This work, in my opinion, has reached the standard of meeting the requirements for the award of the degree of Bachelor of Technology in accordance with the regulations of the Date institute. The microstructural characterization of the milled powder was followed by scanning electron microscopy (SEM) and XRD. XRD and energy dispersive X-ray analysis (EDX) showed the formation of a homogeneous solid solution of the above mixtures after 50 hours of milling.
This indicates that major structural changes and dissolution of the alloying elements were almost complete by 25 hours, and further milling refined the product by MA. This is due to the entry of Si and Ni atoms into the lattice of the Al which causes deformation in it. Since the grain boundaries act as pinning points for dislocation motion, the increase in density helps to strengthen the structure and thus increases the yield strength of the material [2].
The aim of the present work is to synthesize Al-Si-Ni nanostructured material by mechanical alloying and product characterization. Depending on the shape of the crystallities, three categories of nanomaterials can be distinguished; layer-shaped tallities, rod-shaped crystallities and nanostructures composed of nanometer-sized equiaxed crystallites. This is indeed an approximation to keep the power in the range of 0.3-0.7. The smaller the grain size, the greater the number of grains will be, hence the strength of the material increases as grain boundaries act as pinning points for dislocations.
As a result, molecular dynamics simulation is considered a valuable tool in aiding our understanding of their deformation mechanism [12]. The result of atomic simulations has allowed several researchers to suggest different plastic deformation mechanisms as a function of grain size [13]. There appears to be a similarity in this existence of three regimes: (a) grain size d<1 µm regime in which unit dislocations and work hardening control plasticity; (b) regime with the smallest grain size d<10 nm, where limited intragranular dislocation activity occurs and grain boundary shear is assumed to be the mechanism of deformation.
Fatigue in Nanocrystalline materials: There have not been many reports on the fatigue properties of nanocrystalline materials. Among the earliest study is
Relatively high rate sensitivity, coupled with the near-zero post-flow hardening rates in the ultrafine-grained Cu and Ni, up to a high pre-existing dislocation density. The amount of stress is comparable to the creep stress at room temperature previously observed in nanocrystalline copper under a constant stress comparable to the maximum cyclic stress.
PREPARATION METHODS
Inert Gas Condensation : The inert gas condensation technique, conceived by Gleiter [17], consists of evaporating a metal (by resistive heating, radio-frequency,
Nanocrystalline alloys can also be synthesized by evaporating different metals from more than one evaporation source. The maximum densities of the pressed metal samples were measured with values of approximately 98.5% bulk density. However, porosity was found to have a significant effect on mechanical strength, especially in tension.
In recent years, Erbetal [19] has studied the synthesis, structure and properties of nanocrystalline nickel synthesized by pulse electrodeposition. They showed that grain refinement of electroplated nickel into the nanometer range results in unique and, in many cases, improved properties compared to conventional polycrystalline nickel. Electrodeposition of multilayer (1D) metals can be achieved using two separate electrolytes or much more conveniently using one electrolyte by appropriate control of stirring and electrical conditions.
3D nanostructure crystallites can also be prepared by this method using the interface of one ion with the deposition of the other. Electrodeposition has been shown to yield grain sizes in the nanometer range when the electrodeposition variables are chosen to favor nucleation of new grains rather than growth of existing grains.
Crystallization from amorphous solids: The basic principle for the crystallization method from the amorphous state [20] is to control the crystallization kinetics
Mechanical alloying process: The MA process itself starts with mixing the powders in the right amount and loading the powder mixture into the mill together with the grinding medium (generally steel balls). The less agglomerated state of the powder particles in the wet state is also a useful factor. Depending on the type of powder, the amount of the powder and the final constitution, a suitable mill can be selected.
If the material of the grinding container is different from that of the powder, then the powder can be contaminated with the material of the grinding container. But, depending on the design of the mill, there are some limitations to the maximum speed that can be used. Another limitation of the maximum speed is that at high speed (or grinding intensity), the temperature of the vial can reach a high value.
28] reported that when vanadium and carbon powders were milled together at different energy levels (by adjusting the positions of the magnets in the Uni-Ball mill), the final composition of the powder was different. Normally, the time is chosen in such a way that a steady state is reached between breaking and cold welding of the powder particles. The density of the grinding medium must be high enough so that the balls create sufficient impact force on the powder.
However, as with the grinding bowl, some special materials are used for grinding which include copper, titanium, niobium, zirconium (ZrO2), agate, yttrium stabilized zirconium (YSZ), partially stabilized zirconium + yttrium, sapphire , silicon nitride (Si3N4) and Cu-Be. In general, a large size (and high density) of the grinding media is useful, as the heavier weight of the balls will transfer more impact energy to the dust particles. It has also been reported that the final composition of the powder depends on the size of the grind used.
In the initial stage of milling, the powder to be milled is applied to the surface of the milling medium and also cold welded. It is also possible that more heat is generated due to the higher energy and this can also change the composition of the powder. If the amount of balls and powder is very small, then the production rate is very small.
The PCA adsorbs on the surface of the powder particles and reduces cold welding between powder particles and thereby inhibits agglomeration. The force of the impact plastically deforms the powder particles leading to work hardening and fracture.
EXPERIMENTAL
Structural Characterization of Mechanically Alloyed Powders By XRD Analysis
It can be observed that after 20 hours of milling Al and Ni peaks drastically decreased and gradually disappeared. The presence of Al-Ni, Ni-Si intermetallics revealed that they are present in an insignificant amount of the diffraction pattern, suggesting only complete solubility. The significant broadening of the Al peaks may be due to the reduction and refinement of the grain size and the increase in the internal stress caused by the repeated breaking and welding process during mechanical alloying.
The crystallite size, lattice microstrain, and lattice parameter of each alloy powder were calculated from the XRD peak broadening. According to the Scherrer equation, the crystallite size and lattice stress were determined for ground samples. The lattice parameter value was calculated from the peak positions of Al from the XRD pattern by extrapolating ‘aAl’ against the cos2θ/sinθ plot to cos θ =0 [42].
Fig (2) shows variation of (a) crystallite size (b) lattice microstrain and (c) lattice parameter of Al-rich solid solution in Al75Si15Ni10 alloy with meal. There was a continuous reduction in crystallite size during different stages of mechanical milling and it remained almost constant after 25 hours of milling. The crystallite size, estimated from the XRD analysis of the powder, was almost 30 nm.
The induced lattice strain increased rapidly during the initial stages of mechanical milling up to 25 h and slowly became nearly constant with further milling. The lattice parameter of Al-rich solid solution (aAl) was ∼4.045 Å and showed a strongly decreasing value with the progress of grinding varying from 4.049 Å to 4.0381 Å. The smaller Si and Ni atoms were expected to enter the solid solution of Al during the early stage of mechanical alloying.
The formation of nanocrystalline structures during grinding can also improve the dissolution of Si and Ni in Al. The change in the above parameters was determined up to 30 hours of grinding, as Al peaks disappear with further grinding.
Morphology of Powdered Samples
EDX (Energy Dispersive X-ray) analysis was performed along with SEM to see the chemical composition of each powder particle. As a type of spectroscopy, it relies on examining a sample through interactions between electromagnetic radiation and matter. , which analyzes the X-rays emitted by matter in response to impacts with charged particles. I It was used to determine the composition of the powder particles after 50 hours of grinding and to determine whether adequate alloying was achieved or not. EDX analysis of Al75Si15Ni10 powders after 50 hours of grinding is shown in the table.
It shows atom% and weight% showing that a homogeneous chemical composition was obtained for sample milled for 50 hours. The atomic % of Si and Ni were found to be slightly greater and less, respectively, during the process of MA. 1. The mechanical alloy of Al75Si15Ni10 up to 50 hours resulted in partially amorphous structure along with some intermetallic phases.
The lattice parameter of Al-rich solid solution decreased with increase in meal time up to 30 h, indicating the progressive dissolution of Si and Ni in Al. EDX analysis revealed that a homogeneous nanostructure was formed during MA of Al75Si15Ni10 up to 50 hours. The current work leaves much room for future researchers to investigate many other aspects, such as TEM studies, to directly determine the structure formed by MA.
