obtained from the Nb-containing sample austenitized at 1200 ° C. The hardness and tensile strength were 238 Vickers and 859 MPa, respectively. However, this sample showed an elongation of less than 1%. After 50% reduction in the thickness of the as-quenched samples by cold rolling, annealing was carried out at 600 ° C for 90 min. Optical and scanning electron microscopy (SEM) showed that the microstructure of the niobium-free sample was completely annealed and the martensite process effects were eliminated. The tensile strength of this sample was approximately the same as that of the hot rolled specimen. On the other hand, the Nb-containing sample exhibited a fine-grained microstructure. In comparison with the as-quenched condition, the hardness of the microalloyed specimen was essentially preservedafter martensite process. The results of the tensile test also revealed that the martensite process led to a good combination of strength and elongation in the microalloyed specimen because its tensile strength and elongation were higher than 700 MPa and 15%, respectively.

In this study, Co3O4-15 wt. % Fe2O3 composite powder was synthesized by two methods including: A-simultaneous milling of oxide mixtures (0, 1 and 16 h) and B-separately milling of cobalt oxide (0, 1 and 16 h) and iron oxide (0, 0. 5 and 7 h) and then mixing. Average particle size, particle morphology, iron oxide distribution, phase analysis, and heat storage capacity were studied by FE-SEM, XRD and thermogravimetry methods. It was found that preparation method has a significant effect on the above-mentioned parameters. The results showed that decreasing of iron oxide particle size and addition of it to cobalt oxide, increase the heat storage capacity of the material relative to as-received cobalt oxide. It was revealed that samples prepared by method A generally had lower heat storage capacity than samples prepared by method B. In addition, it was found that small-scale particle size of the composite may not necessarily improve heat storage capacity of the material, and spinel phase formation (Fe2O3. CoO) also is a key factor that has a great effect on decreasing the heat storage capacity.

In the present study, the effect of srain hardening exponent of material on flow characteristics, strain and damage distribution in sample and the load requred for the execution of the deformation process was investigated using finite element simulation. Results showed that the corner gap is not formed during deformation of ideally plastic material and the amount of equivalent strain is higher in the bottom side of sample compared with other regions. However, with increassing the work hardening exponent, the size of corner gap increases and the bottom side recieves less amount of strain. Also, damage factor in the sample of idealy plastic material is higher at the top side compared with other regions and the tensile stresses are applied on this area. Whereas, in the strain hardened material the higher damage factor was observed at the lower half of the sample. Finally, it was concluded that the pressing force increases with increassing the work hardening exponent.

This research deals with the effect of heat input on microstructure and mechanical properties of a sheet welded joints made of low alloy quenched and tempered steel (AISI 4340). Samples were prepared by manual gas tungsten arc welding (GTAW) with the low carbon consumable and also pre and post-weld heat treatment has been applied. Macro etch analysis showed that as the heat input increases, the HAZ area widespread. Indeed, the primary austenite grain phase grows up. The microstructure of HAZ consists of bainite and different ferrite morphology. The martensite and ferrite can be seen in the weld section. The volume fraction of ferrite increases as heat input increases. Furthermore, the ferrite morphology changes to acicular ferrite due to reduced cooling rate in the weld. Tensile strength decreases from 1078 Mpa to 970 Mpa correlatively coarsening in the heat-affected zone. Impact energy results vary with heat input correspondingly, the toughness increases in the weld and was reduced in HAZ. However, for conclusion, improved mechanical properties of the weld joints were achieved using low heat input and low carbon consumable.