Influences of grain size and twin boundary on the tensile properties of nanocrystalline face-centered cubic Cu50Ni50 alloy

To address the severe fuel crisis and environmental pollution, the use of lightweight metal materials, such as AZ alloy, represents an optimal solution. This study investigates the mechanical behavior and deformation mechanism of AZ alloys under uniaxial compressive using molecular dynamics (MD) simulations. The influence of various compositions, grain sizes (GSs), and temperatures on the compressive stress, the ultimate compressive strength (UCS), compressive yield stress (CYS), Young's modulus (E), shear strain, phase transformation, dislocation distribution, and total deformation length is thoroughly examined. The results show that although AZ91 has the highest Al content, it exhibits the lowest UCS, CYS, and fraction atoms with shear strain larger than 0.2 (FSS0.2) compared to AZ31 and AZ61. At the same time, the total dislocation length of AZ31 is the largest. The effect of GS and temperature on the mechanical response and deformation mechanism of AZ31 alloy indicates that a GS of 7.6nm is the critical value to determine the mechanical properties and deformation intensity of AZ31 alloy. Moreover, the E, UCS, and CYS values decrease gradually as temperature increases. The compressive stress, E, UCS, CYS, and FSS0.2 of single crystalline AZ31 are higher than those of polycrystalline AZ31 with all GSs. The MD simulation results show that the sample experiences the formation of stacking faults at both single crystalline and polycrystalline AZ31 while forming the shear band at the single crystalline, leading to strong oscillations in compressive stress. In contrast, polycrystalline AZ31, across all GSs, exhibits high shear strain zones, causing oscillations in compressive stress. The ATOMSK program is used to create the polycrystalline AZ structures. The MD method is employed to investigate the influence of various compositions, GSs, and temperatures on the mechanical properties and deformation mechanism of AZ alloys. All the simulations are performed by LAMMPS software. The visualization tool (OVITO) is used to inspect, analyze, and illustrate the simulation results. The EAM potential is applied to the interactions between Al-Zn, Al-Mg, and Zn-Mg.

Severe plastic deformation techniques including high-pressure torsion and equal channel angular pressing have been widely used to refine coarse-grained materials to produce nanocrystalline and ultrafine-grained materials, or manipulate the microstructure of nanocystalline materials for superior mechanical properties. This paper overviews severe plastic deformation induced structural and mechanical property evolutions on bulk nanocrystalline metals, mainly in a nanocrystalline Ni-20%Fe(mass fraction) alloy with a face-centred cubic(fcc) structure processed by high-pressure torsion to different strain values. The structural evolution and mechanical property evolution at different strain values were studied. Comprehensive characterizations on structural evolution during deformation indicate that:(1) grain growth occurred via grain rotation, and is accompanied with changes in dislocation density and twin density;(2) there is a significant grain size effect on deformation induced twinning and de-twinning. There exists an optimum grain size range for the formation of deformation twins. Outside of this grain size range the de- twinning process will dominate to annihilate existing twins;(3) different types of dislocation- twin boundary(TB) interactions occurred during deformation. Dislocation density plays an important role in dislocationTB interactions. In a twinned grain with a low dislocation density, a dislocation may react with a TB to fully or partially penetrate the TB or to be absorbed by the TB via different dislocation reactions. In a twinned grain with a high dislocation density, dislocations tangle with each other and are pinned at the TBs, leading to the accumulation of dislocations at the TBs and raising the local strain energy. In order to release the stress concentration, stacking faults and secondary twins formed by partial dislocation emissions from the other side of the TB;(4) atom probe tomography investigation reveals that C and S atoms, which are the major impurities in the Ni-Fe alloy and segregated at grain boundaries(GBs) of the as-deposited material, migrated from disappearing GBs to the remaining GBs during high-pressure torsion. Investigation on structure-hardness relationship of the Ni-Fe alloy reveals that: strain hardening and strain softening occurred at different deformation stages. Dislocation density evolution plays a major role in the hardness evolution, while other structural evolutions, including twin density and grain size evolutions, play minor roles in the hardness evolution.

Fatigue tests under rotating bending and ultrasonic loading were carried out using plain specimens with different grain sizes of Ni-base super alloy, Inconel 718, in order to investigate the effects of grain size and loading frequency on fatigue properties. Fatigue strength was increased with decreasing in grain size under both tests. Moreover, the fatigue strength under ultrasonic loading was higher than that under rotating bending. The resistance to crack initiation was larger in smaller grain sized alloy under both tests, and larger under ultrasonic loading than under rotating bending. Effects of loading frequency and grain size on crack initiation were explained from the points of view of the effects of those on flow stress. On the other hand, the effect of grain size on crack growth rate was small in both loading conditions. The crack morphology was rougher in the larger grain sized alloys, meaning that the crack growth in the larger grain sized alloys was suppressed by roughness induced crack closure effect. However, flat facets caused by twin boundary cracking and intergranular cracking were observed in the larger grain sized alloys, which inversely led to crack growth acceleration. Consequently, the effect of grain size on crack growth rate was decreased.

Influences of grain size, alloy composition, and temperature on mechanical characteristics of Si100-xGex alloys during indentation process

Effects of grain size and temperature on mechanical properties of nano-polycrystalline Nickel-cobalt alloy

The fatigue behaviour of metals is strongly influenced by the grain size. It is known that the formation of cracks due to cyclic tensile loading occurs the later the smaller the grain size of the metal is. In this work the influence of the grain size of Nickel on the contact fatigue behaviour in cyclic indentation and cyclic sliding is investigated. A (111)-single crystal, microcrystalline, ultra-fine-crystalline and nanocrystalline Nickle where examined. For the cyclic indentation a failure criterion on the basis of the measured contact stiffness was defined. A decrease of 10% in respect of the maximum value of the contact stiffness was defined as failure. The time to failure was the larger the smaller the grain size was. The loss in contact stiffness was identified to be related to the formation of cracks. Microstructural changes have been examined by focused ion beam microscopy. Due to cyclic indentation grain growth occurred in the nanocrystalline Nickel. This was more pronounced with higher load amplitude and duration. In cyclic sliding, grain growth was observed for the nanocrystalline Nickel, a slight grain coarsening took place in ultra-fine-crystalline Nickel and a distinct grain fining took place in microcrystalline Nickel. The resulting grain structure of the examined samples in the area under the cyclic sliding paths exhibited a grain size of a view 100 nm. Measurement of the hardness of the sliding paths revealed a hardening of the microcrystalline samples, a slight softening of the ultra-fine-crystalline samples and a strong softening of the nanocrystalline samples, which became more pronounced with higher number of cycles. This is in agreement with the change in grain structure. During the microscopic examination, a material deposition around the cyclically loaded areas was observed. This deposition became stronger for longer duration and maximum load. The deposition has been identified as carbon, with the source of the carbon being presumably adsorbed carbon monoxide. A comparison between a finite element simulation and a real cyclic indentation experiment was conducted for nanocrystalline Nickel. With the number of cycles the load-displacement curves differed more and more. This has been related to microstructural changes in the material. In addition experiments and a FEM-simulation where conducted to exclude the possibility of grain growth in the nanocrystalline Nickel due to a rise in temperature because of the cyclic loading. The results of the simulation indicate that there is no relevant rise in temperature. So the change in the grain structure is related to the mechanical loading condition.

Incipient plasticity and voids nucleation of nanocrystalline gold nanofilms using molecular dynamics simulation

Effects of grain and twin boundary on friction and contact characteristics of CuZrAl nanocrystallines

The effect of grain size on the deformation induced martensite transformation and mechanical properties in austenitic stainless steel with high amount of Mn was studied. a'-martensite was formed by deformation and deformation induced martensite was formed with surface relief. With increase of grain size, volume fraction of deformation induced martensite was increased. With the increase in degree of cold rolling, hardness, and tensile strength was rapidly increased with linear relationship, while, elongation was decreased rapidly and then decreased slowly. With increase of grain size, hardness and tensile strength was rapidly increased with linear relationship, while elongation was decreased rapidly. The hardness, tensile strengths, and elongation were more strongly influenced by deformation induced martensite than the grain size.

Effects of interfacial defect on deformation and mechanical properties of Cu/Ni bilayer—A molecular dynamics study

This study investigated the mechanical properties and crack propagation behavior of polycrystalline copper using a molecular dynamics simulation. The effects of temperature, grain size, and crack length were evaluated in terms of atomic trajectories, slip vectors, common neighbor analysis, the material’s stress–strain diagram and Young’s modulus. The simulation results show that the grain boundary of the material is more easily damaged at high temperatures and that grain boundaries will combine at the crack tip. From the stress–strain diagram, it was observed that the maximum stress increased as the temperature decreased. In contrast, the maximum stress was reduced by increasing the temperature. With regard to the effect of the grain size, when the grain size was too small, the structure of the sample deformed due to the effect of atomic interactions, which caused the grain boundary structure to be disordered in general. However, when the grain size was larger, dislocations appeared and began to move from the tip of the crack, which led to a new dislocation phenomenon. With regards to the effect of the crack length, the tip of the crack did not affect the sample’s material when the crack length was less than 5 nm. However, when the crack length was above 7.5 nm, the grain boundary was damaged, and twinning structures and dislocations appeared on both sides of the crack tip. This is because the tip of the crack was blunt at first before sharpening due to the dislocation effect.

Equiatomic CoCrNi is a kind of medium-entropy alloy with excellent mechanical properties and wide application potentials in aerospace, nuclear energy, biomedical engineering, etc. In this paper, we investigate the effect of grain boundaries in CoCrNi under the shear loads through molecular dynamics (MD) simulations on 21 specially designed CoCrNi bicrystal specimens with symmetric tilt grain boundaries. The MD models of CoCrNi bicrystals were constructed and simulated using the open-source software LAMMPS. Shear loads parallel to the grain boundary plane were applied to the models after they reach equilibrium. Post-processing is conducted utilising the Common Neighbour Analysis (CNA) algorithm and Dislocation Extraction Algorithm (DXA). The stress–strain curves and microstructural evolution processes were analysed. According to the stress–strain curve and microstructural evolution characteristics, four types of deformation modes were identified, each corresponding to a specific range of misorientation angles. This study provides unique insights into the plastic deformation mechanisms of medium-entropy alloys.

In this work, nanoindentation on a (110) crystal plane with a spherical indenter and (111) twin boundaries at different distances was simulated using molecular dynamics. In addition, the load–displacement curves and mechanical properties were calculated, and the deformation mechanism of the nickel matrix was analysed using a dislocation extraction algorithm (DXA). The results showed that the load decreased in the load–displacement curve, which was caused by the initial nucleation of the dislocations, and the twinning boundary hindered dislocation propagation. Furthermore, Young’s modulus values near the twin boundary were lower than those farther away, and the maximum shear stress near the twin boundary was lower. Therefore, dislocation activity in the nickel matrix during indentation was mainly in the form of Shockley partial dislocations.

The influences of impurity levels, grain size, and tensile strength on in-service temper embrittlement of CrMoV steels have been investigated. The samples for this study were taken from several steam turbine CrMoV rotors which had operated for 15 to 26 years. The effects of grain size and tensile strength on embrittlement susceptibility were separated by evaluating the embrittlement behavior of two rotor forgings, which were made from the same ingot, after giving an extended step-cooling treatment. The results reveal that among the residual elements in the steels, only P produces a significant embrittlement. The variation of P and tensile strength of the steels in the ranges investigated has no effect on in-service temper embrittlement susceptibility, as measured by the shift in fracture appearance transition temperature (FATT). However, the prior austenite grain size plays a major role on in-service embrittlement. The fine grain steels with a grain size of ASTM No. 9 or higher are virtually immune to in-service embrittlement. In steels having duplex grain sizes, the embrittlement susceptibility is controlled by the size of coarser grains. For a given steel chemistry, the coarse grain steel is more susceptible to in-service embrittlement, and a decrease in ASTM grain size number from 4 to 0/1 increases the shift in FATT by 61°C (110°F). It is demonstrated that long-term service embrittlement can be simulated, except in very coarse grain steels, by using the extended step-cooling, treatment. The results of step-cooling studies also show that the coarse grain rotor steels take longer time during service to reach a fully embrittled state than the fine grain rotor steels. This difference in the kinetics of embrittlement is believed to be related to the variations in Mo content in the matrix and the grain size of the steels.

Effects of grain size from micro scale to nanoscales on the yield strain of brass under compressive and tensile stresses using a Kelvin probing technique