Mechanical behaviour of nanocrystalline iron and nickel in the quasi-static and low frequency anelastic regime (original) (raw)
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Mechanical behavior of a bulk nanostructured iron alloy
Metallurgical and Materials Transactions a Physical Metallurgy and Materials Science, 1998
Bulk, fully dense materials were prepared from Fe-10Cu with grain diameters between 45 nm and 1.7 m. The materials were prepared by ball milling of powders in a glove box, followed by hot isostatic pressing (hipping) or powder forging. Larger grain sizes were obtained by thermal treatment of the consolidated powders. The bulk materials were relatively clean, with oxygen levels below 1500 wpm and other contaminants less than 0.1 at. pct. The mechanical behavior of these materials was unique. At temperatures from 77 to 470 K, the first and only mechanism of plastic deformation was intense shear banding, which was accompanied by a perfectly plastic stress-strain response (absence of strain hardening). There was a large tension-compression asymmetry in the strength, and the shear bands did not occur on the plane of maximum shear stress or the plane of zero extension. This behavior, while unusual for metals, has been observed in amorphous polymers and metallic glasses. On the other hand, the fine-grained Fe-10Cu materials behaved like coarse-grained iron in some respects, particularly by obeying the Hall-Petch equation with constants reasonably close to those of pure iron and by exhibiting low-temperature mechanical behavior which was very similar to that of steels. Transmission electron microscopy (TEM) studies found highly elongated grains within shear bands, indicating that shear banding occurred by a dislocation-based mechanism, at least at grain sizes above 100 nm. Similarities and differences between the fine-grained Fe-10Cu and metals, polymers, metallic glasses, radiation-damaged metals, and quench-damaged metals are discussed. I. INTRODUCTION THE mechanical behavior of bulk nanostructured metals is not well understood or documented. A large volume fraction of boundary material might cause anomalous behavior, and for this reason, nanostructured solids have been regarded as composite materials made of a crystalline ''phase'' with long-range order and a more disordered grain boundary phase. [1,2,3] Further, reports on the deformation behavior of nanostructured thin films of gold and silver have indicated that mechanisms other than dislocation motion, for example, grain boundary sliding and grain rotation, may become dominant in thin films, even at room temperature, for grain sizes of 25 nm and lower. [4,5] However, studies of bulk nanostructured metals have been limited by poor densification, grain growth during consolidation, and contamination of materials. The purpose of this study was to investigate the mechanical behavior of a fully dense, ''bulk'' nanostructured metal. The iron-copper system was selected for study in this research for several reasons. First, materials of immiscible systems such as Cu-Nb, Cu-Ta, and Cu-Fe have been
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.
Materials Science and Engineering: A, 2012
Nanocrystalline Cu-0.75 at.%Zr alloy was synthesized by high energy ball milling under cryogenic temperature. To investigate the influence of 0.75 at.%Zr addition on thermal stabilization of nanocrystalline state of Copper, milled powder was annealed up to T/T m = 0.79 for 1h in an inert atmosphere. The microstructural changes of both milled and annealed powders were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Mechanical properties were determined in terms of hardness. It was found that addition of 0.75 at.%Zr can stabilize grain size at higher temperature, i.e., ~ 32 nm at 800 o C (T/Tm = 0.79). The hardness of Cu-0.75 at.%Zr at 800 o C was found to decrease by only ~ 13% as opposed to a 65% decrease in pure copper from cryomilled condition. The thermal stability of Cu-0.75 at.%Zr system at high temperatures was attributed to the kinetic stabilization, i.e., grain boundary pinning by intermetallic phases. Thermal stability contributions were assessed by thermodynamic models elicits added Zr is not sufficient for stabilization, rather kinetic stabilization (by intermetallic pinning of grain boundary) became active at higher annealing temperature.
Grain-size dependent mechanical behavior of nanocrystalline metals
Grain size has a profound effect on the mechanical response of metals. Molecular dynamics continues to expand its range from a handful of atoms to grain sizes up to 50 nm, albeit commonly at strain rates generally upwards of 10 6 s À 1. In this review we examine the most important theories of grain size dependent mechanical behavior pertaining to the nanocrystalline regime. For the sake of clarity, grain sizes d are commonly divided into three regimes: d4 1 μm, 1 μm od o100 nm; and d o100 nm. These different regimes are dominated by different mechanisms of plastic flow initiation. We focus here in the region d o 100 nm, aptly named the nanocrystalline region. An interesting and representative phenomenon at this reduced spatial scale is the inverse Hall–Petch effect observed experimentally and in MD simulations in FCC, BCC, and HCP metals. Significantly, we compare the results of molecular dynamics simulations with analytical models and mechanisms based on the contributions of Conrad and Narayan and Argon and Yip, who attribute the inverse Hall–Petch relationship to the increased contribution of grain-boundary shear as the grain size is reduced. The occurrence of twinning, more prevalent at the high strain rates enabled by shock compression, is evaluated.
Anelasticity and structural stability of nanostructured metals and compounds
Nanostructured Materials, 1995
Internal friction and dynamic elasticity moduli on thin reeds of nanostructured Al, Fe, FeAl and Fe& intermetallics prepared by ball milling have been measured. A relaxational damping peak in pure metals at 350-500 K and a modulus increase at 450-500 K without appreciable grain growth has been detected. Moreover, magnetoelastic coupling dependent on grain size has been observed in iron. In the nanophase intermetallic aluminides a strong relaxational damping peak was observed in the 700-800 K range. These results are briefly discussed with reference to the anelastic behaviour of similar coarse grained materials.
Toward a quantitative understanding of mechanical behavior of nanocrystalline metals
Acta Materialia, 2007
Focusing on nanocrystalline (nc) pure face-centered cubic metals, where systematic experimental data are available, this paper presents a brief overview of the recent progress made in improving mechanical properties of nc materials, and in quantitatively and mechanistically understanding the underlying mechanisms. The mechanical properties reviewed include strength, ductility, strain rate and temperature dependence, fatigue and tribological properties. The highlighted examples include recent experimental studies in obtaining both high strength and considerable ductility, the compromise between enhanced fatigue limit and reduced crack growth resistance, the stress-assisted dynamic grain growth during deformation, and the relation between rate sensitivity and possible deformation mechanisms. The recent advances in obtaining quantitative and mechanics-based models, developed in line with the related transmission electron microscopy and relevant molecular dynamics observations, are discussed with particular attention to mechanistic models of partial/perfect-dislocation or deformation-twin-mediated deformation processes interacting with grain boundaries, constitutive modeling and simulations of grain size distribution and dynamic grain growth, and physically motivated crystal plasticity modeling of pure Cu with nanoscale growth twins. Sustained research efforts have established a group of nanocrystalline and nanostructured metals that exhibit a combination of high strength and considerable ductility in tension. Accompanying the gradually deepening understanding of the deformation mechanisms and their relative importance, quantitative and mechanisms-based constitutive models that can realistically capture experimentally measured and grain-size-dependent stress-strain behavior, strain-rate sensitivity and even ductility limit are becoming available. Some outstanding issues and future opportunities are listed and discussed.
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.
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.
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...
Grain Size Distribution Effect on Mechanical Behavior of Nanocrystalline Materials
MRS Proceedings, 2004
Grain size distribution effect on the mechanical behavior of NiTi and Vitroperm alloys were investigated. Yielding at significantly lower stresses than found in equiaxed counterparts, along with well defined strain hardening was observed in these nanocrystalline materials with large grains embedded in the matrix during tensile deformation at temperatures of 0.4Tm. At higher temperature the effect of grain size distribution on yield stress was not revealed while plasticity was increased in 50% in NiTi alloy with bimodal grain size structure.