Strain softening in nanocrystalline Ni–Fe alloy induced by large HPT revolutions (original) (raw)
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Scripta Materialia, 2011
The structural evolution of a nanocrystalline Ni-Fe alloy induced by high-pressure torsion (HPT) was investigated. HPT-induced grain growth occurred via grain rotation and coalescence, forming three-dimensional small-angle sub-grain boundaries. Further deformation eliminates the sub-grain boundaries from which dislocations glide away on different {1 1 1} planes. A significant number of these dislocations come together to form Lomer-Cottrell locks that effectively increase the dislocation storage capacity of the nanocrystalline material. These observations may help with developing strong and ductile nanocrystalline materials.
Dislocation density evolution during high pressure torsion of a nanocrystalline Ni–Fe alloy
Applied Physics Letters, 2009
High-pressure torsion ͑HPT͒ induced dislocation density evolution in a nanocrystalline Ni-20 wt %Fe alloy was investigated using x-ray diffraction and transmission electron microscopy. Results suggest that the dislocation density evolution is fundamentally different from that in coarse-grained materials. The HPT process initially reduces the dislocation density within nanocrystalline grains and produces a large number of dislocations located at small-angle subgrain boundaries that are formed via grain rotation and coalescence. Continuing the deformation process eliminates the subgrain boundaries but significantly increases the dislocation density in grains. This phenomenon provides an explanation of the mechanical behavior of some nanostructured materials.
Mechanism of grain growth during severe plastic deformation of a nanocrystalline Ni–Fe alloy
Applied Physics Letters, 2009
Deformation induced grain growth has been widely reported in nanocrystalline materials. However, the grain growth mechanism remains an open question. This study applies high-pressure torsion to severely deform bulk nanocrystalline Ni-20 wt % Fe disks and uses transmission electron microscopy to characterize the grain growth process. Our results provide solid evidence suggesting that high pressure torsion induced grain growth is achieved primarily via grain rotation for grains much smaller than 100 nm. Dislocations are mainly seen at small-angle subgrain boundaries during the grain growth process but are seen everywhere in grains after the grains have grown large.
High Plasticity and Substantial Deformation in Nanocrystalline NiFe Alloys Under Dynamic Loading
Advanced Materials, 2009
Bulk nanocrystalline (NC) materials (with an average grain size <100 nm) have been widely reported to exhibit high strength but disappointingly low plasticity. In this Communication, upon dynamically deforming an NC NiFe alloy, we report impressively large plastic deformation: from 8% quasi-static strain to a maximum prescribed dynamic strain of $22% without failure. This large, dynamic plastic deformation is accompanied by a much elevated yield strength (33% increase compared with quasi-static strength). Detailed postmortem microstructure analysis reveals that the dynamic deformation resulted in significant grain coarsening and de-twinning manifested by a great reduction of the twin density vis-à-vis slight grain coarsening without de-twinning in quasi-static deformation. We envisage that such mechanisms are responsible for the unique texture as compared with the conventional deformation texture as uncovered by in-depth texture analysis based on X-ray diffraction (XRD) using synchrotron radiation. Our efforts highlight potential ingenious avenues to exploit the superior behavior of NC materials under extreme conditions by invoking the favorable deformation mechanisms, such as reported herein.
Mechanical properties of nanocrystalline Ni-20%Fe alloy at temperatures from 300 to 4.2K
Materials Science and Engineering: A, 2009
Mechanical properties of the nanocrystalline Ni-20%Fe alloy (the average grain size is 22 nm) have been studied under uniaxial compression at different strain rates (from 10 −5 s −1 to 10 −1 s −1 ) and temperatures from 300 to 4.2 K. Comparison of the strength characteristics, strain and failure peculiarities of the nanocrystalline and coarse-grained structural states of Ni-20%Fe alloys is carried out. Micromechanisms of plastic deformation in this nanocrystalline alloy (including increase of the slip localization at low temperatures) are discussed.
Elemental redistribution in a nanocrystalline Ni–Fe alloy induced by high-pressure torsion
2011
An electrochemically deposited nanocrystalline supersaturated face-centred-cubic Ni-21 at.% Fe alloy with an initial average grain size of ∼21 nm was processed using high-pressure torsion (HPT) that resulted in grain growth via grain rotation and coalescence to an average grain size of ∼53 nm. Atom probe tomography investigations revealed that the supersaturated Ni-Fe solid solution was stable under HPT and that C and S atoms, which are the major impurities in the material and segregated to the grain boundaries (GBs) of the as-deposited material, migrated from disappearing GBs to the remaining GBs during HPT. We propose that the elemental redistribution was facilitated by GB diffusion and the motion of a large volume of HPT-induced defects at the GB regions during the grain growth process. This elemental redistribution process is different from other HPT-induced elemental redistribution processes reported in the literature.
High-pressure torsion-induced grain growth in electrodeposited nanocrystalline Ni
Applied Physics Letters, 2006
Deformation-induced grain growth has been reported in nanocrystalline (nc) materials under indentation and severe cyclic loading, but not under any other deformation mode. This raises an issue on critical conditions for grain growth in nc materials. This study investigates deformation-induced grain growth in electrodeposited nc Ni during high-pressure torsion (HPT). Our results indicate that high stress and severe plastic deformation are required for inducing grain growth, and the upper limit of grain size is determined by the deformation mode and parameters. Also, texture evolution suggests that grain-boundary-mediated mechanisms played a significant role in accommodating HPT strain.
Strain-Dependent Deformation Behavior in Nanocrystalline Metals
Physical Review Letters, 2008
The deformation behavior as a function of applied strain was studied in a nanostructured Ni-Fe alloy using the in situ synchrotron diffraction technique. It was found that the plastic deformation process consists of two stages, undergoing a transition with applied strain. At low strains, the deformation is mainly accommodated at grain boundaries, while at large strains, the dislocation motion becomes probable and eventually dominates. In addition, current results revealed that, at small grain sizes, the 0.2% offset criterion cannot be used to define the macroscopic yield strength any more. The present study also explained the controversial observations in the literature.
Mechanical behaviors of as-deposited and annealed nanostructured Ni–Fe alloys
Scripta Materialia, 2011
The mechanical properties of an electrodeposited nanocrystalline Ni-Fe alloy were studied as a function of the angle between the compression and columnar grain axes. Improved ductility was achieved as the applied load was along the grain column axis. After annealing at 250°C, the yield strength increased and the plastic strain dropped remarkably. However, the annealing embrittlement cannot be attributed to the impurity segregation. The roles of texture and grain boundary relaxation upon annealing on the strength and ductility of nanocrystalline alloys were discussed.
Effect of Annealing on Hardness and the Modulus of Elasticity in Bulk Nanocrystalline Nickel
Metallurgical and Materials Transactions A
Experiments on hardness and the modulus of elasticity were conducted at room temperature on samples of electrodeposited (ED) nanocrystalline (nc) Ni that were annealed at temperatures ranging from 323 to 693K (50 to 420°C). The results showed the presence of three regions: I, II, and III. In region I (300K (27°C)<T<350K (77°C)), the hardness and the elastic modulus remained essentially constant. In region II (350K (77°C)<T<500K (227°C)), both the hardness and the elastic modulus increased. In region III (T>500K (227°C)), the hardness dropped and then decreased with increasing grain size, whereas the modulus of elasticity approached a maximum plateau of ~240GPa. It is suggested that while the increase in hardness in region II can be attributed in part to the formation of annealing twins, which serve as a source of strengthening, the decrease in hardness above 500K (227°C) is due to the occurrence of significant grain growth. The increase in the modulus of elasticity wi...