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Examining, Analyzing, Interpreting, and Understanding the Fracture Resistance of High Entropy Alloys
Published in T.S. Srivatsan, Manoj Gupta, High Entropy Alloys, 2020
At the later stage of deformation, 12<110> type perfect dislocations start to move, but in the form of small segments with extremely low speeds. Due to the difficulty in motion, perfect dislocations tend to form close-packed dislocation arrays and move in localized bands, ultimately causing planar slip. The localized band of the planer slip and the slow-moving perfect dislocation segments inside act as impediments to the fast-moving partial dislocations. Partial often cease to move and start to pile up once they run into a perfect dislocation band. The hindrance to the motion of partial dislocations is deleterious to ductility but it boosts the strong interaction of the perfect and partial dislocations, yielding important strain hardening to plastic deformation. In a nutshell, during the entire process of plastic deformation, the fast motion of partial dislocations, the formation of stacking fault parallelepipeds, and partial dislocation/perfect dislocation interactions work jointly and coordinately to engender the high fracture toughness value of over 200 MPam in the CrMnFeCoNi HEAs [53].
Fracture Simulations Using Molecular Dynamics (MD)
Published in Snehanshu Pal, Bankim Chandra Ray, Molecular Dynamics Simulation of Nanostructured Materials, 2020
Snehanshu Pal, Bankim Chandra Ray
The orientation of the materials with respect to the crack plays a vital role in determining the plasticity of the material. The crack propagation behavior of the SC metallic systems with different orientations has been presented in Figure 7.13 [28]. In case of orientation I, that is, the crack propagation, can be divided into two stages, and the crack propagation takes place in the <1 0 0> direction. In the first stage, crack propagation is rapid, the crack tip remains sharp, and no dislocation emissions are witnessed in the crack tip, indicating brittle behavior. However, in the second stage, partial dislocation emission eventuates, thereby blunting the crack shape and generating stacking faults, thus transitioning from brittle to ductile fracture behavior. As a result, the stiffness of the material also drops in the second stage as compared with the first stage. However, in case of orientation II dislocation, emission occurs faster than the crack propagation, thereby leading to plastic behavior right from the beginning of the deformation process, leading to the blunting of the crack. Dislocation motion is more active in case of blunted tip as compared with the sharp tip. Extensive formation of stacking faults in case of orientation II leads to an interaction of stacking faults; as a result, the atoms corresponding to the intersection of stacking faults serve as a potential site for void nucleation. Under such conditions, in addition to atomic bond cleavage, void nucleation growth and coalescence also play a crucial role in the damage mechanism of the SC metallic system. Such observations are generally witnessed in case of ductile materials. However, in case of orientation III, twin partial dislocation emission is preceding to crack propagation. The twin partial dislocation has identical Burgers vector with the Shockley partial dislocation a/6〈112〉. Dissimilar Shockley partial which leaves the stacking faults behind and an array of twin partial dislocations will form a micro twin behind, typically with several layers of (1 1 1) plane atoms. As opposed to dislocation, the formation of micro twins does not impede the crack propagation effectively. In a nutshell, the emission of dislocations at the crack tip will result in blunting of the crack, thereby slowing down the crack propagation process. However, the formation of micro twins does not have any such beneficial effect on the delaying of the crack propagation. The dislocation process greatly influences the brittle versus ductile behavior of materials. The nucleation of dislocation can be marked as the onset of brittle-to-ductile transition for metallic system, whereas for covalent materials, ductility is governed by the mobility of the dislocations.
Small-scale deformation behaviour of the AlCoCrFeNi2.1 eutectic high entropy alloy
Published in Philosophical Magazine, 2022
Shailesh Kumar Singh, Govind Kumar, Pokula Narendra Babu, Snehanshu Pal, Saurabh Vashistha, M. S. Azam, Saurabh Dixit
The plastic deformation behaviour of AlCoCrFeNi2.1 HEA is investigated at small length scale using the nanoindentation technique and via MD nanoindentation simulation. It helps to identify the phase-specific contribution to the mechanical load. Nano-indentation is a very efficient and effective technique to study the elasto-plastic behaviour of materials. The microstructure is comprised of eutectic lamellae L12 phase and B2 phase. It is observed both experimentally and through computational study that B2 phase exhibited higher hardness than L12 phase. The partial dislocations such as Hirth and Stair-rod partials increase the hardness of the specimen by strain hardening phenomena because these partial dislocations have sessile nature, and these dislocations are examined clearly with dislocation analysis. The B2 phase showed higher wear resistance compared to the L12 phase. The L12 phase is deformed by forming multiple slip lines and significant material pile up during small scale. Proper knowledge of the mechanical properties of a multi-phase material is vital for advanced material design and application.
Designing damping capacity in high strength Fe–Mn based alloys by controlling crystal defect configurations
Published in Philosophical Magazine, 2021
Ji Zhang, Yongning Wang, Qiang Luo, Huabei Peng, Yuhua Wen
To further improve the damping capacity especially under low amplitude loading situations, we investigated the effects of different thermal–mechanical treatments on the damping capacity in a cold-drawn Fe–17.5Mn–0.022C alloy, including the quenchening temperature, ageing temperatures and time, and the combinations of ageing and deformation. The main conclusions to be drawn from the present work were as follows: The combination of ageing and then deformation at room temperature could improve the damping capacity in the Fe–Mn alloys more remarkably than only the ageing or deformation could, especially at low amplitudes. The δ at the strain amplitudes of 4 × 10−4 and 6.5 × 10−4 in the quenched alloy increased by 176% and 74% after ageing and 4% deformation, respectively. The reason can be ascribed to the depinning of the Shockley partial dislocations due to the atoms segregation and the vacancies.The atoms segregation to stacking faults pinned the partial dislocations more strongly than the vacancies did. The strong carbide-forming elements, such as Ti, Nb and V, should be added into the high strength Fe–Mn based alloys to suppress the deterioration in the damping capacity during the service induced by the atoms segregation to stacking faults.
Synthesis and modelling of the mechanical properties of Ag, Au and Cu nanowires
Published in Science and Technology of Advanced Materials, 2019
Nurul Akmal Che Lah, Sonia Trigueros
Nanoindentation is an efficient method to quantify mechanical properties of nanowires within the classical elasticity theory. The time-dependent deformation behaviour of penta-twinned Ag nanowires, which is related to stress relaxation for loading and complete strain recovery for unloading condition, has also been reported. The strain hardening test is performed using a MEMS-based testing system where the load is applied using a thermal actuator and is measured through the differential capacitive sensor on the other side. Carbon is used for displacement markers on the nanowires [3]. Both loads and displacement are measured simultaneously and accurately [232–234]. As the load on the nanowire specimen is decreased, the specimen elongation is increased. The temperature at the end of specimen is usually higher as compared to other areas as the stress is accumulated at the end of the specimen (Figure 6(a,b)) [204]. During the recovery process, the actuator is turned off, and the specimen is retracted to the initial position (Figure 6(c)). The stress relaxation typically emerged from the nucleation of initial partial dislocations created by vacancy diffusion. Meanwhile, the complete strain recovery occurred from the reverse movement of partial dislocations exerted by the repulsive force both from the twin boundaries and the intrinsic stress field of the fivefold twin.