Bulk nanostructured materials from severe plastic deformation

Mechanical properties of nanocrystalline materials

Principles of equal-channel angular pressing as a processing tool for grain refinement

Nanostructured materials: basic concepts and microstructure☆

Nanocrystalline metals--with grain sizes of less than 100 nm--have strengths exceeding those of coarse-grained and even alloyed metals, and are thus expected to have many applications. For example, pure nanocrystalline Cu (refs 1-7) has a yield strength in excess of 400 MPa, which is six times higher than that of coarse-grained Cu. But nanocrystalline materials often exhibit low tensile ductility at room temperature, which limits their practical utility. The elongation to failure is typically less than a few per cent; the regime of uniform deformation is even smaller. Here we describe a thermomechanical treatment of Cu that results in a bimodal grain size distribution, with micrometre-sized grains embedded inside a matrix of nanocrystalline and ultrafine (<300 nm) grains. The matrix grains impart high strength, as expected from an extrapolation of the Hall-Petch relationship. Meanwhile, the inhomogeneous microstructure induces strain hardening mechanisms that stabilize the tensile deformation, leading to a high tensile ductility--65% elongation to failure, and 30% uniform elongation. We expect that these results will have implications in the development of tough nanostructured metals for forming operations and high-performance structural applications including microelectromechanical and biomedical systems.

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High tensile ductility in a nanostructured metal.

The objective of this study was to assess, for the first time, the potential impact of ``Grain Boundary Design and Control`` on the bulk sensitization and intergranular corrosion resistance of a commercial corrosion resistant nickel-based austenitic alloy: Alloy 600 (UNS N06600). Increasing the special grain boundary ({Sigma} {le} 29) frequency in thermomechanically processed Alloy 600 from 37% to 71% has been shown to result in commensurate decreases in bulk intergranular corrosion susceptibility in both the solution annealed and sensitized condition; these findings being attributed to both the intrinsic corrosion resistance, and resistance to solute segregation and precipitation exhibited by structurally-ordered low {Sigma} grain boundaries. These results provide considerable promise for the practical application of grain boundary design and control considerations in the general field of corrosion prevention and control.

Influence of grain boundary character distribution on sensitization and intergranular corrosion of alloy 600

Nanocrystalline materials, having a crystal size less than ~10 rim, have been shown to possess unusual properties [1-3]. These properties are primarily the result of a substantial fraction of the atoms (20 50%) lying in intercrystalline regions [3]. On the basis of x-ray scattering [4], EXAFS [5], hydrogen solubility [6], small angle neutron scattering [7], and self diffusivity measurements [8] conducted on bulk nanocrystalline materials, it has been suggested [3] that the grain boundaries in these materials are more disordered than those in conventional polycrystals. Recent studies[9,10] involving direct observation of nanocrystalline interfaces by HREM, provide contradictory results. Wunderlich and co-workers[9] have shown that interfaces in nanocrystalline Pd show an 'extended' structure not typically observed in conventional systems. However, Thomas et al [10] observed that the interfacial structure of nanocrystalline Pd is consistent with that typically observed in coarse-grained materials.

On the contribution of triple junctions to the structure and properties of nanocrystalline materials

The possibility of a dislocation mechanism in the deformation process of nanocrystalline materials is reviewed and analyzed. The present theoretical calculation, by taking the anisotropic characteristic of crystallographic symmetry and different choices of critical shear strength into account, results in a reasonable limit in grain size for applying dislocation pile-up theory to nanocrystalline materials. The deviation from the Hall—Petch relationship is rationalized in terms of a small number dislocation pile-up mechanism. A composite model is proposed to evaluate the strength of nanocrystalline materials. It is shown that this model can be used for interpreting the various cases observed in Hall—Petch studies. An analytical expression for assessing the creep rate of nanocrystalline materials by a diffusion mechanism, including triple line diffusion, is derived. It is predicted that the creep rate due to triple line diffusion will exhibit a stronger grain size dependence than that due to grain boundary diffusion.

Effect of grain size on mechanical properties of nanocrystalline materials

The effect of grain size on the wear properties of electrodeposited nanocrystalline nickel coatings

2,106,004 1/1938 Inglee .................................. 204/300 R 2,764,540 9/1956 Farin et al. .......................... 204/1297 3,103,235 9/1963 Stringham ................................. 138/97 3,125,464 3/1964 Harmes ................................... 18/105 3,287,248 11/1966 Braithwaite ............................. 204/260 3,618,639 11/1971 Daley et al. ............................ 137/28 3,673,073 6/1972 Tobey et al. ............................ 204/226 3,804,725 4/1974 Haynes .................................... 205/104 4,080,268 3/1978 Suzuki et al. ........................... 205/131 4,120,994 10/1978 Inoue ...................................... 427/239 4,200,674 4/1980 Inoue ...................................... 427/290 4,227,986 10/1980 Loqvist et al. .......................... 204/209 4,280,882 7/1981 Hovey ....................................... 205/50

K.T. Aust

Papers

Influence of grain boundary character distribution on sensitization and intergranular corrosion of alloy 600

On the contribution of triple junctions to the structure and properties of nanocrystalline materials

Effect of grain size on mechanical properties of nanocrystalline materials

The effect of grain size on the wear properties of electrodeposited nanocrystalline nickel coatings

Deviations from hall-petch behaviour in as-prepared nanocrystalline nickel