Showing papers by "Mingwei Chen published in 2002"

High tensile ductility in a nanostructured metal.

Abstract: 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.

Enhanced tensile ductility and toughness in nanostructured Cu

Abstract: Pure copper with ultrafine grain sizes and nanoscale subgrain (dislocation) structures was prepared by using severe plastic deformation through cold rolling at subambient temperatures, with or without subsequent recovery annealing. We report coexisting high strength and tensile ductility (large elongation to failure and ductile fracture). Factors leading to the simultaneous strengthening and toughening with increasing cold deformation and microstructural refinement are discussed.

Nanocrystalline grain structures developed in commercial purity Cu by low-temperature cold rolling

Abstract: Nanocrystalline pure copper was obtained by cold rolling a commercial bulk Cu to very large extensions at subambient temperatures. The eventual formation of nanocrystalline grain structures is attributed to dynamic grain refinement (recrystallization) mechanisms activated by the low-temperature continuous plastic deformation that leads to ultrahigh densities of dislocations.

Annealing embrittlement of Al89Fe10Zr1 amorphous alloy

Abstract: The mechanism of annealing embrittlement of A1 89 Fe 10 Zr 1 amorphous alloy is studied. The activation energies for the ductile–brittle transition ( E b ) and the precipitation of fcc-Al ( E x ) are measured according to Kissinger plot and Arrhenius plot. Their values are E b =168–l79 kJ mol −1 and E x =173–204 kJ mol −1 , respectively. The correlation between the ductile–brittle transition and the precipitation of fcc-Al means that they are the same thermally activated processes. The analysis of solute concentrations in the alloy using atom probe field ion microscope shows that the precipitation of fcc-Al causes the increase of Fe and Zr concentrations in the residual amorphous phase during the annealing process. This is the main reason for the ductile–brittle transition.